More sensitive response of crown conductance to VPD and larger water consumption in tropical evergreen than in deciduous broadleaf timber trees

Abstract

The differences in crown conductance sensitivity (m) to vapor pressure deficit (VPD) and water consumption between tropical evergreen and deciduous tree are not well-understood despite the importance of such information for the development of plantation and water-resource management strategies in regions that experience seasonal drought and increasing water deficits. To this end, during the wet season we measured sap flow in 60 individual trees representing 21 tropical angiosperm broadleaf timber species (11 evergreen and 10 deciduous) growing in plantations in a marginal tropical area of southwestern China. The results showed that m was strongly correlated with reference crown conductance (Gcref) at 1kPa VPD across all species, with a slope value of 0.46, which was significantly different from the proposed theoretical value of 0.6, suggesting the study species as anisohydric. The m was positively correlated with tree diameter at the height of 1.3m (diameter at breast height, DBH) but for a given DBH, m was significantly higher in evergreens than in deciduous trees. Further, whole-tree mean daily water consumption was strongly and positively correlated with DBH, but at a given DBH, evergreen species exhibited larger water consumption than deciduous species. Vessel diameter (related to xylem hydraulic conductance) and sapwood area were also positively correlated with DBH, and evergreen species had significantly larger sapwood area than deciduous species at a given DBH. In conclusion, we found that 1) crown conductance of evergreen trees is more sensitive to VPD compared to deciduous species and 2) evergreen trees consume more water than deciduous trees, partly because of having higher peak transpiration rate and, larger sapwood area (i.e., larger xylem area for water transport and storage). Therefore, we suggest that in tropical regions associated with seasonal drought timber plantations with more deciduous species could potentially be considered as a management possibility for achieving a balance between timber production and water conservation, even in the wet season, which needs to be tested at the plantation level.