In silvicultural trials, the confounding influence of permanent soil properties is assumed to be ... more In silvicultural trials, the confounding influence of permanent soil properties is assumed to be minimal. A covariance analysis using total elemental concentrations of parent material (geochemistry) and soil particle size distribution (texture) was used to understand the role that these variables could play in tree growth and foliar nutrient status, and in the growth response to site preparation of 16- to18-yr-old jack pine plantations growing on seemingly homogeneous glacial outwash deposits. Three treatments were tested in a replicated design: direct plantation, and site preparation with TTS or with Bräcke. The combination of particle size distribution and soil geochemistry explained the site quality index (SQI) in control plots (R2 = 0.94) better than soil texture alone. In all plots, SQI was strongly related to K and Mg concentrations in foliage and in humus and, to a lesser degree, to foliar N concentrations. A covariance analysis using soil texture and geochemical composition indicated that while site preparation had an effect on tree growth, the sites that responded the most to site preparation were the ones with the lowest growth potential as determined by soil texture and geochemistry. Key words: Silviculture, site preparation, site quality index, growth, permanent soil factors, texture, geochemistry, nutrition
Abstract In existing carbon budget models, carbon stocks are not explicitly related to forest suc... more Abstract In existing carbon budget models, carbon stocks are not explicitly related to forest successional dynamics and environmental factors. Yet time-since-last-fire (TSLF) is an important variable for explaining successional changes and subsequent carbon storage. The objective of this study was to predict the spatial variability of aboveground biomass carbon (ABC) as a function of TSLF and other environmental factors across the landscape at regional scales. ABC was predicted using random forest models, both at the sample-plot level and at the scale of 2-km2 cells. This cell size was chosen to match the observed minimum fire size of the Canadian large fire database. The percentage variance explained by the empirical sample-plot level model of ABC was 50%. At that scale, TSLF was not significantly related to ABC. At the 2-km2 scale, ABC was influenced mainly by the proportions of cover density classes, which explained 83% of the variance. Changes in cover density were related to TSLF at the same 2-km2 scale, indicating that the increase in cover density following fire disturbance is a dominant mechanism through which TSLF acts upon ABC at the scale of landscapes.
Poor growth of black spruce (Picea mariana (Mill.) BSP) has been associated with the presence of ... more Poor growth of black spruce (Picea mariana (Mill.) BSP) has been associated with the presence of Ledum groenlandicum L. (Ledum) on some sites in the eastern boreal forest in Canada. To increase black spruce productivity on lowland sites, a study was carried out to test the effect of mechanical control of Ledum (by clipping) on black spruce growth on forested peatland in northwestern Quebec. We compared the growth and foliar nutrient concentrations of advance-regeneration black spruce seedlings with and without Ledum control. Contrary to our expectations, our results showed that 3-year control of aboveground competition by Ledum decreased rather than increased black spruce growth and had no effect on foliar nutrient concentrations. Ledum grows on a variety of site types; therefore, the mere presence of this species does not necessarily indicate that there will be a problem with conifer regeneration (growth and germination). Résumé : Une diminution dans la croissance de l'épinette noire (Picea mariana (Mill.) BSP) a été associée à la présence du Ledum groenlandicum L. (Ledum) sur certains sites dans la forêt boréale de l'est du Canada. Avec comme objectif d'augmenter la productivité de l'épinette noire dans les stations des basses-terres, une étude a été entreprise afin de mesurer l'effet du contrôle mécanique du Ledum sur la croissance de l'épinette noire dans une tourbière boisée située dans le nord-ouest du Québec. Nous avons comparé la croissance et les concentrations foliaires de nutriments de semis d'épinette noire dans des parcelles avec et sans contrôle du Ledum. Contrairement à nos attentes, nos résultats ont montré qu'un contrôle de la compétition épigée du Ledum pendant trois ans résultait en une baisse plutôt qu'une augmentation de la croissance de l'épinette noire et n'occasionnait pas de différence dans les concentrations foliaires de nutriments. Le Ledum croit sur différents types de sites et la présence de cette espèce n'indique pas nécessairement un problème avec la régénération de conifères (croissance et germination).
