The sap flow of six tree species and stand water use of a mangrove forest in Hainan, China

Abstract

Mangrove forests grow in coastal saline environments and, little is known about the stand water use (S) and regulation mechanisms of mangrove forests, particularly in the eastern mangrove group. We measured the xylem sap flow density (SFD) of six tree species for two years in a tropical mangrove forest on Hainan Island, China, which is under a monsoon climate. Bimodal daily SFD patterns were observed in all six species in spring and summer. The sap flow of all species were suppressed in winter strongly and also in spring. The annual water use per tree was positively correlated with diameter at breast height (DBH) (R2 = 0.81–0.89, P < 0.05). By scaling up from individual trees, we found that the S values of two mangrove plots ranged from 269.19 to 357.27 mm y−1 between the two study years, accounting for 18–22% of the annual rainfall. These S values are similar to those of other mangrove and tropical forests with similar stand basal areas. The crown conductance sensitivity (mc) to ln VPD (vapor pressure deficit) was strongly correlated with reference crown conductance (GCref) at 1 kPa VPD across all species. The slope value between mc and GCref divided the six species into two groups, suggesting Bruguiera sexangula, Rhizophora apiculata and Sonneratia alba were isohydric, Excoecaria agallocha, B. s. var. rhymchopetala and Xylocarpus granatum were anisohydric. The results of this study will be useful for modeling ecosystem services of mangrove ecosystems facing climate change.