Patterns in intraspecific variation in root traits are species-specific along an elevation gradient

Intraspecific trait variation is an important driver of plant performance in different environments. Although roots acquire essential resources that vary with the environment, most studies have focused on intraspecific variation in leaf traits, and research on roots is often restricted to a few species. It remains largely unclear how and to what extent root traits vary with the environment and whether general intraspecific patterns exist across species. We compared intraspecific variation in specific root length (SRL), root diameter, root tissue density (RTD) and root branching density of 11 species along a 1,000 m elevation gradient in the French Alps. We tested (a) the extent of intraspecific versus interspecific root trait variation along the gradient, (b) whether intraspecific trait patterns with elevation were consistent among species and (c) whether environmental variables better explained intraspecific variation in root traits than elevation. Specifically, we hypothesised that within a species, root trait values would adjust to enhance resource acquisition (either through an increase in SRL or root diameter, and/or in branching density) and/or conservation (increased RTD) at higher elevations. Species identity explained most of the overall variation in root traits. Elevation explained only a minor proportion of intraspecific root trait variation, which was larger within than between elevations. Also, trait relationships with elevation rarely agreed with our hypotheses, varied strongly across species and were often differently related to environmental variation. Generally, climate, soil and vegetation properties better explained intraspecific root variation than elevation, but these relationships were highly species-dependent. Along complex environmental gradients where multiple properties simultaneously change, roots of different species vary in different ways, leading to species-specific patterns in intraspecific root trait variation. The lack of support for our hypotheses may be caused by the multiple interactions between environmental properties, small-scale soil heterogeneity, species phylogeny and changing plant-plant interactions. Our findings suggest that, to enhance our understanding of the effects of environmental change on plant performance, we need to better integrate the multiple dimensions of plant responses to change and measure a broader set of root traits and environmental variables. A free Plain Language Summary can be found within the Supporting Information of this article.

Automated summary

Intraspecific trait variation in roots is influenced more by species identity than elevation, with root traits showing different responses to environmental changes across species. The relationships between root traits and elevation rarely matched the hypotheses, indicating complex species-specific patterns in response to environmental gradients. Climate, soil, and vegetation properties explain intraspecific root variation better than elevation, but these relationships are highly dependent on species.