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Salvage logging during spruce budworm outbreaks increases defoliation of black spruce regeneration
Forest Ecology and Management 430 (2018) 421–430
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Salvage logging during spruce budworm outbreaks increases defoliation of black spruce regeneration
T
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Anne Cotton-Gagnona, Martin Simarda, , Louis De Grandpréb, Daniel Kneeshawc a
Centre for Forest Research and Dept. of Geography, Université Laval, Québec, Québec G1V 0A6, Canada Natural Resources Canada, Canadian Forest Service, Laurentian Forestry Centre, 1055 rue du PEPS, P.O. Box 10380, Québec, QC G1V 4C7, Canada c Centre for Forest Research and Dept. of Biological Sciences, Université du Québec à Montréal, CP 8888, Succ. Centre-Ville Montréal, Québec H3C 3P8, Canada b
A R T I C LE I N FO
A B S T R A C T
Keywords: Spruce budworm (Choristoneura fumiferana) Advance regeneration Balsam fir (Abies balsamea) Black spruce (Picea mariana) Defoliation Salvage logging Boreal forest
Although advance regeneration abundance and vigor are critical factors determining future forest composition and productivity, very few studies have focused on how they are affected by spruce budworm (SBW) outbreaks even though they affect millions of hectares of boreal forest on a cyclical basis. Post-SBW salvage logging is often used to reduce economic losses but could interact with the outbreak to affect advance regeneration. This study aims to determine the impact of SBW outbreaks and post-outbreak salvage logging on the defoliation of advance regeneration in mixed coniferous stands of northeastern Canada. Specifically, we assessed the effect of regeneration height and species (balsam fir or black spruce), as well as canopy composition, on the defoliation of advance regeneration. We then evaluated the effect of salvage logging on defoliation sustained by advance regeneration and compared it to the one observed in stands only affected by the SBW. Regeneration height and species, canopy composition and salvage logging all significantly affected defoliation and showed multiple interactions. Taller balsam fir seedlings were three times as defoliated as smaller ones, whereas it was 2.3 times for black spruce. Balsam fir seedlings were 15% more defoliated than black spruce. Seedlings of both species located beneath a balsam fir canopy were also more defoliated (> 50% defoliation) than seedlings found under black spruce trees (about 26% defoliation). Salvage logging in black spruce-dominated stands resulted in a ∼25% increase in defoliation of tall (2.5 m) black spruce regeneration when compared to non-harvested sites. We speculate that this could increase the fir component in spruce-dominated stands, leading to forests that are more susceptible to future SBW outbreaks. To protect spruce advance regeneration from increased defoliation, salvage harvesting of spruce-dominated stands may thus be delayed until the outbreak has subsided. Long-term studies are needed to determine whether a compositional change occurs or not, particularly in spruce-dominated stands. As a precautionary measure, changes in salvage logging practices may be implemented immediately to avoid potential problems such as decreased black spruce abundance and increased susceptibility to future SBW outbreaks.
1. Introduction In temperate and boreal biomes, recurrent insect outbreaks are an important component of forest ecosystems, influencing biogeochemical cycles, vegetation dynamics, and resource availability (Hunter, 2001; Martin et al., 2006; Edburg et al., 2012). This co-evolution of forestinsect systems induced a long-term biological resilience of forests to native insect outbreaks, i.e. their ability to absorb change and return to their original state (Drever et al., 2006; Thompson et al., 2009). For instance, in almost all outbreak-forming insect taxa (mostly defoliators and bark beetles), the insects primarily attack mature trees, which
allows small understory trees, called advance regeneration, to grow and form the next stand (Mattson and Addy, 1975; Morin and Laprise, 1997; Greene et al., 1999; Astrup et al., 2008; Boggs et al., 2008; Kayes and Tinker, 2012). A typical example for this resilience pattern is that of the spruce budworm (SBW; Choristoneura fumiferana), a Lepidopteran defoliator native to the North American boreal forest. Baskerville (1975) labeled the SBW a super silviculturist, as it kills the overstory layer of forests thus releasing the understory regeneration. Since spruce budworm outbreaks last several years, during which establishment by seeds is low (Batzer and Popp, 1985; Astrup et al., 2008; Man and Rice, 2010; Moulinier et al., 2011), pre-established regeneration is the primary
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Corresponding author at: Dept. of Geography, Pavillon Abitibi-Price, 2405, rue de la Terrasse, Laval University, Québec, Québec G1V 0A6, Canada. E-mail addresses: [email protected] (A. Cotton-Gagnon), [email protected] (M. Simard), [email protected] (L. De Grandpré), [email protected] (D. Kneeshaw). https://doi.org/10.1016/j.foreco.2018.08.011 Received 6 May 2018; Received in revised form 3 August 2018; Accepted 5 August 2018 0378-1127/ Crown Copyright © 2018 Published by Elsevier B.V. All rights reserved.
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that area, provided the perfect opportunity to address these questions (Fig. 1). The study area is located on the North Shore of the St. Lawrence River and east of the Saguenay fjord in Quebec. The sub-arctic and sub-humid climate is characterized by a mean annual temperature of 1.7 °C (mean January temperature: −14.3 °C and mean July temperature: 15.6 °C), and mean annual precipitation of 1040.5 mm, 33% of which falls as snow (Environment Canada, 2016). The landscape is characterized by tall hills that rarely exceed 500 m, some rock cliffs and many valleys, while the metamorphic bedrock is covered mainly by thin tills. Forests are characterized by a mixture of balsam fir and black spruce, the latter increasing in abundance northward and inland (Robitaille and Saucier, 1998; De Grandpré et al., 2009).
mechanism of stand replacement. However, it has also been reported that defoliation of regeneration occurs at high SBW densities, when mature trees are completely defoliated and larvae spin down on silk threads in search of food (Nealis and Régnière, 2004; Cooke et al., 2007). Given that advance regeneration abundance and quality are critical for post-disturbance stand recovery, i.e. their resilience, an evaluation of the conditions under which advance regeneration is defoliated will help understand when future dynamics will be affected. Salvage logging to recover mature trees killed by insects could affect advance regeneration and lead to a compositional shift following outbreaks if it increases the vulnerability of one species over another (Burton et al., 2015). Similarly, future productivity could be reduced if the overall number of stems of advance regeneration is reduced (but see Griffin et al., 2013). A closer look into how salvage logging influences the defoliation of advance regeneration will thus inform us as to the future resilience of these forests to the combined effect of both SBW and salvage logging disturbances. Using the boreal forest of eastern Canada, the general objective of this study was to acquire insight into the regeneration process of stands affected by a SBW outbreak, and how salvage logging influences it. At least three factors could affect the defoliation of advance regeneration. First, larger understory trees should be more defoliated than smaller ones because larvae have a greater probability of falling onto large stems than onto small ones, as observed by Nie et al. (2018). Second, the different species (balsam fir [Abies balsamea (L.) Mill] vs. black spruce [Picea mariana (Mill.) B.S.P]) of advance regeneration could experience differential defoliation levels because of the timing of their budburst, which occurs sooner in balsam fir, explaining its greater vulnerability than black spruce, as observed in the canopy (Nealis and Régnière, 2004). However, differential defoliation among host species in the understory might be different from that observed in the canopy depending on the moment of the spinning down of larvae. Third, stands with a higher percentage of balsam fir in the canopy should have higher SBW populations than in black spruce-dominated stands, increasing the probability of larvae falling onto advance regeneration in fir-dominated stands. Salvage logging could either protect advance regeneration or increase its risk of being defoliated. Salvage logging could reduce the defoliation of advance regeneration because removal of the overstory would reduce the abundance of larvae spinning down and feeding on saplings and seedlings. Alternatively, salvage logging could increase defoliation of advance regeneration because the remaining saplings and seedlings would be the only egg-laying sites for SBW moths and subsequently the only food available for the larvae. In this study, we specifically aimed (1) to determine the effects of height and species of advance regeneration and of canopy composition on the severity of the defoliation sustained by advance regeneration and (2) to determine the effects of salvage logging on the defoliation of advance regeneration. For the first objective, we hypothesized that (1) taller individuals would be more defoliated than smaller ones, that balsam fir regeneration would sustain more defoliation than black spruce regeneration, and that advance regeneration in balsam firdominated stands would experience more defoliation than in mixed or spruce-dominated stands. For the second objective, we considered two alternative hypotheses, i.e., that advance regeneration located in salvage logged stands would either sustain less or more defoliation than that located in non-harvested stands.
