Fine roots are of immediate importance for tree nutrient uptake, critical for ensuring species co-existence in natural forests. Their biomass, tissue density and distribution along soil profile have been viewed as proxy for nutrient foraging strategy. However, their contribution to trees underground interactions has been rarely quantified and their effects on productivity remain to be demonstrated. We used root weight density (RWD, soil volume filled by fine roots) and root dry matter content (RDMC, the amount of structural material invested by unit volume of roots) respectively as proxies of biomass and root tissue density to characterize root foraging strategies of black spruce (Picea mariana [Mill] B.S.P.) and trembling aspen (Populus tremuloides (Michx.)) in three soil horizon layers (organic, top and bottom mineral layers taken respectively at 0-15 cm and 15-30 cm under the organic layer) in pure and mixed stands to determine whether the two species partition their resource uptake with soil depth in boreal mixedwoods of eastern Canada.
Results/Conclusions
Spruce fine root biomass significantly decreased by 23-45% in mixed compared to pure stands which were generally more nutrient poor than mixed stands. Aspen rooting patterns did not show significant differences in the organic and the bottom mineral layers between pure and mixed stands, except in the top mineral horizon layer where the RWD increase by 26% in mixed stands. The results demonstrated a negative and positive mixing effect on underground biomass production of spruce and aspen, respectively. In both types of stands spruce had a shallower rooting as compared to aspen. The results indicated a contrasted and stable root foraging strategies respectively for spruce and aspen between pure and mixed stands. Spruce rooting patterns allowed a more efficient resource use (intensive strategy) in mixed than in pure stands, conferring it a competitive advantage over aspen in the organic horizon. The competitive pressure of spruce in the organic horizon led to an increased aspen competiveness deeper in the soil in mixed stands; which might act towards stronger partitioning of their nutrients uptake between the two soil horizons. We speculate that spruce nutrient uptake may benefit from the presence of aspen in the stand and do not limit aspen nutrients acquisition.