Understory vegetation plays an important role in maintaining biodiversity, soil nutrient cycling and carbon stocks, yet the ability of understory functional traits to predict forest productivity after harvesting disturbance is unknown. Our objective was to investigate the utility of individual traits (the community-weighted trait mean) and combined information from multiple traits (functional diversity indices) of understory plants in predicting tree diameter growth of black spruce (Picea mariana (Mill.) B.S.P.) after two harvesting methods (clearcutting and careful logging). We used vegetation and tree growth data from a network of field plots (171 plots on 43 sites) established in black spruce dominated forests across Ontario and Quebec, in northeastern Canada. Functional traits (11 traits) of dominant understory species that reflect plant morphology, regeneration strategy, and resource utilization were collected. We then used generalized linear mixed models (GLMMs) to model the response of tree diameter increment to functional diversity indices, community-weighted trait means and their interactions with harvesting method. Our results showed that: 1) at the functional diversity level, functional richness had a positive relationship with tree growth in clear-cut forests, while functional evenness showed a negative and functional divergence a positive relationship with tree growth in careful logging forests; 2) at the individual trait level, understory species with relatively large morphological trait values often promoted tree growth in both forest types, with stronger relationships in careful logging than in clear-cut forests. For regeneration traits, heavier seed weight or permanent seed bank persistence of understory plants had negative relationships with tree growth, and those relationships were either only found in careful logging forests or did not depend on harvesting type. A positive relationship was found between vegetative propagation species and tree growth, which was stronger in careful logging than in clear-cut forests. Species preferring either humid or xeric habitat had positive relationships with tree growth; both shade-tolerant and shade-intolerant species also positively correlated to tree growth. In conclusion, our study shows the important roles of understory traits in predicting tree growth based on both niche complementarity and mass-ratio hypotheses. Moreover, disturbance severity influenced the balance or degree of the positive and negative interactions that occurred between understory plants and black spruce tree growth. We suggest that identifying the threshold of harvesting disturbance severity that can activate facilitation interactions between understory plants and crop trees would be an important topic for future study.

La maîtrise de la végétation concurrente est essentielle afin que les plantations présentent les rendements escomptés, mais ses effets dépendent de son moment d’application. Ainsi, notre objectif était d’évaluer l’effet, après 15–20 ans, d’un délai d’application du dégagement mécanique par rapport au moment requis selon les procédures opérationnelles au Québec (Canada) pour des plantations de Picea glauca et Picea mariana. Nous avons utilisé trois dispositifs expérimentaux de reboisement hâtif comprenant les traitements suivants : i) témoin, non dégagé; ii) dégagement mécanique l’année requise (Requis); iii) dégagement mécanique avec un délai d’une année par rapport à Requis (Requis+1); et, iv) dégagement mécanique avec un délai de deux années (Requis+2). Nos résultats montrent que dans un scénario de reboisement hâtif comprenant un traitement de nettoiement au stade gaulis, il n’y a pas d’impact significatif sur la croissance à retarder l’application d’un dégagement mécanique jusqu’à deux années après le moment requis. Cette marge opérationnelle devrait être utilisée afin d’assurer la réalisation des dégagements; l’omission du dégagement a un effet négatif marqué sur la production aux échelles de l’arbre et du peuplement. Le dégagement a en effet augmenté la hauteur et le diamètre des arbres plantés ainsi que la surface terrière totale des peuplements.

In the boreal forest, ground-layer composition may modulate the effects of precommercial thinning (PCT) on stand productivity by affecting tree growth conditions. Based on data from 15 years of PCT monitoring in black spruce (Picea mariana) and jack pine (Pinus banksiana) stands, the objectives of this study were to investigate the effects of PCT on ground-layer composition and the way ground-layer composition is related to tree growth, stand productivity, and the PCT impact on stand productivity. PCT favored lichen expansion in xeric sites. The positive impact of PCT on stand productivity after 15 years was lower in sites with high year-one lichen cover, suggesting that the aboveground positive effect of PCT on growth may have been mitigated by a belowground negative feedback resulting from lichen expansion in xeric sites. Although Sphagnum spp. cover was not affected by PCT, 15-year increase in stand productivity was lower in sites with high year-one Sphagnum spp. cover. These results suggest that xeric stands with high lichen cover should not be targeted for PCT because of either null or negative effects on stand productivity. Subhydric stands with high Sphagnum spp. cover should also be avoided because of lower potential stand productivity. © 2017 Society of American Foresters.

Ecosystem-based management, now a dominant forestry paradigm, implies reducing the gap between variability of natural and managed forests (i.e. ecological distance) to reconcile ecological issues with production of socioeconomic services. Here, we tested whether a trade-off exists between conserving key ecosystem attributes of natural forests and maintaining and/or increasing merchantable wood production at the stand scale in humid boreal stands. Using 20-y data from an experimental design comparing silviculture scenarios of increasing intensity, (i) careful logging around advance growth (CLAAG); (ii) CLAAG followed by pre-commercial thinning; (iii) plantation followed by mechanical release; and (iv) plantation followed by chemical release, we examined plant community composition, stand structure and the quantity and the quality of snags. We also assessed timber productivity by comparing scenarios in terms of conifer and merchantable (diameter at breast height > 9 cm) tree dimensions. We used data from stands originating from a spruce budworm outbreak as a baseline to understand scenario impacts on variability of key attributes and productivity. Our results showed increasing differences in these attributes between natural and managed stands with increasing silviculture intensity: the diameter structure became more homogenized, light demanding species richness and abundance increased and the quantity and the quality of snags decreased. Therefore, our results showed that the ecological distance from naturally disturbed stands was lower after CLAAG than after the other silviculture scenarios. However, CLAAG favored an increase in the density of deciduous trees and a decrease of conifer snag density that have the potential to affect resilience of mature stands. Pre-commercial thinning resulted in crop trees reaching larger diameter than following CLAAG only and in the decrease of birch tree density, with no effect on deciduous regeneration density ? 60 cm in height. We measured higher basal area of merchantable trees in plantations than in stands originating from natural regeneration scenarios, with mechanical and chemical release scenarios resulting in similar crop tree productivity. Globally, our study confirmed a general antagonism between the impacts of silviculture on key ecosystem attributes and forest productivity, posing a challenge for reconciling ecological issues with the production of socioeconomic services. At the stand level, results support that retention forestry could emulate natural disturbances by conserving biological legacies during harvest in humid boreal forests. Further research is needed to determine retention parameters to achieve expected wood production while maintaining variability of key attributes in humid boreal forests. © 2017 Elsevier B.V.

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 on the provisioning of goods and services from these forests and on 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 (Abies spp.) and spruce (Picea spp.). 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. While spruce budworm outbreak centres 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.