The role of rock fractures as a water source for trees growing in karst

Global warming has led to an accelerated dry-wet transition, causing forests to experience more water stress and water use strategy alterations. This could take a great effect on trees in karst region due to tremendous spatial and temporal variability of soil and rock moistures. In this study, we monitored and compared transpiration (sap flow) responses to meteorological variables, soil moisture content and rock moisture content at five sites with a variety of plant-soil-rock compositions in the karst region of southwest China. Results show that the soil-rock composition generally controlled tree growth and transpiration amount, and over 80% transpiration was concentrated in wet growing period. The thin soils can only offer a limited soil moisture and rock moisture dominated transpiration variability and physiological strategies of tree water-use. High and steady rock moisture in appropriate rock fractures enabled tree to exhibit isohydric behavior that can substantially reduce transpiration and seasonal variability. Conversely, low rock moisture made tree tend to anisohydric behavior that increased transpiration in the wet period for resisting drought stress in the dry period. The transition from isohydric to anisohydric behavior for tracking varying environment could reduce tree transpiration response to meteorological variations, such as vapor pressure deficit, and even results in alteration of tree size dominant transpiration. Since tree physiological behavior is extremely sensitive to climate variations and soil-rock compositions, the future acceleration of wet-dry transition is highly possible to increase vulnerability of ecosystems in the region.