Summary• Because the phenology of trees is strongly driven by environmental factors such as temperature, climate change has already altered the vegetative and reproductive phenology of many species, especially in the temperate zone. Here, we aimed to determine whether projected levels of warming for the upcoming decades will lead to linear changes in the phenology of trees or to more complex responses.• We report the results of a 3-yr common garden experiment designed to study the phenological response to artificial climate change, obtained through experimental warming and reduced precipitation, of several populations of three European oaks, two deciduous species (Quercus robur, Quercus pubescens) and one evergreen species (Quercus ilex), in a Mediterranean site.• Experimental warming advanced the seedlings' vegetative phenology, causing a longer growing season and higher mortality. However, the rate of advancement of leaf unfolding date was decreased with increasing temperature. Conversely, soil water content did not affect the phenology of the seedlings or their survival.• Our results show that the phenological response of trees to climate change may be nonlinear, and suggest that predictions of phenological changes in the future should not be built on extrapolations of current observed trends.

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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 (LNC) was similar in both life-forms; expressed on an area basis, leaf nitrogen concentration (LNC) was significantly higher in perennials. The correlation between SLA and LNC was negative in annuals and positive in perennials, while that between the inverse of specific leaf area (1/SLA) and LNC 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 LNC 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.

The relationships between leaf structure, nitrogen concentration and CO # assimilation rate (A) were studied for 14 grass species grown in the laboratory under non-limiting nutrient conditions. Structural features included leaf thickness and density, and the proportion of leaf volume occupied by different types of tissue (mesophyll, epidermis, vessels and sclerenchyma). Relationships were assessed for data expressed per unit leaf area and fresh mass. The latter was found to be closely related to leaf volume, which allowed us to use A per unit leaf fresh mass (A fm ) as a surrogate of A per unit leaf volume. Assimilation rate per unit leaf area (A a ) was positively correlated with leaf thickness and with the amount of mesophyll per unit leaf area ; the relationship with leaf nitrogen content per unit area was only marginally significant. A fm was negatively correlated with leaf thickness and positively with fresh mass-based leaf organic nitrogen concentration. A multiple regression involving these two variables explained 81% of the variance in A fm . The value of A fm was also significantly related to the proportion of mesophyll in the leaf volume, but surprisingly the correlation was negative. This was because thin leaves with high A fm and nitrogen concentration had proportionally more mechanically supportive tissues than thick ones ; as a consequence, they also had a lower proportion of mesophyll. These data suggest that, in addition to leaf nitrogen, leaf thickness has a strong impact on CO # assimilation rate for the grass species studied.