The contribution of intraspecific trait variation to changes in functional community structure along a stress gradient

Question We considered two possibilities related to the contribution of intraspecific trait variation (ITV) to changes in functional community structure along a stress gradient in tundra vegetation. First, ITV could contribute to the success of plant species across the stress gradient by promoting optimal trait values for each condition along the gradient; thus, ITV enhances convergence toward optimal traits. Second, ITV of only a few dominant species aligns with the optimal trait, and ITV of other species promotes trait diversification within communities. Location Salluit, Quebec, Canada (62 degrees 12 ' N, 75 degrees 39 ' W). Methods Vegetation was surveyed under three different conditions (harsh, intermediate, and competitive) representing an environmental stress gradient. We assessed ITV across the gradient for four plant functional traits: leaf carbon (C) and nitrogen (N) contents, specific leaf area (SLA), and plant height. We assessed community-weighted means (CWMs) and functional dispersion (FDis) for each of the four traits. These indices were calculated from mean trait values from all individuals across all habitats (mean-CWM and mean-FDis), and from each habitat (ITV-CWM and ITV-FDis). Results For the four traits, increasing trends (leaf N content, SLA, and plant height) and decreasing trends (leaf C content) in mean-CWM along the stress gradient were maintained and pronounced for ITV-CWM. Comparisons between ITV-FDis and mean-FDis suggested that, except for leaf C content, ITV tends to reduce and increase FDis values at the abiotically stressful and competitive ends of the environmental stress gradient, respectively. Conclusions ITV along the abiotic stress-competition gradient appears to promote community-level changes towards optimal traits. In particular, ITV in three traits (with the exception of leaf C content) may contribute to equalization of optimal trait values, and to "passage through environmental filtering" in harsh habitats. However, when species deal with competition for richer resources, only some dominant species may show optimal trait values. Therefore, ITV may allow for trait diversification within communities under competitive rather than harsh abiotic conditions.