Variation in leaf traits through seasons and N-availability levels and its consequences for ranking grassland species

Question: Are leaf dry matter content, specific leaf area and leaf life span relevant plant traits to discriminate the fertility gradient in species-rich natural grasslands? In other words, is species ranking conserved when nitrogen availability or growing periods change? Location: Toulouse Research Centre, France; 150 m a.s.l. Methods: Fifteen grasses and nine dicotyledons were sown in pure stands in a random block design with three replicates. Each species was cultivated at two levels of nitrogen supply, limiting and non-limiting for growth, with three replications per nitrogen level. Leaf traits were measured across both levels of nitrogen supply and growing periods over the year. Results: Leaf dry matter content values separated the species into three life-form classes (grasses, rosette forbs and upright forbs, P < 0.001). This was not the case for specific leaf area and leaf life span. The three leaf traits were variable across growing periods and nitrogen levels, but the ranking of species was conserved over N-levels and growth periods. Furthermore leaf dry matter content was always less variable than the other leaf traits. Conclusion: We conclude that leaf dry matter content measured only on grasses could be used as an indicator to describe the N-richness of the habitat where native herbaceous vegetation develops. Abbreviations: CV = Coefficient of variation; INN = Index of nitrogen nutrition; LDMC = Leaf dry matter content; LLS = Leaf life span; LSD = Least significant difference; RGR = Relative growth rate; SLA = Specific leaf area.