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Although the importance of coarse woody debris (CWD) to understory species diversity has been recognized, the combined effects of CWD decay and substrate species on abundance and species diversity of epixylic vegetation have received little attention. We sampled a wide range of CWD substrate species and decay classes, as well as forest floors in fire- origin boreal forest stands. Percent cover, species richness, and evenness of epixylic vegetation differed significantly with both CWD decay class and substrate species. Trends in cover, species richness, and evenness differed significantly between nonvascular and vascular taxa. Cover, species richness, and species evenness of nonvascular species were higher on CWD, whereas those of vascular plants were higher on the forest floor. Epixylic species composition also varied significantly with stand ages, overstory compositions, decay classes, substrate species, and their interactions. Our findings highlight strong interactive influences of decay class and substrate species on epixylic plant communities and suggest that conservation of epixylic diversity would require forest managers to maintain a diverse range of CWD decay classes and substrate species. Because stand development and overstory compositions influence CWD decay classes and substrate species, as well as colonization time and environmental conditions in the understory, our results indicate that managed boreal landscapes should consist of a mosaic of different successional stages and a broad suite of overstory types to support diverse understory plant communities. Key words: boreal forest, coarse woody debris, decay class, epixylic plants, substrate species. Bien que l'importance des debris ligneux grossiers (DLG) pour la diversite des especes de sous-bois ait ete demontree, les effets combines de la decomposition des DLG et de l'espece du substrat sur l'abondance et la diversite des especes de vegetaux epixyliques ont ete peu etudies. Nous avons echantillonne une grande variete d'especes du substrat de DLG et de classes de decomposition ainsi que de couvertures mortes dans des peuplements de foret boreale issus de feu. Le pourcentage de couvert, la richesse en especes et l'homogeneite de la vegetation epixylique etaient significativement differents selon la classe de decomposition et l'espece du substrat de DLG. Il y avait des differences significatives entre les taxons vasculaires et non vasculaires en ce qui a trait aux tendances dans le couvert, la richesse en especes et l'homogeneite des especes. Ces attributs prenaient davantage d'ampleur sur les DLG dans le cas des plantes non vasculaires et sur la couverture morte dans le cas des plantes vasculaires. La composition en especes epixyliques variait egalement de facon significative selon l'age du peuplement, la composition de l'etage dominant, la classe de decomposition, l'espece du substrat et leurs interactions. Nos resultats mettent en evidence de fortes influences interactives de la classe de decomposition et de l'espece du substrat sur les communautes vegetales epixyliques et indiquent que la conservation de la diversite epixylique exigerait que les gestionnaires forestiers maintiennent un eventail diversifie de classes de decomposition des DLG et d'especes du substrat. Etant donne que le developpement du peuplement et la composition de l'etage dominant influencent les classes de decomposition des DLG et les especes du substrat, ainsi que le moment de la colonisation et les conditions environnementales dans le sous-bois, nos resultats indiquent que le paysage boreal amenage devrait etre constitue d'une mosaique de differents stades successionnels et d'une vaste suite de types d'etage dominant pour supporter diverses communautes vegetales de sous-bois. [Traduit par la Redaction] Mots-cles: foret boreale, debris ligneux grossiers, classe de decomposition, plantes epixyliques, especes du substrat., Introduction Coarse woody debris (CWD) plays a key role in biodiversity and ecosystem functioning as it provides habitat for many plants and animals (Bunnell and Houde 2010; Stokland et al. [...]

