Stem hydraulic supply is linked to leaf photosynthetic capacity: evidence from New Caledonian and Tasmanian rainforests

A strong relationship between hydraulic supply of water to leaves and maximum photosynthetic capacity was found in a group of seven conifers and 16 angiosperm species, including two vessel-less taxa, from similar rainforest communities in New Caledonia and Tasmania (Australia). Stem hydraulic supply was expressed as the hydraulic conductivity of branches in terms of leaf area supplied (ΚL) and leaf photosynthetic capacity was measured as the mean quantum yield of PSII (Φ/PSII) in leaves exposed to full sun, as determined by chlorophyll fluorescence analysis. A single, highly significant linear regression (r2 = 0.74) described the relationship between hydraulic conductivity and quantum yield in all species. This suggests that the maximum photosynthetic rate of leaves is constrained by their vascular supply. In both rainforest locations, the ΚL of conifer wood overlapped broadly with that of associated vessel-bearing and vessel-less angiosperms indicating a degree of hydraulic convergence in these forests.