Recent studies on phenotypic plasticity of plant traits indicate that within-species variation in litter quality may be a significant factor that feeds back on litter decomposition and nutrient cycling rates at the stand level. These findings may be especially significant for understanding biodiversity-stability relationships in species-poor ecosystems that have little functional redundancy among primary producers. We tested the null hypothesis that black spruce and Kalmia were functional equivalents with respect to their structuring roles of subordinate vegetation and their influence on site biogeochemistry. The purpose of the study was to determine the degree to which forest cover exerts top-down control on community structure and function of Kalmia-black spruce communities. This community type dominates much of the forest understory and unforested heathlands in Atlantic Canada. We intensively studied a representative stand of Kalmia heath in Terra Nova National Park in eastern Newfoundland. Thirty-two 0.5 m × 0.5 m sample plots were randomly distributed among five transects bisecting gradients in dominance of black spruce and Kalmia. Light levels, species composition, vascular plant cover and soil respiration rate were determined for each plot. Tissue samples of litter, mature and current year leaves of Kalmia were collected and analyzed for nutrient status. Herbaceous species richness and cover peaked at intermediate light levels. Kalmia foliar N concentration and above-ground biomass increased with increasing shade. Soil respiration rates were strongly related to the light gradient and increased with increasing quality of Kalmia litter inputs. Our data indicate that Kalmia's vigour and foliar nitrogen concentrations are greater under black spruce canopy as opposed to heath condition and that the shaded phenotype has relatively benign feedbacks on soil productivity compared to the open-habitat phenotype. In the absence of functional diversity at the species level in these species-poor habitats, phenotypic plasticity in Kalmia appears to be an important dimension of the biodiversity-stability relationship in these communities since our data suggest that this species has the potential either to inhibit or facilitate carbon cycling and the pathway is strongly linked to the presence or absence of overstory cover. The role of forest regeneration as an indirect control of forest soil processes such as carbon and nitrogen cycling in this ecosystem is discussed.
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