Increasing environmental concerns with forestry have led to mounting interest in mixedwood management as a possible strategy to achieve a diversity of ecological and productivity goals within the context of sustainable forest management. This review focuses on the effects of mixedwoods on biodiversity including understory plants, birds, soil fauna, and ectomycorrhizae (ECM). It examines stand-level species diversity and composition but particularly focuses on the regional scale (gamma diversity) by examining evidence for the occurrence of mixedwood-indicator species. The main conclusions are as follows: (i) The existence of different tree species in the canopy is associated with provision of a greater diversity of microhabitats allowing for the addition of understory plant species associated with each canopy species. However, there is little evidence for the existence of understory plant species uniquely associated with mixedwoods. (ii) Some bird species require or prefer the presence of different tree species within a landscape or within a stand. Mixed forest stands and landscapes are thus critical to support populations of these species. (iii) A few studies showed a positive effect of mixed stands or mixed litters on some groups of soil organisms, but high variability makes it difficult to draw firm conclusions about any potential benefits of mixedwoods for biodiversity of soil fauna. (iv) Some ECM taxa are associated with multiple hosts, and could thus benefit from availability of multiple possible host species found in mixed forest stands. Several studies confirmed that there was a greater abundance of multiple-host ECM taxa in mixedwoods. Key words: mixedwoods, gamma diversity, understory vegetation, birds, soil fauna, ectomycorrhizae. Les preoccupations environnementales croissantes en foresterie ont provoque un interet pour l'amenagement mixte comme une strategie possible dans un contexte d'amenagement forestier durable. Cette revue de litterature se concentre sur les effets des peuplements mixtes sur la biodiversite, incluant les plantes de sous-bois, la faune aviaire, la faune du sol, et les ectomycorrhizes (ECM). Elle examine la diversite et la composition specifiques a l'echelle des peuplements mais focalise particulierement sur l'echelle du paysage (diversite gamma) en recherchant la presence d'especes indicatrices des peuplements mixtes. Les principales conclusions sont les suivantes : (i)L'existence de differentes especes d'arbres dans la canopee est associee avec une plus grande diversite de microhabitats, permettant l'addition des especes de plantes de sous-bois associees a chacune des especes de la canopee. Il n'y a toutefois que peu d'indications quant a l'existence d'especes de plantes de sous-bois associees exclusivement aux peuplements mixtes. (ii) Certaines especes d'oiseaux exigent ou preferent la presence de differentes especes d'arbres dans un paysage ou un peuplement. Les peuplements ou paysages forestiers mixtes sont donc critiques pour la conservation de telles especes. (iii) Quelques etudes ont montre un effet positif des peuplements ou litieres melanges sur certains groupes d'organismes du sol, mais la forte variabilite de ces resultats rend toute conclusion hasardeuse. (iv) Certains taxa d'ECM sont associes a des hotes multiples, et pourraient donc beneficier de la disponibilite de plusieurs hotes dans les peuplements mixtes. Plusieurs etudes ont confirme la plus grande abondance de ces taxa a hotes multiples dans les peuplements mixtes. Mots-cles : peuplements mixtes, diversite gamma, vegetation du sous-bois, faune aviaire, faune du sol, ectomycorrhizes., 1. Introduction 1.1. Mixedwoods Canopy trees play a central role in forest ecosystem processes such as net primary productivity, hydrology and nutrient cycling. They also moderate the forest microclimate and [...]

What maintains plant species diversity has been the subject of much debate with no general consensus. In forest ecosystems in which understory plants account for the majority of floristic diversity, a crucial question is whether understory plant diversity is driven by resource quantity or resource heterogeneity. This study sought to reconcile the two hypotheses in relation to their effects on understory plant diversity in forest ecosystems. A database of studies that investigated the effects of resources on understory plant diversity was compiled and analyzed using log-linear models. Whether resource quantity or resource heterogeneity is the determinant of understory plant diversity in individual studies was dependent on stand successional stage(s), presente or absence of intermediate disturbance, and forest biome within which the studies were conducted. Resource quantity was found to govern species diversity in both young and mature stands, whereas resource heterogeneity dominated in old-growth stands. Resource quantity remained the important driver in both disturbed and undisturbed forests, but resource heterogeneity played an important role in disturbed forests. We argue that neither resource quantity nor heterogeneity alone structures species diversity in forest ecosystems, but rather their influences on understory plant diversity vary with stand development and disturbances in forest ecosystems. Key words: biome; intermediate disturbance; neutral theory; old-growth forest; plant diversity; resource heterogeneity; resource quantity; understory.

