Balsam Fir Forest Research Articles

Soil solution chemistry weak response to long-term N addition points towards a strong resilience of northeastern American forests to past and future N deposition

Northern temperate and boreal forests are large biomes playing crucial ecological and environmental roles, such as carbon sequestration. Despite being generally remote, these forests were exposed to anthropogenic nitrogen (N) deposition over the last two centuries and may still experience elevated N deposition as human activities expand towards high latitudes. However, the impacts of long-term high N deposition on these N-limited forest ecosystems remain unclear. For 18 years, we simulated N deposition by chronically adding ammonium nitrate at rates of 3 (LN treatment) and 10 (HN treatment) times the ambient N deposition estimated at the beginning of the experiment at a temperate sugar maple and a boreal balsam fir forest site, both located in northeastern America. LN and HN treatments corresponded respectively to addition of 26 kgN·ha−1·yr−1 and 85 kgN·ha−1·yr−1 at the temperate site and 17 kgN·ha−1·yr−1 and 57 kgN·ha−1·yr−1 at the boreal site. Between 2002 and 2018, soil solution was collected weekly during summer and concentrations of NO3−, NH4+, Ca2+ and pH were measured, totalling ~12,700–13,500 observations per variable on the study period. N treatments caused soil solution NO3−, NH4+ and Ca2+ concentrations to increase while reducing its pH. However, ion responses manifested through punctual high concentration events (predominantly on the HN plots) that were very rare and leached N quantity was extremely low at both sites. Therefore, N addition corresponding to 54 years (LN treatment) and 180 years (HN treatment) of accelerated ambient N deposition had overall small impacts on soil solution chemistry. Our results indicate an important N retention of northeastern American forests and an unexpected strong resilience of their soil solution chemistry to long-term simulated N deposition, potentially explained by the widespread N-limitation in high latitude ecosystems. This finding can help predict the future productivity of N-limited forests and improve forest management strategies in northeastern America.

Carbon balance of forest management and wood production in the boreal forest of Quebec (Canada)

Management of boreal forests can increase terrestrial carbon sinks and reduce greenhouse gas (GHG) emissions to the atmosphere. A case study was conducted in the boreal balsam fir forests of Quebec (Canada), a commercially important region for forestry, to identify optimal management and wood production solutions that contribute to reducing GHG emissions to the atmosphere. Scenarios were based on a steady level of harvest and silvicultural activities over time and a stable flow of wood products to markets. Scenarios included: increases and decreases in the volume of harvested timber; the transition of harvesting activities from clearcuts (the most common practice in the region) to partial cuts; and changes in the rate of natural regeneration (the usual mode of regeneration) vs. plantations. All scenarios provided a carbon sink regardless of the time frame. Compared with other scenarios, reducing harvest levels increased the forest carbon sink in the short (10 to 20 years) and medium (20 to 50 years) terms. Also, for a similar harvest level, the increased use of partial cutting and planting increased the forest carbon sink. In the long term (over 50 years), strategies with low harvesting levels resulted in lower ecosystem carbon sequestration, even though they still had the lowest cumulative emissions. Nevertheless, higher harvesting levels could not be justified because the long-term increase in the forest ecosystem carbon sink could not offset higher emissions from wood products, particularly from short-lived paper products. Sensitivity analyses showed that improving sawmill efficiency and thus increasing the proportion of long-lived products was an important factor that can greatly reduce emissions. On the other hand, transportation distances between forest stands and sawmills had a relatively marginal impact on the overall carbon balance of forest management and wood production scenarios.

