Trembling aspen (Populus tremuloides) is widely distributed in North American forests. Increased stand productivity with resource availability has been reported, but the relationship between soil microbial community structure and stand productivity remains unclear. To examine soil microbial composition of 4 aspen stand productivity classes, we assessed soil properties, microbial biomass and respiration, and bacterial and ectomycorrhizal diversity. Most variables showed no significant differences between productivity classes. However, mean values for basal respiration (0.05 to 27.99 µg CO2-C·g-1 soil·h-1), bacterial biomass, and metabolic quotient (0.08 to 5.22 CO2-C·mg-1 Cmic·h-1) were lowest in low productivity (Class 1) sites. Bacteria to fungi ratios were significantly lower (P = 0.05) in Class 1 compared to other classes. Microbial biomass ranged from 1.39 to 8.11 mg Cmic·g-1soil. Thirty-seven distinct aspen ectomycorrhizas (ECM) were characterized, 21 were considered rare (from ≤3 trees). ECM richness did not differ significantly between classes, although relative abundance for some types did. Canonical correspondence analysis showed productivity class explained most microbial community variation, e.g., ECM fungi (80% explained) and soil bacteria (46%). Despite some differences, we could not identify statistically significant bacterial or ECM assemblages linked to stand productivity. Results may reflect a strong association between microbial processes and the dominant host, aspen. Aspen associated with widely distributed fungi common to all classes, possibly facilitating its survival and growth, including on sites exhibiting low pH and low soil fertility.

Sustainable management of boreal ecosystems involves the establishment of vigorous tree regeneration after harvest. However, two groups of understory plants influence regeneration success in eastern boreal Canada. Ericaceous shrubs are recognized to rapidly dominate susceptible boreal sites after harvest. Such dominance reduces recruitment and causes stagnant conifer growth, lasting decades on some sites. Additionally, peat accumulation due to Sphagnum growth after harvest forces the roots of regenerating conifers out of the relatively nutrient rich and warm mineral soil into the relatively nutrient poor and cool organic layer, with drastic effects on growth. Shifts from once productive black spruce forests to ericaceous heaths or paludified forests affect forest productivity and biodiversity. Under natural disturbance dynamics, fires severe enough to substantially reduce the organic layer thickness and affect ground cover species are required to establish a productive regeneration layer on such sites. We succinctly review how understory vegetation influences black spruce ecosystem dynamics in eastern boreal Canada, and present a multi-scale research model to understand, limit the loss and restore productive and diverse ecosystems in this region. Our model integrates knowledge of plant-level mechanisms in the development of silvicultural tools to sustain productivity. Fundamental knowledge is integrated at stand, landscape, regional and provincial levels to understand the distribution and dynamics of ericaceous shrubs and paludification processes and to support tactical and strategic forest management. The model can be adapted and applied to other natural resource management problems, in other biomes.

Abstract: Litter quality is often considered the main driver of decomposition rate. The objective of this study was to investigate the relative contribution of two other tree-driven mechanisms, litter mixing and forest floor conditions, to foliar litter decomposition and nutrient dynamics for trembling aspen (Populus tremuloides Michx.) and black spruce (Picea mariana (Mill.) BSP) using a microcosm approach. Results based on mixed linear models show that the greater influence over these processes was obtained through litter quality followed by forest floor conditions and litter mixing. Specifically, the results indicate that significantly more C and nutrients were mineralized (i) from aspen than from spruce litter, (ii) from spruce litter in mixture with aspen litter than from spruce litter applied singly, and (iii) from litter incubated on forest floor from the aspen stand rather than from the spruce stand, except for nutrients in the spruce litter. Collectively, our results show that the litter and forest floor material from aspen both favour decomposition and nutrient mineralization processes. Hence, we provide evidence that the effect of tree species on litter decomposition may not only be caused by the properties of its litter but also, indirectly, by the specific conditions and the decomposer community that tree species develop in their forest floor.

Résumé : La qualité de la litière est souvent considérée comme le facteur déterminant de la décomposition. L'objectif de cette étude était d'étudier la contribution relative de deux autres mécanismes contrôlés par l'arbre, soit le mélange de litière et les propriétés de la couverture morte, à la décomposition et à la dynamique des nutriments de la litière foliaire du peuplier faux-tremble (Populus tremuloides Michx.) et de l'épinette noire (Picea mariana (Mill.) BSP) en utilisant une approche en microcosme. Les résultats basés sur des modèles mixtes linéaires montrent que l'influence la plus grande sur ces processus a été obtenue via la qualité de la litière, suivie des propriétés de la couverture morte et de mélange de litière. Spécifiquement, les résultats indiquent que significativement plus de C et de nutriments ont été minéralisés (i) dans la litière de peuplier que dans la litière d'épinette, (ii) dans la litière d'épinette en mélange avec la litière de peuplier que dans la litière d'épinette seule et (iii) dans les litières incubées sur la couverture morte provenant du peuplement de peuplier que sur celle provenant du peuplement d'épinette, excepté pour les nutriments de la litière d'épinette. Collectivement, nos résultats montrent que la litière et la couverture morte de peuplier favorisent les processus de décomposition et de minéralisation des nutriments. Ainsi, cette étude prouve que l'influence des essences forestières sur la décomposition de la litière est non seulement reliée aux propriétés de leur litière, mais aussi, indirectement, aux conditions spécifiques et à la communauté de décomposeurs que les essences forestières développent dans leur couverture morte.

The occurrence of aspen (Populus tremuloides Michx.) patches within stands dominated by black spruce (Picea mariana Mill. BSP) has been shown to increase litter decomposition and nutrient cycling rates by improving soil physical and chemical properties. It is well known, however, that these processes are also influenced by the structure of the soil biota, but this factor has received less attention. In this study, relationships between forest floor properties and soil invertebrates were studied along black spruce-trembling aspen gradients in three stands of the eastern boreal forest of Canada. The forest floor layer of 36 plots differing in aspen basal area was sampled and analyzed to determine physical and chemical properties, the rates of decomposition of standard substrates, net N mineralization, as well as microbial basal respiration and metabolic quotient. Soil invertebrates were also collected using funnel-extraction and pitfall trapping methods. Based on redundancy analyses, we found that forest floor properties, the abundance and composition of soil invertebrates, and the rates of belowground processes changed along the spruce-aspen gradient. The increase in aspen basal area was associated with a reduction in forest floor thickness, moisture content and microbial biomass, and with an increase in the concentration of nutrients. It was also accompanied by changes in soil faunal communities, as soil invertebrates were associated with specific soil properties. In general, macroinvertebrates (i.e., Lumbricidae, Formicidae, Carabidae, Staphylinidae and Gastropoda) were related to the nutrient-rich forest floor associated with aspen, whereas microarthropods and Enchytraeidae tended to be negatively related to aspen basal area. According to mixed linear models, decomposition rates of standard substrates and net ammonification significantly increased along the spruce-aspen gradient. Given the functional significance of macroinvertebrates in soils, these results suggest that aspen favours the elaboration of a macrofaunal community, which in turn accelerates the rate of soil processes by having either direct or indirect influence on microbial activity. Moreover, this study shows that the changes in soil processes and in the biodiversity of soil organisms related to the presence of mixed stands can operate only in the immediate surroundings of a given tree species. Therefore, coarse-scale tree species mixing in a forest stand may have a different effect on soil biodiversity and soil processes than fine-scale mixing.