Seasonal water use strategy of Cyclobalanopsis glauca in a karst area of southern China

Subtropical evergreen and deciduous broadleaved mixed forests occur on shallow soils or even barren areas with carbonate bedrock in karst areas of southern China. The nature of seasonal water sources is poorly understood in the complex environment of these deeply rooted forests; these forests grow well despite frequent seasonal droughts and limited water availability. The authors hypothesized that trees here mainly adsorb water from deep root systems that also recharge the surface soil at least during the dry season. To test this hypothesis, seasonal isotopic hydrogen and oxygen compositions (δ18O and δD) of surface, different subsurface, well and xylem water available to Cyclobalanopsis glauca were compared in a subtropical mixed forest near Guilin, Guangxi, China. Levels of δ18O and δD in soil water decreased with depth in the 0–30 cm soil horizon and increased below 30 cm depth in March, July and October. However, in January the soil water δ18O and δD values decreased generally with soil depth. Deep root systems of trees were suspected to redistribute groundwater to shallow surface soils providing the main stable water source for adult C. glauca during the growing season (March–October). However, IsoSource outputs indicated that C. glauca extracts water mainly from shallow soil throughout the sampling year. In the cold and dry season (January), winter rains recharging soil water may provide most of the water available for transpiration. The two-tier water structure in this karst environment connected by deep roots appears to help form a closed water circulation of groundwater–soil–plant–atmosphere.