Silviculture

Abstract

Silviculture is the art and science of managing forest stands to meet landowner goals and objectives; traditional examples of goals and objectives include managing for timber production, improved wildlife habitat, fuels reduction, and maintenance or improvement of forest health.

20.1 Introduction

Silviculture is the art and science of managing forest stands to meet landowner goals and objectives (see Box 20.1); traditional examples of goals and objectives include managing for timber production, improved wildlife habitat, fuels reduction, and maintenance or improvement of forest health. Within forest health, objectives often involve mitigating negative impacts of forest insects while recognizing the beneficial role of insects in provision of ecosystem services. Goals tend to be broad, encompassing perspective on desired conditions at large scales, such as the forest or landscape. Objectives are more specific, and often target specific outcomes (e.g. reduced levels of insect-caused mortality following treatment) and are typically focused at the stand-scale. In this chapter, we have focused on the stand-scale unless explicitly noted otherwise. Silviculture, through prescriptions and treatment implementation (see Box 20.1 for definitions) can be used to manipulate the species composition, vertical and horizontal structure of the stand, individual and stand-level tree vigor, and numerous other stand characteristics that might influence susceptibility to insects. Numerous silvicultural treatments exist (e.g. prescribed fire); however, mechanical removal of trees is perhaps the most common association people make with silvicultural treatments to meet management objectives. The outcomes targeted by silvicultural prescriptions will depend upon the site, existing stand characteristics, specific insect(s) of concern, and any other management objectives.

Box 20.1: Silviculture definitions used in this chapter. From The Dictionary of Forestry (Helms 1998) unless otherwise indicated

Term	Definition
Silviculture	The art and science of controlling the establishment, growth, composition, health, and quality of forests and woodlands to meet the diverse needs and values of landowners and society on a sustainable basis
Silviculture Prescription	A planned series of treatments designed to change current stand structure to one that meets management goals
Silvicultural Treatment	A management intervention conducted to achieve desired goals (definition by authors)
Stand	A contiguous group of trees sufficiently uniform in age-class distribution, composition and structure and growing on a site of sufficiently uniform quality to be a distinguishable unit
Even-aged stand	A stand of trees composed of a single age class
Uneven-aged stand	A stand of trees of three or more distinct age classes, either intimately mixed or in small groupings
Multi-aged stand	A stand of trees with two or more distinct age classes
Regeneration	Seedlings or saplings existing in a stand
Residual tree(s)	A tree or snag remaining after an intermediate or partial cutting of a stand
Stand density	A quantitative measure of stocking expressed either absolutely in terms of number of trees, basal area, or volume per unit area or relative to some standard condition
Stand development	Changes in forest stand structure over time
Stand structure	The horizontal and vertical distribution of components of a forest stand including the height, diameter, crown layers, and stems of trees, shrubs, herbaceous understory, snags, and down woody debris
Intermediate treatment	Any treatment or tending designed to enhance growth, quality, vigor and composition of the stand after establishment or regeneration and prior to final harvest
Thinning	An intermediate treatment made to reduce stand density of trees primarily to improve growth, enhance forest health, or recover potential mortality. Variations on the most common types of thinning (defined below) are common •Low thinning: removal of trees in the suppressed/overtopped crown class in order to favor those in the upper crown classes. Syn thin from below •Mechanical thinning: thinning of trees involving removal of trees in rows, strips or by using fixed spacing intervals. Syn geometric thinning •Crown thinning: removal of trees from the dominant or co-dominant crown classes in order to favor the best trees of those same crown classes •Dominant thinning: removal of trees in the dominant crown class in order to favor the lower crown classes. Syn selection thinning; thin from above
Sanitation cutting	The removal of trees to improve stand health by stopping or reducing the actual or anticipated spread of insects and disease
Salvage cutting	The removal of dead trees or trees damaged or dying because of injurious agents other than competition, to recover economic value that would otherwise be lost
Regeneration method	A cutting procedure by which a new age class is created. Traditional methods are: •Clearcut: the cutting of essentially all trees, producing a fully exposed microclimate for the development of a new age class •Coppice: All trees from the previous stand are cut and the majority of regeneration is from sprouts or root suckers. Syn. clearfell •Seed tree: the cutting of all trees except for a small number of widely dispersed trees retained for seed production and to produce a new age class in fully exposed microenvironments •Shelterwood: the cutting of most trees, leaving those needed to produce sufficient shade to produce a new age class in a moderated environment. Modifications are numerous, and include group shelterwood with non-uniform spacing of residual trees post-harvest and shelterwoods with reserves, in which the residual trees are not removed, creating a two-aged stand •Group Selection: trees are removed and new age classes are established in small groups •Single tree selection: individual trees of all size classes are removed more or less uniformly throughout the stand, to promote growth of remaining trees and to provide space for regeneration

The goals of this chapter are:

• 1. To identify broad approaches and specific silvicultural strategies and tools managers can use to alleviate or prevent forest insect problems such as mortality or reduced growth and vigor; and

• 2. To discuss the impact of silvicultural strategies and tools on forest structure, stand development, and other biotic and abiotic factors as well as forest insect population dynamics.

20.2 Silvicultural Strategies for Management of Forest Insects

From a silvicultural perspective, managing forest insects can be considered in two broad approaches: (1) those that increase resistance, and/or (2) those that increase resilience (DeRose and Long 2014; Table 20.1). Resistance is the ability of a system to withstand change; that is, a resistant forest stand will have the same condition, structure, and species composition before and after a disturbance (Walker et al. 2004). Resilience is the ability of a system to change but maintain its basic attributes; a resilient forest stand subjected to disturbance will return to conditions similar to those present prior to the disturbance but may have changes in structure (Walker et al. 2004). A more entomological perspective would place silvicultural strategies into the categories of reducing susceptibility or vulnerability along with increasing regeneration potential (Muzika and Liebhold 2000). This chapter takes the silvicultural perspective in terminology, but the underlying theoretical basis for treatments between the two perspectives is highly compatible.

Table 20.1 Approaches to increasing resistance and resilience using silvicultural strategies to adjust vertical and horizontal stand structure, their impact on residual trees and potential impact on bark beetle and defoliator damage. Assumes species composition is not altered during treatment. See Box 20.1 for definitions

Strategies designed to increase stand resistance focus on the influence of structure and species composition on the potential severity of a given insect disturbance (DeRose and Long 2014). Severity is principally determined by how much mortality or die-back is associated with an insect outbreak. Strategies to increase stand resilience are longer-term and focus on how the disturbance influences stand structure and species composition (DeRose and Long 2014). Silviculture can be used in both approaches to mitigate anticipated negative impacts, with prescriptions based on characteristics of, and predictions for, individual stands.

Resistance and resilience strategies can be applied separately or as complementary short- and long-term treatments to ensure that live trees remain in a stand over longer time horizons. For example, the spruce beetle (Dendroctonus rufipennis) in the western United States tends to target mature overstory spruce (Picea spp.) and may cause extensive mortality in stands dominated by large spruce (>90%) (DeRose and Long 2007, and references therein). In the short-term, reducing overstory density may increase resistance of existing trees to spruce beetle attack, thus maintaining similar stand conditions by preventing extensive overstory mortality. Over a longer time period, resilience is necessary to maintain healthy stand conditions. Windmuller-Campione and Long (2015) defined resilience of spruce-fir stands to spruce beetle outbreaks as adequate stocking of Engelmann spruce (Picea engelmannii) regeneration following an outbreak. Resilience is provided through the use of young spruce to replace overstory spruce trees lost during the outbreak, providing for live trees in the stand over a long time period. Silviculture can be used proactively (prior to an outbreak) to create conditions conducive to Engelmann spruce regeneration, thus increasing long-term resilience.

Silvicultural treatments are also commonly categorized according to whether they target direct or indirect control of forest insects, primarily in bark beetle management (Fettig et al. 2014). Direct control strategies are meant to immediately reduce current insect populations. Indirect strategies focus on proactive management meant to reduce the potential for future tree damage. Most silvicultural strategies are indirect and consequently the primary focus of this chapter. However, a few common direct control tools are identified where appropriate.

