Together but different: co-occurring dune plant species differ in their water- and nitrogen-use strategies.

Stress factors may severely constrain the range of plant physiological responses in harsh environments. Convergence of traits is expected in coastal dunes because of environmental filtering imposed by severe abiotic factors. However, the wide range of morphological and phenological traits exhibited by coexisting dune species suggests considerable variation in functional traits. We hypothesized that the constraints imposed by structural traits ought to translate into physiological differences. Five dominant species with different morphological traits, but coexisting in a homogeneous dune area in Northwest Spain, were selected for study. Soil characteristics and leaf functional traits were measured in April, June and November 2008. Integrated water-use efficiency (assessed by C isotope discrimination) and N acquisition and use strategies (estimated by N isotope composition) varied significantly among species and the differences changed over time. Species differences in specific leaf area, relative water content, leaf N and C:N ratio, also varied over time. The species differed in stomatal density but not in soil characteristics, with the exception of pH. Species differences in functional traits related to the use of resources suggest species niche segregation. Species-specific temporal effects on the use of these resources support temporal niche differentiation. Somewhat in contrast to the findings of previous studies on harsh environments, this study revealed a considerable level of functional diversity and complexity, suggesting that dune plant species have evolved species-specific strategies to survive by partitioning growth-limiting resources.