This study investigates the potential of mixed forest stands as better aboveground carbon sinks than pure stands. According to the facilitation and niche complementarity hypotheses, we predict higher carbon sequestration in mature boreal mixedwoods. Aboveground carbon contents of black spruce (Picea mariana (Mill.) Britton. Sterns, Poggenb.) and trembling aspen (Populus tremuloides Michx.) mixtures were investigated in the eastern boreal forest, whereas jack pine (Pinus banksiana Lamb.) and trembling aspen were used in the central boreal forest. No carbon gain was found in species mixtures; nearly pure trembling aspen stands contained the greatest amount of aboveground carbon, black spruce stands had the least, and mixtures were intermediate with amounts that could generally be predicted by linear interpolation with stem proportions. These results suggest that for aspen, the potentially detrimental effect of spruce on soils observed in other studies may be offset by greater light availability in mixtures. On the other hand, for black spruce, the potentially beneficial effects of aspen on soils could be offset by greater competition by aspen for nutrients and light. The mixture of jack pine and trembling aspen did not benefit any of these species while inducing a loss in trembling aspen carbon at the stand level. Resume: Cette etude vise a determiner si les peuplements forestiers mixtes peuvent etre des puits de carbone aeriens plus efficaces que les peuplements purs. En accord avec les hypotheses de facilitation et de separation des niches ecologiques. nous predisons une plus grande sequestration du carbone dans les peuplements mixtes. Nous avons done determine les stocks de carbone aeriens dans des melanges d'epinettes noires (Picea mariana (Mill.) Britton. Sterns. Poggenb.) et de peupliers faux-tremble (Populus tremuloides Michx.) situes dans la foret boreale de l'est, tandis que des melanges de pin gris (Pinus banksiana Lamb.) et de peupliers faux-tremble furent plutot utilises dans la foret boreale centrale. Les melanges d'especes ne presentaient aucun gain en carbone. Les peuplements domines par le tremble contenaient la plus grande quantite de carbone aerien. les pessieres la plus petite quantite. et les melanges en contenaient des quantites intermediaires qui pouvaient generalement etre predites par interpolation lineaire en fonction de la proportion de tiges de chaque espece. Ces resultats suggerent que dans le cas du tremble, l'effet potentiellenient nefaste de 1'epinette sur les sols peut etre com-pense par une meilleure disponibilite de la lumiere dans les peuplements mixtes. Parallelement, dans le cas de 1'epinette noire, l'effet potentiellement benefique du peuplier faux-tremble sur les sols pourrait etre contrebalance par une competition accrue pour la lumiere et les nutriments en sa presence. Le melange de pin gris et de peuplier faux-tremble n'a apporte aucun benefice pour aucune des deux especes tout en induisant une perte en carbone chez le tremble a l'echelle du peuplement., Introduction With the increasing attention given to the problem of global climate change, many researchers have focused on vegetation carbon sequestration as part of a possible solution (IPCC 2007). The [...]

Understory vegetation plays a critical role in boreal ecosystems. Despite this, quantitative evaluation of the factors controlling understory vegetation abundance, diversity, and composition in the most diverse boreal forest region in North America is lacking. This study examined the dynamics of understory vegetation of stands of fire origin and tested effects of overstory composition and logging vs. fire on the understory vegetation dynamics in Ontario, Canada. Understory vegetation communities were sampled in 68 stands of conifer, mixed-wood, and deciduous overstory type ranging from 7 to 201 years post-fire for stands of fire origin, and from 7 to 31 years for stands of logging origin. For stands of fire origin, total cover and species richness followed similar trends for the three overstory types and were highest in the intermediate-aged stands (72-90 years). Trends in cover and richness, however, differed significantly for vascular and nonvascular plant groups. Vascular cover and species richness were generally higher under deciduous stands, and lower on older stands, while nonvascular species richness was highest under conifer stands and increased with time since fire. Neither species richness nor compositional turnover was higher under mixed-wood stands; mixed-wood stands were compositionally intermediate to conifer and deciduous stands. Multivariate analysis using multiple-response permutation procedures indicated that understory communities were compositionally distinct for all overstory types and showed no convergence with increasing time since fire. Compared with postfire stands of similar ages, post-logged stands had similar total understory cover and richness. Vascular cover and richness, however, were higher on post-logged stands, and nonvascular cover and richness were lower. Stands of logging and fire origin were compositionally distinct for all overstory types and ages. Compositional differences appeared to be driven by higher preestablished rhizomatous species and few pyrophilic species on post-logged sites. Understory vegetation communities in the central boreal shield appear to support the intermediate disturbance hypothesis. Understory richness, however, was not negatively associated with high cover values as predicted by the intermediate disturbance hypothesis. Moreover, richness appears to be highest on sites with high light availability, suggesting that boreal understory communities are influenced more by plant tolerances for low resources, rather than by competition. Key words: Canadian boreal forest; environmental heterogeneity hypothesis; forest management; intermediate disturbance hypothesis; resource ratio hypothesis; timber harvesting; thole since fire or logging; understory vegetation.

