Seasonal differences in leaf attributes in Australian tropical tree species: family and habitat comparisons

1. An individual evergreen leaf in the northern Australian savanna biome experiences seasonal extremes in rainfall, raising questions of whether there is a corresponding shift in the physiology of these leaves, and whether estimates of leaf function for the biome differ between wet and dry seasons. 2. A range of leaf attributes was measured for 21 tree species in four contrasting habitats during the wet season, and again during the dry season for the 14 species that retained leaves. 3. The 14 leaf-retaining species showed decreases in assimilation rates, foliar saturated water content and chlorophyll content, and increases in leaf mass per area and leaf density between wet and dry seasons. Species means were strongly correlated between seasons for attributes other than assimilation and stomatal conductance. 4. Seasonal variation in CO2 assimilation rates was larger in non-Myrtaceous species than in the leaf-retaining Myrtaceous species that largely dominate the biome, and also varied considerably among the four habitats, which had different edaphic conditions. Assimilation per unit mass decreased between the wet and dry seasons by only 5% in the Melaleuca swamp and 17% in the Eucalyptus open forest, compared with 47% in the dry monsoon forest and 57% in the mixed woodland. 5. Biome means differed between seasons because wet-season means included measurements of fully deciduous species which tended to have large, thin, nitrogen-rich leaves.