Biogeography of intraspecific trait variability in matgrass (Nardus stricta): High phenotypic variation at the local scale exceeds large scale variability patterns

•Functional biogeography of intraspecific trait variability (ITV) remains largely unknown.•We measured plant height and leaf traits in populations of Nardus stricta distributed from the Pyrenees to the Balkans.•Traits showed large ITV and higher between- than within-population variability. ITV between mountain ranges was generally low.•The traits of the leaf economics spectrum (LES) were dependent on height, mostly reflecting the growing season length.•Local factors are pivotal to model ITV at biogeographic scales. Intraspecific trait variability (ITV) is an important component of functional ecology studies. While functional biogeography requires us to understand broad-scale patterns of trait distribution, ITV remains inadequately studied. Due to isolation of ranges, habitat fragmentation and sharp topoclimatic gradients over short distances, temperate mountains offer opportunities to study the ITV in widespread species. In this study, we explored ITV in geographically separated populations of matgrass (Nardus stricta), a clonal dominant plant species of mountain pastures. We measured plant height and leaf traits in 55 populations distributed across six European mountain ranges (from the Pyrenees to the Carpathians) to examine the structure of ITV. We hypothesised that the leaf economics spectrum (LES) and plant size exhibit a strong convergence across a prevailing gradient of growing season length (GSL). We then partitioned the ITV into between ranges, between populations nested in ranges and between individuals nested in populations, and examined the role of climate and local variables on the spatial variation and covariation of traits. Traits showed large ITV and higher between- than within-population variability. Leaf traits displayed a high dependency on height reflecting the growing phase climate and other local factors, while range identity alone (differentiation between ranges) explained a low to moderate amount of the variability in the traits. The strength of coordination between several of the LES traits was higher under shorter GSL due to late snowmelt, suggesting higher phenotypic integration. In the context of functional biogeography of mountain ecosystems, allometric effects and growing phase climate are both key to understanding ITV. We discuss the implications of these findings as they relate to future comparative studies of ITV in seasonal environments.