ABSTRACT Hybrid poplar plantations were fertilized with hog manure at different manure supply rat... more ABSTRACT Hybrid poplar plantations were fertilized with hog manure at different manure supply rates for 6 years. In order to evaluate this practice in agricultural landscapes, we studied the effects of increasing manure supply on the growth and nutrition of three hybrid poplar clones and on soil solution chemistry at depths of 20 and 40 cm. More specifically, experimental plantations were established in nine fallow fields, each of which was divided into four plots fertilized with one of four levels of hog manure supply (i.e. 0, 40, 60 or 80 kg ha−1 in total N). Manure application significantly increased tree growth and C sequestration, although growth was not correlated with manure supply rates. Vector analysis of the nutritional status of trees suggests that N and K likely were limiting nutrients for tree growth on unfertilized plots. Our results further suggest that manure application in hybrid poplar plantations did not cause important changes in soil solution NO3− and PO43− concentrations. From a manure management perspective, this could indicate that hog manure can be used to increase the productivity of tree plantations without creating major concerns about potential damage to aquatic ecosystems, especially if plantations are located away from water bodies.
Journal Of Geophysical Research: Biogeosciences, May 1, 2019
Identifying geochemical paleo-proxies of vegetation type in watersheds could become a powerful to... more Identifying geochemical paleo-proxies of vegetation type in watersheds could become a powerful tool for paleoecological studies of ecosystem dynamics, particularly when commonly used proxies, such as pollen grains, are not suitable. In order to identify new paleological proxies to distinguish ecosystem types in lake records, we investigated the differences in the sediment geochemistry of lakes surrounded by two boreal forest ecosystems dominated by the same tree species: closed-canopy black spruce-moss forests (MF) and open-canopy black spruce-lichen woodlands (LW). This study was designed as a first calibration step between terrestrial modern soils and lacustrine sediments (0-1000 cal yr BP) on six lake watersheds. In a previous study, differences in the physical and geochemical properties of forest soils had been observed between these two modern ecosystems. Here we show that the geochemical properties of the sediments varied between the six lakes studied. While we did not identify geochemical indicators that could solely distinguish both ecosystem types in modern sediments, we observed intriguing differences in concentrations of C:N ratio, carbon isotopic ratio, and aluminum oxide species, and in the stabilization of their geochemical properties with depth. The C accumulation rates at millennial scale were significantly higher in MF watersheds than in LW watersheds. We suggest that these variations could result from organic matter inflows that fluctuate depending on forest density and ground vegetation cover. Further investigations on these highlighted geochemistry markers need to be performed to confirm whether they can be used to detect shifts in vegetation conditions that have occurred in the past.
Over long time periods, paludification reduces aboveground productivity resulting in forest retro... more Over long time periods, paludification reduces aboveground productivity resulting in forest retrogression. Paludified forests are typified by intense accumulation of the soil organic layer and a reduction in soil temperatures and nutrient availability. En route to paludification, early successional forests experience large inputs of deadwood biomass during the senescence of the post-fire cohort, much of which may be entombed in this rapidly growing soil organic layer. Here we examined the effects of paludification across a .2000-year chronosequence of black spruce forests on wood decomposition using three complementary approaches. We (1) repeatedly measured wood density of logs through time, (2) utilize a time-series of logs that varied in time since death, and (3) estimate woody biomass at the stand level as it progresses from live trees to snags, logs and ultimately to buried or decomposed deadwood. Together these approaches demonstrated a 6-7-year delay before the onset of rapid decomposition. We also found strong evidence that paludification results in a large proportion of logs becoming buried in the soil organic layer. Stand level modeling indicates that the rates of accumulation of buried deadwood were greatest following the senescence of the post-fire cohort when both soil organic layer build-up and creation of deadwood peaked. Following this period of high deadwood creation, stands enter a retrogressive state whereby productivity continues to decline albeit more slowly. Continued losses in woody carbon biomass from trees during this retrogressive state are offset by lower wood decomposition rates and a high biomass of accumulated buried deadwood, essentially stabilizing the wood based carbon budget in these ecosystems. We recommend that partial cutting be conducted prior to or near the senescence of the post-fire cohort to improve emulation of natural forest succession in terms of both live tree and deadwood biomass. Furthermore, deadwood during this period has an extremely short residence time and the dynamics of deadwood should recover much quicker than if harvesting is conducted later in succession when there is less live tree biomass and deadwood has longer residence times.