The post-fire shift of temperate white pine-birch forest to boreal balsam fir forest in eastern Canada: climate-fire implications

Extensive 14C dating and botanical identification of charcoal fragments located in the organic surface soil layer and buried in the mineral podzolic solum were used to reconstruct the successional pathways of a balsam fir forest site. The studied forest site developed in a context of continuous fire disturbance over the last 9000 years with at least 26 fires occurring at a mean interval of 330 years. Tree vegetation of the site followed a four-step trajectory consisting of an early-Holocene spruce forest and a late-Holocene mixedwood balsam fir forest. Boreal-like spruce-birch and temperate-like white pine-birch forests dominated the site between 7900 and 5900 cal. B.P. and 5600 and 1275 cal. B.P., respectively. Because all forest types developed repeatedly after fire since early deglaciation, changes in forest composition, in particular the shift of white pine forest to balsam fir forest, and concurrent decline of birch (yellow birch and/or paper birch) and pine populations were most likely related to progressive cooler and wetter conditions from mid- to late Holocene. Fire disturbance on this part of the southern boreal biome has been a continuous, positive regenerative process over the Holocene, allowing the successional turnover of boreal and temperate trees under the influence of climatic change.

Effects of a warmer climate and forest composition on soil carbon cycling, soil organic matter stability and stocks in a humid boreal region

Abstract. The maintenance of the large soil organic carbon (SOC) stocks of the boreal forest under climate change is a matter of concern. In this study, major soil carbon pools and fluxes were assessed in 22 closed-canopy forests located along an elevation and latitudinal climatic gradient expanding 4 ∘C in mean annual temperature (MAT) for two important boreal conifer forest stand types: balsam fir (Abies balsamea), a fire avoider, and black spruce (Picea mariana), a fire-tolerant species. SOC stocks were not influenced by a warmer climate or by forest type. However, carbon fluxes, including aboveground litterfall rates, as well as total soil respiration (Rs) and heterotrophic (Rh) and autotrophic soil respiration (Ra), were linearly related to temperature (cumulative degree days >5 ∘C). The sensitivity of soil organic matter (SOM) degradation to temperature, assessed by comparing Q10 (rate of change for a T increase of 10 ∘C) of soil respiration and Rs10 (soil respiration rates corrected to 10 ∘C), did not vary along the temperature gradient, while the proportion of bioreactive carbon and nitrogen showed higher values for balsam fir and for warmer sites. Balsam fir forests showed a greater litterfall rate, a better litter quality (lower C : N ratio) and a higher Rs10 than black spruce ones, suggesting that their soils cycle a larger amount of C and N under a similar climate regime. Altogether, these results suggest that a warmer climate and a balsam fir forest composition induce a more rapid SOC turnover. Contrary to common soil organic matter stabilisation hypotheses, greater litter input rates did not lead to higher total SOC stocks, and a warmer climate did not lead to the depletion of bioreactive soil C and N. Positive effects of warming both on fluxes to and from the soil as well as a potential saturation of stabilised SOC could explain these results which apply to the context of this study: a cold and wet environment and a stable vegetation composition along the temperature gradient. While the entire study area is subject to a humid climate, a negative relationship was found between aridity and SOM stocks in the upper mineral soil layer for black spruce forests, suggesting that water balance is more critical than temperature to maintain SOM stocks.

Cumulative effects of spruce budworm and moose herbivory on boreal forest ecosystems

Abstract Ungulate herbivory is a key natural disturbance in many ecosystems. In forest ecosystems, ungulate herbivory often co‐occurs with other disturbances yet there are few studies looking at the cumulative impacts of ungulates and other natural disturbances on forest dynamics. We report on an 18‐year experiment to investigate the combined effects of introduced, non‐native moose and native spruce budworm herbivory on balsam fir forest recruitment and regeneration. Specifically, we measured inputs into early life‐history components of balsam fir dominant boreal forest ecosystems such as seed density, seedbed quality, seedling and sapling height and density, as well as adult density in eight paired moose exclosure and control sites across a spruce budworm disturbance gradient (i.e. no outbreak, recent outbreak, old outbreak) that resulted in closed, partially open and open canopies. Piecewise structural equation models (SEMs) revealed a strong negative effect of spruce budworm disturbance on balsam fir adult density and indirect effects of this budworm disturbance on balsam fir seedling and sapling density mediated through adult density. We observed a similar pattern of budworm disturbance effects on balsam fir seed density with the lowest densities in sites with open canopy, then partially open and closed canopies. As expected, the SEM showed a positive effect of bryophyte seedbed cover on balsam fir seedling density. The SEM showed support for a positive effect of moose exclusion on balsam fir sapling height and density. Specifically, saplings were on average taller and denser in exclosure than control plots and these differences are most pronounced at sites with partially open canopies created by spruce budworm disturbance and undisturbed or closed canopy sites. Overall, we show evidence for cumulative and interactive effects of spruce budworm and moose herbivory on all life stages of balsam fir and such effects are negatively affecting regeneration of balsam fir forests. Human activities are creating conditions that facilitate the expansion and increased impacts of herbivores on boreal ecosystem functioning. A greater understanding of how ungulate herbivory interacts with other disturbances is needed to improve our predictions of forest ecosystem dynamics under global change. A free Plain Language Summary can be found within the Supporting Information of this article.

