Complémentarités nutritionnelles entre l'épinette noire et le peuplier faux-tremble dans la forêt boréale mixte de l'est du Canada

Les études montrent que les coupes totales, utilisées sur plus de 85% des forêts affectées à l’exploitation du bois au Canada, pourraient impacter négativement la biodiversité et la productivité futures des forêts boréales du pays. L’épinette noire et le peuplier faux-tremble sont les deux espèces les plus abondantes à haute valeur marchande dans la forêt boréale de l'est du Canada. La valeur marchande plus élevée de l'épinette noire par rapport à celle du peuplier faux-tremble, la supposé exclusion compétitive du tremble par l’épinette au cours de la succession forestière, ainsi que l’absence de consensus sur le type de relation (positive, négative ou non significative) diversité-productivité dans le biome forestier boréal, incitent les aménagistes forestiers à orienter les opérations sylvicoles (éclaircie commerciale et pré-commerciale) de façon à exclure le peuplier faux-tremble des peuplements. Les relations dans les sols entre les espèces, généralement omises des études diversité-productivité en forêt boréale, semblent être la clé pour une meilleure compréhension des interactions entre ces deux espèces. Cette thèse explore les interactions dans les sols entre l’épinette noire et le peuplier faux-tremble afin de déterminer si elles sont complémentaires vis-à-vis de l’utilisation des nutriments du sol dans les peuplements mixtes de la forêt boréale du Nord-Ouest du Québec. Dans un premier temps (Chapitres II et III), je caractérise les stratégies d'acquisition des ressources des deux espèces le long du profil du sol. J’insiste par la suite (Chapitre IV) sur l’azote (N) qui représente la ressource la plus limitante à la croissance des arbres en forêt boréale. A cet effet, les attributs racinaire (biomasse, densité de tissus et symbioses mycorrhiziennes) et foliaire (teneur en N, rapports isotopiques du N (δ15N)) de l’épinette et du tremble ont été mesurés pour déterminer si les deux espèces absorbe les nutriments dans des horizons de sol différents. Une expérience in situ d’ajouts de deux formes d’isotopes stables de l’azote (ammonium et nitrate) a également été mise en place pour déterminer si les deux espèces ont des préférences différentes pour les deux formes minérales de l’azote. L’analyse des attributs racinaire et foliaire a montré que l’épinette tire l’essentiel de sa nutrition azotée dans l’horizon organique tandis que celle du peuplier provient de l’horizon minéral. Les résultats montrent que l'épinette bénéficie d'un effet facilitateur du tremble sur les propriétés chimiques du sol pour adopter une stratégie intensive d'absorption des nutriments. Cette stratégie confère à l'épinette un avantage concurrentiel sur le tremble dans la couche organique. La diversité et l'abondance plus élevées des communautés mycorhiziennes associées aux racines de l'épinette que celles du peuplier faux-tremble dans l'horizon organique des peuplements mixtes démontrent une domination compétitive des communautés associées à l’épinette sur celle du peuplier faux-tremble; suggérant une exclusion compétitive du tremble par l’épinette dans l'horizon organique. Le peuplier faux-tremble maintenait la même biomasse de racines fines dans l'horizon organique et l'augmentait de 25% dans l'horizon minéral des peuplements mixtes par rapport aux peuplements purs. J’ai interprété ce patron comme un mécanisme d'évitement de la compétition, car l'augmentation de la biomasse de racines fines dans le sol minéral n'était pas le résultat de l'effet du mélange d'espèces sur les propriétés chimiques du sol. La diversité et l'abondance plus élevées des communautés mycorhiziennes du tremble que celles de l'épinette dans l'horizon minéral des peuplements mixtes suggéraient une domination compétitive du tremble sur l'épinette dans le sol minéral des peuplements mixtes. L’expérience d’ajouts d’isotopes a révélé une claire préférence de l’épinette pour l’ammonium et du peuplier faux-tremble pour le nitrate dans leur peuplement pur respectif. Ces préférences étaient inexistantes dans les peuplements mixtes où la cinétique d’absorption ou le facteur de fractionnement des deux formes d’isotopes de l’azote ajoutées ne différait pas entre les deux espèces. Contrairement aux précédentes études, cette étude montre que l’épinette bénéficie de la présence du tremble sans impacter négativement l’acquisition des ressources de ce dernier. Les résultats suggèrent une exclusion compétitive du tremble par l'épinette qui ne se produit probablement que dans la couche organique du sol dans les peuplements mixtes. Une séparation spatiale plus forte des racines de l'épinette et du tremble dans les peuplements mixtes contribue à un partitionnement spatial de leur absorption de nutriments le long du profil du sol. La biomasse de racines fines plus faible de l'épinette dans les peuplements mixtes que purs a montré que l'épinette alloue plus de carbone (C) à sa croissance aérienne dans les peuplements mixtes que dans les peuplements purs où le C est principalement alloué dans le sol pour la recherche de nutriments. Ces résultats sont conformes à l'observation précédente selon laquelle les peuplements mixtes d’épinettes et de trembles ont un volume plus élevé de biomasse marchande avec un volume d'épinettes dont les tiges sont plus grosses que les peuplements purs d’épinettes (Légaré et al., 2004). Ainsi, cette thèse suggère aux aménagistes forestiers du Québec de diversifier les pratiques sylvicoles à travers le paysage forestier selon les types de peuplements. Plus spécifiquement, je recommande de maintenir le tremble pendant l'éclaircie pré-commerciale dans les peuplements à dominés par l'épinette. La présence de trembles assurera une certaine stabilité aux peuplements dominés par l’épinette et réduira leur susceptibilité aux pertes de productivité liée à la paludification des sols. Dans les peuplements de trembles purs et mixtes, je recommande de promouvoir le mélange d'épinettes et de trembles. L’étude suggère qu’au-delà de leur coexistence en raison du partitionnement de leur absorption de nutriments dans les horizons de sol différents, l'épinette et le tremble utiliseraient aussi efficacement les nutriments disponibles dans le sol

