Disentangling the role of inter and intraspecific trait variations in plant community assembly processes

The analysis of functional trait variation in plant assemblages along environmental gradients may provide fundamental information to understand community assembly. Differences in trait dominance among assemblages, commonly expressed by the community-weighted mean, can arise from either the intraspecific variability alone, a change in species composition (species turnover, that is, interspecific variation), or, most often, a combination of these two sources of variation. Although differences among plant communities in response to environmental changes are mostly due to interspecific variations, there is increasing evidence that intraspecific variation may also play an important role. Disentangling the relative contribution of inter and intraspecific variation is particularly useful to investigate the relative importance of “internal” and “external” filters in shaping plant community responses to the environment, and hence to understand community assembly. Investigating the magnitude of the inter and intraspecific sources of variation may be useful to shed light on community resistance, since a low intraspecific variability indicates low capabilities of plant communities to adapt to environmental changes. Inter and intraspecific variations along gradients can occur in the same direction (positive covariance) or in opposite directions (negative covariance). Positive covariation indicates that the environmental factors that favor the occurrence of plant species with some particular traits also favor the same traits at the individual level, whereas negative covariation indicates that individuals with trait values that deviate most from the predominant species-mean trait values have some selective advantage at the local scale. Investigating intraspecific variation may be difficult because of the large number of measures that must be taken, yet assessing its role for multiple traits is of pivotal importance in community ecology.