Chapitre 11 Un aménagement écosystémique de la pessière du nord de la ceinture d'argile québ... more Chapitre 11 Un aménagement écosystémique de la pessière du nord de la ceinture d'argile québécoise* Gérer la forêt mais surtout les sols Martin Simard, Nicolas Lecomte, Yves Bergeron, Pierre Y. Bernier et David Paré * Nous remercions les organismes ...
Following a wildfire, organic carbon (C) accumulates in boreal-forest soils. The long-term patter... more Following a wildfire, organic carbon (C) accumulates in boreal-forest soils. The long-term patterns of accumulation as well as the mechanisms responsible for continuous soil C stabilization or sequestration are poorly known. We evaluated post-fire C stock changes in functional reservoirs (bioreactive and recalcitrant) using the proportion of C mineralized in CO 2 by microbes in a long-term lab incubation, as well as the proportion of C resistant to acid hydrolysis. We found that all soil C pools increased linearly with the time since fire. The bioreactive and acid-insoluble soil C pools increased at a rate of 0.02 and 0.12 MgC ha −1 yr −1 , respectively, and their proportions relative to total soil C stock remained constant with the time since fire (8 % and 46 %, respectively). We quantified direct and indirect causal relationships among variables and C bioreactivity to disentangle the relative contribution of climate, moss dominance, soil particle size distribution and soil chemical properties (pH, exchangeable manganese and aluminum, and metal oxides) to the variation structure of in vitro soil C bioreactivity. Our analyses showed that the chemical properties of podzolic soils that characterize the study area were the best predictors of soil C bioreactivity. For the O layer, pH and exchangeable manganese were the most important (model-averaged estimator for both of 0.34) factors directly related to soil organic C bioreactivity, followed by the time since fire (0.24), moss dominance (0.08), and climate and texture (0 for both). For the mineral soil, exchangeable aluminum was the most important factor (model-averaged estimator of −0.32), followed by metal oxide (−0.27), pH (−0.25), the time since fire (0.05), climate and texture (∼ 0 for both). Of the four climate factors examined in this study (i.e., mean annual temperature, growing degree-days above 5 • C, mean annual precipitation and water balance) only those related to water availability-and not to temperature-had an indirect effect (O layer) or a marginal indirect effect (mineral soil) on soil C bioreactivity. Given that predictions of the impact of climate change on soil C balance are strongly linked to the size and the bioreactivity of soil C pools, our study stresses the need to include the direct effects of soil chemistry and the indirect effects of climate and soil texture on soil organic matter decomposition in Earth system models to forecast the response of boreal soils to global warming.
Forest management strongly influences the carbon (C) budget of boreal forests and their potential... more Forest management strongly influences the carbon (C) budget of boreal forests and their potential to mitigating greenhouse gas emissions. A better quantification of the net changes of carbon pools with time since harvesting is necessary to guide the development of climate-friendly forest management practices. The objective of this study was to assess the evolution of forest C pools, with a special focus on detrital biomass, in an 80-year postharvesting chronosequence consisting of 36 very homogenous stem-only harvested plots from a humid boreal balsam fir forest of eastern Canada. Dead wood C stocks comprised of snags, stumps, downed woody debris and buried wood averaged 37 Mg C ha −1 and evolved according to an upward-facing «boomerang» shape pattern throughout the chronosequence (rapid decrease in the first years followed by a constant increase until the end of the time horizon). In contrast, soil C stocks (FH and mineral) averaged 156 Mg C ha −1 and remain constant through time. Stand C sequestration increased rapidly in the early stages up to age 50 when it reached about 250 Mg C ha −1 , and then continued to accumulate at a slower rate. The temporal trends observed in C pools suggest that C originating from aboveground dead wood (snags, stumps, downed woody debris) is either leaving the system (respired or leached) or transferred into buried wood, and does not appear to influence the C stocks of the fine fraction of the organic and mineral soil horizons. However, the ultimate fate of dead wood C is still poorly understood and further research is needed in this field. We recommend fixing the length of harvest rotation at a minimum of 50 years for this ecosystem to allow the build-up of its dead wood capital, and to promote dead wood retention on site. We also recommend including buried wood in carbon inventories as this pool represents an important share of the detrital C stock in these humid boreal forests.
Vegetation-soil interactions are complex and constitutive processes of ecosystem dynamics, materi... more Vegetation-soil interactions are complex and constitutive processes of ecosystem dynamics, materialised by functional feedback roles between plant communities and the soil sys-Published by Copernicus Publications on behalf of the European Geosciences Union.