20.2.1 Structural Strategies

Silvicultural strategies that adjust the vertical or horizontal stand structure can target both increased resistance or resilience at the stand-scale. Such strategies attempt to reduce the potential for large-scale insect infestations and can include a number of silvicultural treatments that result in a wide range of vertical and horizontal stand structures. Adjustments to vertical and horizontal stand structure can be effective because some stand structures are more susceptible to damage from forest insect pests. Silviculture can be used to shift stand structures from more susceptible to less susceptible states. Susceptible stand structures vary depending upon the insect pest species, corresponding tree host species characteristics and underlying site conditions.

Two common guilds of forest insect pests are bark beetles (see Chapter 10, Bark Beetles) and foliage feeders (defoliators; see Chapter 9, Foliage Feeders). Susceptible forest structures associated with damage by some of the most damaging agents within these guilds can be quite different, leading to trade-offs between structures: a structure that creates resistance or resilience to a bark beetle may lessen these attributes when considering a defoliator, for example. It is important to understand the mechanisms driving these relationships and why shifting structures can be an effective management strategy.

Bark beetles need to successfully find host trees and overcome tree defenses; they also require bark with thick enough phloem to complete their development and ensure reproductive success. Some bark beetle species require relatively large trees as hosts (e.g. mountain pine beetle (Dendroctonus ponderosae) in lodgepole pine (Pinus contorta)) while other bark beetle species need smaller diameters to successfully complete their life cycle (e.g. pine engraver (Ips pini) in ponderosa pine (Pinus ponderosa)). Defoliators need to find appropriate host trees, but some species are limited to relatively short distance dispersal, often by wind and gravity from upper to lower crowns or trees, or by crawling between individual trees. Hence, complex, multilayered vertical structures are more conducive to defoliator success than simple, single canopy layers. Conversely, bark beetle populations are favored by simple structures of even-sized trees.

Silvicultural treatments that remove trees alter stand structure immediately, and the indirect control of insect damage is based on changes to the microclimate within the stand and the ability for insects to find appropriate host trees. Microclimatic changes include disruption of the chemical signals used by insects to find host trees and mates (Progar et al. 2014) and changing individual tree microclimates enough to reduce the suitability of host trees (e.g. by creating warmer conditions along the tree bole). Microclimates within the stand may also be altered enough to affect insect success. For example, increased temperatures or insolation may result in increased mortality during the dispersal phase and/or the early larval stage. In order to reduce the ability of insects to find appropriate host trees, managers can reduce the number of host trees available, change the average tree size, and/or create a vertical or horizontal structure not conducive to successful host location by the insect (Fettig et al. 2014).

Tree vigor in general refers to the overall health of trees, and can be assessed qualitatively, by visually rating tree crowns (Miller and Keen 1960) or quantitatively, by comparing growth rates of trees to each other and their potential to succumb to insect attack. Quantitative assessments of tree vigor require additional field measurements, and may be assessed along with qualitative crown ratings, typically through the use of tree cores to measure annual or periodic basal area growth, sapwood area (water conducting tissue) and density or size of defensive structures (resin ducts) (Kane and Kolb 2010). While early research often related sapwood area to leaf area (photosynthetic capacity of the tree) to define vigor (Waring and Pitman 1980), other researchers have found a simple measure of basal area increment adequately captures individual tree vigor (defined by increased resin flow) (McDowell et al. 2007). Trees that produce less sapwood per unit leaf area typically require fewer bark beetle attacks for successful colonization (Waring and Pittman 1980) and Mitchell et al. (1983) related this to stand density, finding that reducing tree density was an effective method for increasing relative resistance to bark beetle attack by increasing tree growth per unit of leaf area. Ultimately, silviculture can shift stand structure to increase resistance and/or resilience, with the underlying cause of the increase likely a combination of multiple factors working together (Fig. 20.1).

Fig. 20.1

Susceptibility to defoliation damage increases as structural complexity increases, from A through C. From Brookes et al. (1987)

Silvicultural treatments to reduce structural complexity include thinning from below and traditional even-aged regeneration methods (Table 20.1). Silvicultural strategies to reduce defoliation and its impacts have not been researched as thoroughly as strategies for bark beetle management and damage mitigation. The lack of empirical studies documenting post-treatment reductions in defoliation and/or defoliation damage means treatment effects are largely hypothetical, based on expected stand responses (Muzika and Liebhold 2000). Additionally, increasing tree vigor through density reduction may not alleviate defoliation severity, but may enable trees to recover more quickly following defoliation (Fajvan and Gottschalk 2012). A wide variety of traditional and modified silvicultural treatments are used to alter vertical and horizontal stand structure, many of which are identified, along with the anticipated impact of treatment on bark beetle and defoliator damage (Table 20.1). Additionally, each strategy is placed into either the resistance or resilience approach.

Traditional thinning results in a regular spatial pattern, creating similar spacing between residual trees. This pattern may be more resilient to bark beetle outbreaks from a tree vigor perspective, than leaving trees irregularly spaced where inter-tree competition remains high within groups of trees. However, inter-tree distance can also influence microclimate and negatively affect dispersal, and mate- and host-finding ability; a factor to consider when designing thinning regimes and spatial patterns of residual (leave) trees.

Much research has focused on the use of thinning to prevent bark beetle outbreaks in the United States, and the majority of research indicates that thinning can be effective at reducing tree mortality during outbreaks (i.e. thinned stands have less mortality than denser, unthinned stands) (Fettig et al. 2007). Dense, unthinned stands are generally considered to be at high hazard of bark beetle infestation and subsequent tree mortality, and hazard rating systems include metrics such as stand basal area or trees per unit area as an indicator variable. While thinning may reduce the probability of future mortality from bark beetles in most conifer species, some tree mortality should be expected when bark beetle populations rise to very high levels and pressure on the stand is high, except at low to moderate stand densities (15–20 m² ha⁻¹) (McGregor et al. 1987; Schmid and Mata 2005; Hansen et al. 2010). However, different bark beetle species, sites, and host species may have different thresholds. For example, stand susceptibility to southern pine beetle (Dendroctonus frontalis) decreases when stands are thinned to under 7.5 m² ha⁻¹ basal area (Nowak et al. 2008 and references within, Nowak et al. 2015). Additionally, bark beetle mortality may create conditions more resilient to future outbreaks by increasing the proportion of unfavorable size classes or host species (Kashian et al. 2011). Ultimately, the reduction of stand density to a critical threshold that is site and species specific is more important than whether silviculture, bark beetles, or some other damaging agent causes the density reduction. In stands impacted by defoliators, thinning can improve the ability of defoliated trees to recover to previous rates of growth (Fajvan and Gottschalk 2012).

Regeneration methods fall into both the resistance and resilience categories given their effects on the overstory and understory over both short- and long-term time frames (Table 20.1). Most even- and uneven-aged regeneration methods reduce overstory density and stand susceptibility while providing for regeneration, which is not an objective of intermediate treatments, including thinning (see Box 20.1 for definitions). The exceptions are clearcuts, which reduce density to zero, do not increase vigor because no overstory trees remain, alter the microclimate dramatically, and provide for regeneration when implemented correctly. Traditional seed tree and shelterwood regeneration methods result in the same stand structure as a clearcut, and all three eliminate the potential for bark beetle-caused mortality until the newly regenerated trees reach a susceptible size.

Even-aged regeneration methods can be modified (e.g. group shelterwood or any even-aged system with reserves; Table 20.1) to provide additional structure by leaving residual overstory trees. These trees would have increased vigor and experience an altered microclimate, both factors which can influence bark beetle attacks. These methods result in two-aged or multi-aged stands (Table 20.1) and can also be resistant and resilient to bark beetle outbreaks. The large overstory trees will be at a low density and, depending on spatial pattern, spaced at a distance far enough from each other to reduce inter-tree competition and create conditions less conducive to successful insect mating, dispersal, and host-finding. Until the youngest age class reaches a susceptible size and density, extensive mortality from bark beetles is unlikely. Regeneration methods can also be used to enhance development of a new age class of trees, creating long-term resilience by providing for young trees if bark beetles kill the overstory (Windmuller-Campione and Long 2015). Group shelterwood methods may be useful in promoting such resilience in spruce stands dominated by large diameter, even-aged trees. These stands are highly susceptible to spruce beetle, which is a particularly aggressive bark beetle that may kill the entire overstory during an outbreak. Prior to an outbreak, implementing a group shelterwood to create conditions for a new spruce age class in the understory results in a stand that will have live trees, albeit young and small, following the outbreak (Windmuller-Campione and Long 2015).

