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 maxim um photosynthetic capacity was found in a group of seven conifers and 16 an giosperm species, including two vessel-less taxa, from similar rainforest c ommunities in New Caledonia and Tasmania (Australia). Stem hydraulic supply was expressed as the hydraulic conductivity of branches in terms of leaf a rea supplied (K-L) and leaf photosynthetic capacity was measured as the mea n quantum yield of PSII (O-PSII) in leaves exposed to full sun, as determin ed by chlorophyll fluorescence analysis. A single, highly significant linea r regression (r(2) = 0.74) described the relationship between hydraulic con ductivity and quantum yield in all species. This suggests that the maximum photosynthetic rate of leaves is constrained by their vascular supply. In b oth rainforest locations, the K-L of conifer wood overlapped broadly with t hat of associated vessel-bearing and vessel-less angiosperms indicating a d egree of hydraulic convergence in these forests.