2.2. Site selection Eight sites, hereafter referred to as “natural sites”, were selected in old uneven-aged stands (120+ years) defoliated by the SBW but not harvested, representing a composition gradient ranging from balsam fir-dominated to black spruce-dominated stands. Six sites, hereafter referred to as “harvested sites”, were selected in stands that were salvage logged in 2011 (one stand), in 2012 (four stands), or in 2013 (one stand). In all sites, salvage harvest was carried out using careful logging standards (Cutting with protection of regeneration and soils or CPRS), which is the main type of logging in Quebec’s forests. This type of harvesting removes the overstory trees but leaves the naturally established regeneration. Prior to harvesting, these stands were similar to the natural sites (i.e., old uneven-aged stands growing on till deposits) and their composition was selected to have a codominance of balsam fir and black spruce. All sites were at least 25 m from a road and showed no sign of other disturbances. To verify that there were no differences between the defoliation history in the natural and harvested sites, we used annual aerial detection survey data to compute cumulative defoliation in each stand (MFFP, 2015). Starting in 2006 (the first year of the outbreak), we added the defoliation values (1 = low, 2 = moderate, and 3 = severe) to obtain a defoliation severity index (Simard and Lajeunesse, 2015) representing cumulative defoliation since the beginning of the outbreak. Stand defoliation history was variable but comparable between the natural and harvested sites, except for one of the natural sites (site N3) that was more defoliated than the others (Fig. 2). Thus, any differences between the defoliation of advance regeneration in natural sites versus harvested sites should not be a result of differences in defoliation history. 2.3. Sampling 2.3.1. Advance regeneration During the summer of 2015, one (in natural sites) or two (in harvested sites) 100-m long transects were sampled at each site to evaluate the abundance, composition and defoliation of advance regeneration. Natural sites were installed in 2013 whereas harvested sites were installed in 2015. Two transects were installed in the harvested sites to account for spatial heterogeneity due to skid trails and unsalvaged patches. At each site, 60 live balsam fir and 60 live black spruce regenerating stems were randomly selected along the transects, in each of the five following height classes (total of 300 stems per species): 8 to 14.9 cm, 15 to 49.5 cm, 50 cm to 1.29 m, 1.3 to 2.29 m, and > 2.3 m but with a dbh < 5 cm (diameter at breast height; 1.3 m above the ground). We tagged and mapped each stem, and noted their status (live or dead; all stems were alive in 2013 in natural sites and in 2015 in harvested sites) and cumulative defoliation caused by the SBW according to the following categories: 0 to 5%, 6 to 25%, 26 to 50%, 51 to 75%, 76 to 95%, and 96 to 100%. At each site, more precise measurements were made of a subsample of 10 regenerating stems of each species and of each height class, for a
2. Methods 2.1. Study area A SBW outbreak which started in 2006 in the eastern boreal forest of Quebec (Canada), along with ongoing salvage logging operations in
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Fig. 1. Defoliation map based on aerial survey data (MFFP, 2015) showing the state of the outbreak during the sampled year (2015) and the study sites. Natural sites refer to defoliated but unharvested sites.
2.3.2. Mature trees In natural sites, all living and dead mature trees (dbh ≥ 9.0-cm) were identified and mapped in a 40 × 100-m quadrat, and their crown width and dbh were measured. In harvested sites, pre-logging stand composition was reconstructed by tallying all live trees, standing dead trees and cut stumps in two 2-m × 100-m transects positioned on the regeneration transects, for a total of 400 m2 per site. Species, status, decay class (Hunter, 1990) and dbh were noted for all trees, whereas species (determined based on bark left on the stumps), decay class, cause of death (harvested or not), and basal diameter were noted for all stumps. Basal diameters were converted to diameter at breast height using allometric relationships developed from 104 balsam fir trees and 98 black spruce trees (balsam fir: R2 = 0.97; black spruce: R2 = 0.98; Appendix A). Pre-logging basal area per species was computed using live trees, recently dead trees (decay class of 4 or less), and harvested stumps, all with a dbh ≥ 9 cm. 2.4. Statistical analyses
Fig. 2. Defoliation history in natural sites (circles) and harvested sites (triangles). When symbols of the same type overlap, they are represented as open symbols.
2.4.1. Stand-scale vs. local-scale (neighbourhood) canopy composition Stand-level canopy composition was used as an explanatory variable in the two regressions (natural sites only and natural + logged sites), and was simply calculated as the percentage of balsam fir basal area relative to total basal area of all tree species in the site. In addition, for natural sites only, we calculated a local-scale canopy composition, based on the rationale that because SBW larvae fall from the canopy onto understory stems, the defoliation of a given sapling or seedling may be influenced by the composition of the canopy directly above the stem. Therefore, the effect of canopy composition was also assessed at the local scale, i.e. in a stems’ neighbourhood (in a 5-m
total of 50 balsam fir stems and 50 black spruce stems. The exact height (for the first four size classes) or dbh (for the fifth size class) of these stems were measured, as well as the width of their crown where the branches were the longest. For stems in the fifth height class (> 2.3-m tall but < 5-cm dbh), dbh was converted to height using allometric equations developed on a sample of 144 balsam fir and 74 black spruce saplings (balsam fir: R2 = 0.70; black spruce: R2 = 0.75; Appendix A).
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Fig. 3. Relative (left) and absolute (right) stand composition of the canopy (top) and advance regeneration (bottom) for the natural (N) and harvested (H) sites (preand post-harvest). Note that the scale is smaller for the post-harvest canopy composition in absolute abundance. N.A. = not available.
radius around each regenerating stem). Neighbourhood composition was determined for each stem of advance regeneration by overlaying the map of georeferenced mature trees and that of sampled advance regeneration in ArcGIS (ESRI, 2012). First, we created a 5-m radius buffer around each regeneration stem to represent the local composition in their neighbourhoods. Second, we created buffers representing the horizontal extent of the crown of each mature tree using speciesspecific allometric relationships between dbh and crown width (balsam fir: R2 = 0.50, P < 0.0001, n = 1044; black spruce: R2 = 0.28, P < 0.0001, n = 2169; Appendix A). The two buffer layers were then intersected to calculate, for each regenerating stem, the proportion of balsam fir, black spruce, white spruce (Picea glauca (Moench) Voss), and open space in the canopy in a 5-m radius. Neighbourhood composition was calculated as the percentage of the area occupied by balsam fir over the total area of the advance regeneration’s surroundings, including the area unoccupied by canopy trees.
mixed-effects linear model in R (package nlme, version 3.2.2; R Core Team, 2015). This model contained only the natural sites (n = 2195 regeneration stems). Then, to test if harvest and the number of years since harvest influenced cumulative defoliation of advance regeneration (second hypothesis), we built a second mixed-effects linear model, combining all natural and harvested sites (n = 7577 stems). Mixed-effects linear modeling allowed us to account for environmental differences among sites, which were treated as a random factor. In all cases, the median of each regenerating stem’s defoliation class was used as the response variable. Because height classes were available for the entire sample, we used this variable instead of exact heights or crown widths in the models, which were measured on subsamples only. All numerical variables were centered on their means before analysis to facilitate interpretation of coefficient.
2.4.2. Hypothesis testing To test whether the species and height of advance regeneration, stand-scale and neighbourhood composition had an effect on cumulative defoliation of advance regeneration (first hypothesis), we built a
3.1. Stand composition
3. Results
Relative balsam fir basal area in the canopy ranged from 2.6% to 72.2% in natural sites, from 11.8% to 63.9% in harvested sites prior to
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Fig. 4. Predicted defoliation of advance regeneration according to regeneration height and species for three stand-scale canopy compositions (top; in black sprucedominated stands, balsam fir basal area was < 2%, averaged 42% in mixed stands, and was > 72% in balsam fir-dominated stands), and three neighbourhood canopy compositions (bottom; balsam fir basal area was 10% in black spruce-dominated neighbourhoods, averaged 50% in mixed neighbourhoods, and was > 90% in balsam fir-dominated neighbourhoods). Shaded areas indicate the 95th confidence interval calculated on fixed effects. Stand-scale composition graphs were calculated based on the mean value of neighbourhood compositions and vice versa.