Other insects less common than bark beetles and defoliators can also cause stand-scale damage. White pine weevils (Pissodes stobi) infest the leaders of seedlings, resulting in multiple forks and stem deformities. White pine weevils are most abundant in open areas that promote higher temperatures in the understory and thicker leader diameters in seedlings (Ostry et al. 2010; Pitt et al. 2016). Group shelterwood or shelterwood with reserves methods (Table 20.1) that leave the residual trees intact can be used to successfully regenerate eastern white pine (Pinus strobus) while mitigating white pine weevil damage. The overstory cover provided through these systems (50–75% full sunlight or up to 26 m² ha⁻¹) provides enough cover to moderate the microclimate and reduce eastern white pine regeneration leader diameters, thus reducing damage from the white pine weevil (Stiell and Berry 1985; Pitt et al. 2016).

Multi-aged regeneration methods can result in structures that are both resistant and resilient to bark beetle outbreaks due to the vertical complexity that results (O’Hara 2014). However, resistance may vary across the stand, as a complex horizontal structure can also result in dense groups of trees that are competing heavily under a similar microclimate as pre-treatment. Such pockets of trees may remain susceptible to bark beetle attack. However, Kollenberg and O’Hara (1999) found multiaged stands tended to have higher leaf area indices and basal area increment compared to even-aged stands.

The benefits of structural complexity and the overall increased resistance and resilience are likely to outweigh the consequences of small-scale pockets of lower vigor trees. In uneven-aged, single-species stands, treatments that reduce density only marginally are not likely to alter the microclimate or tree vigor enough to reduce bark beetle hazard and may have the opposite effect. For example, a low thinning that removes only suppressed/overtopped trees increases average tree size—a factor that could increase bark beetle hazard. However, if the stand is being converted from a simple structure to a more complex structure, resistance and resilience will increase to bark beetles while decreasing to defoliators following harvest. The opposite would be expected if a stand is shifted from a more complex structure to a simplified vertical and / or horizontal structure. It therefore requires a careful balancing of objectives to arrive at a vertical and horizontal structure that is both resistant and resilient to bark beetles and defoliators while also meeting other objectives, such as timber production or fire hazard reduction. In the western United States, timber production is becoming less of a societal value and healthy forested landscapes resilient to large-scale mortality events that provide biodiversity and wildlife habitat are taking precedence. In these forests, reducing overall stand density to a low basal area (~35% of carrying capacity) has the potential to meet these new objectives without creating increased insect susceptibility or wildfire hazard.

Sanitation is an intermediate treatment and direct control approach used to reduce insect population levels in a stand (Box 20.1). The objective of sanitation treatment is to improve stand health by removing trees infested or likely to be infested by insects. Controlling a bark beetle population using sanitation is not considered a viable option, with the exception of the southern pine beetle. Spot infestations (Fig. 20.2) of southern pine beetle can be controlled, thus avoiding a landscape-scale outbreak, using either cut-and-remove or cut-and-leave strategies. If trees can be removed and handled appropriately following removal from the site, cut-and-remove strategies are preferred (Fig. 20.3; Fettig et al. 2007). However, cut-and-leave strategies, in which cut trees are left onsite, can also be effective and do not appear to increase the hazard of attacks in nearby trees (Fettig et al. 2007 and references therein).

Fig. 20.2

Spot infestation of southern pine beetle, from above (left) and below (right). Modified from Asaro et al. (2017)

Fig. 20.3

Illustration of an expanding spot infestation (A) and the cut-and-leave sanitation treatment implemented to control southern pine beetle (B). From Fettig et al. (2007)

Southern pine beetle outbreaks have decreased in frequency since the 1950’s despite a concomitant increase in the acreage of pine plantations. One hypothesis related to the decline in outbreaks is that intensive silviculture practices have resulted in less susceptible stands (lower density, higher average tree vigor) than were present in earlier decades (Asaro et al. 2017). Widespread use of sanitation strategies may also have a role in outbreak frequency reduction, as cut-and-remove and cut-and-leave strategies are implemented quickly in new spot infestations (Asaro et al. 2017).

Coniferous forests composed of a single species, size and age class will be highly susceptible to bark beetle outbreaks when these factors align with insect pest host preferences. When bark beetle populations are high, even trees in lower density stands may be attacked and overcome by beetles. In these situations, sanitation may be the best option if attacked and dead trees need to be removed. For example, dead trees in recreation areas are hazard trees and pose a safety threat to visitors and should be removed using a sanitation treatment. Numerous dead trees in more remote locations may not warrant removal if they do not pose a safety issue and recovering economic value from these trees is not an objective.

20.2.2 Strategies to Adjust Species Composition

Many forest insect pests are considered specialists, preferring specific tree host species over others. In some insects, this host preference is quite strong and attacks on non-preferred species are rare (e.g. spruce beetle). Other insects have a range of tree hosts, with one generally preferred over others but finding several species attacked in a stand would not be considered unusual. Bark beetles tend to have narrower host ranges than defoliators. Defoliators frequently infest a range of host species, with an order of preference. For example, western spruce budworm (Choristoneura freemanii) is an unfortunately named species, as it preferentially attacks true fir (Abies spp.). Infestation of spruce (Picea spp.) occurs, but damage and mortality may be less severe or occur after the true fir have been fully infested and are dead or declining from multiple, successive defoliation events (Polinko 2014; Vane et al. 2017). Western spruce budworm inhabits a wide geographic range across western North America, and preferentially feeds on tree hosts in order of tree shade tolerance patterns (Brookes et al. 1978). Other defoliators vary more in their host preferences; Douglas-fir tussock moth (Orgyia pseudotsugata) primarily feeds on white fir (Abies concolor) in the southwestern US, switching to a preference for either grand fir (Abies grandis) or Douglas-fir (Pseudotsuga mensiezii) in the northern Rocky Mountains, depending on site conditions, and even further north, in Canada’s Rocky Mountains, feeds primarily on Douglas-fir (Brookes et al. 1985). However, even with changing geographic host tree preference (i.e. when a species’ host tree preference differs throughout its range), preferred host tree species still exhibit higher shade tolerance than less preferred species in the same stand (e.g. pine species (Pinus spp.). Under certain circumstances, such as at high larval population levels or when non-preferred tree hosts are surrounded by more preferred tree hosts, feeding will occur on all tree species in the area. Defoliator damage to host trees ranges from short- and long-term growth reductions to widespread mortality following multiple, recurring defoliation events (Naidoo and Lechowicz 2001; Vane et al. 2017; Rapp 2017).

Silvicultural strategies designed to adjust species composition are primarily used to mitigate defoliator damage and mortality but could also be used to prevent or mitigate other insect infestations, particularly if tree host preference is known. Defoliators disperse from upper to lower tree canopies; the most susceptible stands are dense with a species composition composed primarily of the most preferred host species in multiple vertical canopy layers. Abiotic site factors, including warmer, drier sites that are more prone to drought (e.g. upper ridges), can also play a pre-disposing role in defoliator hazard. If one or more, less preferred host tree species are present or planned for after treatment, silviculture can be an effective indirect control method of reducing the potential for future insect damage. Intermediate treatments or regeneration methods can be used (Box 20.1, Table 20.1); the prescription should remove dead and dying infested trees and live trees of the most preferred tree host species. Such a prescription should also adjust the vertical and horizontal stand structure in a complementary manner to increase both resistance and resilience. Additionally, other stand objectives are typically accounted for in the prescription, including fire hazard reduction, timber production, and wildlife habitat.

