Components of leaf dry mass per area - thickness and density - alter leaf photosynthetic capacity in reverse directions in woody plants

The relationships of foliage assimilation capacity per unit area (P-max(P)) with leaf dry mass per unit area (LMA) and nitrogen content per unit area (N-P) differ between species and within species grown in different habitats . To gain a more mechanistic insight into the dependencies of P-max(p) on L MA and N-P, this literature study based on 597 species from a wide range of earth biomes with woody vegetation examines the relations between leaf pho tosynthetic capacity and the components of LMA (leaf density (D, dry mass p er volume) and thickness (T)), and also the correlations of D and T with le af nitrogen content and fractional leaf volumes in different tissues. Acros s all species, P-max(P) varied 12-fold and photosynthetic capacity per unit dry mass (P-max(m)) 16-fold, N-P 12-fold, and nitrogen per unit dry mass ( N-m) 13-fold, LMA 16-fold, D 13-fold, and T 35-fold, indicating that foliar morphology was more plastic than foliar chemistry and assimilation rates. Although there were strong positive correlations between P-max(P) and N-P, and between P-max(m) and N-m, leaf structure was a more important determina nt of leaf assimilation capacities. P-max(P) increased with increasing LMA and T, but was independent of D. By contrast, P-max(m) scaled negatively wi th LMA because of a negative correlation between P-max(m) and D, and was po orly related to T. Analysis of leaf nitrogen and tissue composition data in dicated that the negative relationship between D and P-max(m) resulted from negative correlations between D and N-m, D and volumetric fraction of leaf internal air space, and D and symplasmic leaf fraction. Thus, increases in leaf density bring about (1) decreases in assimilative leaf compounds, and (2) extensive modifications in leaf anatomy that may result in increases i n intercellular transfer resistance to CO,. Collectively, (1) and (2) lead to decreased P-max(m), and also modify P-max(p) versus LMA relationships.