Table 1 Model parameter estimates of the mixed linear model for cumulative defoliation (%) of advance regeneration in natural sites. Model included seedling height, species, stand-scale canopy composition, neighbourhood canopy composition and three-way interactions among seedling height, species and either stand-scale or neighbourhood canopy composition (n = 2195). N.S. = non-significant (alpha = 5%); CC = canopy composition. Coefficient
Standard error
t-value
p-value
28.6905 0.0998 15.1594 0.5074 0.0184 0.1259 0.0805 −0.0009 0.0023 −0.1184 0.0022 −0.0023
1.7599 0.0075 0.9495 0.0810 0.0520 0.0006 0.0106 0.0006 0.0005 0.0551 0.0008 0.0006
16.3025 13.2714 15.9657 6.2634 0.3545 1.7193 7.6242 −1.7026 5.1128 −2.1492 2.7706 −3.6032
< 0.0001 < 0.0001 < 0.0001 0.0008 N.S. N.S. < 0.0001 N.S. < 0.0001 0.0317 0.0056 0.0003
Fixed effects Intercept Height Species (reference: black spruce) Stand-scale CC (% balsam fir) Neighbourhood CC (% balsam fir) Species * Neighbourhood CC Height * Species Height * Neighbourhood CC Height * Stand-scale CC Species * Stand-scale CC Height * Species * Neighbourhood CC Height * Species * Stand-scale CC Random effects
Inter-group standard deviation Site
4.5884
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and neighbourhood canopy composition; Fig. 4a, b and c). Taller balsam fir seedlings were three times as defoliated as the smallest ones, whereas it was 2.3 ftimes or black spruce. However, defoliation of black spruce regeneration in black spruce-dominated stands was very low and did not change with stem height (Fig. 4a), but as the proportion of balsam fir increased in the canopy, black spruce defoliation also increased (from 6% in black spruce stands to > 40% in balsam fir stands, when adjusted for mean seedling height and neighbourhood canopy composition), especially for larger stems (Fig. 4a, b and c). Neighbourhood (5-m radius) canopy composition had a significant but weaker effect (Height * Species * Neighbourhood CC: P < 0.01; Table 1) than stand-scale canopy composition (Height * Species * Standscale CC: P < 0.001; Table 1). Whereas the relationships between defoliation and canopy composition were similar at the neighbourhood vs. stand scale in mixed composition stands (Fig. 4b and e), they were different in pure compositions for black spruce regeneration. Notably, at the local scale, black spruce regeneration was more defoliated in black spruce-dominated neighbourhoods (16–47%; Fig. 4d) than in balsam fir-dominated neighbourhoods (25–36%; Fig. 4f), whereas the opposite was observed at the stand scale. To determine the height above which regenerating stems were most likely to be severely defoliated, we used a logistic regression between defoliation severity (light: defoliation < 50% vs. severe: defoliation ≥50%) and stem height (Fig. 5). Balsam fir stems > 1.2-m tall had a 0.5 probability of sustaining severe defoliation and the probability reached 0.95 above 2 m in height. For black spruce, the probability of sustaining severe defoliation reached 0.5 only in saplings taller than 3.5 m. Using the exact height or crown width (measured in the subsample), instead of height class medians, gave identical results. Therefore, using height classes, which saves times in the field and is less prone to variability among people making the measurements, is sufficient for assessing advance regeneration vulnerability (height classes vs. exact height: r = 0.97; height classes vs. crown width: r = 0.85; exact height vs. crown width: r = 0.88; n = 675; data not shown otherwise).
Fig. 5. Probability of sustaining severe (≥50%) defoliation as a function of stem height. Shaded areas indicate the 95th confidence interval.
salvage logging and from 27.3% to 57.0% following salvage logging (Fig. 3, Appendix B). Black spruce was the other dominant species, while white spruce and hardwood species (mainly white birch [Betula papyrifera Marsh]) were occasionally found.
3.2. Factors influencing defoliation of advance regeneration in natural sites Defoliation increased with stem height and this relationship differed significantly by seedling species and with stand-scale canopy composition (three-way interaction; Fig. 4, Table 1). Balsam fir was consistently more defoliated than black spruce (15% more defoliation when adjusted for mean seedling height, neighbourhood and standscale canopy composition; Table 1), and the positive relationship between regeneration height and defoliation increased as the proportion of balsam fir in the canopy increased (from 26% in black spruce stands to > 50% in balsam fir stands, when adjusted for mean seedling height
3.3. Effect of salvage harvesting on defoliation of advance regeneration Time since harvest did not affect defoliation of advance
Table 2 Model parameter estimates of the mixed linear model for cumulative defoliation (%) of advance regeneration in natural and logged sites. Model included seedling height, species, stand-scale canopy composition, status (logged or natural), and all interactions (n = 7577). N.S. = non-significant (alpha = 5%); CC = canopy composition. Coefficient
Standard error
t-value
p-value
24.8721 0.0620 16.8091 0.5795 −0.6685 0.0945 0.0018 −0.1301 −0.0013 0.0547 −5.8683 −0.3963 −0.0289 −0.0018 0.3318 0.0026
1.5163 0.0057 0.7075 0.0615 2.3317 0.0077 0.0003 0.0305 0.0003 0.0079 1.0379 0.1039 0.0110 0.0004 0.0471 0.0005
16.4030 10.8019 23.7571 9.4194 −0.2867 12.2748 6.7931 −4.2580 −3.6283 6.9158 −5.6540 −3.8151 −2.6249 −4.7316 7.0443 5.1479
< 0.0001 < 0.0001 < 0.0001 < 0.0001 N.S. < 0.0001 < 0.0001 < 0.0001 0.0003 < 0.0001 < 0.0001 0.0034 0.0087 < 0.0001 < 0.0001 < 0.0001
Fixed effects Intercept Height Species (reference: black spruce) CC (Stand-scale % balsam fir) Logging (reference = natural sites) Height * Species Height * CC Species * CC Height * Species * CC Logging * Height Logging * Species Logging * CC Logging * Height * Species Logging * Height * CC Logging * Species * CC Logging * Height * Species * CC Random effects
Inter-group standard deviation Site
3.7389
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Fig. 6. Predicted defoliation of advance regeneration in natural and harvested sites by stem height for balsam fir (a–c) and black spruce (d–f) and three stand-scale canopy compositions (for black spruce-dominated stands, relative basal area in balsam fir was 10%, it was 40% for mixed stands, and 65% for balsam fir-dominated stands). Shaded areas indicate the 95th confidence interval calculated on fixed effects.
First, defoliation increased with height of advance regeneration, which supports our first hypothesis and the results of Nie et al. (2018). This can be explained by the fact that taller stems have wider crowns, and thus a higher probability of intercepting larvae spinning down from the canopy, as well as potentially being a suitable site for females to lay eggs. Smaller regenerating stems may also benefit from the sheltering effect provided by taller stems, which would intercept falling larvae and in turn reduce the probability of larvae landing on smaller stems. Ruel and Huot (1993) also observed that balsam fir regeneration over two meters in height was practically absent from balsam fir-dominated stands that were severely defoliated. Similar to forests defoliated by the gyspy moth (Lymantria dispar dispar), stand recovery may be ensured by smaller established seedlings, as tall advance regeneration sustains greater defoliation and mortality (Hix et al., 1991). Also consistent with our hypothesis, balsam fir regeneration was more defoliated than black spruce. Even though this should not be a surprise, as mature balsam fir trees are also more susceptible to the SBW, the mechanisms driving susceptibility of understory tree species may not be the same as for mature trees. While SBW larvae are present in the canopy of natural forests because it is their preferred oviposition site, their presence in the understory is mostly due to spinning down from the canopy when food is scarce in the canopy or when populations are high. The greater defoliation of balsam fir regeneration may depend
regeneration (P = 0.2919). The best model included a four-way interaction among all effects (Table 2). It showed that in both harvested sites and natural sites, defoliation was greater on balsam fir regeneration than it was on black spruce (16.8% more defoliation when adjusted for mean seedling height and stand-scale canopy composition). Like we observed in natural sites, defoliation increased with regeneration height and with the proportion of balsam fir in the canopy (Fig. 6, Table 2). However, in stands that were dominated by black spruce before logging, the effect of harvesting greatly increased defoliation of black spruce regeneration after harvesting (Fig. 6d). Whereas defoliation of tall (> 2.5 m) black spruce regeneration reached 30% in harvested sites, it did not even attain 10% in natural sites (Fig. 6d). 4. Discussion 4.1. Factors influencing defoliation of regeneration in natural sites SBW outbreaks have affected the boreal forest for centuries, shaping its dynamics, regeneration processes, and resilience (Morin, 1994), yet little is known about the effects of defoliation on advance regeneration. This study provides insight into how SBW outbreaks affect different size classes of regeneration of balsam fir and black spruce in natural and salvage logged sites.