Contrasting Root System Structure and Belowground Interactions between Black Spruce (\u3cem\u3ePicea mariana\u3c/em\u3e (Mill.) B.S.P) and Trembling Aspen (\u3cem\u3ePopulus tremuloides\u3c/em\u3e Michx) in Boreal Mixedwoods of Eastern Canada

This study explored the underground interactions between black spruce and trembling aspen in pure and mixed stands to understand how their soil resource use help these species coexist in the boreal mixedwoods of Western Quebec. We analyzed species-specific fine root foraging strategies (root biomass and root tissue density) along three soil layers (organic, top 0–15 cm, and bottom 15–30 cm mineral soil), using 180 soil cores. We collected cores in three sites, each containing three 20 × 50 m2 plots of pure spruce, pure aspen, and mixed spruce and aspen stands. Spruce had a shallow rooting, whereas aspen had a deep rooting in both types of stands. Compared to pure spruce stands, spruce had a lower fine root biomass (FRB) and a higher root tissue density (RTD) in the organic layer of mixed stands. Both patterns were indicative of spruce’s more intensive resource use strategy and competitive advantage over aspen in that layer. Aspen FRB in the organic soil did not differ significantly between pure and mixed stands, but increased in the mineral soil of mixed stands. Since we did not observe a significant difference in the nutrient content of the mineral soil layer between pure aspen and mixed stands, we concluded that aspen may experience competitive exclusion in the organic layer by spruce. Aspen exhibited an extensive nutrient uptake strategy in the organic layer of mixed stands: higher FRB and lower RTD than spruce. In mixed stands, the differences in aspen rooting patterns between the organic and mineral layers suggested the use of contrasting nutrient uptake strategies along the soil profile. We speculate that the stronger spatial separation of the roots of spruce and aspen in mixed stands likely contribute to a higher partitioning of their nutrient uptake along the soil profile. These results indicate the competitive exclusion of aspen by spruce in boreal mixedwoods, which likely occurs in the soil organic layer

Contrasting Root System Structure and Belowground Interactions between Black Spruce (Picea mariana (Mill.) B.S.P) and Trembling Aspen (Populus tremuloides Michx) in Boreal Mixedwoods of Eastern Canada

This study explored the underground interactions between black spruce and trembling aspen in pure and mixed stands to understand how their soil resource use help these species coexist in the boreal mixedwoods of Western Quebec. We analyzed species-specific fine root foraging strategies (root biomass and root tissue density) along three soil layers (organic, top 0-15 cm, and bottom 15-30 cm mineral soil), using 180 soil cores. We collected cores in three sites, each containing three 20 × 50 m2 plots of pure spruce, pure aspen, and mixed spruce and aspen stands. Spruce had a shallow rooting, whereas aspen had a deep rooting in both types of stands. Compared to pure spruce stands, spruce had a lower fine root biomass (FRB) and a higher root tissue density (RTD) in the organic layer of mixed stands. Both patterns were indicative of spruce's more intensive resource use strategy and competitive advantage over aspen in that layer. Aspen FRB in the organic soil did not differ significantly between pure and mixed stands, but increased in the mineral soil of mixed stands. Since we did not observe a significant difference in the nutrient content of the mineral soil layer between pure aspen and mixed stands, we concluded that aspen may experience competitive exclusion in the organic layer by spruce. Aspen exhibited an extensive nutrient uptake strategy in the organic layer of mixed stands: higher FRB and lower RTD than spruce. In mixed stands, the differences in aspen rooting patterns between the organic and mineral layers suggested the use of contrasting nutrient uptake strategies along the soil profile. We speculate that the stronger spatial separation of the roots of spruce and aspen in mixed stands likely contribute to a higher partitioning of their nutrient uptake along the soil profile. These results indicate the competitive exclusion of aspen by spruce in boreal mixedwoods, which likely occurs in the soil organic layer

Intraspecific variability in growth response to environmental fluctuations modulates the stabilizing effect of species diversity on forest growth