At the northernmost extent of the managed forest in Quebec, the boreal forest is currently underg... more At the northernmost extent of the managed forest in Quebec, the boreal forest is currently undergoing an ecological transition between two forest ecosystems. Open lichen woodlands (LW) are spreading southward at the expense of more productive closed-canopy black spruce-moss forests (MF). The objective of this study was to investigate whether soil properties could distinguish MF from LW in the transition zone where both ecosystem types coexist. All the soils studied were typical podzolic soil profiles evolved from glacial till deposits that shared a similar texture of the C layer. However, soil humus and the B layer varied in thickness and chemistry between the two forest ecosystems at the pedon scale. Multivariate analyses of variance were used to evaluate how soil properties could help distinguish the two types at the site scale. MF humus (FH horizons) showed significantly higher concentrations of organic carbon and of the main exchangeable base cations (Ca, Mg) than LW soils, whic...
h i g h l i g h t s Eastern Canada has large wood supply from natural disturbances that is under-... more h i g h l i g h t s Eastern Canada has large wood supply from natural disturbances that is under-utilized by sawmills. Despite high variability, biomass from fire-killed trees can contribute to a profitable pellet supply chain. Ecological and operational constraints have little impact on biomass supply. Uncertainties due to the variability of natural disturbances need to be accounted for.
In the context increasing atmospheric CO2 concentration, there is a growing interest for using af... more In the context increasing atmospheric CO2 concentration, there is a growing interest for using afforestation and reforestation to claim emissions reductions. Establishment of plantations necessarily involves silvicultural treatments that affect environmental conditions and hence soil microbial communities, influencing carbon and nitrogen cycles and potential soil carbon sequestration. We have monitored soil microbial indicators during a twenty year period of ecosystem recovery from disturbance at the Petawawa Research Forest (Ontario, Canada), in plantations of white pine (Pinus strobus L.) and white spruce (Picea glauca). The first ten years was marked by important changes in Cmic:Corg and Cmic:Nmic, related to vegetation control that provoked modification of bacteria:fungi ratios, accelerated N cycling and loss, as well as shifts in vegetation composition that are still quite marked after twenty years. After another 10 years (at 20 yrs) we note a closing of the N cycle and again i...
In silvicultural trials, the confounding influence of permanent soil properties is assumed to be ... more In silvicultural trials, the confounding influence of permanent soil properties is assumed to be minimal. A covariance analysis using total elemental concentrations of parent material (geochemistry) and soil particle size distribution (texture) was used to understand the role that these variables could play in tree growth and foliar nutrient status, and in the growth response to site preparation of 16- to18-yr-old jack pine plantations growing on seemingly homogeneous glacial outwash deposits. Three treatments were tested in a replicated design: direct plantation, and site preparation with TTS or with Bräcke. The combination of particle size distribution and soil geochemistry explained the site quality index (SQI) in control plots (R2 = 0.94) better than soil texture alone. In all plots, SQI was strongly related to K and Mg concentrations in foliage and in humus and, to a lesser degree, to foliar N concentrations. A covariance analysis using soil texture and geochemical composition indicated that while site preparation had an effect on tree growth, the sites that responded the most to site preparation were the ones with the lowest growth potential as determined by soil texture and geochemistry. Key words: Silviculture, site preparation, site quality index, growth, permanent soil factors, texture, geochemistry, nutrition
Abstract In existing carbon budget models, carbon stocks are not explicitly related to forest suc... more Abstract In existing carbon budget models, carbon stocks are not explicitly related to forest successional dynamics and environmental factors. Yet time-since-last-fire (TSLF) is an important variable for explaining successional changes and subsequent carbon storage. The objective of this study was to predict the spatial variability of aboveground biomass carbon (ABC) as a function of TSLF and other environmental factors across the landscape at regional scales. ABC was predicted using random forest models, both at the sample-plot level and at the scale of 2-km2 cells. This cell size was chosen to match the observed minimum fire size of the Canadian large fire database. The percentage variance explained by the empirical sample-plot level model of ABC was 50%. At that scale, TSLF was not significantly related to ABC. At the 2-km2 scale, ABC was influenced mainly by the proportions of cover density classes, which explained 83% of the variance. Changes in cover density were related to TSLF at the same 2-km2 scale, indicating that the increase in cover density following fire disturbance is a dominant mechanism through which TSLF acts upon ABC at the scale of landscapes.
