Top-down effects, like predation, are drivers of insect outbreaks, but bottom-up effects, like host nutritional quality, also influence outbreaks and could in turn be altered by insect-caused defoliation. We evaluated the prediction that herbivory leads to a positive feedback on outbreak severity as nutrient concentration in plant tissues increases through improved soil nutrient availability from frass and litter deposition. Over seven years of a spruce budworm outbreak, we quantified litter nutrient fluxes, soil nitrogen availability, and host tree foliar nutrient status along a forest susceptibility gradient. As the outbreak progressed, both soil nutrient fluxes and availability increased which, in turn, improved foliage quality in surviving host trees. This is consistent with boosted insect fitness and increased population density and defoliation as outbreaks grow. Our results suggest that a positive bottom-up feedback to forest ecosystems from defoliation may result in conditions favorable to self-amplifying population dynamics in insect herbivores that can contribute to driving broad-scale outbreaks.

 

1. As major alterations are occurring in climate and pest ranges, it is imperative to evaluate their combined contribution to tree mortality in order to propose mitigation measures and limit losses in forest productivity. The objective of this study was to explore the association between declines in tree growth resulting from climatic and biotic (spruce budworm) disturbances, and their interactions on tree mortality of two dominant tree species, Abies balsamea and Picea mariana, of the eastern North?American boreal forest.
2. We disentangle the influences of abiotic and biotic components on growth through a combination of model?data comparison techniques. First, we characterized the variability in tree growth and mortality in the study area using a network of tree?ring width measurements collected from living and dead trees. Subsequently, a bioclimatic simulation model was used to estimate the past annual, nonlinear, responses of stand?level net primary production (NPP) to climate variability (period 1902–2012). From these two data sources, we defined the biotic stress events as the variance in the tree?ring data unexplained by the bioclimatic forest growth simulation.
3. Throughout the 20th century, two periods of adverse climatic conditions preceded spruce budworm outbreaks episodes and induced tree mortality. Climatic stress events were associated with cold springs, warmer than average summers. We found that past stress history in interaction with tree characteristics and species predisposed trees to mortality. In addition, co?occurring events (climatic and biotic) increased the severity of mortality episodes.
4. Synthesis. Our study challenges the belief that spruce budworm outbreak is the primary driver of broad?scale tree mortality in eastern boreal forest. Rather, tree mortality is the result of cumulative events that combine unfavourable conditions for growth, resulting in loss of tree vigour and subsequently, mortality. Co?occurrence of stresses in the future may lead to more severe episodes of mortality, as extreme climatic events become more frequent.

Climate change is projected to increase fire severity and frequency in the boreal forest, but it could also directly affect post-fire recruitment processes by impacting seed production, germination, and seedling growth and survival. We reviewed current knowledge regarding the effects of high temperatures and water deficits on post-fire recruitment processes of four major tree species (Picea mariana, Pinus banksiana, Populus tremuloides and Betula papyrifera) in order to anticipate the effects of climate change on forest recovery following fire in the boreal biome. We also produced maps of future vulnerability of post-fire recruitment by combining tree distributions in Canada with projections of temperature, moisture index and fire regime for the 2041–2070 and 2071–2100 periods. Although our review reveals that information is lacking for some regeneration stages, it highlights the response variability to climate conditions between species. The recruitment process of black spruce is likely to be the most affected by rising temperatures and water deficits, but more tolerant species are also at risk of being impacted by projected climate conditions. Our maps suggest that in eastern Canada, tree species will be vulnerable mainly to projected increases in temperature, while forests will be affected mostly by droughts in western Canada. Conifer-dominated forests are at risk of becoming less productive than they currently are, and eventually, timber supplies from deciduous species-dominated forests could also decrease. Our vulnerability maps are useful for prioritizing areas where regeneration monitoring efforts and adaptive measures could be developed.

1.As major alterations are occurring in climate and pest ranges, it is imperative to evaluate their combined contribution to tree mortality in order to propose mitigation measures and limit losses in forest productivity. The objective of this study was to explore the association between declines in tree growth resulting from climatic and biotic (spruce budworm) disturbances, and their interactions on tree mortality of two dominant tree species, Abies balsamea and Picea mariana, of the eastern North?American boreal forest.

2.We disentangle the influences of abiotic and biotic components on growth through a combination of model?data comparison techniques. First, we characterized the variability in tree growth and mortality in the study area using a network of tree?ring width measurements collected from living and dead trees. Subsequently, a bioclimatic simulation model was used to estimate the past annual, non?linear, responses of stand?level net primary production (NPP) to climate variability (period 1902?2012). From these two data sources, we defined the biotic stress events as the variance in the tree?ring data unexplained by the bioclimatic forest growth simulation.

