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.
Highlights
Understanding the relationship between diversity and productivity in forest ecosystems is critical to quantifying ecosystem responses to past and future environmental variability
Aspen root biomass in the organic and the bottom mineral layers did not differ significantly between pure and mixed stands, the 25% increase in fine root biomass (FRB) recorded in the top mineral layer of mixed stands indicated a positive effect of mixing on aspen root biomass, supporting hypothesis H3
We found an increase of spruce FRB in pure stands in response to lower nutrient availability, and its decline in mixed stands as a result of increased nutrient availability compared to pure stands (Figure 5a)
Summary
Understanding the relationship between diversity and productivity in forest ecosystems is critical to quantifying ecosystem responses to past and future environmental variability. A few studies have highlighted an insignificant or even negative effect of species diversity on forest productivity [17,18] and tree growth [14]. Earlier studies of biodiversity–productivity relationships (BPR) in boreal mixedwoods focused primarily on the effects of variability in climate [14,19] and light conditions [17]. Evaluating variability in tree diameter and volume growth, these studies indicated the potential role of competition, facilitation, and niche complementarity in shaping tree productivity [12,20,21]. Apart from theoretical interest, a mechanistic model explaining BPR in the boreal forest could be of critical importance in developing silvicultural practices in boreal mixedwoods
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