Aerial spraying of bacterial insecticides to control spruce budworm defoliation leads to reduced carbon losses

AbstractSpruce budworm (SBW) outbreaks are a major natural disturbance in boreal forests of eastern North America. During large‐scale infestations, aerial spraying of bacterial insecticides is used to suppress local high‐density SBW populations. While the primary goal of spraying is the protection of wood volume for later harvest, it should also maintain carbon stored in trees. This study provides the first quantitative analysis of the efficacy of aerial spraying against SBW on carbon dynamics in balsam fir, spruce, and mixed fir–spruce forests. In this study, we used the TRIPLEX‐Insect model to simulate carbon dynamics with and without spray applications in 14 sites of the boreal forest located in various regions of Québec. We found that the efficacy of aerial spraying on reducing annual defoliation was greater in the early stage (<5 yr since the outbreak began) of the outbreak than in later (5–10 yr since the outbreak began) stage. Our results showed that more net ecosystem productivity is maintained in balsam fir (the most vulnerable species) than in either spruce or mixed fir–spruce forests following spraying. Also, average losses in aboveground biomass due to the SBW following spraying occurred more slowly than without spraying in balsam fir forests. Our findings suggest that aerial spraying could be used to maintain carbon in conifer forests during SBW disturbances, but that the efficacy of spray programs is affected by host species and stage of the SBW outbreak.

Early responses of bark and wood boring beetles to an outbreak of the hemlock looper Lambdina fiscellaria (Guenée) (Lepidoptera: Geometridae) in a boreal balsam fir forest of North America

AbstractWe investigated the early responses of bark and wood boring beetles to an extensive supply of balsam fir (Abies balsamea) trees, stressed or recently killed during an outbreak of the hemlock looper (Lambdina fiscellaria) in Quebec, Canada. The present study aimed to (i) examine beetle assemblages across all levels of defoliation and (ii) assess the relative influences of defoliation, tree mortality, stand structure and spatial context on beetle assemblages. We used three types of traps in each of 36 plots: multidirectional flight interception traps measured adult beetle flight activity within stands, trunk window traps measured visitation rates at trees and emergence traps measured beetle colonization of trees. Flight traps in nondefoliated plots allowed characterization of ‘pre‐outbreak’ beetle assemblages, which were not discernable in emergence traps because few insects colonize heathy trees. ‘Pre‐outbreak’ assemblages, characterized by a mix of polyphagous beetle species (e.g. Evodinus monticola, Dryocoetes autographus), shifted to those dominated by Trypodendron lineatum after extensive defoliation. Two Pissodes species colonized heavily defoliated trees but not dead ones. Defoliation level, conifer basal area, basal area of dead balsam firs and the extent of recently logged areas around sampling plots were positively correlated with the abundance of T. lineatum.