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on the timing of the dispersal. Early in the spring, phenology could explain a difference in defoliation between the host species, as bud burst occurs earlier in fir. Later in the season, differences in defoliation could be due to mechanisms other than phenology, such as (i) black spruce’s greater foliage biomass (approximately 25–50% greater than balsam fir for the studied range of DBH), which would allow black spruce to better sustain defoliation caused by a given density of larvae (Lambert et al., 2005; Ung et al., 2008), (ii) the greater propensity of black spruce to produce epicormic shoots, allowing the replacement of consumed foliage (Simard and Payette, 2003), (iii) differences in foliage quality (nutrient contents, chemical and/or physical defenses, etc.) between the two studied species (Blais, 1957; Hudes and Shoemaker, 1984), or some combination of all of these factors. Seedling defoliation also increased as the proportion of balsam fir in the canopy increased, which supports our third hypothesis and is consistent with the resource concentration hypothesis (Bognounou et al., 2017). For black spruce seedlings, this effect was stronger in fir-dominated stands than in fir-dominated neighbourhoods (Fig. 4a). This may be due to associational susceptibility at high SBW population density in balsam fir stands leading to greater defoliation of all stems in the understory, irrespective of their species. Whereas in neighbourhoods, ballooning of down-spinning threads may lead to larvae not descending directly below infested trees. In addition to this spatial variability, Bognounou et al. (2017) also suggest that there may be temporal variability in neighbourhoods and stand-level defoliation patterns between species.
growth rate and increases its mortality rate, stand composition may shift away from black spruce and towards balsam fir, which may increase forest susceptibility to future infestations of the SBW. Even modest increases in balsam fir abundance may have disproportionate effects on defoliation of black spruce because of the associational susceptibility of black spruce (Atsatt and O’Dowd, 1976; Bognounou et al., 2017).
4.2. Effect of harvesting on defoliation of regeneration
Conflict of interest
4.3. Implications for sustainable forest management The effect of salvage harvesting on black spruce defoliation has important landscape-scale consequences in the study area, where the totality of the annual allowable cuts are currently carried out in defoliated stands. The results of this study suggest that forest management may be locally adapted in response to broad-scale infestations, similar to results found on salvage logging following a spruce beetle (Dendroctonus rufipennis Kirby) outbreak (Boucher and Mead, 2006). To protect spruce advance regeneration from increased defoliation, salvage harvesting of spruce-dominated stands may thus be delayed until the outbreak has subsided. Long-term studies are needed to determine whether a compositional change towards balsam fir occurs or not, particularly in spruce-dominated stands. We argue however that as a precautionary measure, changes in salvage logging practices may be implemented immediately to avoid potential problems such as decreased black spruce abundance and increased susceptibility to future SBW outbreaks.
Our study shows that the natural regeneration process of a boreal forest affected by the SBW may be compromised by salvage logging in black spruce-dominated stands. Under natural dynamics, black spruce is maintained during outbreaks because of the preference of SBW for balsam fir; as a corollary, stands in which black spruce is abundant decrease the future impact of the SBW (Burton et al., 2015). This phenomenon was also found in oak (Quercus spp.) forests attacked by the gypsy moth, as they underwent an increase in ligneous species diversity that reduced their susceptibility to future attacks by that insect (Hix et al., 1991). However, in our case, regeneration processes in black spruce-dominated stands are likely compromised as the black spruce component of the overstory that is removed is much less vulnerable than balsam fir (black spruce sustains less than a third of the defoliation sustained by balsam fir; Hennigar et al., 2008) while the opening of the canopy exposes tall black spruce regeneration to defoliation (6 times more defoliation in harvested sites (30%) than in natural sites (5%)). We speculate that if increased defoliation of black spruce reduces its
None.
Acknowledgments This study was conducted in collaboration with the Canadian Forest Service. We are grateful to M. Marchand (Nat. Res. Can., LFC) and J.-C. Ruel (Laval University) for their counseling throughout the project, and to G. Cervello, H. Dorion, M. Gauvin and J. Moisan Perrier for their dedication and hard work collecting data on the field. We also want to thank the Centre for Forest Research. Funding: This work was supported by the Fonds de recherche du Québec – Nature et technologies (FRQNT, Programme de financement de la recherche et développement en aménagement forestier, #165640) and the Natural Sciences and Engineering Research Council of Canada (NSERC, #RGPIN-2017-06616).
Appendix A. Allometric equations Allometric equations used to convert diameter at breast height (dbh, in cm) to stem height (cm) in the fifth height class (> 2.3-m tall but < 5cm dbh) using a sample of 144 balsam fir and 74 black spruce saplings (balsam fir: R2 = 0.70; black spruce: R2 = 0.75):
Balsam fir
Black spruce
height = 36.89∗dbh + 114.62
(A.1)
height = 37.145∗dbh + 139.34
(A.2)
Allometric equations used to convert basal diameter (bd, in cm) to diameter at breast height (dbh, in cm) for stumps in salvage logged sites using a sample of 104 balsam fir trees and 98 black spruce trees (balsam fir: R2 = 0.97; black spruce: R2 = 0.98): (A.3)
Balsam fir dbh = 0.0002∗bd2−0.011∗bd + 6.3713
Black spruce
dbh = 0.0002∗bd2−0.0384∗bd + 10.741
(A.4)
Allometric equations used to convert diameter at breast height (dbh, in cm) to crown width (cw, in cm) for mature trees in the local composition analysis (balsam fir: R2 = 0.50, P < 0.00001, n = 1044; black spruce: R2 = 0.28, P < 0.00001, n = 2169):
Balsam fir
Black spruce
(A.5)
cw = 10.163∗dbh + 128.91
cw = 5.7987∗ dbh + 124.21
(A.6) 428
2011-ongoing 2007-ongoing 2009-ongoing 2011-ongoing 2010-ongoing 2010-ongoing 2011-ongoing 2011-ongoing 2009-ongoing 2012-ongoing 2009-ongoing 2010-ongoing 2010-ongoing 2011-ongoing
−68.1569 −68.0294 −68.1231 −67.8251 −67.7824 −67.8032 −68.1482 −68.1401 −68.1270 −68.0718 −68.0602 −68.0700 −68.2299 −68.2489
N1 N2 N3 N4 N5 N6 N7 N8 H1 H2 H3 H4 H5 H6
50.1219 49.4052 49.7051 49.5785 49.5756 49.5710 50.1610 49.7153 49.7044 49.7023 49.7490 50.1637 49.6995 49.7092
Defoliation period
Longitude (DD)
Site Latitude (DD) – – – – – – – – 2013 2012 2012 2012 2012 2011
Year of harvest
17.9 18.4 17.8 14.3 14.4 15.9 17.6 18.5 14.0 10.8 10.3 10.9 13.0 16.4
Avg DBH (cm)
72.2/N.A. 69.1/N.A. 68.8/N.A. 60.7/N.A. 48.0/N.A. 35.9/N.A. 26.1/N.A. 2.60/N.A. 63.9/51.1 42.8/57.0 32.6/54.8 14.3/34.5 12.7/27.3 11.8/27.4
Bf (pre/post logging) 14.7/N.A. 5.40/N.A. 11.1/N.A. 37.4/N.A. 50.3/N.A. 64.1/N.A. 73.9/N.A. 96.9/N.A. 29.9/17.7 57.2/43.0 67.4/45.2 85.7/65.5 84.3/18.4 88.2/72.6
Bs (pre/post logging)
Relative abundance (%)
Canopy composition (basal area)
5.7 21.8 7.1 0.0 1.5 0.0 0.0 0.0 0.0 0.0 0.0 0.0 2.8 0.0
Ws
7.4 3.7 13.0 1.9 0.2 0.0 0.0 0.5 6.2 0.0 0.0 0.0 0.1 0.0
Hw
27.6/N.A. 19.8/N.A. 13.2/N.A. 8.30/N.A. 8.90/N.A. 6.60/N.A. 3.80/N.A. 0.80/N.A. 11.4/0.11 6.90/0.06 8.70/0.10 3.80/0.06 5.10/0.04 2.10/0.02
Bf (pre/post logging)
5.6/N.A. 1.6/N.A. 2.1/N.A. 5.1/N.A. 9.3/N.A. 11.7/N.A. 10.7/N.A. 31.0/N.A. 5.3/0.04 9.2/0.04 18.0/0.08 22.8/0.10 33.8/0.03 15.6/0.06
Bs (pre/post logging)
Absolute abundance (m2/ha)
Avg DBH: Average diameter at breast height; Bf: Balsam fir; Bs: Black spruce; Ws: White spruce; Hw: Hardwoods; N.A.: Not available.
