Specific leaf area and leaf nitrogen concentration in annual and perennial grass species growing in Mediterranean old-fields.

Specific leaf area (the ratio of leaf area to leaf dry mass) and leaf nitrogen concentration were measured on ten annual and nine perennial grass species growing in two old-fields of southern France, under a sub-humid Mediterranean climate. Specific leaf area (SLA) was found to be significantly higher in annuals than in perennials, but leaf nitrogen concentration expressed on a dry mass basis (LNCm) was similar in both life-forms; expressed on an area basis, leaf nitrogen concentration (LNCa) was significantly higher in perennials. The correlation between SLA and LNCm was negative in annuals and positive in perennials, while that between the inverse of specific leaf area (1/SLA) and LNCa was positive in annuals and not significant in perennials. It is hypothesized that these contrasting patterns depend on whether the two components of SLA - leaf thickness and density - vary in opposite directions. For nine of the species studied (six annuals and three perennials), relative growth rate data obtained in the laboratory under non-limiting nutrient supply were available; positive correlations were found between these values and both SLA and LNCm obtained in the field, suggesting that the interspecific differences in structural and chemical characteristics of leaves are maintained under a wide range of growing conditions.