Eastern spruce budworm (Choristoneura fumiferana) prefers balsam fir (Abies balsmaea) over white spruce (Picea glauca) and black spruce (Picea mariana) in eastern Canada, and also tends to cause the highest levels of mortality in dense mature balsam fir stands. A silvicultural prescription that both reduces density and preferentially removes mature balsam fir will result in a stand with a lower probability of future damage (DeGroot et al. 2005). Similar strategies are being implemented to reduce western spruce budworm damage in the southwestern US; the prescription reduces density to increase overall tree vigor and shifts species composition towards less preferred host trees such as quaking aspen (Populus tremuloides) and ponderosa pine. White fir and defoliated Douglas-fir are preferentially removed (Fig. 20.4). When developing silvicultural prescriptions, it is important to understand differences in the ecology of insect species. For example, while eastern spruce budworm causes mortality in mature balsam fir first, western spruce budworm mortality tends to occur first in the smaller size classes (Brookes et al. 1978; DeGroot et al. 2005).

Fig. 20.4

Silviculture used to reduce western spruce budworm damage and mortality on the Kaibab National Forest, Arizona, USA. The treatment reduced stand density, created openings to promote regeneration, and favored less preferred host species as residual trees. Photo by K. Waring

Another opportunity to shift species composition occurs during the regeneration phase. Silviculture can be used to encourage certain species to naturally regenerate over others or artificial regeneration can be used to select a specific species composition and density for the new age class. Ensuring adequate natural regeneration can be challenging following widespread overstory mortality if live trees are not available to provide a seed source. In the case of defoliators, heavily defoliated live trees will often have limited capacity for seed production following defoliation (Brookes et al. 1978). In these stands, natural regeneration of less preferred host tree species is more likely than regeneration of the most susceptible host species. A shift toward less preferred host tree species can be encouraged even more by removing preferred host trees from the overstory and leaving only less preferred host trees to regenerate the stand. Such a composition shift may or may not be desirable, depending upon the objectives of the silvicultural treatment.

Planting is the best way to ensure regeneration by less preferred host trees. In most situations, complete replacement of preferred host tree species with less preferred host tree species will not be desirable, as this represents a stand conversion. Single-species plantations may also be vulnerable to a different suite of insect and/or disease problems but may be warranted to meet landowner goals and objectives, such as timber production. Generally, planting will entail a subtle shift from dominance by preferred host tree species to dominance by less preferred host tree species by planting a reduced density of the preferred tree host species.

20.2.3 Potential Drawbacks to the Use of Silviculture

It is possible to create conditions more conducive to insect damage and mortality through silviculture. For example, regenerating eastern white pine under full sun will lead to white pine weevil problems in certain regions (Ostry et al. 2010). It is the responsibility of the silviculturist to know and understand the silvics and ecology of the trees and their pests in a given stand to avoid creating these problems. Silviculturists frequently rely on forest health experts to provide information about specific, stand-level insect or disease issues that may be a concern before or after treatment. Pruning large live branches during bark beetle flight periods can result in attacks leading to mortality, thus pruning treatments should be timed to occur outside of these flight periods whenever possible. Generally, the objectives of pruning for wood quality will not create conditions conducive to bark beetle attack as the stands targeted for pruning treatments are young, and small live branch removal from conifers has not been found to increase bark beetle susceptibility. Hadfield and Flanagan (2000) found pruning increased susceptibility to Douglas-fir beetle attack in campgrounds where large live branches were pruned to meet a hazard tree objective (removal of dwarf mistletoe (Arceuthobium douglasii)-infected branches with large brooms).

Prescribed burning, even at low intensity and severity that does not outright kill the overstory trees, can increase susceptibility to bark beetle attacks through crown scorch and injuries to the cambium (McHugh et al. 2003; Billings et al. 2004). Post-fire tree mortality due to bark beetle attack tends to be short-term (Kane et al. 2017) but as we increase the use of prescribed fire as a management tool, caution is warranted (Bentz et al. 2009). Frequent use of prescribed fire also reduces stand resilience by removing tree regeneration. Central American forests were subject to management practices that reduced both resistance and resilience, resulting in a large, landscape-scale southern pine beetle outbreak (Billings et al. 2004).

The interactions between tree physiology (including tree defenses), herbivory, and abiotic stresses are complex and a review of these is beyond the scope of this chapter (see Massad and Dyer (2010) and Ryan et al. (2015) and literature cited within, for a review and overview of these concepts).

From a silvicultural perspective, thinning has the potential to not just increase tree vigor, but also increases residual tree growth, leading to thicker phloem. Very dense stands have small individual trees with thin phloem that limits bark beetle development and reproduction. Such stands may have reduced susceptibility to bark beetle attacks; thinning may increase susceptibility by increasing average tree size and phloem thickness (Anhold et al. 1996). Very low stand densities have historically been resistant to bark beetle attacks (as described previously in this chapter). Recent research indicates that individual trees in such stands may be less resilient to drought, possibly due to an inability to maintain large crowns when water is limiting (D’Amato et al. 2013).

Drought stress has been linked to increased insect activity in multiple tree species (Savage 1994; Gaylord et al. 2013; Anderegg et al. 2015; Kolb et al. 2016;). Very low and very high stand densities may not be conducive to long-term resistance or resilience given this interaction. A recent study suggests that drought lowers tree resistance to infection by some bark beetle fungal symbionts (Klutsch et al. 2017). During drought conditions, stress is often manifested within individual trees as reduced growth (Fischer et al. 2010; Thomas and Waring 2015; Sohn et al. 2016).

The ability of individual trees within a stand to recover to pre-drought growth rates can be an indicator of susceptibility to bark beetles. Fischer et al. (2010) found that at high stand densities (~14 m² ha⁻¹) ponderosa pine trees that failed to return to pre-drought growth rates were preferentially attacked by the rounded pine beetle (Dendroctonus adjunctus). Douglas-fir tussock moth and western spruce budworm damage tends to be higher on sites more prone to drought conditions (Brookes et al. 1978, 1985). This effect is likely linked to the preferred host species being among the least drought tolerant at these sites. Thinning may also change the chemical composition of residual tree foliage, leading to increased susceptibility. In spruce-fir stands of northeastern North America, thinning altered the foliar monoterpene concentrations of both spruce and fir, making them more susceptible to defoliation from eastern spruce budworm (Fuentealba and Bauce 2011). Due to the complex interactions described above, the response to thinning is not always predictable, nor does it always lead to reductions in herbivory.

Implementing silvicultural treatments can result in logging damage to residual trees and increases slash on the forest floor. To avoid increasing residual tree susceptibility to bark beetle attack, logging operations should be timed to occur when bark beetle flights are low or not occurring, and care should be taken to avoid damaging live trees. Slash piles can serve as suitable host material for many Ips species, which may then ‘spill-over’ into the tops of neighboring trees (Kegley et al. 1997). Slash piles should be removed, chipped or burned in a timely manner to avoid this problem. Freshly cut logs and log decks of large trees can result in fast build-up of certain bark beetles as well (such as the spruce beetle), which then move on to attack live trees nearby (Reynolds and Holsten 1994). Logging activities may damage the soil, increasing compaction, erosion, and/or rutting. Soil damage can lead to increased tree stress, and susceptibility to insect damage, such as the Douglas-fir tussock moth (Brookes et al. 1978).

20.2.4 Linkages with Integrated Pest Management

As discussed in Chapter 17, Integrated Pest Management (IPM) is an integrated approach, which considers multiple strategies and tactics to manage pests efficiently while incorporating economic, social and ecological components. In forest entomology, IPM has primarily focused on efforts to reduce or describe more targeted approaches for land managers using insecticides, and silviculture adds another tool to help reduce potentially environmentally damaging chemical agents on the landscape (McIntire 1988).

It should be noted that silviculture is an IPM tactic. Generally considered cultural strategies, these tactics are generally defined as any treatment that involves a modification of established practices to make a host less favorable for pests or minimize the loss of a particular commodity. Concepts of preventative management are readily applicable to silvicultural strategies. In stands where pest outbreaks are a concern, using management tactics to foster resistance and/or resilience in the resulting stand is crucial (Table 20.1). Silvicultural tactics can be used in tandem with other management activities to increase resistance and/or resilience, while also providing opportunities for other, more immediate tactics to be implemented should pest populations increase. In this section, we cover the use of silviculture in combination with monitoring, chemical control, biological control, and genetic selection.