Abstract: Stand structure diversity is hypothesized (i) to increase with stand development and (ii) to be greater in mixedwood stands than in conifer and broadleaf stands. We examined the effects [...]

Red spruce (Picea rubens Sarg.) forests are an ecologically and economically important forest type in eastern Canada. We quantified the carbon (C) stocks of natural red spruce dominated stands in central Nova Scotia. Twenty- four stands over a 140 year chronosequence were sampled. Within each stand, major C pools including above- and belowground tree biomass, shrub and herb vegetation, dead organic matter, and upper (0-10 cm) mineral soil were measured. A nonlinear four-parameter logistic function was fitted to the total site C stock data to describe the change in total ecosystem C storage over time. Total site C storage increased throughout stand development in a general sigmoidal pattern, increasing from 94.4 Mg C*[ha.sup.-1] in the youngest age-class to a maximum of 247.0 Mg C*[ha.sup.-1] in the 81- to 100- year-old age-class, then decreasing in the oldest age-classes. Carbon pools of live vegetation, standing dead trees, and downed woody debris displayed recognizable changes in C storage throughout stand development, conforming to some of the fundamental ideas on forest stand dynamics. Overall, above- and below-ground tree biomass had the greatest influence on total site C storage dynamics. These results are likely to be integrated into further forest management plans and generalized in other contexts to evaluate carbon stocks at the regional scale. Dans l'est du Canada, les forets d'epinette rouge (Picea rubens Sarg.) constituent un type de foret important du point de vue ecologique et economique. Nous avons quantifie les stocks de carbone (C) des peuplements naturels domines par l'epinette rouge dans le centre de la Nouvelle-E cosse, au Canada. Nous avons echantillonne 24 peuplements couvrant une chronosequence de 140 ans. Les principaux reservoirs de C ont ete mesures dans chaque peuplement, incluant la biomasse aerienne et souterraine des arbres, les arbustes et la vegetation herbacee, la matie re organique morte et la partie superieure (0-10 cm) du sol mineral. Une fonction logistique non lineaire a quatre parame tres a ete ajustee aux donnees correspondant au stock total de C emmagasine dans l'ecosyste me en fonction du temps. Le stock total de C emmagasine dans la station augmentait tout au long du developpement du peuplement en suivant une courbe generalement sigmoide, passant de 94,4 Mg C*[ha.sup.-1] dans la classe d'age la plus jeune jusqu'a un maximum de 247,0 Mg C*[ha.sup.-1] dans la classe d'age de 81-100 ans et decroissait par la suite dans les classes d'age les plus vieilles. Les reservoirs de C dans la vegetation vivante, les arbres morts debout et les debris ligneux au sol ont connu des changements reconnaissables dans le stockage du C tout au long du developpement du peuplement. Ces changements etaient conformes a certaines des notions fondamentales de la dynamique des peuplements forestiers. Dans l'ensemble, la biomasse aerienne et souterraine des arbres avait la plus forte influence sur la dynamique du stockage total de C de la station. Ces resultats pourraient etre integres aux futurs plans d'amenagement forestier et generalises dans d'autres contextes pour evaluer les stocks de carbone a l'echelle regionale. [Traduit par la Redaction], Introduction Carbon (C) storage in forest ecosystems has been the focus of much research in recent years because of the role of C[O.sub.2] in global climate change. Papadopol (2000) and [...]