Poor growth of black spruce (Picea mariana (Mill.) BSP) has been associated with the presence of ... more Poor growth of black spruce (Picea mariana (Mill.) BSP) has been associated with the presence of Ledum groenlandicum L. (Ledum) on some sites in the eastern boreal forest in Canada. To increase black spruce productivity on lowland sites, a study was carried out to test the effect of mechanical control of Ledum (by clipping) on black spruce growth on forested peatland in northwestern Quebec. We compared the growth and foliar nutrient concentrations of advance-regeneration black spruce seedlings with and without Ledum control. Contrary to our expectations, our results showed that 3-year control of aboveground competition by Ledum decreased rather than increased black spruce growth and had no effect on foliar nutrient concentrations. Ledum grows on a variety of site types; therefore, the mere presence of this species does not necessarily indicate that there will be a problem with conifer regeneration (growth and germination). Résumé : Une diminution dans la croissance de l'épinette noire (Picea mariana (Mill.) BSP) a été associée à la présence du Ledum groenlandicum L. (Ledum) sur certains sites dans la forêt boréale de l'est du Canada. Avec comme objectif d'augmenter la productivité de l'épinette noire dans les stations des basses-terres, une étude a été entreprise afin de mesurer l'effet du contrôle mécanique du Ledum sur la croissance de l'épinette noire dans une tourbière boisée située dans le nord-ouest du Québec. Nous avons comparé la croissance et les concentrations foliaires de nutriments de semis d'épinette noire dans des parcelles avec et sans contrôle du Ledum. Contrairement à nos attentes, nos résultats ont montré qu'un contrôle de la compétition épigée du Ledum pendant trois ans résultait en une baisse plutôt qu'une augmentation de la croissance de l'épinette noire et n'occasionnait pas de différence dans les concentrations foliaires de nutriments. Le Ledum croit sur différents types de sites et la présence de cette espèce n'indique pas nécessairement un problème avec la régénération de conifères (croissance et germination).
ABSTRACT Hybrid poplar plantations were fertilized with hog manure at different manure supply rat... more ABSTRACT Hybrid poplar plantations were fertilized with hog manure at different manure supply rates for 6 years. In order to evaluate this practice in agricultural landscapes, we studied the effects of increasing manure supply on the growth and nutrition of three hybrid poplar clones and on soil solution chemistry at depths of 20 and 40 cm. More specifically, experimental plantations were established in nine fallow fields, each of which was divided into four plots fertilized with one of four levels of hog manure supply (i.e. 0, 40, 60 or 80 kg ha−1 in total N). Manure application significantly increased tree growth and C sequestration, although growth was not correlated with manure supply rates. Vector analysis of the nutritional status of trees suggests that N and K likely were limiting nutrients for tree growth on unfertilized plots. Our results further suggest that manure application in hybrid poplar plantations did not cause important changes in soil solution NO3− and PO43− concentrations. From a manure management perspective, this could indicate that hog manure can be used to increase the productivity of tree plantations without creating major concerns about potential damage to aquatic ecosystems, especially if plantations are located away from water bodies.
Journal Of Geophysical Research: Biogeosciences, May 1, 2019
Identifying geochemical paleo-proxies of vegetation type in watersheds could become a powerful to... more Identifying geochemical paleo-proxies of vegetation type in watersheds could become a powerful tool for paleoecological studies of ecosystem dynamics, particularly when commonly used proxies, such as pollen grains, are not suitable. In order to identify new paleological proxies to distinguish ecosystem types in lake records, we investigated the differences in the sediment geochemistry of lakes surrounded by two boreal forest ecosystems dominated by the same tree species: closed-canopy black spruce-moss forests (MF) and open-canopy black spruce-lichen woodlands (LW). This study was designed as a first calibration step between terrestrial modern soils and lacustrine sediments (0-1000 cal yr BP) on six lake watersheds. In a previous study, differences in the physical and geochemical properties of forest soils had been observed between these two modern ecosystems. Here we show that the geochemical properties of the sediments varied between the six lakes studied. While we did not identify geochemical indicators that could solely distinguish both ecosystem types in modern sediments, we observed intriguing differences in concentrations of C:N ratio, carbon isotopic ratio, and aluminum oxide species, and in the stabilization of their geochemical properties with depth. The C accumulation rates at millennial scale were significantly higher in MF watersheds than in LW watersheds. We suggest that these variations could result from organic matter inflows that fluctuate depending on forest density and ground vegetation cover. Further investigations on these highlighted geochemistry markers need to be performed to confirm whether they can be used to detect shifts in vegetation conditions that have occurred in the past.