1. Differences between species in their response to environmental fluctuations cause asynchronized growth series, suggesting that species diversity may help communities buffer the effects of environmental fluctuations. However, within-species variability of responses may impact the stabilizing effect of growth asynchrony. 2. We used tree ring data to investigate the diversity-stability relationship and its underlying mechanisms within the temperate and boreal mixed woods of Eastern Canada. We worked at the individual tree level to take into account the intraspecific variability of responses to environmental fluctuations. 3. We found that species diversity stabilized growth in forest ecosystems. The asynchrony of species' response to climatic fluctuations and to insect outbreaks explained this effect. We also found that the intraspecific variability of responses to environmental fluctuations was high, making the stabilizing effect of diversity highly variable. 4. Synthesis. Our results are consistent with previous studies suggesting that the asynchrony of species' response to environmental fluctuations drives the stabilizing effect of diversity. The intraspecific variability of these responses modulates the stabilizing effect of species diversity. Interactions between individuals, variation in tree size and spatial heterogeneity of environmental conditions could play a critical role in the stabilizing effect of diversity

Data from: Intraspecific variability in growth response to environmental fluctuations modulates the stabilizing effect of species diversity on forest growth

1.Differences between species in their response to environmental fluctuations cause asynchronized growth series, suggesting that species diversity may help communities buffer the effects of environmental fluctuations. However, within-species variability of responses may impact the stabilizing effect of growth asynchrony. 2.We used tree ring data to investigate the diversity-stability relationship and its underlying mechanisms within the temperate and boreal mixed woods of Eastern Canada. We worked at the individual tree level to take into account the intraspecific variability of responses to environmental fluctuations. 3.We found that species diversity stabilized growth in forest ecosystems. The asynchrony of species’ response to climatic fluctuations and to insect outbreaks explained this effect. We also found that the intraspecific variability of responses to environmental fluctuations was high, making the stabilizing effect of diversity highly variable. 4.Synthesis. Our results are consistent with previous studies suggesting that the asynchrony of species’ response to environmental fluctuations drives the stabilizing effect of diversity. The intraspecific variability of these responses modulates the stabilizing effect of species diversity. Interactions between individuals, variation in tree size and spatial heterogeneity of environmental conditions could play a critical role in the stabilizing effect of diversity

Contrasting Structure of Root Mycorrhizal Communities of Black Spruce and Trembling Aspen in Different Layers of the Soil Profile in the Boreal Mixedwoods of Eastern Canada

Purpose Mycorrhizal fungi are critical for the growth and survival of trees although the knowledge on the extent of their association with different tree species in the boreal forest remains limited. Methods We examined the vertical distribution and composition of the root mycorrhizal communities of black spruce (Picea mariana (Mill.) B.S.P) and trembling aspen (Populus tremuloides Michx) along three soil layers (organic, minerals top 0–15 cm and bottom 15–30 cm) in pure and mixed stands, using next generation sequencing. Results We found that spruce and aspen differ in the composition of their mycorrhizal communities in respective pure stands. The difference was present also in mixed stands, despite a shift in the composition of species-specific mycorrhizal communities between pure and mixed stands. In mixed stands, the relative abundance of spruce-specialist mycorrhizae in the organic layer was higher than that of aspen-specialists. The opposite pattern was observed in the mineral soil. The mixed stands exhibited lower richness and abundance of generalist mycorrhizae in the organic and in the mineral soil layers. Conclusion The results suggest that it is the soil chemistry that structure species-specific mycorrhizal communities between pure stands and along different soil depth within stands. However, in mixed stands, it is the identity of tree species that determines the structure of mycorrhizae communities within soil layers. We speculate that the differences in the richness and abundance of individual mycorrhizal communities of spruce and aspen along the soil profile would likely contribute to stronger partitioning of tree nutrient uptake between these two species in mixed stands

Climatic data

01 - Climatic data for each site. File names in the 01_CLIM folder: climABI.csv, climBIC.csv, climD1823D1847.csv (given their geographical proximity, D1823 and D1847 received the same climatic data), climSUT.csv. These files contain climatic variables for the 1953-2013 period. Climatic variables are coded with letters: T: monthly mean temperatures P: total monthly precipitation S: total monthly snowfall DD: total monthly degree day DC: monthly mean drought-code GSL: growth season length. Numbers following the variables letters indicate the number of the month associated with the variable. Negative values refer to a month of the previous year. GSL6 refers to the current GSL and GSL-6 refers to the previous GSL

Tree ring data

06 - Tree ring data. 20 files in the 06_TRD folder. File names are coded with species initials followed by the name of the site. These files contain ring width measurement for all trees kept for the analyses. Trees ID are coded with a site code (1: D1823, 2: D1847, 3: ABI, 4:BIC, 5: SUT) followed by their species code, followed by a number

Insect outbreak dates

03 - Insect outbreak dates. File name: 03_outbreaks_dates.xlsx. This file contains the forest tent caterpillar and spruce budworm outbreak dates for each site

Distance among trees

04 - Distances among trees on each site. File names in the 04_DIST folder: distABI.csv, distBIC.csv, distD1823.csv, distD1847.csv, distSUT.csv. These files contain the distances (< 20 m) between the cored trees (rows) and all their neighbours (columns). Trees ID are coded with their species code followed by a number