20.2.5 Silviculture and Monitoring

As discussed previously in Chapter 19, effective monitoring of insect activity is the critical first step of developing an appropriate IPM response. Monitoring should be conducted in a way that is both regular and economically feasible, in order to continually update information on insect population sizes and activity. Management actions should be based on regular assessments of both the insect pest population size and their potential to inflict damage. Conducting regular stand assessments for insect activity, in addition to more stationary and passive approaches, i.e. insect traps, should be both conducted annually, and monitored frequently, to best identify areas where insect activity is increasing. Land managers use this information to prioritize stands for management and abate potential large-scale insect damage or mortality.

Proactive management entails preparing unaffected areas such that if the problem occurs (i.e. non-native invasive expands its non-native range) stands are better able to cope with these changes (e.g. Schoettle and Sniezko 2007). Monitoring pest spread is a key component of proactive management facilitating the identification of high-risk areas (i.e. as characterized by stand conditions, species compositions, vertical/horizontal structure, edaphic and abiotic features of the landscape). Silvicultural actions triggered through monitoring demonstrate the potential of the combination of these two strategies to better prepare forested stands for potential or imminent pest expansion and movement.

In long-term forestry projects, regular monitoring is crucial to determine if silvicultural approaches are warranted (i.e. the identification of emerging threats). Post-treatment, they can be used to evaluate treatment impact on target pest populations. Favorable environmental conditions, or certain disturbances (wind-throw events, storm damage, etc.) can lead to rapid insect population growth. Regular monitoring facilitates the identification of both changes in insect populations and above-threshold population levels [levels above which severe economic damage occurs (see Chapter 19)], both of which are critical to maintaining the health and vigor of forest stands.

Monitoring is critical for effective management of non-native, invasive insects. For example, the sirex woodwasp (Sirex noctilio), an invasive insect of pines that recently established in northeastern North America (Hoebeke et al. 2005), the combined approach of proper silvicultural management and monitoring population expansion, whether through trapping or categorizing infestations aerially, helps land managers determine a proper course of action. Stand resistance to sirex woodwasp can be increased through thinning prior to insect invasion. Maintenance of both host tree vigor through basal area reductions (for eastern white and red (Pinus resinosa) pines these are reported between 9.3 and 14 m² ha⁻¹), creates stands that are optimal for tree growth and therefore production of defensive compounds (Gilmore and Palik 2006; Dodds et al. 2007). Monitoring allows managers to prioritize treatment of pre-invasion stands while considering location of those stands across the landscape.

Monitoring is also an important consideration for native insect pests. Bark beetles are especially damaging during epidemic population cycles. Due to their ubiquity in the Northern Hemisphere, methods such as aerial detection, trapping, ground surveys and remote sensing have been developed and implemented widely for monitoring, and newer technologies, such as unmanned aerial vehicles, are being considered (Wulder et al. 2005; Fettig and Hilszczanski 2015; Morris et al. 2017). Ultimately, proactive monitoring in combination with silvicultural strategies, such as direct control of potential infestations, can be effective preventative measures to make stands and landscapes less susceptible to widespread mortality from the activity of both non-native and native pests.

20.2.6 Silviculture and Chemical Control (Insecticides)

As discussed previously the impetus for the development of IPM was largely generated by an over-reliance on insecticides and the subsequent development of insecticide resistance. However, chemical control is still a large part of any IPM strategy, and proper timing of applications and insecticide selection can yield multiple benefits. For example, the North Carolina Department of Agriculture recommends spraying Fraser fir (Abies fraseri) plantations with a number of pyrethroid insecticides during specific times of the year to control for multiple pests such as balsam woolly adelgid (Adelges piceae), balsam twig aphid (Mindarus abietinus) and hemlock rust mite (Nalepella tsugifoliae) (Sidebottom 2009). The timing of the applications, coupled with adequate tree spacing in these plantation settings, highlights an effective IPM strategy combining silviculture (spacing, tree growth) with insecticide use. Pest populations are reduced when problematic, while minimizing the number of insecticide applications required to reach the management goal.

Effective and economical use of chemicals cannot always be achieved in forest settings. Chemical control is expensive and difficult to apply at landscape-scales or in remote areas, highlighting the necessity of having multiple management strategies to manage pests. Imidacloprid, a neonicotinoid systemic pesticide, has been used by the National Park Service to protect eastern hemlock (Tsuga canadensis) from damage caused by the invasive insect, hemlock woolly adelgid (Adelges tsugae) in both trunk and soil applications (NPS Environmental Assessment 2007). Current research is showing that hemlock woolly adelgid responds negatively to increased light and that releasing these shade-tolerant species using silviculture (e.g. crown thinning, where eastern hemlock are the favored residual trees, with the objective of sustaining the species) may be a strategy to reduce pest populations through stand manipulations. This strategy may be particularly useful for releasing understory hemlock, especially in riparian areas and other areas not feasibly sprayed with insecticides (Brantley et al. 2017).

Carlson et al. (1983) suggest simplifying stand vertical structure (i.e. single-canopy or two-aged), and varying species composition are viable silvicultural strategies to mitigate damage and potential population increase of western spruce budworm in spruce-fir forests. By simplifying canopy strata/altering composition, land managers build natural barriers to population expansion on longer time scales, while using insecticides in untreated and susceptible stands. These examples highlight how insecticide use can be minimized by the creation of less susceptible stand conditions through active IPM management strategies.

Targeted insecticide use can reduce impacts on non-target species and can effectively reduce pest populations during outbreaks. When coupled with regeneration methods (Table 20.1), chemical control can be utilized to protect the future stand. For example, Gottschalk (1993) recommended shelterwood regeneration methods in stands vulnerable to spongy moth (Lymantria dispar), followed by aerial application of insecticides. This strategy reduces insect population numbers while building resilience through the regenerating trees. While chemical control may still be an effective management tool to reduce pest numbers during outbreaks, using silviculture to maintain tree vigor and maintain or enhance understory species diversity and abundance [as habitat for potential biological control agents (e.g. natural predators and parasitoids)], can provide useful components of IPM programs that help to alleviate the need for chemical control (Elek and Wardlaw 2013).

20.2.7 Semiochemicals

Chemical control also includes the use of semiochemicals, organic molecules produced by plants or animals that mediate behavioral interactions between organisms. Semiochemicals involved with intraspecific (within species) communication are pheromones, and those involved with interspecific (between species) communication are allelochemicals. Synthetic copies of these signals and cues can be used in monitoring and management programs for forest insects. For example, verbenone, an anti-aggregation pheromone released by both mountain pine beetle and western pine beetle (Dendroctonus brevicomis), has been utilized to directly protect many different species of western North American conifers (e.g. Gillette et al. 2012; Borden et al. 2006; Fettig and Munson 2020). Site factors such as lower stand densities and higher temperatures diminish its efficacy on a stand-scale when deployed as individual slow-release packets (Fettig et al. 2009), while area-wide deployment on the forest floor in flake releasing formulations effectively reduce beetle mass-attacks on individuals (Gillette et al. 2014).

These strategies, referred to as push/pull strategies, exploit bark beetle behavior to repel pests from the desired resource (e.g. a stand or individual tree) and pull them towards a resource that can then be managed to explicitly eradicate attracted individuals (Cook et al. 2007). Push strategies use numerous tactics including but not limited to semiochemicals (both host- and pest-derived) such as anti-feedants (host-derived chemicals that deter insect feeding activities), anti-aggregants (such as verbenone) and alarm pheromones (pest-derived pheromones that elicit fight-or-flight responses) (Cook et al. 2007).

