Foliar N Content Parallels Increasing Aridity in a Mediterranean‐Saharan Transition Zone: Evidence From Regional and Global Trends

ABSTRACT Aim Warm deserts are characterised by water shortages and high temperature extremes. A commonly reported adaptive strategy in such environments is maximisation of photosynthetic capacity, which allows plants to achieve positive carbon budgets by taking advantage of short periods of water availability and non‐inhibitory temperatures. Considering the well‐supported interspecific covariation between photosynthetic capacity and leaf N concentration, we tested the hypothesis that environmental aridity is related to an elevated leaf nitrogen content. Location 53 locations in the transitional zone spanning the Mediterranean and the Sahara Desert in Morocco. The mean maximal temperature ( T max ) within the area varied between 35.7°C and 43.5°C, and the mean annual precipitation (MAP) was between 12 and 246 mm. Taxon 225 vascular species representative of local vegetation. Methods Leaf samples were collected along a regional aridity gradient and preserved in herbarium presses. The leaf mass per area (LMA) and N concentrations expressed on leaf mass ( N mass ) and area ( N area ) basis were determined. We also obtained LMA and N mass values for 6711 species from a worldwide database for comparative analysis. Results Significant increases in mean LMA, N mass and N area accompanied the increase in T max and the decrease in MAP in woody species and in non‐graminoid herbs, but not in graminoids. Considering the overall aridity of our sampling area, we compared the N mass values of Moroccan plants with those from a worldwide database. We found that at a common LMA, the Moroccan plants showed on average elevated N mass relative to global values. Main Conclusions These two lines of evidence: regional gradient and global comparison confirm that hot deserts select for high leaf N content. This result suggests the direction of natural selection that will accompany future climate warming and habitat aridification.