Resistance, Resilience or Change: Post-disturbance Dynamics of Boreal Forests After Insect Outbreaks

Understanding and measuring forest resistance and resilience have emerged as key priorities in ecology and management, particularly to maintain forest functioning. The analysis of the factors involved in a forest’s ability to cope with disturbances is key in identifying forest vulnerability to environmental change. In this study, we apply a procedure based on combining pathway analyses of forest composition and structure with quantitative indices of resistance and resilience to disturbances. We applied our approach to boreal forests affected by a major spruce budworm outbreak in the province of Quebec (Canada). We aimed to identify the main patterns of forest dynamics and the environmental factors affecting these responses. To achieve this goal, we developed quantitative metrics of resistance and resilience. We then compared forests with different pre-disturbance conditions and explored the factors influencing their recovery following disturbance. We found that post-outbreak forest dynamics are determined by distinct resistance and resilience patterns according to dominant species and stand composition and structure. Black spruce forests are highly resistant to spruce budworm outbreaks, but this resistance is conditioned by the length of the defoliation period, with long outbreaks having the potential to lead the system to collapse. In contrast, balsam fir forests easily change to a different composition after outbreaks but are highly resilient when mixed with hardwood species. Overall, the severity of the disturbance and the tree species affected are the main drivers contributing to boreal forest resistance and resilience. Our procedure is valuable to understand post-disturbance dynamics of a broad range of communities and to guide management strategies focused on enhancing the resistance and resilience of the system.

Spatially explicit climate change projections for the recovery planning of threatened species: The Bicknell's Thrush (Catharus Bicknelli) as a case study

With climate change, natural resource managers are faced with the challenging task of planning the conservation of habitat for threatened species. Classified as "threatened" under the Species at Risk Act in Canada, the Bicknell's Thrush (Catharus Bicknelli - BITH) is a migratory bird whose range is highly restricted. Bioclimatic models project more than 50% loss of suitable habitat - high elevation dense balsam fir (Abies balsamea) forests - in the northeastern USA by 2050 due to climate change. We used BITH as a case study to demonstrate the value of forest landscape models (e.g., LANDIS-II) to support decision-making on conservation of habitat of threatened species. We modeled the impacts of forest management and natural disturbances, as well as climate-induced changes, on forest stand and landscape structure. Under RCP climate forcing scenarios 2.6, 4.5 and 8.5, simulations projected significant changes in dominant tree species biomass, from coniferous to broadleaved deciduous, implying important losses of mature balsam fir forest. Climate change was projected to have severe effects with major changes projected to occur after 2080 and losses of more than half of BITH suitable habitat by 2100 for “worst-case” climate change scenarios. This contrasts with results from bioclimatic models which do not capture the expected lags in vegetation responses to changing climate. Our results also suggest that innovative forest management strategies could help maintain BITH habitat abundance under RCP 2.6 and RCP 4.5 climate forcing. Such results can provide guidance for considering effective long-term conservation of habitat for threatened species in a changing world.

Solar radiation transmittance of a boreal balsam fir canopy: Spatiotemporal variability and impacts on growing season hydrology

Forest canopies act as permeable barriers between the atmosphere and the ground, reflecting and absorbing solar radiation. In the boreal forest, the large number of gaps and heterogeneities further complicates these processes. Several studies have adequately measured and modeled the transmittance of solar radiation through forest canopies in western North America and Scandinavia, but few have addressed those of Eastern North America. Furthermore, most of these studies have assessed the effects of solar radiation transmittance on snowpack energetics, but few have focused on the hydrological impacts during the growing season. This paper addresses this knowledge gap with precise measurements of sub-canopy solar radiation in a juvenile balsam fir forest located in the Montmorency Forest, Quebec, Canada. Twenty (20) sub-canopy stations were deployed in a 200 m by 150 m gridded box around a flux tower measuring above canopy radiation and eddy covariance fluxes during late summer and early fall 2016. Results show that the heterogeneous forest has substantial spatial variability of transmittance, with site-specific seasonal averages ranging between 0.07 and 0.69. Canopy gaps of size relative to tree height (H) between 0.1H and H had a temporal influence on solar radiation transmittance in canopy gaps at the sub-daily scale, but do not influence seasonal trends. This is attributed to very frequent cloudiness at the site, which renders the solar radiation mostly diffuse. As a result, a Beer-Lambert extinction law proved adequate at modeling site-specific or spatially averaged transmittance on a seasonal basis. We complement the observations by modeling canopy and soil moisture balances at 20 sites using the Canadian Land Surface Scheme (CLASS). The modeling results exhibit the following trend: a thicker (thinner) vegetation leads to more (less) evapotranspiration, because there is more (less) evaporation of intercepted precipitation and more (less) transpiration, but less (more) ground evaporation. During drier periods, the latter leads to wetter soil conditions for the thicker vegetation. These modeling results of sensitivity to vegetation density, while informative, still need to be confirmed with observations.

