Vertical patterns of soil water acquisition by non-native rubber trees (Hevea brasiliensis) in Xishuangbanna, southwest China

The rubber tree (Hevea brasiliensis) has been extensively cultivated in Xishuangbanna, southwest (SW) China. It shows strong synchronicity for flushing and shedding, displaying a very different phenology to the native vegetation. However, little is known about the water-use patterns of the plant in this area. We assessed seasonal water-use strategies of rubber trees over the course of a rainy/dry season cycle. Stable isotope compositions of water in xylem, soil, rain and groundwater were sampled on seasonally distinct dates, and soil water content, root distribution and leaf water potential on sunny days were measured in order to determine the proportion of water derived from different soil layers. Midday leaf water potential of rubber trees was relatively stable throughout the year and did not drop significantly during the late dry season, displaying isohydric behaviour. Soil and stem water isotope signatures along with rooting distributional patterns revealed that rubber trees extracted their water mostly from the top 30 cm and less from below 70 cm of the soil profile during the late rainy season when soil water was plentiful. During the late dry season, as the moisture in the middle soil layers (30-70 cm) was gradually depleted, the depth of water uptake shifted to deeper soil levels. Model calculations showed that the proportion of water uptake from the shallow soil layer (<30 cm) increased markedly after the most recent rainfall in the late dry season and the early rainy season (varying between 65% and 71%), indicating significant plasticity in sources of water uptake in this dimorphic-rooted species. This ability to take up a large proportion of shallow soil water after rainfall is likely the key feature enabling rubber trees to thrive through the period of greatestwater demand. Our results suggest that rubber trees are able to adjust the allocation of resources and thus acclimate to the spatiotemporal changes to water conditions in the soil profile. Copyright (C) 2013 John Wiley & Sons, Ltd.