Increasing soil organic carbon (SOC) storage is one of the promising solutions to mitigate climate change. Fast-growing trees are a potential tool in this context as they rapidly accumulate C in their biomass and could transfer more organic matter (OM) into the soil. However, the relationship between aboveground productivity and SOC storage remains poorly understood. Five clones with different growth rates were selected from a 14-year-old hybrid poplar plantation located in New Liskeard, ON, Canada. We collected soil cores at 87.5 and 175.0 cm distance from the stem and at 0–20, 20–40 and 40–60 cm soil depth for soil C concentration analysis. The most productive clone DN2 (Populus deltoides × P. nigra) stored less SOC (83 Mg ha−1) between 0 and 60 cm depth than the mid-productive clones 1079 (Populus × jackii (P. balsamifera × P. deltoides)) and 915005 (P. maximowiczii × P. balsamifera) (95 and 96 Mg ha−1 respectively), while the least productive clone 747210 (P. balsamifera × P. trichocarpa) also had a lower SOC stock (85 Mg ha−1) compared to the other clones, but not significantly. There was no relationship between aboveground productivity and SOC stocks and total SOC stocks increased by 6 % when the sampling distance was closer to the tree stems. The difference in SOC stocks between clones was mostly observed at the 20–40 cm depth suggesting the significant effect of roots on SOC storage. Soil C/N ratios were significantly different between clones at 0–20 and 20–40 cm depths suggesting differences in OM decomposition rates between clones. There could be a trade-off between aboveground productivity and litter decomposition rate to increase SOC storage.

Plant species mixtures are often seen as being able to achieve higher productivity and carbon (C) sequestration than their single-species counterparts, but it is unclear whether this is true in natural forests. Here, we investigated whether naturally-regenerated mixtures of common North American boreal tree species were more productive and stored more C than single-species stands. We also examined how closely the different C pools and fluxes were interrelated and whether these relationships varied with species composition. Single- and mixed-species stands of trembling aspen, black spruce and jack pine on mesic sites were selected in two regions of the Canadian boreal forest to assess aboveground and belowground productivity and C storage. Although previous studies conducted in these stands found synergistic effects of tree species mixtures on specific C pools and fluxes, such as higher organic layer C stocks and higher fine root productivity in some mixtures, no effects were detected on combined C pools or fluxes at the ecosystem level in the current study. Aspen abundance was linked with higher aboveground tree productivity, higher aboveground living biomass and higher soil heterotrophic respiration, indicating that aspen acts as a key driver of ecosystem C storage and fluxes in these natural forest ecosystems, more so than species richness. However, our results do not rule out the possibility of increased productivity and C storage in mixed stands under environmental conditions or stand developmental stages that are different from the ones studied here. Furthermore, when the entire forest ecosystem is considered (not only tree parts), synergistic effects of tree species mixtures may be more difficult to observe because the beneficial effect of species mixing on one specific C pool may be counterbalanced by a negative effect on another pool.

Changes in forest composition as a result of forest management, natural disturbances, and climate change may affect the accumulation of soil organic carbon (SOC). We examined the influence of common boreal tree species (trembling aspen, black spruce, and jack pine), either in pure stands or in conifer-broadleaf mixtures, on the amount, distribution, and quality of SOC in two regions of the Canadian boreal biome. Long-term laboratory incubations were used to assess SOC quality by quantifying proportions of fast carbon (C) (that is, proportion of total C released during the first 100 days of incubation) and active C (that is, modeled proportion of total C that can be potentially released). Total amounts of SOC did not differ between stand types, but the effects of stand type on SOC stocks and quality differed with soil depth. Among stand types, aspen stands had the greatest relative proportion of total SOC in deeper mineral layers and the lowest amount of active C in the organic layer. For these reasons, the SOC stock that developed under aspen was more stable than in the other stand types. Although black spruce stands allowed a greater accumulation of SOC in surface layers, these stocks, however, might become more vulnerable to extra losses if environmental conditions are to become more favorable to decomposition in the future. Our work highlights that boreal forest composition influences the stability of SOC stocks and how climate change could alter this large C pool.