Dynamics of detrital carbon pools following harvesting of a humid eastern Canadian balsam fir boreal forest

Forest management strongly influences the carbon (C) budget of boreal forests and their potential to mitigating greenhouse gas emissions. A better quantification of the net changes of carbon pools with time since harvesting is necessary to guide the development of climate-friendly forest management practices. The objective of this study was to assess the evolution of forest C pools, with a special focus on detrital biomass, in an 80-year post-harvesting chronosequence consisting of 36 very homogenous stem-only harvested plots from a humid boreal balsam fir forest of eastern Canada. Dead wood C stocks comprised of snags, stumps, downed woody debris and buried wood averaged 37 Mg C ha−1 and evolved according to an upward-facing «boomerang» shape pattern throughout the chronosequence (rapid decrease in the first years followed by a constant increase until the end of the time horizon). In contrast, soil C stocks (FH and mineral) averaged 156 Mg C ha−1 and remain constant through time. Stand C sequestration increased rapidly in the early stages up to age 50 when it reached about 250 Mg C ha−1, and then continued to accumulate at a slower rate. The temporal trends observed in C pools suggest that C originating from aboveground dead wood (snags, stumps, downed woody debris) is either leaving the system (respired or leached) or transferred into buried wood, and does not appear to influence the C stocks of the fine fraction of the organic and mineral soil horizons. However, the ultimate fate of dead wood C is still poorly understood and further research is needed in this field. We recommend fixing the length of harvest rotation at a minimum of 50 years for this ecosystem to allow the build-up of its dead wood capital, and to promote dead wood retention on site. We also recommend including buried wood in carbon inventories as this pool represents an important share of the detrital C stock in these humid boreal forests.

Anticosti Island: a hot spot for Neospondylis upiformis (Coleoptera: Cerambycidae) in eastern Canada?

BackgroundDuring an inventory of insect diversity on Anticosti Island in 1993, we caught unprecedented numbers of Neospondylis upiformis (Mannerheim), a longhorned beetle rarely observed in eastern North America. All specimens were caught using 12-funnel Lindgren traps baited with 95% ethanol and α-pinene. This longhorned beetle was captured again in 2007 on Anticosti with the same traps. Other than that, seven specimens of N. upiformis were caught elsewhere in Quebec between 1993 and 2015. Only 14 specimens were found in the 45 most important insect collections of the province, the most recent specimen dating back to 1964.New informationAt least 90% of the captures came from old-growth balsam fir stands of the south-central part of the island. Seasonal flight activity ranged from early June to late July, but adult captures peaked in early July. Results suggest that Anticosti Island might be a hot spot for N. upiformis in eastern North America, particularly in its south-central part where old-growth balsam fir forests still exist.

Macrocharcoal-Based Chronosequences Reveal Shifting Dominance of Conifer Boreal Forests Under Changing Fire Regime

Balsam fir (Abies balsamea) and black spruce (Picea mariana) forests are the main conifer forest types in the North American boreal zone. The coexistence of the two species as well as their respective canopy dominance in distinct stands raises questions about the long-term evolution from one forest type to the other in relation to environmental factors including climate and stand disturbance. We tested the hypothesis that repetitive fire events promote the succession of balsam fir forest to black spruce forest and vice versa. Postfire chronosequences of one black spruce (BSP) and one balsam fir (BFI) sites were reconstructed based on the botanical composition and 14C-dated soil macrocharcoals. The results support the hypothesis of a successional dynamics. The BSP site has been affected by fires for the last 7600 years, whereas the BFI site, after having been impacted by several fires during the first half of the Holocene, evolved in a fire-free environment for the last 4400 years. Periods of fire activity facilitated the dominance of black spruce forests. The cessation of fires around 4400 cal. years BP on BFI site marks the beginning of the transition from black spruce to balsam fir stands. This succession is a long process, due to the ability of black spruce to regenerate by layering in the absence of fire. The resulting balsam fir stands are ancient and precarious ecosystems, since fire generally leads to the return of black spruce. The increase in balsam fir to the detriment of black spruce in boreal forests is a response to a decrease in fire frequency.