Push strategies emphasize keeping the pest away from resources (e.g. host trees), while pull strategies tend to use attractants to concentrate individuals in an area. Trap trees represent a common tactic used as a pull strategy in controlling endemic and epidemic bark beetle populations (e.g. Fettig et al. 2007). Felled trees, which mimic windthrown trees, are targeted by some species of bark beetle, therefore felling and baiting trap trees with an aggregation pheromone can be an effective pull strategy (e.g. Schmid and Frye 1977). Trap trees then need to be removed from the stand in a sanitation operation to limit population build-up in stands. Combined with silvicultural strategies such as harvesting infested individuals (as in sanitation treatments; Table 20.1), trap trees (both baited and non-baited) are effective at controlling endemic populations of beetles (e.g. Bentz and Munson 2000).

Generally, large diameter trees tend to be more attractive to infestation by bark beetles, indicating the usefulness of selecting trap trees that are most likely to become infested (Mezei et al. 2014) and effectively timing treatments for greatest impact. Use of felled or standing trap trees is a common sanitation tactic, but their effective use is dependent upon the environment (e.g. Fettig and Hilszczanski 2015). For example, during warm, dry winters with low snowpack, Holusa et al. (2017) recommend land managers fell trap trees just before bark beetle emergence in the spring to maximize efficacy, but during cooler, wetter winters with more snowpack, trap trees can be felled earlier in the winter, as these conditions maintain characteristics of the trap trees attractive to emerging beetles. Coupling push–pull strategies with silvicultural strategies designed to maintain vigorous trees and favoring less susceptible host trees for retention can aid in reducing pest population growth.

20.2.8 Silviculture and Biological Control

Biological control involves utilizing natural enemies (parasites, parasitoids, pathogens etc.) to achieve a reduction or control of pest populations. Increasing the size of established natural enemy populations (parasitoids, predators etc.) by releasing large numbers of individuals as defense against pests is referred to as augmentative biological control (Hoy 2004a). In contrast, classical biological control (Hoy 2004b) involves introducing non-native natural enemies to establish populations to reduce non-native, invasive pest populations. A third option, conservation biological control, involves altering the vertical or horizontal structure, including species composition, of a given land unit to provide more habitat for natural enemies and thus maintain a reserve of beneficial insects within your forested stand.

Silviculture can actively promote conservation biological control, by manipulating the overstory composition or structure to increase understory growth or shift species composition to increase habitat reservoirs of beneficial natural enemy species, illustrating the direct link between silvicultural strategies and biological control in pest management. Classical and augmentative biological control can be used in concert with silvicultural treatments designed to promote individual tree vigor or increase or maintain horizontal and vertical stand structural complexity, including the use of species mixtures. For example, Perez-Alvarez et al. (2019) found classical and augmentative biological control to be more effective in complex than in simple landscapes. This highlights the potential for creation of complex forest structures, and landscape heterogeneity, to potentially increase the impact of biological control programs.

Traditional silviculture practice to meet timber production objectives has primarily utilized monocultures and even-aged regeneration methods (clearcut, seed tree and shelterwood methods) and thus result in reduced stand complexity. Even-aged monocultures can be more susceptible to insect outbreaks and large-scale damage and mortality. Increasing stand structural and compositional complexity increases natural enemy populations and relatively low pest populations (Klapwijk et al. 2016) while also enhancing resilience. For example, single-tree selection in uneven-aged stands increases shading of cut stumps, lowering the temperature of the stump surface and increasing development times for the large pine weevil larvae (Hylobius abietis), making them more vulnerable to predation (Inward et al. 2012). Predator population increases help to prevent the buildup of pest populations and thus can aid in preventing epidemic outbreaks (Klapwijk et al. 2016).

Warzée et al. (2006) calculated predator/prey ratios for the native European spruce bark beetle, Ips typographus, which primarily attacks spruce species, and the predator, the ant beetle (Thanasimus formicarius), in stands of different species compositions. They found that these ratios were significantly greater in mixed species stands, especially those stands with a substantial pine component, as the ant beetle finds more favorable pupation sites on thick barked pines compared to thinner barked spruce (Warzée et al. 2006). In this study, pine species were present on two sites, one composed of 26% pine, the other 80% pine, suggesting that pine as either a minor or major component can positively influence predator/prey ratios for this species (Warzée et al. 2006). Similarly, promotion of certain flowering species in agricultural settings can increase longevity of parasitoids, showing promise for similar use in forested stands (Russell 2015). Mixed species management can influence the life cycle and population levels of natural enemies thus additionally impacting pest species populations (Klapwijk et al. 2016). Incorporating native biodiversity into the silvicultural prescription allows for multiple objectives to be met in a single treatment.

When considering biological control of invasive species, natural enemies from their native habitat are often used as biological control agents in their introduced ranges (e.g. Cheah 2011; Bauer et al. 2015; Kenis et al. 2017). The abundance of invasive species is often greater in their invaded ranges, potentially due to their release from predation, and as such, invasive species often do not have natural enemies in their new habitats (the Enemy Release Hypothesis; Williamson 1996). This generally means that within their native ranges, populations are controlled by natural enemies and tree host defenses, however, when freed from natural predation and host defenses, they become much more damaging as populations rise. Many recent insect invasions around the world exhibit population growth supportive of this hypothesis, including the recent invasion of the emerald ash borer (Agrilus planipennis) in the eastern United States. This invasive beetle kills overstory ash species (Fraxinus spp.), significantly altering forest succession, and causing economic losses. Researchers and managers, as part of a classical biological control program, released a parasitoid, Tetrastichus planipennisi, of the emerald ash borer, which effectively reduced sapling mortality in emerald ash borer-infested stands in Michigan (Duan et al. 2017). While most ash species show little to no resilience to emerald ash borer, green ash (Fraxinus pennsylvanica) regenerates quickly after disturbance and reaches reproductive maturity relatively quickly (Kashian 2016). There is potential to sustain green ash by combining classical biological control with silviculture, creating stand conditions conducive to maintaining or increasing populations of the biological control agent and regeneration of green ash.

The sirex woodwasp has been an established non-native invasive pest for decades, recently arriving in the northeastern United States. Current silvicultural strategies involve thinning stands and removing smaller and suppressed size classes (Dodds et al. 2014). Establishment of the parasitic nematode, Deladenus siricidicola, for biological control has been successfully utilized in Australia (and elsewhere in the southern hemisphere) and shows promise, albeit with serious reservations, for expansion to North America (Haugen 1990; Bedding and Iede 2005; Bittner et al. 2019). Pines are introduced to Australia, meaning the risk to non-target organisms is minimal as insects in Australia did not co-evolve with pines and are rarely associated with the trees. In North America, there are communities of native insects associated with pines and, consequently, there are potential negative impacts for non-target organisms that warrant pause in applying this strategy. In a recent study, Bittner et al. (2019) evaluated strains of these nematodes within North America, and observed that native nematodes may both positively and negatively influence the sterilization success of sirex woodwasp. Other invasive insects, such as the balsam and hemlock woolly adelgids, have both been successfully preyed upon by a single species of beetle, Sasajiscymnus tsugae, in a laboratory setting, showing potential for this agent to be released as a classic biological control agent and further advance IPM strategies for both invasive species (Jetton et al. 2011).

Elkinton et al. (1996) found evidence that increased white-footed mouse (Peromyscus leucopus) density resulted in reduced spongy moth population density. Further, they also found a strong positive association between acorn density and the white-footed mouse population, indicating the importance of acorns for over-wintering populations of white-footed mice. Strategies aimed at maximizing acorn production (e.g. low thinning, crop-tree release), as well as species composition manipulation, especially in low-risk stands, may help to maintain conditions less conducive to high spongy moth populations. This example illustrates how silviculture can promote conditions conducive to native predators (conservation biocontrol) for the control of non-native invasive insects, thereby aiding in reduction of pest populations and maintaining forest health.

