Contrasting patterns in leaf traits of Mediterranean shrub communities along an elevation gradient: measurements matter

We assessed the changes in community-weighted mean (CWM) and variability of specific leaf area (SLA) and leaf area (LA) of different Mediterranean shrub communities along an elevation gradient in the island of Sardinia (Italy). Furthermore, we explored the relative contribution of species turnover and intraspecific variation to shifts in CWM values along the gradient. Forty sampling units (5 × 5 m) were selected in a probabilistic way along a 1300 m elevation gradient which crossed four thermotypes (thermometric belts). Leaf traits were measured in each sampling unit. ANOVA and a trend test for monotonic changes in variance were used to assess, respectively, CWM differences and variability in both the leaf traits across thermotypes. Variance decomposition of CWM values was used to identify the role of inter- and intraspecific variation. SLA and LA responded differently along the studied gradient in terms of abundance-weighted mean values and variability: CWM of SLA showed the lowest values in the driest thermotype, while LA in the more humid one; SLA variability showed a significant increasing trend with increased water availability, while LA variability did not show any pattern. The contribution of intraspecific trait variation was significant for both the leaf traits, but higher for SLA, where negative covariation between inter- and intraspecific variation was detected. We highlight the importance of simultaneously considering measurements of both leaf traits to understand the functional response of communities in Mediterranean environments. Moreover, neglecting intraspecific variation in leaf traits, even along steep gradients with relevant species compositional changes, can result in the underestimation of the amount of trait variation in response to environmental changes.