Simplicity is key: restoration protocols for nonregenerating forests degraded by overabundant herbivores

AbstractGlobal forests are being lost and degraded at an alarming rate; hence ecological restoration becomes an integral component ensuring future forest health. Beneficial effects of restoration will depend on scientifically based practices within an adaptive management framework. On the island of Newfoundland, moose (Alces alces) have become overabundant since their introduction in the early 1900s causing regeneration failure of the balsam fir (Abies balsamea) forest. Intensive selective browsing by moose on this foundation species within naturally insect‐generated gaps has created “spruce‐moose meadows.” Experimental restoration to support Parks Canada Ecological Integrity targets was implemented in the boreal forest of Terra Nova National Park (Newfoundland, Canada), along a gradient of disturbance from closed canopy forest to large insect‐disturbed gaps. Seedling planting was carried out under various ground preparation treatments (field planting, aboveground suppression, and scarification). Seedling performance (survival, growth, and browsing occurrence) was monitored over 2 years and mixed‐effects models were constructed to determine seedling responses, which form the template of future forests. Results show minimally positive effects of the ground treatments along the gradient of disturbance, while environmental conditions and seedling individual traits explained the majority of seedling responses. Better growth, lower survival, and higher browsing intensity were observed with increasing forest disturbance, with taller seedlings at planting performing the best. Considering that no substantial biological benefits were detected following ground treatments, which are costly and time‐consuming to implement, active restoration in boreal forest can be implemented using standard forestry planting protocols, without any ground preparation, independently of the forest degradation state.

Fungal community composition in the gut of rove beetles (Coleoptera: Staphylinidae) from the Canadian boreal forest reveals possible endosymbiotic interactions for dietary needs

Abstract The goal of this study was to investigate the fungal community composition in the gut of Staphylinidae from boreal forest in order to better understand the diversity and the complexity of fungus-insect relationships. DNA gut content analyses of nine abundant rove beetle species (Coleoptera, Staphylinidae) living in the boreal balsam fir forest ecosystem (Montmorency Forest, Quebec, Canada) were performed to identify the fungal taxa present either as endosymbiotic taxa or as a source of nutrition. A total of 42 fungal operational taxonomic units (OTUs) were recorded from the analysis of 441 fungal ITS rDNA sequences recovered from gut extracts. The OTU richness per species ranged between four in Tachinus quebecensis and 16 in Atheta ventricosa . The fungal mycobiota in posterior gut extracts was dominated by Saccharomycetales (12 OTUs), followed by Sordariomycetes (nine OTUs). No significant difference was observed between the OTU richness recorded within each of the three subfamilies of rove beetles investigated. The core mycobiome of the posterior gut extracts was dominated by three OTUs related to yeasts, with ITS sequences having pairwise similarities equal to or greater than 99% with Candida mesenterica , Debaryomyces spp. and Ophiostoma pluriannulatum . These results provide some evidence of the consumer-resource relationships of these beetles. Predominance of yeast and fungal spores in the posterior gut of rove beetles suggests that they may play an important role in their dietary requirements and as endosymbionts.

