Sapwood to heartwood ratio affects whole-tree water use in dry forest legume and non-legume trees

We investigated vegetation structure, seasonal water use and leaf deciduousness in a seasonally dry forest of DzibilchaltA(0)n, Mexico. Legumes, species which tend to dominate these forests, have an array of water-saving traits. We explored whether legume species had reduced water use under similar growth conditions as other non-legume species of this seasonally dry forest. Sap flux and conductive sapwood area were measured for eight legume and 12 non-legume species. Species abundance, diameter at breast height (DBH), wood density and seasonal leaf cover were characterized in 16, 10 x 10 m(2) plots. Seasonal stand water use was calculated using the sap flux and ecological data. As predicted, legumes presented lower whole-tree water use compared with sympatric non-legume species. This difference, however, was related to a higher allocation to non-conductive heartwood in legumes and not to differences in sap flux density. Differences in allocation were higher in wider stems (> 10 cm DBH); legumes above 25 cm DBH presented nearly half the daily water use of non-legumes of similar size. Wet (July) and dry (March) season stand water use was 629,000 and 156,000 kg ha(-1) month(-1), respectively. During the wet season three non-legume species with high basal area dominated the stand water use, but due to early leaf fall in these species, dry season stand water use was dominated by the legumes.