Over long time periods, paludification reduces aboveground productivity resulting in forest retro... more Over long time periods, paludification reduces aboveground productivity resulting in forest retrogression. Paludified forests are typified by intense accumulation of the soil organic layer and a reduction in soil temperatures and nutrient availability. En route to paludification, early successional forests experience large inputs of deadwood biomass during the senescence of the post-fire cohort, much of which may be entombed in this rapidly growing soil organic layer. Here we examined the effects of paludification across a .2000-year chronosequence of black spruce forests on wood decomposition using three complementary approaches. We (1) repeatedly measured wood density of logs through time, (2) utilize a time-series of logs that varied in time since death, and (3) estimate woody biomass at the stand level as it progresses from live trees to snags, logs and ultimately to buried or decomposed deadwood. Together these approaches demonstrated a 6-7-year delay before the onset of rapid decomposition. We also found strong evidence that paludification results in a large proportion of logs becoming buried in the soil organic layer. Stand level modeling indicates that the rates of accumulation of buried deadwood were greatest following the senescence of the post-fire cohort when both soil organic layer build-up and creation of deadwood peaked. Following this period of high deadwood creation, stands enter a retrogressive state whereby productivity continues to decline albeit more slowly. Continued losses in woody carbon biomass from trees during this retrogressive state are offset by lower wood decomposition rates and a high biomass of accumulated buried deadwood, essentially stabilizing the wood based carbon budget in these ecosystems. We recommend that partial cutting be conducted prior to or near the senescence of the post-fire cohort to improve emulation of natural forest succession in terms of both live tree and deadwood biomass. Furthermore, deadwood during this period has an extremely short residence time and the dynamics of deadwood should recover much quicker than if harvesting is conducted later in succession when there is less live tree biomass and deadwood has longer residence times.
Chapitre 11 Un aménagement écosystémique de la pessière du nord de la ceinture d'argile québ... more Chapitre 11 Un aménagement écosystémique de la pessière du nord de la ceinture d'argile québécoise* Gérer la forêt mais surtout les sols Martin Simard, Nicolas Lecomte, Yves Bergeron, Pierre Y. Bernier et David Paré * Nous remercions les organismes ...
Following a wildfire, organic carbon (C) accumulates in boreal-forest soils. The long-term patter... more Following a wildfire, organic carbon (C) accumulates in boreal-forest soils. The long-term patterns of accumulation as well as the mechanisms responsible for continuous soil C stabilization or sequestration are poorly known. We evaluated post-fire C stock changes in functional reservoirs (bioreactive and recalcitrant) using the proportion of C mineralized in CO 2 by microbes in a long-term lab incubation, as well as the proportion of C resistant to acid hydrolysis. We found that all soil C pools increased linearly with the time since fire. The bioreactive and acid-insoluble soil C pools increased at a rate of 0.02 and 0.12 MgC ha −1 yr −1 , respectively, and their proportions relative to total soil C stock remained constant with the time since fire (8 % and 46 %, respectively). We quantified direct and indirect causal relationships among variables and C bioreactivity to disentangle the relative contribution of climate, moss dominance, soil particle size distribution and soil chemical properties (pH, exchangeable manganese and aluminum, and metal oxides) to the variation structure of in vitro soil C bioreactivity. Our analyses showed that the chemical properties of podzolic soils that characterize the study area were the best predictors of soil C bioreactivity. For the O layer, pH and exchangeable manganese were the most important (model-averaged estimator for both of 0.34) factors directly related to soil organic C bioreactivity, followed by the time since fire (0.24), moss dominance (0.08), and climate and texture (0 for both). For the mineral soil, exchangeable aluminum was the most important factor (model-averaged estimator of −0.32), followed by metal oxide (−0.27), pH (−0.25), the time since fire (0.05), climate and texture (∼ 0 for both). Of the four climate factors examined in this study (i.e., mean annual temperature, growing degree-days above 5 • C, mean annual precipitation and water balance) only those related to water availability-and not to temperature-had an indirect effect (O layer) or a marginal indirect effect (mineral soil) on soil C bioreactivity. Given that predictions of the impact of climate change on soil C balance are strongly linked to the size and the bioreactivity of soil C pools, our study stresses the need to include the direct effects of soil chemistry and the indirect effects of climate and soil texture on soil organic matter decomposition in Earth system models to forecast the response of boreal soils to global warming.