Prioritizing boreal forest restoration sites based on disturbance regime

Selection of high priority sites for ecological restoration becomes essential with globally decreasing forest cover following natural and anthropogenic disturbances. A disturbance gradient resulting from native insect (Choristoneura fumiferana) outbreaks and non-native moose (Alces alces) browsing was studied in balsam fir forest stands in Newfoundland (Canada) to inform land managers where active vs. passive restoration would be most appropriate. The disturbance gradient ranged from small gaps (area <5ha) to medium and large gaps created by insects, in addition to a control (i.e., “no gap”) closed canopy mature balsam fir forest. In all areas seedlings and saplings (<2m) were browsed by moose causing failed forest regeneration. Differences in the vegetation community, environmental conditions and species functional groups were quantified across the disturbance regime using non-metric multidimensional analysis and nested analysis of variance. In this study, the closed canopy stand retained the optimal conditions for balsam fir forest regeneration and does not need active restoration to regenerate if moose densities are low. Insect-disturbed areas <5ha retained conditions that would allow them to regenerate naturally under conditions of low herbivory (i.e., they would not need active restoration); they were dominated by late succession forbs, retained low densities of balsam fir adults and optimal abiotic conditions for seedlings, with an increase in sunlight and decline in the quality of feathermoss seedbed. In contrast, a complete shift was observed in open areas >5ha, which were dominated by grasses, early succession forbs and had abiotic conditions closer to early succession boreal forest, indicating an ecological threshold had been crossed, confirming the need for active restoration. To allow regeneration regardless of gap size, reduction of moose numbers must be continued. Threshold identification in the boreal forest should be based on disturbance regime and used to inform critical areas for future forest restoration strategies. Based on our study, insect outbreak areas >5ha should be prioritized for active restoration, as the crossing of a biotic threshold indicates that such sites cannot naturally return to pre-disturbance balsam fir forests.

Fire history of the central boreal forest in eastern North America reveals stability since the mid-Holocene

Past and present fire frequencies are key factors to evaluate fire-mediated changes in climate, but this metric is difficult to evaluate realistically in paleoecological and climatic reconstructions. Here, we applied charcoal analysis of forest soils to test from direct evidence the stability and resilience of the eastern North-American boreal forest at the Holocene timescale. Current knowledge indicates that the boreal forest is not so stable and resilient in several parts of its range, particularly at its northern and southern edges where it is converted to tundra and woodland communities, respectively. However, it is not known to what degree the structure and composition of the boreal forest situated at the central core of its range (FCR), far from the climate-sensitive edges, have been modified during the Holocene. To address the long-term status of the boreal forest vis-a-vis the Holocene climate, we have used a large dataset composed of 14C-dated and botanically identified charcoal. Long-term fire frequencies in several sites of the FCR were calculated to assess the stability of the main forest ecosystems. The mean fire interval over the last 5000 years of the two principal ecosystems composed of black spruce–jack pine forests and black spruce–balsam fir forests was ~200 and &gt;300 years, respectively. Fires occurred repetitively during this period without fire gaps longer than 400 years (ecosystem 1) and 600 years (ecosystem 2), an indication that the fire regime of the FCR remained stable since the mid-Holocene. Unchanged forest ecosystems during this period also suggest the maintenance of both fire-prone and less fire-prone tree species in their respective sites and the ability of the boreal biome to maintain its structure and function in a repetitive fire-disturbance regime.

Climate‐induced changes in host tree–insect phenology may drive ecological state‐shift in boreal forests

Climate change is altering insect disturbance regimes via temperature‐mediated phenological changes and trophic interactions among host trees, herbivorous insects, and their natural enemies in boreal forests. Range expansion and increase in outbreak severity of forest insects are occurring in Europe and North America. The degree to which northern forest ecosystems are resilient to novel disturbance regimes will have direct consequences for the provisioning of goods and services from these forests and for long‐term forest management planning. Among major ecological disturbance agents in the boreal forests of North America is a tortricid moth, the eastern spruce budworm, which defoliates fir (Abiesspp.) and spruce (Piceaspp.). Northern expansion of this defoliator in eastern North America and climate‐induced narrowing of the phenological mismatch between the insect and its secondary host, black spruce (Picea mariana), may permit greater defoliation and mortality in extensive northern black spruce forests. Although spruce budworm outbreak centers have appeared in the boreal black spruce zone historically, defoliation and mortality were minor. Potential increases in outbreak severity and tree mortality raise concerns about the future state of this northern ecosystem. Severe spruce budworm outbreaks could decrease stand productivity compared with their occurrence in more diverse, southern balsam fir forest landscapes that have coevolved with outbreaks. Furthermore, depending on the proportion of balsam fir and deciduous species present and fire recurrence, changes in regeneration patterns and in nutrient cycling could alter ecosystem dynamics and replace black spruce by more productive mixed‐wood forest, or by less productive ericaceous shrublands. Long‐term monitoring, manipulative experiments, and process modeling of climate‐induced phenological changes on herbivorous insect pests, their host tree species, and natural enemies in northern forests are therefore crucial to predicting species range shifts and assessing ecological and economic impacts.