• Although fine roots (< 2 mm in diameter) account for a major share of the production of terrestrial ecosystems, diversity effects on fine root productivity and their mechanisms remain unclear.
• We hypothesized that: (i) fine root productivity increases with tree species diversity, (ii) higher fine root productivity is a result of greater soil volume filling due to species-specific patterns of root placement and proliferation, and (iii) differences in fine root productivity and soil volume filling associated with tree species diversity are more pronounced in summer when plants are physiologically active and demand for water and nutrients is at its greatest.
• We investigated the effects of tree species diversity on fine root productivity and soil volume filling of boreal forest stands that have grown naturally for 85 years on similar sites.
• Annual fine root production was 19–83% higher in evenly mixed- than single-species-dominated stands, and increased with tree species evenness, but not tree species richness. Fine root biomass was higher in evenly mixed- than single-species-dominated stands in summer months, but not in spring or fall. Higher fine root productivity in evenly mixed- than single-species-dominated stands was realized by filling more soil volume horizontally and vertically in the forest floor in the mixtures of deep- and shallow-rooted species vs. the deeper mineral soil in the mixtures of deep-rooted species.
• Synthesis. Our results provide some of the first direct evidence for below-ground species complementarity in heterogeneous natural forests, by demonstrating that tree species evenness increases fine root productivity by filling/exploiting the soil environment more completely in space and time, driven by differences in the inherent rooting traits of the component species and variations of root growth within species.

Les forêts du Bouclier boréal stockent de grandes quantités de carbone (C) sous forme de C organique du sol (COS). Toutefois, l'intégrité de ces stocks de COS est menacée par les changements climatiques, mais aussi par la modification de la composition forestière à l'échelle du paysage. En effet, chaque espèce d'arbre possède des traits fonctionnels différents lui permettant d'avoir une influence singulière sur son environnement et sur des processus comme l'accumulation, le stockage et la libération du COS. L'acquisition de connaissances sur la dynamique du COS en forêt boréale est fondamentale pour prédire et atténuer les conséquences de tels changements. L’objectif de cette thèse était d'explorer l'influence de la composition forestière (1) sur la quantité, la distribution et la qualité du COS et sa stabilité face au réchauffement, (2) sur la répartition du COS dans des fractions physiques de sol qui varient en stabilité, et (3) sur les pertes annuelles de CO2 in situ et les facteurs responsables de la variation entre les types de forêts. Collectivement, les résultats montrent que la composition forestière affecte grandement la dynamique du COS en forêt boréale. L'influence de la composition forestière sur les pertes de CO2 se faisait surtout via des variations dans la température du sol et dans la qualité biochimique du C produit dans les types de forêts étudiées. En général, les forêts contenant du peuplier faux-tremble accumulaient moins de COS en surface que les forêts contenant des conifères (pin gris et épinette noire), mais il y en avait davantage en profondeur, endroit où le C est moins susceptible d'être libéré dans l'atmosphère et donc plus stable. Les forêts contenant du peuplier avaient aussi davantage de COS situé dans les fractions physiques du sol les plus stables, c'est-à-dire celles étroitement associées aux particules minérales. Bien que les forêts d'épinettes noires soient celles qui possédaient la plus grande quantité de COS de surface de tous les types de forêts étudiées, ce COS était le plus sensible à l'augmentation de la température de sorte que sa réponse au réchauffement se traduisait par une libération de CO2 plus élevée. De plus, comme le stock de COS de surface des conifères est en partie liée à un microclimat du sol plus froid induit par le peuplement, la libération du CO2 suite à des perturbations qui ouvriraient le couvert ou en changeraient la composition pourrait être plus importante que dans les forêts de feuillus. En conclusion, les résultats de cette thèse suggèrent que le rajeunissement des forêts et notamment l'augmentation du couvert de peuplier faux-tremble dans le paysage diminuerait la taille des stocks de COS, mais en augmenterait la stabilité. Par ailleurs, les forêts d'épinettes noires, qui possèdent d'importants stocks de COS et qui dominent la forêt boréale canadienne, pourraient perdre de grandes quantités de COS suite au réchauffement du climat ou à une transformation du couvert. 

 

Processes governing tree interspecific interactions, such as facilitation and competition, may vary in strength over time. This study tried to unveil them by performing dendrometrical analyses on black spruce Picea mariana, trembling aspen Populus tremuloides and jack pine Pinus banksiana trees from pure and mixed mature boreal forest stands in the Clay Belt of northwestern Quebec and on the tills of northwestern Ontario. We cored 1430 trees and cut 120 for stem analysis across all stand composition types, tree species and study regions. Aspen annual growth rate was initially higher when mixed with conifers, but then progressively decreased over time compared to pure aspen stands, while jack pine growth rate did not differ with black spruce presence throughout all stages of stand development. When mixed with aspen, black spruce showed a contrary response to aspen, i.e. an initial loss in growth but a positive gain later. On the richer clay soil of the Quebec Clay Belt region, however, both aspen and spruce responses in mixed stands reversed between 37 and 54 years. Overall, our results demonstrate that interspecific interactions were present and tended to change with stand development and among species. Our results also suggest that the nature of interspecific interactions may differ with soil nutrient availability. © 2011 The Authors.