Appendix B. Site description
2.2 6.2 1.4 0.0 0.3 0.0 0.0 0.0 0.0 0.0 0.0 0.0 1.1 0.0
2.8 1.1 2.5 0.3 0.0 0.0 0.0 0.2 1.1 0.0 0.0 0.0 0.1 0.0
68,350 28,650 58,900 37,450 6750 18,350 55,600 17,000 56,950 10,150 19,600 24,450 18,500 8300
Ws Hw Bf
750 16,450 21,450 11,150 26,400 28,350 61,350 17,250 20,000 22,450 14,000 66,250 62,850 38,800
Bs
50 650 0 1000 100 50 500 100 14,300 1400 5350 0 9800 4950
Hw
density (stems/ha)
Regeneration
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Salvage logging during spruce budworm outbreaks increases defoliation of black spruce regeneration
T
⁎
Anne Cotton-Gagnona, Martin Simarda, , Louis De Grandpréb, Daniel Kneeshawc a
Centre for Forest Research and Dept. of Geography, Université Laval, Québec, Québec G1V 0A6, Canada Natural Resources Canada, Canadian Forest Service, Laurentian Forestry Centre, 1055 rue du PEPS, P.O. Box 10380, Québec, QC G1V 4C7, Canada c Centre for Forest Research and Dept. of Biological Sciences, Université du Québec à Montréal, CP 8888, Succ. Centre-Ville Montréal, Québec H3C 3P8, Canada b
A R T I C LE I N FO
A B S T R A C T
Keywords: Spruce budworm (Choristoneura fumiferana) Advance regeneration Balsam fir (Abies balsamea) Black spruce (Picea mariana) Defoliation Salvage logging Boreal forest
Although advance regeneration abundance and vigor are critical factors determining future forest composition and productivity, very few studies have focused on how they are affected by spruce budworm (SBW) outbreaks even though they affect millions of hectares of boreal forest on a cyclical basis. Post-SBW salvage logging is often used to reduce economic losses but could interact with the outbreak to affect advance regeneration. This study aims to determine the impact of SBW outbreaks and post-outbreak salvage logging on the defoliation of advance regeneration in mixed coniferous stands of northeastern Canada. Specifically, we assessed the effect of regeneration height and species (balsam fir or black spruce), as well as canopy composition, on the defoliation of advance regeneration. We then evaluated the effect of salvage logging on defoliation sustained by advance regeneration and compared it to the one observed in stands only affected by the SBW. Regeneration height and species, canopy composition and salvage logging all significantly affected defoliation and showed multiple interactions. Taller balsam fir seedlings were three times as defoliated as smaller ones, whereas it was 2.3 times for black spruce. Balsam fir seedlings were 15% more defoliated than black spruce. Seedlings of both species located beneath a balsam fir canopy were also more defoliated (> 50% defoliation) than seedlings found under black spruce trees (about 26% defoliation). Salvage logging in black spruce-dominated stands resulted in a ∼25% increase in defoliation of tall (2.5 m) black spruce regeneration when compared to non-harvested sites. We speculate that this could increase the fir component in spruce-dominated stands, leading to forests that are more susceptible to future SBW outbreaks. To protect spruce advance regeneration from increased defoliation, salvage harvesting of spruce-dominated stands may thus be delayed until the outbreak has subsided. Long-term studies are needed to determine whether a compositional change occurs or not, particularly in spruce-dominated stands. As a precautionary measure, changes in salvage logging practices may be implemented immediately to avoid potential problems such as decreased black spruce abundance and increased susceptibility to future SBW outbreaks.
1. Introduction In temperate and boreal biomes, recurrent insect outbreaks are an important component of forest ecosystems, influencing biogeochemical cycles, vegetation dynamics, and resource availability (Hunter, 2001; Martin et al., 2006; Edburg et al., 2012). This co-evolution of forestinsect systems induced a long-term biological resilience of forests to native insect outbreaks, i.e. their ability to absorb change and return to their original state (Drever et al., 2006; Thompson et al., 2009). For instance, in almost all outbreak-forming insect taxa (mostly defoliators and bark beetles), the insects primarily attack mature trees, which
allows small understory trees, called advance regeneration, to grow and form the next stand (Mattson and Addy, 1975; Morin and Laprise, 1997; Greene et al., 1999; Astrup et al., 2008; Boggs et al., 2008; Kayes and Tinker, 2012). A typical example for this resilience pattern is that of the spruce budworm (SBW; Choristoneura fumiferana), a Lepidopteran defoliator native to the North American boreal forest. Baskerville (1975) labeled the SBW a super silviculturist, as it kills the overstory layer of forests thus releasing the understory regeneration. Since spruce budworm outbreaks last several years, during which establishment by seeds is low (Batzer and Popp, 1985; Astrup et al., 2008; Man and Rice, 2010; Moulinier et al., 2011), pre-established regeneration is the primary
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Corresponding author at: Dept. of Geography, Pavillon Abitibi-Price, 2405, rue de la Terrasse, Laval University, Québec, Québec G1V 0A6, Canada. E-mail addresses: [email protected] (A. Cotton-Gagnon), [email protected] (M. Simard), [email protected] (L. De Grandpré), [email protected] (D. Kneeshaw). https://doi.org/10.1016/j.foreco.2018.08.011 Received 6 May 2018; Received in revised form 3 August 2018; Accepted 5 August 2018 0378-1127/ Crown Copyright © 2018 Published by Elsevier B.V. All rights reserved.
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that area, provided the perfect opportunity to address these questions (Fig. 1). The study area is located on the North Shore of the St. Lawrence River and east of the Saguenay fjord in Quebec. The sub-arctic and sub-humid climate is characterized by a mean annual temperature of 1.7 °C (mean January temperature: −14.3 °C and mean July temperature: 15.6 °C), and mean annual precipitation of 1040.5 mm, 33% of which falls as snow (Environment Canada, 2016). The landscape is characterized by tall hills that rarely exceed 500 m, some rock cliffs and many valleys, while the metamorphic bedrock is covered mainly by thin tills. Forests are characterized by a mixture of balsam fir and black spruce, the latter increasing in abundance northward and inland (Robitaille and Saucier, 1998; De Grandpré et al., 2009).
mechanism of stand replacement. However, it has also been reported that defoliation of regeneration occurs at high SBW densities, when mature trees are completely defoliated and larvae spin down on silk threads in search of food (Nealis and Régnière, 2004; Cooke et al., 2007). Given that advance regeneration abundance and quality are critical for post-disturbance stand recovery, i.e. their resilience, an evaluation of the conditions under which advance regeneration is defoliated will help understand when future dynamics will be affected. Salvage logging to recover mature trees killed by insects could affect advance regeneration and lead to a compositional shift following outbreaks if it increases the vulnerability of one species over another (Burton et al., 2015). Similarly, future productivity could be reduced if the overall number of stems of advance regeneration is reduced (but see Griffin et al., 2013). A closer look into how salvage logging influences the defoliation of advance regeneration will thus inform us as to the future resilience of these forests to the combined effect of both SBW and salvage logging disturbances. Using the boreal forest of eastern Canada, the general objective of this study was to acquire insight into the regeneration process of stands affected by a SBW outbreak, and how salvage logging influences it. At least three factors could affect the defoliation of advance regeneration. First, larger understory trees should be more defoliated than smaller ones because larvae have a greater probability of falling onto large stems than onto small ones, as observed by Nie et al. (2018). Second, the different species (balsam fir [Abies balsamea (L.) Mill] vs. black spruce [Picea mariana (Mill.) B.S.P]) of advance regeneration could experience differential defoliation levels because of the timing of their budburst, which occurs sooner in balsam fir, explaining its greater vulnerability than black spruce, as observed in the canopy (Nealis and Régnière, 2004). However, differential defoliation among host species in the understory might be different from that observed in the canopy depending on the moment of the spinning down of larvae. Third, stands with a higher percentage of balsam fir in the canopy should have higher SBW populations than in black spruce-dominated stands, increasing the probability of larvae falling onto advance regeneration in fir-dominated stands. Salvage logging could either protect advance regeneration or increase its risk of being defoliated. Salvage logging could reduce the defoliation of advance regeneration because removal of the overstory would reduce the abundance of larvae spinning down and feeding on saplings and seedlings. Alternatively, salvage logging could increase defoliation of advance regeneration because the remaining saplings and seedlings would be the only egg-laying sites for SBW moths and subsequently the only food available for the larvae. In this study, we specifically aimed (1) to determine the effects of height and species of advance regeneration and of canopy composition on the severity of the defoliation sustained by advance regeneration and (2) to determine the effects of salvage logging on the defoliation of advance regeneration. For the first objective, we hypothesized that (1) taller individuals would be more defoliated than smaller ones, that balsam fir regeneration would sustain more defoliation than black spruce regeneration, and that advance regeneration in balsam firdominated stands would experience more defoliation than in mixed or spruce-dominated stands. For the second objective, we considered two alternative hypotheses, i.e., that advance regeneration located in salvage logged stands would either sustain less or more defoliation than that located in non-harvested stands.
