Dry season water uptake by two dominant canopy tree species in a tropical seasonal rainforest of Xishuangbanna, SW China

Radiation fog has been suggested as an important source of moisture in tropical seasonal rainforest of Xishuangbanna during 6 months of annual drought, yet its importance for the plants which inhabit this ecosystem is virtually unknown. We assessed patterns of water uptake by two major seasonal rainforest canopy tree species in Xishuangbanna, SW China, during two consecutive dry seasons (2005 and 2006). Stable oxygen isotope compositions of water in xylem, soil, fog, rain, and groundwater were analyzed, and soil water content and leaf water potential were measured concurrently in order to determine the proportion of water deriving from shallow soil by mature trees and from fog water by seedlings during the pronounced dry season. Our results indicated that evergreen Gironniera subaequalis tree appeared to be acquiring water preferentially from the upper 50 cm of the soil profile, with around 53–72% of its water from shallow soil. In contrast, brevi-deciduous Pometia tomentosa tree seemed to be tapping water mostly from depths greater than 60 cm or from groundwater, with about 28–46% of its water from shallow soil, suggesting that it relies predominantly on its deep taproot for water uptake. During the dry season, when fog was most frequent, 23–59% of the water used by P. tomentosa seedling came from fog water after it had dripped from tree foliage into the soil, indicating that fog water was an important source for seedling growth, especially at the peak of the dry season. Since both seedlings and shallow-rooted understory species require moisture and cool conditions to regenerate, it might be expected that some other tree seedlings and understory species also partially rely on fog moisture during the pronounced dry season, but further research is needed.