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.

Deforestation usually results in significant losses of soil organic carbon (SOC). The rate and factors determining the recovery of this C pool with afforestation are still poorly understood. This paper provides a review of the influence of afforestation on SOC stocks based on a meta-analysis of 33 recent publications (totaling 120 sites and 189 observations), with the aim of determining the factors responsible for the restoration of SOC following afforestation. Based on a mixed linear model, the meta-analysis indicates that the main factors that contribute to restoring SOC stocks after afforestation are: previous land use, tree species planted, soil clay content, preplanting disturbance and, to a lesser extent, climatic zone. Specifically, this meta-analysis (1) indicates that the positive impact of afforestation on SOC stocks is more pronounced in cropland soils than in pastures or natural grasslands; (2) suggests that broadleaf tree species have a greater capacity to accumulate SOC than coniferous species; (3) underscores that afforestation using pine species does not result in a net loss of the whole soil-profile carbon stocks compared with initial values (agricultural soil) when the surface organic layer is included in the accounting; (4) demonstrates that clay-rich soils (> 33%) have a greater capacity to accumulate SOC than soils with a lower clay content (< 33%); (5) indicates that minimizing preplanting disturbances may increase the rate at which SOC stocks are replenished; and (6) suggests that afforestation carried out in the boreal climate zone results in small SOC losses compared with other climate zones, probably because trees grow more slowly under these conditions, although this does not rule out gains over time after the conversion. This study also highlights the importance of the methodological approach used when developing the sampling design, especially the inclusion of the organic layer in the accounting.

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.

Dans la lutte contre les émissions de gaz à effet de serre, l’afforestation a le potentiel de réduire la concentration atmosphérique en CO₂ sachant le C contenu dans les écosystèmes forestiers représente une proportion majeure du bilan de C planétaire. Toutefois, peu d’études ont réussi à montrer de véritables gains de C du sol (COS) suite à l’afforestation et les facteurs contrôlant la régénération des stocks de COS ne font toujours pas l’objet d’un consensus au sein de la communauté scientifique. La présente étude fait la synthèse de la littérature traitant de l’influence de l’afforestation sur les stocks de COS et fait la méta-analyse de 33 publications récentes (totalisant 120 sites et 189 observations) dans le but de répondre à la question suivante : Qu’est-qui conditionne le retour vers des conditions de sol forestier, particulièrement au niveau des gains de C, après l’afforestation? Basée sur un modèle linéaire mixte, la méta-analyse indique que les facteurs majeurs contrôlant la régénération des stocks de COS après l’afforestation sont : l’espèce d’arbre plantée, la teneur en argile du sol, l’affectation précédente des terres et, dans une moindre mesure, la zone climatique. Plus spécifiquement, elle suggère que 1) les espèces d’arbres feuillues ont une meilleure capacité d’accumuler du COS, 2) elle ne supporte pas l’idée que l’afforestation effectuée avec des pins se solde par des pertes nettes de COS par rapport aux valeurs initiales (sol agricole), 3) elle montre que les sols riches en argile (>33%) ont une capacité près de 10 fois plus élevée pour accumuler du COS que les sols plus pauvre en argile (<33%), 4) elle indique que l’impact positif de l’afforestation sur les stocks de COS est plus marqué dans les sols en culture que les pâturages ou les prairies, et que la taille de cet impact pourrait être prédit en comparant les différents systèmes au niveau de leurs composantes systémiques et, finalement, 5) elle suggère que l’afforestation effectué dans la zone climatique boréale génère des pertes de COS par rapport aux autres zones climatiques, probablement à cause de la faible croissance des arbres soumis à de telles conditions, sans toutefois exclure des gains au fil du temps après la conversion. D’autre part, cette étude souligne l’importance de l’approche méthodologique employée lors de la conception du dispositif d’échantillonnage et notamment l’inclusion de la couche organique dans le calcul des stocks de COS, l’âge de la plantation et le design d’étude © 2008 UQAM tous droits réservés.