2.2. Site selection Eight sites, hereafter referred to as “natural sites”, were selected in old uneven-aged stands (120+ years) defoliated by the SBW but not harvested, representing a composition gradient ranging from balsam fir-dominated to black spruce-dominated stands. Six sites, hereafter referred to as “harvested sites”, were selected in stands that were salvage logged in 2011 (one stand), in 2012 (four stands), or in 2013 (one stand). In all sites, salvage harvest was carried out using careful logging standards (Cutting with protection of regeneration and soils or CPRS), which is the main type of logging in Quebec’s forests. This type of harvesting removes the overstory trees but leaves the naturally established regeneration. Prior to harvesting, these stands were similar to the natural sites (i.e., old uneven-aged stands growing on till deposits) and their composition was selected to have a codominance of balsam fir and black spruce. All sites were at least 25 m from a road and showed no sign of other disturbances. To verify that there were no differences between the defoliation history in the natural and harvested sites, we used annual aerial detection survey data to compute cumulative defoliation in each stand (MFFP, 2015). Starting in 2006 (the first year of the outbreak), we added the defoliation values (1 = low, 2 = moderate, and 3 = severe) to obtain a defoliation severity index (Simard and Lajeunesse, 2015) representing cumulative defoliation since the beginning of the outbreak. Stand defoliation history was variable but comparable between the natural and harvested sites, except for one of the natural sites (site N3) that was more defoliated than the others (Fig. 2). Thus, any differences between the defoliation of advance regeneration in natural sites versus harvested sites should not be a result of differences in defoliation history. 2.3. Sampling 2.3.1. Advance regeneration During the summer of 2015, one (in natural sites) or two (in harvested sites) 100-m long transects were sampled at each site to evaluate the abundance, composition and defoliation of advance regeneration. Natural sites were installed in 2013 whereas harvested sites were installed in 2015. Two transects were installed in the harvested sites to account for spatial heterogeneity due to skid trails and unsalvaged patches. At each site, 60 live balsam fir and 60 live black spruce regenerating stems were randomly selected along the transects, in each of the five following height classes (total of 300 stems per species): 8 to 14.9 cm, 15 to 49.5 cm, 50 cm to 1.29 m, 1.3 to 2.29 m, and > 2.3 m but with a dbh < 5 cm (diameter at breast height; 1.3 m above the ground). We tagged and mapped each stem, and noted their status (live or dead; all stems were alive in 2013 in natural sites and in 2015 in harvested sites) and cumulative defoliation caused by the SBW according to the following categories: 0 to 5%, 6 to 25%, 26 to 50%, 51 to 75%, 76 to 95%, and 96 to 100%. At each site, more precise measurements were made of a subsample of 10 regenerating stems of each species and of each height class, for a
2. Methods 2.1. Study area A SBW outbreak which started in 2006 in the eastern boreal forest of Quebec (Canada), along with ongoing salvage logging operations in
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Fig. 1. Defoliation map based on aerial survey data (MFFP, 2015) showing the state of the outbreak during the sampled year (2015) and the study sites. Natural sites refer to defoliated but unharvested sites.
2.3.2. Mature trees In natural sites, all living and dead mature trees (dbh ≥ 9.0-cm) were identified and mapped in a 40 × 100-m quadrat, and their crown width and dbh were measured. In harvested sites, pre-logging stand composition was reconstructed by tallying all live trees, standing dead trees and cut stumps in two 2-m × 100-m transects positioned on the regeneration transects, for a total of 400 m2 per site. Species, status, decay class (Hunter, 1990) and dbh were noted for all trees, whereas species (determined based on bark left on the stumps), decay class, cause of death (harvested or not), and basal diameter were noted for all stumps. Basal diameters were converted to diameter at breast height using allometric relationships developed from 104 balsam fir trees and 98 black spruce trees (balsam fir: R2 = 0.97; black spruce: R2 = 0.98; Appendix A). Pre-logging basal area per species was computed using live trees, recently dead trees (decay class of 4 or less), and harvested stumps, all with a dbh ≥ 9 cm. 2.4. Statistical analyses
Fig. 2. Defoliation history in natural sites (circles) and harvested sites (triangles). When symbols of the same type overlap, they are represented as open symbols.
2.4.1. Stand-scale vs. local-scale (neighbourhood) canopy composition Stand-level canopy composition was used as an explanatory variable in the two regressions (natural sites only and natural + logged sites), and was simply calculated as the percentage of balsam fir basal area relative to total basal area of all tree species in the site. In addition, for natural sites only, we calculated a local-scale canopy composition, based on the rationale that because SBW larvae fall from the canopy onto understory stems, the defoliation of a given sapling or seedling may be influenced by the composition of the canopy directly above the stem. Therefore, the effect of canopy composition was also assessed at the local scale, i.e. in a stems’ neighbourhood (in a 5-m
total of 50 balsam fir stems and 50 black spruce stems. The exact height (for the first four size classes) or dbh (for the fifth size class) of these stems were measured, as well as the width of their crown where the branches were the longest. For stems in the fifth height class (> 2.3-m tall but < 5-cm dbh), dbh was converted to height using allometric equations developed on a sample of 144 balsam fir and 74 black spruce saplings (balsam fir: R2 = 0.70; black spruce: R2 = 0.75; Appendix A).
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Fig. 3. Relative (left) and absolute (right) stand composition of the canopy (top) and advance regeneration (bottom) for the natural (N) and harvested (H) sites (preand post-harvest). Note that the scale is smaller for the post-harvest canopy composition in absolute abundance. N.A. = not available.
radius around each regenerating stem). Neighbourhood composition was determined for each stem of advance regeneration by overlaying the map of georeferenced mature trees and that of sampled advance regeneration in ArcGIS (ESRI, 2012). First, we created a 5-m radius buffer around each regeneration stem to represent the local composition in their neighbourhoods. Second, we created buffers representing the horizontal extent of the crown of each mature tree using speciesspecific allometric relationships between dbh and crown width (balsam fir: R2 = 0.50, P < 0.0001, n = 1044; black spruce: R2 = 0.28, P < 0.0001, n = 2169; Appendix A). The two buffer layers were then intersected to calculate, for each regenerating stem, the proportion of balsam fir, black spruce, white spruce (Picea glauca (Moench) Voss), and open space in the canopy in a 5-m radius. Neighbourhood composition was calculated as the percentage of the area occupied by balsam fir over the total area of the advance regeneration’s surroundings, including the area unoccupied by canopy trees.
mixed-effects linear model in R (package nlme, version 3.2.2; R Core Team, 2015). This model contained only the natural sites (n = 2195 regeneration stems). Then, to test if harvest and the number of years since harvest influenced cumulative defoliation of advance regeneration (second hypothesis), we built a second mixed-effects linear model, combining all natural and harvested sites (n = 7577 stems). Mixed-effects linear modeling allowed us to account for environmental differences among sites, which were treated as a random factor. In all cases, the median of each regenerating stem’s defoliation class was used as the response variable. Because height classes were available for the entire sample, we used this variable instead of exact heights or crown widths in the models, which were measured on subsamples only. All numerical variables were centered on their means before analysis to facilitate interpretation of coefficient.
2.4.2. Hypothesis testing To test whether the species and height of advance regeneration, stand-scale and neighbourhood composition had an effect on cumulative defoliation of advance regeneration (first hypothesis), we built a
3.1. Stand composition
3. Results
Relative balsam fir basal area in the canopy ranged from 2.6% to 72.2% in natural sites, from 11.8% to 63.9% in harvested sites prior to
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Fig. 4. Predicted defoliation of advance regeneration according to regeneration height and species for three stand-scale canopy compositions (top; in black sprucedominated stands, balsam fir basal area was < 2%, averaged 42% in mixed stands, and was > 72% in balsam fir-dominated stands), and three neighbourhood canopy compositions (bottom; balsam fir basal area was 10% in black spruce-dominated neighbourhoods, averaged 50% in mixed neighbourhoods, and was > 90% in balsam fir-dominated neighbourhoods). Shaded areas indicate the 95th confidence interval calculated on fixed effects. Stand-scale composition graphs were calculated based on the mean value of neighbourhood compositions and vice versa.