20.2.9 Silviculture and Genetic Selection

Genetic selection, or selecting host trees that show promise of resistance to insect attack, and the establishment of breeding programs to propagate these “plus” trees, is a widely researched topic (Kinloch and Stonecypher 1969; McKeand et al. 2003; Roberds et al. 2003). Outside of traditional tree breeding programs, selection of trees in natural settings requires managers to select trees based on their phenotype; the underlying genotype is usually unknown. Exploiting these pre-adapted traits through the utilization of existing genetic variation in breeding programs is an effective method of characterizing resistance mechanisms within species, and then propagating progeny that show increased defensive capabilities. For example, Zas et al. (2017) characterized existing genetic variation in Norway spruce (Picea abies) traits related to increased resistance to the large pine weevil.

Land mangers currently use silvicultural strategies to minimize damage from this pest on artificial regeneration, including soil preparation and shelterwood treatments (Nordlander et al. 2011), however these may be difficult to apply or expensive. Therefore, the decision-making process land managers use to select treatment options is important. Consider Fig. 20.5, which highlights a general decision model including both silvicultural strategies and genetic selection can be utilized to manage plantations, as well as the research requirements for IPM (Alfaro et al. 1995). This demonstrates how genetic selection of putatively resistant and susceptible individuals, and subsequent silvicultural interventions along with other IPM strategies (biological control, etc.) create a framework to help guide land managers in establishing productive plantations that demonstrate the core principles of IPM. By including both resistant and susceptible individuals, one can assess how alternative management strategies (pruning, spacing, biological control) can reduce infestation levels.

Fig. 20.5

Decision key for integrated pest management of Sitka spruce plantations to mitigate damage from the white pine weevil. Modified from Alfaro et al. (1995)

Genetic host tree resistance can be categorized as constitutive or inducible. Constitutive defenses are those defenses that are always expressed, whereas induced defenses are those defenses a plant expresses in response to herbivory (Larsson 2002). Antibiosis indicates that some aspect of the host plant (chemical composition of tissues, defenses) has a negative impact on the pest biology (i.e. survival, development) (Painter 1958). For example, Bucholz et al. (2017) found that without direct contact, volatile organic compounds associated with the resistant Veitch fir (Abies veitchii) compared to a susceptible species, Fraser fir, resulted in significantly reduced eclosion success of balsam woolly adelgid eggs. This suggests an antibiotic effect of the constitutive chemicals released by Veitch fir on balsam woolly adelgid eggs.

Antixenosis or non-preference (e.g. Painter 1958), occurs when some aspect of the host, either chemical or morphological, results in reduced interaction (e.g. feeding, oviposition) of the herbivore with the host. An example of this can be seen with the codling moth (Cydia pomonella) and leaf tissue metabolites from resistant and susceptible cultivars of apple trees (Malus spp). Significantly more oviposition occurred on cloth containing metabolites from susceptible than resistant cultivars, indicating oviposition preference based on chemical cues (Lombarkia and Derridj 2008).

Tolerance describes hosts that can sustain insect feeding activity without serious loss in productivity and is therefore subtly different from resistance (Painter 1958). Strauss and Agrawal (1999) defined tolerance as the degree to which plant fitness is impacted by herbivore damage relative to an undamaged state, whereas resistance was defined as any plant trait which reduced herbivore preference or performance. Tolerance mechanisms are therefore related to increased net photosynthetic rate after damage or compensatory action, high relative growth rates, root carbon storage for above-ground reproduction, and increased branching and resource allocations after damage (Strauss and Agrawal 1999 and references therein). An example of tolerance involves tannin concentration in quaking aspen leaves that does not serve as a defensive compound (i.e. resistance) but one that facilitates nutrient uptake post-defoliation (Madritch and Lindroth 2015). This is viewed as a tolerance mechanism, as the production of greater amounts of these types of secondary metabolites influences nutrient recoveries that may be hindered by defoliation damage.

Other resistance mechanisms, such as oleoresin production, are quantitative genetic traits that can be selected for during tree breeding programs. These traits are complex in that they are composed of many different “small effect” loci that contribute to tree phenotype (e.g. Mundt 2014). Ultimately, the goal is to breed host varieties resistant to certain pest species. Oleoresin flow, along with number of canals or preformed defensive (resin) ducts, has been shown to be positively correlated with survival following bark beetle attack. Bark beetle feeding activity slices through these canals or ducts, releasing their resin, which may envelop or remove the beetles; the more resin ducts a tree has, the more likely it is to successfully eject the attacking beetles (Strom et al. 2002; Kane and Kolb 2010).

In addition, seasonality, and its impact on physiological processes is an important consideration. Lorio (1986) used the framework of Loomis’ (1932) growth-differentiation balance hypothesis to examine conditions ideal for southern pine beetle population expansion. He concluded that this hypothesis was useful in explaining seasonal demand for photosynthate, ultimately driving seasonal vulnerability to southern pine beetle. The trade-off between spring growth and defense suggests that fast-growing trees can be susceptible during these periods when growth processes use more available photosynthate, leaving less allocated towards defense production. The variance in this trait among populations of loblolly pine (Pinus taeda) is heritable.

Westbrook et al. (2013) developed a genomic prediction model across the range of loblolly pine, identifying specific genetic regions associated with increased oleoresin production. This work yielded a guide for making genetic selections to provide increased resistance to southern pine beetle. Trees with increased resistance can be incorporated into silviculture when regenerating stands. Planting all or part of the stand with more resistant trees in anticipation of future insect herbivory, coupled with ongoing silvicultural strategies that promote tree vigor (thinning, adequate spacing etc.) would be an approach to increase resilience (Table 20.1) and may be an important step forward for bark beetle IPM.

Resistance within stands can be promoted by maintenance of stand density index [SDI; a measure of relative density using the relationship between average tree size and stand density (Reineke 1933)] below certain thresholds. For example, Long and Shaw (2005) review strategies associated with size/density relationships surrounding mountain pine beetle, and found that maintaining a SDI below 250 minimizes susceptibility to mountain pine beetle attack. The difficult aspect of management is prioritizing stands for treatment and connecting treatments across landscapes to decrease susceptibility. Reforestation, including planting with genetically improved genotypes where available and economically feasible, aids in contributing to decreased landscape-level susceptibility.

Earlier in the chapter, we discussed using silviculture to shift vertical and horizontal stand structure, with one outcome being increased vigor of residual trees. Individual tree response to reduced competition and increased resource availability is related to the genetic profile of the tree and the surrounding abiotic site conditions (the environment). Growth response to treatment can be optimized through appropriate silviculture in combination with genetic selection by retaining high vigor trees (those that allocate more stem wood per square meter leaf area), with the assumption that this trait is partially determined by genetics. Selecting trees with high growth rates prior to treatment can be challenging (Fischer et al. 2010) but may be possible through additional measurements of tree cores and crown area. Remote sensing applications can detect thinned stands with increased growth rates and therefore resistance to bark beetle attack, establishing a relatively easy method of monitoring overall stand resistance at large scales and across multiple land ownerships (Coops et al. 2009). Silviculturists need to consider the evolutionary adaptation occurring in the stand between bark beetles, host trees, and climate; for example bark beetles may be able to select for host trees least adapted to the changing climate (e.g. Six et al. 2018), resulting in a more resilient stand following bark beetle mortality.

Abiotic site conditions also play a key role in determining phenotypes. Abiotic site conditions (e.g. slope, aspect, topography, soil conditions) tend to change slowly through time or not at all. The abiotic capacity of a site to produce vegetation is often used as a proxy for site quality; high (good) sites produce more vegetation than low (poor) sites. Vegetation production is less on low sites due to limiting resources for plant growth, often related to poor soil resources. The relationship between site conditions and resistance to insects is highly dependent upon the tree host species and corresponding pest species. Slow-growing individuals may be more susceptible to attack by certain insects (i.e. subalpine fir (Abies lasiocarpa) by the western balsam bark beetle (Dryocoetes confusus), therefore considerations of site quality and genetics of growing stock (whether natural or artificial) are important for decreasing susceptibility to insect attack (Bleiker et al. 2005). For example, slower-growing Eucalypts (Eucalyptus spp.) are recommended for low quality sites, since slower growing tree foliage may be better defended against defoliation (Stone 2001). Managers face a complexity of decisions related to the interactions between silviculture, genetic selection, and underlying site factors. After carefully considering these interactions, silviculture and genetic selection can be important components of successful IPM programs.