3.Throughout the 20th century, two periods of adverse climatic conditions preceded spruce budworm outbreaks episodes and induced tree mortality. Climatic stress events were associated with cold springs, warmer than average summers. We found that past stress history in interaction with tree characteristics and species predisposed trees to mortality. In addition, co?occurring events (climatic and biotic) increased the severity of mortality episodes.

4.Synthesis: Our study challenges the belief that spruce budworm outbreak is the primary driver of broad?scale tree mortality in eastern boreal forest. Rather, tree mortality is the result of cumulative events that combine unfavourable conditions for growth, resulting in loss of tree vigour and subsequently, mortality. Co?occurrence of stresses in the future may lead to more severe episodes of mortality, as extreme climatic events become more frequent.

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Context

Forest management alters patterns generated by natural disturbances, particularly in ecosystems with infrequent fires. Management effects can differ according to spatial scale and affect ecological processes.

Objectives

To assess the effect of 80 years of forest management at both the landscape and burn/harvest scales on forest age, composition, density, spatial pattern and heterogeneity.

Methods

Forest inventory maps and satellite images were used to compare two contiguous landscapes, respectively managed and unmanaged, of the eastern boreal forest of Canada, in a region with infrequent fires. Burns and harvests occurring from 1920–1950 were also compared.

Results

In addition to reducing the proportion of old-growth stands in the landscape, forest management changed forest composition at both scales, favouring the late-successional species balsam fir. Landscape metrics indicated that old-growth forests and spruce-dominated ones were more fragmented, less connected, and confined to smaller patches in the managed landscape than in the unmanaged one. Forest management increased heterogeneity at the landscape scale, but decreased it at the burn/harvest scale. Logging had a homogenizing effect at the burn/harvest scale by attenuating the effect of the physical environment on forest density.

Conclusions

This study provides knowledge to help reduce effects of forest management at both scales. In this forest region with low fire recurrence, the goal should be to manage for greater forest heterogeneity at the burn/harvest scale whereas, at the landscape scale, restoration strategies should aim to create large contiguous patches of coniferous forests to increase spatial continuity as these were reduced by past management activities.

Avec l’intérêt croissant à travers le monde envers les forêts du nord tant pour la production de fibre que pour l’atténuation des changements climatiques, des travaux explorant la productivité forestière peu après le feu et les facteurs qui la déterminent sont requis. Nous avons étudié la productivité des forêts, définie en termes de qualité du coefficient de distribution (stocking) et de la croissance, dans 116 sites de 10–30 années après feu distribués dans une zone de 90 000 km2 au nord de la limite d’attribution des forêts commerciales du Québec et dominé par Picea mariana (Mill.) B.S.P. et Pinus banksiana Lamb. Soixante-douze pour cent des sites ont été classés comme improductifs, principalement en raison de la faible croissance. Puisque la croissance est fortement liée à des facteurs climatiques, le boisement seul ne serait pas suffisant pour augmenter la productivité des peuplements dans notre zone d’étude. En outre, nos résultats montrent que P. banksiana sur des sites secs pourrait être moins résilient au feu qu’on ne le pensait, probablement à cause de mauvaises conditions de sites et du climat. Finalement, notre étude est l’une des premières à explorer les enjeux de productivité en bas âge dans les forêts naturelles du nord. Le schéma d’analyse proposé, en définissant la productivité des peuplements sur la base de la croissance et du coefficient de distribution, pourrait fournir un outil intéressant pour évaluer des problèmes similaires de manière précoce dans d’autres systèmes.

