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

Abstract

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.

Role of Mixed-Species Stands in Attenuating the Vulnerability of Boreal Forests to Climate Change and Insect Epidemics

“…Higher average growth of black spruce in mixed stands with aspen may be due to lower stand densities than in PBS, although mixed stands had higher basal areas ( Table 3 ), and due to niche partitioning and type of leaf litter ( Cavard et al, 2011 ). Black spruce in mixed stands may exploit a complementary crown packaging with trembling aspen ( Burns and Honkala, 1990a , b ; Cavard et al, 2011 ), a partitioning of the soil profile between the different root systems ( Mekontchou et al, 2020 ), and higher nutrient availability due to aspen litterfall, which is less acidic and decomposes faster than black spruce needle litter ( Légaré et al, 2005 ; Laganière et al, 2010 ). These beneficial effects were reduced following the SBW epidemic and dry conditions during the 1970s, resulting in a negative mean BAI long-term linear trend for spruce in mixed stands ( Figures 4 , 5B ) and in a positive trend for aspen in the same stands ( Figure 5H ), although aspen showed a growth decline after the mid-1980s ( Figure 4 ; Supplementary Figure 4 ).…”

Insect defoliation modulates influence of climate on the growth of tree species in the boreal mixed forests of eastern Canada

“…Differences in xylem anatomy, plant allometry, stomatal behavior, rooting strategies, stand density, and forest management can all modify the response of tree species to environmental variability. For example, deciduous trees such as white birch and trembling aspen have a deeper root system than conifers, enabling them to access water from deeper soil layers to better overcome seasonal water deficits, as compared to coniferous species (Mekontchou et al, 2020; Oltchev et al, 2002).…”

Sequential droughts: A silent trigger of boreal forest mortality

“…Therefore, the mortality rate in the whole plot could be reduced by the higher resistance of broadleaf trees in relation to balsam fir (selection effects). Second, the generally deeper root systems of broadleaf species in comparison with balsam fir may reduce competition for soil moisture due to the rooting stratification (resource partitioning, Ghotsa Mekontchou et al, 2020). Finally, broadleaf species could exert a facilitative effect on balsam fir by reducing solar irradiance in the understorey and mediating water stress (Cavard et al, 2011; St. Clair et al, 2013).…”