Table 1 Model parameter estimates of the mixed linear model for cumulative defoliation (%) of advance regeneration in natural sites. Model included seedling height, species, stand-scale canopy composition, neighbourhood canopy composition and three-way interactions among seedling height, species and either stand-scale or neighbourhood canopy composition (n = 2195). N.S. = non-significant (alpha = 5%); CC = canopy composition. Coefficient
Standard error
t-value
p-value
28.6905 0.0998 15.1594 0.5074 0.0184 0.1259 0.0805 −0.0009 0.0023 −0.1184 0.0022 −0.0023
1.7599 0.0075 0.9495 0.0810 0.0520 0.0006 0.0106 0.0006 0.0005 0.0551 0.0008 0.0006
16.3025 13.2714 15.9657 6.2634 0.3545 1.7193 7.6242 −1.7026 5.1128 −2.1492 2.7706 −3.6032
< 0.0001 < 0.0001 < 0.0001 0.0008 N.S. N.S. < 0.0001 N.S. < 0.0001 0.0317 0.0056 0.0003
Fixed effects Intercept Height Species (reference: black spruce) Stand-scale CC (% balsam fir) Neighbourhood CC (% balsam fir) Species * Neighbourhood CC Height * Species Height * Neighbourhood CC Height * Stand-scale CC Species * Stand-scale CC Height * Species * Neighbourhood CC Height * Species * Stand-scale CC Random effects
Inter-group standard deviation Site
4.5884
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and neighbourhood canopy composition; Fig. 4a, b and c). Taller balsam fir seedlings were three times as defoliated as the smallest ones, whereas it was 2.3 ftimes or black spruce. However, defoliation of black spruce regeneration in black spruce-dominated stands was very low and did not change with stem height (Fig. 4a), but as the proportion of balsam fir increased in the canopy, black spruce defoliation also increased (from 6% in black spruce stands to > 40% in balsam fir stands, when adjusted for mean seedling height and neighbourhood canopy composition), especially for larger stems (Fig. 4a, b and c). Neighbourhood (5-m radius) canopy composition had a significant but weaker effect (Height * Species * Neighbourhood CC: P < 0.01; Table 1) than stand-scale canopy composition (Height * Species * Standscale CC: P < 0.001; Table 1). Whereas the relationships between defoliation and canopy composition were similar at the neighbourhood vs. stand scale in mixed composition stands (Fig. 4b and e), they were different in pure compositions for black spruce regeneration. Notably, at the local scale, black spruce regeneration was more defoliated in black spruce-dominated neighbourhoods (16–47%; Fig. 4d) than in balsam fir-dominated neighbourhoods (25–36%; Fig. 4f), whereas the opposite was observed at the stand scale. To determine the height above which regenerating stems were most likely to be severely defoliated, we used a logistic regression between defoliation severity (light: defoliation < 50% vs. severe: defoliation ≥50%) and stem height (Fig. 5). Balsam fir stems > 1.2-m tall had a 0.5 probability of sustaining severe defoliation and the probability reached 0.95 above 2 m in height. For black spruce, the probability of sustaining severe defoliation reached 0.5 only in saplings taller than 3.5 m. Using the exact height or crown width (measured in the subsample), instead of height class medians, gave identical results. Therefore, using height classes, which saves times in the field and is less prone to variability among people making the measurements, is sufficient for assessing advance regeneration vulnerability (height classes vs. exact height: r = 0.97; height classes vs. crown width: r = 0.85; exact height vs. crown width: r = 0.88; n = 675; data not shown otherwise).
Fig. 5. Probability of sustaining severe (≥50%) defoliation as a function of stem height. Shaded areas indicate the 95th confidence interval.
salvage logging and from 27.3% to 57.0% following salvage logging (Fig. 3, Appendix B). Black spruce was the other dominant species, while white spruce and hardwood species (mainly white birch [Betula papyrifera Marsh]) were occasionally found.
3.2. Factors influencing defoliation of advance regeneration in natural sites Defoliation increased with stem height and this relationship differed significantly by seedling species and with stand-scale canopy composition (three-way interaction; Fig. 4, Table 1). Balsam fir was consistently more defoliated than black spruce (15% more defoliation when adjusted for mean seedling height, neighbourhood and standscale canopy composition; Table 1), and the positive relationship between regeneration height and defoliation increased as the proportion of balsam fir in the canopy increased (from 26% in black spruce stands to > 50% in balsam fir stands, when adjusted for mean seedling height
3.3. Effect of salvage harvesting on defoliation of advance regeneration Time since harvest did not affect defoliation of advance
Table 2 Model parameter estimates of the mixed linear model for cumulative defoliation (%) of advance regeneration in natural and logged sites. Model included seedling height, species, stand-scale canopy composition, status (logged or natural), and all interactions (n = 7577). N.S. = non-significant (alpha = 5%); CC = canopy composition. Coefficient
Standard error
t-value
p-value
24.8721 0.0620 16.8091 0.5795 −0.6685 0.0945 0.0018 −0.1301 −0.0013 0.0547 −5.8683 −0.3963 −0.0289 −0.0018 0.3318 0.0026
1.5163 0.0057 0.7075 0.0615 2.3317 0.0077 0.0003 0.0305 0.0003 0.0079 1.0379 0.1039 0.0110 0.0004 0.0471 0.0005
16.4030 10.8019 23.7571 9.4194 −0.2867 12.2748 6.7931 −4.2580 −3.6283 6.9158 −5.6540 −3.8151 −2.6249 −4.7316 7.0443 5.1479
< 0.0001 < 0.0001 < 0.0001 < 0.0001 N.S. < 0.0001 < 0.0001 < 0.0001 0.0003 < 0.0001 < 0.0001 0.0034 0.0087 < 0.0001 < 0.0001 < 0.0001
Fixed effects Intercept Height Species (reference: black spruce) CC (Stand-scale % balsam fir) Logging (reference = natural sites) Height * Species Height * CC Species * CC Height * Species * CC Logging * Height Logging * Species Logging * CC Logging * Height * Species Logging * Height * CC Logging * Species * CC Logging * Height * Species * CC Random effects
Inter-group standard deviation Site
3.7389
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Fig. 6. Predicted defoliation of advance regeneration in natural and harvested sites by stem height for balsam fir (a–c) and black spruce (d–f) and three stand-scale canopy compositions (for black spruce-dominated stands, relative basal area in balsam fir was 10%, it was 40% for mixed stands, and 65% for balsam fir-dominated stands). Shaded areas indicate the 95th confidence interval calculated on fixed effects.
First, defoliation increased with height of advance regeneration, which supports our first hypothesis and the results of Nie et al. (2018). This can be explained by the fact that taller stems have wider crowns, and thus a higher probability of intercepting larvae spinning down from the canopy, as well as potentially being a suitable site for females to lay eggs. Smaller regenerating stems may also benefit from the sheltering effect provided by taller stems, which would intercept falling larvae and in turn reduce the probability of larvae landing on smaller stems. Ruel and Huot (1993) also observed that balsam fir regeneration over two meters in height was practically absent from balsam fir-dominated stands that were severely defoliated. Similar to forests defoliated by the gyspy moth (Lymantria dispar dispar), stand recovery may be ensured by smaller established seedlings, as tall advance regeneration sustains greater defoliation and mortality (Hix et al., 1991). Also consistent with our hypothesis, balsam fir regeneration was more defoliated than black spruce. Even though this should not be a surprise, as mature balsam fir trees are also more susceptible to the SBW, the mechanisms driving susceptibility of understory tree species may not be the same as for mature trees. While SBW larvae are present in the canopy of natural forests because it is their preferred oviposition site, their presence in the understory is mostly due to spinning down from the canopy when food is scarce in the canopy or when populations are high. The greater defoliation of balsam fir regeneration may depend
regeneration (P = 0.2919). The best model included a four-way interaction among all effects (Table 2). It showed that in both harvested sites and natural sites, defoliation was greater on balsam fir regeneration than it was on black spruce (16.8% more defoliation when adjusted for mean seedling height and stand-scale canopy composition). Like we observed in natural sites, defoliation increased with regeneration height and with the proportion of balsam fir in the canopy (Fig. 6, Table 2). However, in stands that were dominated by black spruce before logging, the effect of harvesting greatly increased defoliation of black spruce regeneration after harvesting (Fig. 6d). Whereas defoliation of tall (> 2.5 m) black spruce regeneration reached 30% in harvested sites, it did not even attain 10% in natural sites (Fig. 6d). 4. Discussion 4.1. Factors influencing defoliation of regeneration in natural sites SBW outbreaks have affected the boreal forest for centuries, shaping its dynamics, regeneration processes, and resilience (Morin, 1994), yet little is known about the effects of defoliation on advance regeneration. This study provides insight into how SBW outbreaks affect different size classes of regeneration of balsam fir and black spruce in natural and salvage logged sites.