Salvage logging following fire has become increasingly used during the last few decades as a way to mitigate economic losses caused by fire. The removal of burned mature trees including their aerial seedbanks immediately after fire could have impacts on post-fire tree recruitment but specific effects are still unknown. We conducted a study of a fire in the boreal forest of Québec, Canada, to answer the following questions: Does fire severity influence tree compositional changes after fire? Does salvage logging affect these changes by favouring species that can reproduce vegetatively? Does salvage logging reduce among-sites heterogeneity relative to natural post-fire forests? To address these questions, we measured pre- and post-fire tree stocking (2. years after fire) of burned forest of different pre-fire stand composition types both in salvaged and non-salvaged sites, whereas fire severity was measured in the latter only. Species composition was evaluated using a Principal Component Analysis, and mixed models were used to test the effects of canopy fire severity, residual organic layer thickness, stand type and salvage logging on pre- to post-fire composition changes. In non-salvaged sites, fire severity had a significant effect on composition changes, and the effect depended on stand type. Low fire severity favoured species that can reproduce vegetatively such as Populus tremuloides, whereas moderate and high fire severity favoured the aerial seedbank species such as Pinus banksiana and Picea mariana. We found no effect of residual organic layer thickness on post-fire composition changes. Salvage treatment had a significant effect on post-fire composition changes and the effect depended on stand type. Overall, salvage logging tended to favour species relying upon vegetative reproduction more than fire alone, which favoured P. banksiana. Although among-site heterogeneity after salvage logging was not smaller than after fire only, salvage alters species composition more than does natural fire alone. Since salvage logging and low severity portions of the burn, which are generally not salvaged, both favour tree species with vegetative reproduction, our results suggest that these species will increase their dominance throughout the landscape if burned forests are systematically salvaged. We suggest strategies that can help to attenuate this divergence from natural post-fire conditions. © 2014 .

Question: In the boreal forest of eastern Canada, how does forest vegetation change in the sustained absence of fire?

Location: Eastern boreal forest in Quebec's North Shore region, Canada (49°30'–50°00'N; 67°30'–68°35'W).

Methods: Aerial photos from three different periods (1930, 1965 and 1987) were used to characterize changes in vegetation composition in 23 scenes of 200 ha. Time since fire, presence of secondary disturbances and data on soil and topographic variables were obtained. Ordination and clustering techniques were used to define compositional trajectories of change over the 57-yr period. These trajectories were further grouped into pathways based on compositional changes, time since fire and preferential deposit-drainage types.

Results: Among the 26 compositional trajectories, three successional pathways were distinguished. Two start post-fire succession with a dominance of intolerant hardwood. In one of these, this is followed by an increase in Abies balsamea, while in the second the importance of Picea mariana increases with time. In the third pathway P. mariana is an important component from the outset. In this pathway, we observed modest fluctuation in the relative dominance of P. mariana and A. balsamea and variation in stand structure.

Conclusion: The boreal forest vegetation of Eastern Canada is diverse and dynamic even in the absence of fire, notably under the influence of partial disturbances. Such disturbances can be associated with changes in composition or stand structure. The development of management strategies aimed at maintaining stand diversity by emulating a broader variety of partial and secondary disturbances should be encouraged.

The influence of stand age and site conditions on the structure of coniferous stands was studied in the boreal forest of Québec North Shore, a region with a low fire recurrence. Stand diameter diversity was measured in 2202 forest inventory plots in black spruce (Picea mariana), balsam fir (Abies balsamea) and mixed stands, using the Shannon-Wiener diversity index. A relative productivity index was developed, based on the relationship between height and age of dominant trees. A stepwise regression analysis indicated that this productivity index best explains stand structure variation in all composition types, while stand age seems to influence structure more at the beginning of stand development. The results suggest that productive stands become uneven-sized earlier than unproductive stands and also maintain a greater diameter diversity. These contrasting structural dynamics may be explained by (i) a higher growth rate in richer stands that likely induces earlier senescence and thus an earlier passage to an uneven-sized structure, and (ii) a restricted maximum tree diameter in poor stands caused by a scarcity of resources, which in turn reduces the diameter diversity of these stands, even after their breakup time.

L’influence de l’âge et des conditions de site sur la structure des peuplements résineux a été étudiée dans la forêt boréale de la Côte-Nord du Québec, une région à faible récurrence des feux. La diversité diamétrale des peuplements a été mesurée à l’aide de l’indice de diversité de Shannon-Wiener dans 2202 placettes d’inventaire forestier incluant des pessières (Picea mariana), des sapinières (Abies balsamea) et des peuplements mixtes. Un indice de productivité relative a été développé à partir de la relation âge-hauteur des arbres dominants. Une analyse de régression pas-à-pas a révélé que cet indice de productivité expliquait le mieux la variation de la structure des peuplements, et ce pour tous les types de composition, alors que l’âge du peuplement semblait influencer la structure davantage au début du développement. Nos résultats suggèrent que les peuplements productifs deviennent irréguliers plus hâtivement que les peuplements peu productifs et maintiennent une diversité diamétrale plus élevée. Ces dynamiques contrastées peuvent être expliquées (i) par un taux de croissance supérieur chez les peuplements les plus riches, entraînant vraisemblablement une sénescence plus hâtive et donc un passage plus hâtif à une structure irrégulière, et (ii) par un diamètre maximum des arbres qui est restreint par la rareté des ressources chez les peuplements pauvres, restreignant leur diversité diamétrale, même après l’âge de bris.