20.3 Silviculture Over Long Temporal and Large Spatial Scales

The impacts of silviculture extend over long temporal and large spatial scales. Understanding the role of individual stands at these large scales is an important consideration when selecting the appropriate silvicultural strategy because individual stands are connected to form a landscape. Silvicultural treatments necessitate understanding and predicting patterns of tree and stand growth at large spatial and long temporal scales, including interactions with various disturbances and incorporating uncertainty into predictive models. However, understanding these predictions and then designing appropriate strategies that meet multiple goals and objectives is a necessary component of building resistant and resilient landscapes.

Building resistance to insect pests at smaller spatial scales highlights the difficulties faced by silviculturists by both long time scales and scaling up to a landscape. For example, stands at risk to spruce bark beetle have common structural characteristics that can be manipulated through management thereby reducing risk. At the stand-scale, this would entail reducing the relative proportion of overstory basal area in host spruce, reducing the average size of spruce in the stand, or reducing stand basal area (Schmid and Frye 1976). However, while an individual stand may be treated, building this resistance at landscape-scales in practice has proved unrealistic due to economic and political constraints (DeRose and Long 2014). Having adjacent stands that are left untreated provides environments capable of allowing pest insect populations to grow. Once populations have reached epidemic levels, resistant stands become susceptible. Strategically placed area treatments (SPLATs, Finney 2001) are useful in reducing fire severity while only treating ~ 20% of the landscape, however, the efficacy of this practice for insect outbreak remains untested. Resistance is a temporally defined window that changes continually as stands grow and develop after treatment. In the case of spruce bark beetle, the maintenance of resistance at a stand-scale would require multiple treatments to maintain vigor of residual spruce, eventually resulting in structures susceptible to spruce beetle outbreak (Schmid and Frye 1976; DeRose and Long 2014). Therefore, focusing solely on building resistance to a pest may be unproductive. Instead, land managers should focus on a dual approach of targeted treatments in high-risk stands, as well as building resilience through maintaining diversity in both age class structure and species composition across the landscape. In many instances where public and private lands are interspersed, training and shared stewardship programs can help bring private landowners and other stakeholders into the decision-making process alongside silviculturists and other land managers (e.g. Neely et al. 2011).

Building resilience across larger temporal and spatial scales adds complexity to assessing silvicultural strategies at smaller scales. For example, quaking aspen across the western and southwestern United States has experienced large-scale droughts over the past decade. As a result, Sudden Aspen Decline, which is a complex progression of physiological stress, insect infestation and disease, has degraded and caused wide-spread mortality in many stands (Worrall et al. 2010). This decline is complex because it involves multiple agents of mortality, starting with abiotic stress (drought) creating conditions conducive to attack by mostly secondary pests. Increased numbers of susceptible, stressed host trees have allowed an increase in secondary mortality-causing pest populations, and therefore their increased ability to be a major driver of mortality within these stands. Insects such as bronze poplar borer (Agrilus liragus) and aspen bark beetles (e.g. Trypophloeus populi) are viewed as contributing agents in this decline complex, where abiotic factors are considered inciting events (Worrall et al. 2010). Therefore, assessing site characteristics that predispose stands to drought may help managers prioritize stands for silvicultural strategies designed to increase the ability to recover following abiotic disturbance. Landscape-scale resilience can be increased by reducing the proportion of drought-susceptible stands in the landscape. Examples of strategies to increase drought resistance and resilience include thinning to increase individual tree vigor, clearfelling the overstory to regenerate the stand, or shifting species composition toward more drought tolerant species.

The rate at which climate change is occurring highlights the challenge in adapting management. Understanding how abiotic conditions can both cause mortality and stress, therefore creating conditions conducive for attack by biotic agents, is an important concept in promoting resilience at a landscape scale. Although trees have the ability to cope with climate stressors (e.g. stomatal regulation, migration to new areas), rapid climatic change and the concomitant alteration of insect pest populations creates uncertainty in tree host species acclimation potential (Rehfeldt 2006). Evolutionary adaptation and migration work on much slower scales in perennial woody species than in annual species. Generation times are slower in forested ecosystems, and therefore large-scale abiotic changes, along with accompanying biotic changes (e.g. native/invasive species ranges, increased reproductive generations) may inhibit their natural abilities to adapt to altered conditions. Concepts like assisted migration (Sensu Aitken et al. 2008) and assisted gene flow (Sensu Aitken and Whitlock 2013) exist to represent this human-aided transition of species to new areas currently outside their range, but require adequate forethought and forecasting to help determine where to move species and how to genetically bolster species in situ.

20.3.1 Adaptive Silviculture for Climate Change

Studies aimed at developing ecologically-based silvicultural treatments for the future in different ecosystems are needed to understand the complex interactions between ecological components under rapid climatic change. An ongoing effort in the United States, referred to as the Adaptive Silviculture for Climate Change (ASCC) program, is one such study (Nagel et al. 2017). As a result of the continuing impact of climate change and the primarily unknown effects of interactions between climate change and both native and non-native insect pests (Weed et al. 2013), there is a need to develop silvicultural strategies now that can benefit forests in the future. The overarching goal of the ASCC is to understand different silvicultural strategies focused around three central approaches: Resistance, Resilience and Transition (Nagel et al. 2017). Figure 20.6 details how each of the above categories fits into management, and the level of change associated with each (Nagel et al. 2017).

Fig. 20.6

Silvicultural strategies being investigated in the adaptive silviculture for climate change program. From Nagel et al. (2017)

Given the uncertainty of climate change predictions, as well as the heterogeneous impact of various abiotic and biotic stressors at different locations, ASCC attempts to address how different silvicultural strategies can be used to meet land management goals at varying time scales and across regions and ecotypes. The three approaches represent an increasing scale of change. The resistance approach maintains the “status quo”, the resilience approach maintains overstory tree vigor while opening growing space for natural regeneration and the transition approach focuses on shifting composition toward trees considered better suited for an uncertain climatic future. The resistance approach increases the ability of current stands to withstand change, while the resilience and transition approaches attempt to accommodate a moderate-to-large amount of change and a shift away from the current structure and/or species composition. This large-scale research project will yield valuable information for silviculturists attempting to sustain healthy stands and forests under an increasingly uncertain and complex future.

20.4 Synthesis and Conclusion

Use of silviculture to manipulate either vertical and horizontal structure or species composition will also impact the trajectory of stand development and the timing of changes within the stand (stand dynamics) (Oliver and Larson 1996). Silviculture results in a disturbance, and depending upon the number and pattern of trees removed, can effectively shift stands in different directions along a stand development continuum. For example, a dense, even-aged stand under high competition that has the overstory density reduced to below full site occupancy will shift from stem exclusion into understory re-initiation as a new age class develops in the understory. While this transition would occur naturally without silvicultural intervention, with silviculture, a stand can shift overnight from one stage to another, greatly increasing the rate of change and altering the process of stand development.

Silviculturists must be able to predict changes to stand development patterns following treatment. This is most frequently achieved using models (e.g. the Forest Vegetation Simulator; Dixon 2002) and before-after monitoring data. Analysis of before-after data allows the silviculturist to adapt the treatment plan as necessary through time. Silviculturists must also understand and watch for the interactions between silviculture, forest insects and diseases, and other disturbances and provide for appropriate mitigation strategies where necessary.

The approach of managing forest insects through increased resistance and/or resilience can be effectively met using silvicultural strategies. These include strategies developed in conjunction with other management tools in an IPM program. Specific silvicultural prescriptions will vary depending upon stand conditions, site factors, and host tree and pest ecology. However, research and experience indicate that similar results can be expected under specific stand vertical and horizontal structures (Table 20.1) and species composition. From simple to quite complex, using silviculture to manage forest insects can be challenging. Only those (e.g. forest health specialists, forest managers) with advanced training should attempt to resolve forest insect problems in multiaged, mixed species stands without aid from a more experienced silviculturist. Silviculture continues to be an important addition to most forest insect management strategies, and approaching it from a resistance and resilience framework is likely to be successful under rapidly changing environmental and social conditions.