Short‐term effect of selection cutting in boreal balsam fir forest on cerambycid and scolytid beetles

AbstractThe ecology and management of boreal forest ecosystems are drawing greater attention worldwide as their importance is being increasingly recognized for carbon sequestration or for harbouring the world's largest remaining intact forests. Selection cuts have been introduced as a more socially acceptable silvicultural method to improve the maintenance of habitat structure and functions as they mimic aspects of boreal forest succession dynamics. Many studies have shown that selection cutting helps maintaining arthropod communities in mature forests, but few have examined the increased risks of damage by bark‐ and wood‐boring insects in boreal forests of eastern North America. We used multidirectional flight‐interception traps to quantify the response of these beetles to 25 and 40% selection cutting in a balsam fir–white birch forest of Québec, Canada. The abundance and species number of both cerambycid and scolytid beetles were 5–6 times larger in selectively cut stands than in controls the year following treatments. Analyses revealed that bark‐ and wood‐boring beetles’ response was mostly associated with increased canopy openness in selectively cut stands (and sun‐exposed locations within them) and residual tree injuries caused by harvesting operations. These conditions attracted beetles such as Trypodendron lineatum (Scolytinae) and Rhagium inquisitor (Cerambycidae), two species known for their ability to attack weakened, dying and dead hosts. Most species were more abundant in selection cuts, except for Evodinus m. monticola (Cerambycidae) whose abundance was strongly reduced after treatment. Some beetles can have detrimental effects on residual trees and thus could reduce timber value, but most species found in treated stands do not represent a high risk for healthy trees. Thus, selection cuts do not seem favourable to the establishment of tree‐killing beetles. However, as they were found more active/abundant after selection cutting, it would be wise to further study their population dynamics over mid‐ and long‐term periods, along with the ecological and economic implications associated with this silvicultural treatment.

Cumulative effects of chronic deer browsing and clear-cutting on regeneration processes in second-growth white spruce stands

Interactions between multiple disturbances can alter resilience mechanisms, thereby triggering alternative successional pathways. Regeneration processes are important mechanisms of forest resilience because they drive successional pathways. On Anticosti Island, chronic browsing by introduced white-tailed deer (Odocoileus virginianus) shifted composition of understory regeneration of overmature balsam fir (Abies balsamea) forests toward dominance by white spruce (Picea glauca). Historic clear-cutting of these altered forests generated mature second-growth white spruce stands. However, the cumulative effect of chronic deer browsing and recent clear-cutting on regeneration processes of mature second-growth white spruce stands has not yet been evaluated. Our objective is to evaluate if regeneration processes would enable white spruce stands to recover from the cumulative effects of these two disturbances. We studied regeneration in relation to seed availability, substrate suitability for seedling establishment, and substrate availability in mature second-growth white spruce stands and recent clear-cuts of mature second-growth white spruce stands. Our results indicate regeneration failure in both ecosystems, which can be explained by a lack of suitable rotten logs for sufficient establishment of white spruce seedlings. Hence, the cumulative effects of chronic deer browsing and clear-cutting of mature second-growth white spruce stands have altered regeneration processes and triggered an alternative successional pathway toward parklands, i.e., partial deforestation. We propose shelterwood cuttings that create nurse logs should be investigated to maintain white spruce stands without planting.