Functional trait variation along environmental gradients in temperate and Mediterranean trees

Aim Characterizing the variation of functional traits in nature is a first step towards linking environmental changes to changes in ecosystem function. Here we aim to characterize the spatial variability of major plant functional traits along wide environmental gradients in Mediterranean and temperate forests, and assess to what extent this variability differs between two dominant families in Northern Hemisphere forests: Fagaceae and Pinaceae. Location Catalonia (north-east Iberian Peninsula). Methods Four functional traits were selected to incorporate information on both the leaf and the wood economic spectra: maximum tree height (Hmax), wood density (WD), leaf mass per area (LMA) and nitrogen content of leaves (Nmass). We quantified the variance distribution of each functional trait across three nested ecological scales: population, species and family. Through such scales, we explored the spatial variation of functional traits through climatic and biotic gradients, as well as the covariation among traits. Results Functional trait variability was distributed across all the ecological scales considered, but mostly at the family level, with functional traits differing markedly between Fagaceae and Pinaceae. Within families, variation in functional traits was similar or higher within species than between species. The spatial variability in functional traits was related to biotic and abiotic gradients, although this effect was quantitatively small compared with differences between families. Covariation among functional traits was not necessarily conserved across ecological scales. Trait covariation across all species was structured along the Hmax−WD and LMA−Nmass axes, but this structure was partially lost within families, where variation was mostly structured along the Hmax−LMA and WD−Nmass axes. Main conclusions Intraspecific variation emerges as a fundamental component of functional trait structure along wide environmental gradients. Understanding the sources of intraspecific variation, as well as how it contributes to community assembly and ecosystem functioning, thus becomes a primary research question.