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on the timing of the dispersal. Early in the spring, phenology could explain a difference in defoliation between the host species, as bud burst occurs earlier in fir. Later in the season, differences in defoliation could be due to mechanisms other than phenology, such as (i) black spruce’s greater foliage biomass (approximately 25–50% greater than balsam fir for the studied range of DBH), which would allow black spruce to better sustain defoliation caused by a given density of larvae (Lambert et al., 2005; Ung et al., 2008), (ii) the greater propensity of black spruce to produce epicormic shoots, allowing the replacement of consumed foliage (Simard and Payette, 2003), (iii) differences in foliage quality (nutrient contents, chemical and/or physical defenses, etc.) between the two studied species (Blais, 1957; Hudes and Shoemaker, 1984), or some combination of all of these factors. Seedling defoliation also increased as the proportion of balsam fir in the canopy increased, which supports our third hypothesis and is consistent with the resource concentration hypothesis (Bognounou et al., 2017). For black spruce seedlings, this effect was stronger in fir-dominated stands than in fir-dominated neighbourhoods (Fig. 4a). This may be due to associational susceptibility at high SBW population density in balsam fir stands leading to greater defoliation of all stems in the understory, irrespective of their species. Whereas in neighbourhoods, ballooning of down-spinning threads may lead to larvae not descending directly below infested trees. In addition to this spatial variability, Bognounou et al. (2017) also suggest that there may be temporal variability in neighbourhoods and stand-level defoliation patterns between species.
growth rate and increases its mortality rate, stand composition may shift away from black spruce and towards balsam fir, which may increase forest susceptibility to future infestations of the SBW. Even modest increases in balsam fir abundance may have disproportionate effects on defoliation of black spruce because of the associational susceptibility of black spruce (Atsatt and O’Dowd, 1976; Bognounou et al., 2017).
4.2. Effect of harvesting on defoliation of regeneration
Conflict of interest
4.3. Implications for sustainable forest management The effect of salvage harvesting on black spruce defoliation has important landscape-scale consequences in the study area, where the totality of the annual allowable cuts are currently carried out in defoliated stands. The results of this study suggest that forest management may be locally adapted in response to broad-scale infestations, similar to results found on salvage logging following a spruce beetle (Dendroctonus rufipennis Kirby) outbreak (Boucher and Mead, 2006). To protect spruce advance regeneration from increased defoliation, salvage harvesting of spruce-dominated stands may thus be delayed until the outbreak has subsided. Long-term studies are needed to determine whether a compositional change towards balsam fir occurs or not, particularly in spruce-dominated stands. We argue however that as a precautionary measure, changes in salvage logging practices may be implemented immediately to avoid potential problems such as decreased black spruce abundance and increased susceptibility to future SBW outbreaks.
Our study shows that the natural regeneration process of a boreal forest affected by the SBW may be compromised by salvage logging in black spruce-dominated stands. Under natural dynamics, black spruce is maintained during outbreaks because of the preference of SBW for balsam fir; as a corollary, stands in which black spruce is abundant decrease the future impact of the SBW (Burton et al., 2015). This phenomenon was also found in oak (Quercus spp.) forests attacked by the gypsy moth, as they underwent an increase in ligneous species diversity that reduced their susceptibility to future attacks by that insect (Hix et al., 1991). However, in our case, regeneration processes in black spruce-dominated stands are likely compromised as the black spruce component of the overstory that is removed is much less vulnerable than balsam fir (black spruce sustains less than a third of the defoliation sustained by balsam fir; Hennigar et al., 2008) while the opening of the canopy exposes tall black spruce regeneration to defoliation (6 times more defoliation in harvested sites (30%) than in natural sites (5%)). We speculate that if increased defoliation of black spruce reduces its
None.
Acknowledgments This study was conducted in collaboration with the Canadian Forest Service. We are grateful to M. Marchand (Nat. Res. Can., LFC) and J.-C. Ruel (Laval University) for their counseling throughout the project, and to G. Cervello, H. Dorion, M. Gauvin and J. Moisan Perrier for their dedication and hard work collecting data on the field. We also want to thank the Centre for Forest Research. Funding: This work was supported by the Fonds de recherche du Québec – Nature et technologies (FRQNT, Programme de financement de la recherche et développement en aménagement forestier, #165640) and the Natural Sciences and Engineering Research Council of Canada (NSERC, #RGPIN-2017-06616).
Appendix A. Allometric equations Allometric equations used to convert diameter at breast height (dbh, in cm) to stem height (cm) in the fifth height class (> 2.3-m tall but < 5cm dbh) using a sample of 144 balsam fir and 74 black spruce saplings (balsam fir: R2 = 0.70; black spruce: R2 = 0.75):
Balsam fir
Black spruce
height = 36.89∗dbh + 114.62
(A.1)
height = 37.145∗dbh + 139.34
(A.2)
Allometric equations used to convert basal diameter (bd, in cm) to diameter at breast height (dbh, in cm) for stumps in salvage logged sites using a sample of 104 balsam fir trees and 98 black spruce trees (balsam fir: R2 = 0.97; black spruce: R2 = 0.98): (A.3)
Balsam fir dbh = 0.0002∗bd2−0.011∗bd + 6.3713
Black spruce
dbh = 0.0002∗bd2−0.0384∗bd + 10.741
(A.4)
Allometric equations used to convert diameter at breast height (dbh, in cm) to crown width (cw, in cm) for mature trees in the local composition analysis (balsam fir: R2 = 0.50, P < 0.00001, n = 1044; black spruce: R2 = 0.28, P < 0.00001, n = 2169):
Balsam fir
Black spruce
(A.5)
cw = 10.163∗dbh + 128.91
cw = 5.7987∗ dbh + 124.21
(A.6) 428
2011-ongoing 2007-ongoing 2009-ongoing 2011-ongoing 2010-ongoing 2010-ongoing 2011-ongoing 2011-ongoing 2009-ongoing 2012-ongoing 2009-ongoing 2010-ongoing 2010-ongoing 2011-ongoing
−68.1569 −68.0294 −68.1231 −67.8251 −67.7824 −67.8032 −68.1482 −68.1401 −68.1270 −68.0718 −68.0602 −68.0700 −68.2299 −68.2489
N1 N2 N3 N4 N5 N6 N7 N8 H1 H2 H3 H4 H5 H6
50.1219 49.4052 49.7051 49.5785 49.5756 49.5710 50.1610 49.7153 49.7044 49.7023 49.7490 50.1637 49.6995 49.7092
Defoliation period
Longitude (DD)
Site Latitude (DD) – – – – – – – – 2013 2012 2012 2012 2012 2011
Year of harvest
17.9 18.4 17.8 14.3 14.4 15.9 17.6 18.5 14.0 10.8 10.3 10.9 13.0 16.4
Avg DBH (cm)
72.2/N.A. 69.1/N.A. 68.8/N.A. 60.7/N.A. 48.0/N.A. 35.9/N.A. 26.1/N.A. 2.60/N.A. 63.9/51.1 42.8/57.0 32.6/54.8 14.3/34.5 12.7/27.3 11.8/27.4
Bf (pre/post logging) 14.7/N.A. 5.40/N.A. 11.1/N.A. 37.4/N.A. 50.3/N.A. 64.1/N.A. 73.9/N.A. 96.9/N.A. 29.9/17.7 57.2/43.0 67.4/45.2 85.7/65.5 84.3/18.4 88.2/72.6
Bs (pre/post logging)
Relative abundance (%)
Canopy composition (basal area)
5.7 21.8 7.1 0.0 1.5 0.0 0.0 0.0 0.0 0.0 0.0 0.0 2.8 0.0
Ws
7.4 3.7 13.0 1.9 0.2 0.0 0.0 0.5 6.2 0.0 0.0 0.0 0.1 0.0
Hw
27.6/N.A. 19.8/N.A. 13.2/N.A. 8.30/N.A. 8.90/N.A. 6.60/N.A. 3.80/N.A. 0.80/N.A. 11.4/0.11 6.90/0.06 8.70/0.10 3.80/0.06 5.10/0.04 2.10/0.02
Bf (pre/post logging)
5.6/N.A. 1.6/N.A. 2.1/N.A. 5.1/N.A. 9.3/N.A. 11.7/N.A. 10.7/N.A. 31.0/N.A. 5.3/0.04 9.2/0.04 18.0/0.08 22.8/0.10 33.8/0.03 15.6/0.06
Bs (pre/post logging)
Absolute abundance (m2/ha)
Avg DBH: Average diameter at breast height; Bf: Balsam fir; Bs: Black spruce; Ws: White spruce; Hw: Hardwoods; N.A.: Not available.
Appendix B. Site description
2.2 6.2 1.4 0.0 0.3 0.0 0.0 0.0 0.0 0.0 0.0 0.0 1.1 0.0
2.8 1.1 2.5 0.3 0.0 0.0 0.0 0.2 1.1 0.0 0.0 0.0 0.1 0.0
68,350 28,650 58,900 37,450 6750 18,350 55,600 17,000 56,950 10,150 19,600 24,450 18,500 8300
Ws Hw Bf
750 16,450 21,450 11,150 26,400 28,350 61,350 17,250 20,000 22,450 14,000 66,250 62,850 38,800
Bs
50 650 0 1000 100 50 500 100 14,300 1400 5350 0 9800 4950
Hw
density (stems/ha)
Regeneration
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