Forest management strongly influences the carbon (C) budget of boreal forests and their potential... more Forest management strongly influences the carbon (C) budget of boreal forests and their potential to mitigating greenhouse gas emissions. A better quantification of the net changes of carbon pools with time since harvesting is necessary to guide the development of climate-friendly forest management practices. The objective of this study was to assess the evolution of forest C pools, with a special focus on detrital biomass, in an 80-year postharvesting chronosequence consisting of 36 very homogenous stem-only harvested plots from a humid boreal balsam fir forest of eastern Canada. Dead wood C stocks comprised of snags, stumps, downed woody debris and buried wood averaged 37 Mg C ha −1 and evolved according to an upward-facing «boomerang» shape pattern throughout the chronosequence (rapid decrease in the first years followed by a constant increase until the end of the time horizon). In contrast, soil C stocks (FH and mineral) averaged 156 Mg C ha −1 and remain constant through time. Stand C sequestration increased rapidly in the early stages up to age 50 when it reached about 250 Mg C ha −1 , and then continued to accumulate at a slower rate. The temporal trends observed in C pools suggest that C originating from aboveground dead wood (snags, stumps, downed woody debris) is either leaving the system (respired or leached) or transferred into buried wood, and does not appear to influence the C stocks of the fine fraction of the organic and mineral soil horizons. However, the ultimate fate of dead wood C is still poorly understood and further research is needed in this field. We recommend fixing the length of harvest rotation at a minimum of 50 years for this ecosystem to allow the build-up of its dead wood capital, and to promote dead wood retention on site. We also recommend including buried wood in carbon inventories as this pool represents an important share of the detrital C stock in these humid boreal forests.
Vegetation-soil interactions are complex and constitutive processes of ecosystem dynamics, materi... more Vegetation-soil interactions are complex and constitutive processes of ecosystem dynamics, materialised by functional feedback roles between plant communities and the soil sys-Published by Copernicus Publications on behalf of the European Geosciences Union.
At the northernmost extent of the managed forest in Quebec, the boreal forest is currently underg... more At the northernmost extent of the managed forest in Quebec, the boreal forest is currently undergoing an ecological transition between two forest ecosystems. Open lichen woodlands (LW) are spreading southward at the expense of more productive closed-canopy black spruce-moss forests (MF). The objective of this study was to investigate whether soil properties could distinguish MF from LW in the transition zone where both ecosystem types coexist. All the soils studied were typical podzolic soil profiles evolved from glacial till deposits that shared a similar texture of the C layer. However, soil humus and the B layer varied in thickness and chemistry between the two forest ecosystems at the pedon scale. Multivariate analyses of variance were used to evaluate how soil properties could help distinguish the two types at the site scale. MF humus (FH horizons) showed significantly higher concentrations of organic carbon and of the main exchangeable base cations (Ca, Mg) than LW soils, whic...
h i g h l i g h t s Eastern Canada has large wood supply from natural disturbances that is under-... more h i g h l i g h t s Eastern Canada has large wood supply from natural disturbances that is under-utilized by sawmills. Despite high variability, biomass from fire-killed trees can contribute to a profitable pellet supply chain. Ecological and operational constraints have little impact on biomass supply. Uncertainties due to the variability of natural disturbances need to be accounted for.
In the context increasing atmospheric CO2 concentration, there is a growing interest for using af... more In the context increasing atmospheric CO2 concentration, there is a growing interest for using afforestation and reforestation to claim emissions reductions. Establishment of plantations necessarily involves silvicultural treatments that affect environmental conditions and hence soil microbial communities, influencing carbon and nitrogen cycles and potential soil carbon sequestration. We have monitored soil microbial indicators during a twenty year period of ecosystem recovery from disturbance at the Petawawa Research Forest (Ontario, Canada), in plantations of white pine (Pinus strobus L.) and white spruce (Picea glauca). The first ten years was marked by important changes in Cmic:Corg and Cmic:Nmic, related to vegetation control that provoked modification of bacteria:fungi ratios, accelerated N cycling and loss, as well as shifts in vegetation composition that are still quite marked after twenty years. After another 10 years (at 20 yrs) we note a closing of the N cycle and again i...
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