Hevea brasiliensis (rubber tree) is native to the tropical rainforest of the Amazon Basin. It has been extensively cultivated in Xishuangbanna, SW China. A reliable assessment of the hydrological threat caused by rapid land-cover conversion to rubber tree requires new data, and studies of evapotranspiration and water use patterns in rubber trees are becoming increasingly important.
Prof. LIU Wenjie and his colleagues of Xishuangbanna Tropical Botanical Garden (XTBG) conducted a study in XTBG (21055'39''N, 101015'55''E)to investigate the seasonal water-use patterns of non-native rubber trees by sampling the stable isotopic ratios (δ18O) of water in soil, rain, plant tissue(stem and superficial root) and groundwater, and by measuring soil water content, root distribution and leaf water potential over the course of a rainy/dry season cycle (2008–2009).Their objectives were to determine the proportional contribution of water from different soil depths to total water uptake by rubber trees and to enhance understandings of how changes in land use in Xishuangbanna has altered plant water uptake dynamics and the implications for hydrologic functioning in Xishuangbanna.
The researchers hypothesized that: (i) rubber trees utilize water primarily from greater depths in the soil profile and greatly deplete the subsoil water, (ii) the water source will differ between seasons, i.e., switch from shallow (in the rainy season) to deep (in the dry season)soil water, and (iii) trees growing on upslope tap deeper soil water than those growing on the downslope, thus circumventing the problem of limited water supply from the shallow soil layer during the pronounced dry season.
The study found that rubber trees greatly relied their water (varying between 49% and 71%) on the shallow soil water (< 30 cm) and extracted only a small fraction of water (varying between 14% and 30%) from the deep soil layers (>70 cm). During the late dry season as the soil moisture in the middle layers (30–70 cm) was gradually depleted, water utilization from those layers decreased sharply (< 15%). However, the proportion of water uptake from the shallow soil layer increased markedly after the most recent rainfall during 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. The ability to take up a large proportion of shallow soil water after rainfall was likely the key feature enabling rubber trees to thrive through the period of greatest water demand.
Their results suggested that rubber trees in Xishuangbanna are able to adjust the allocation of resources and thus acclimate to the spatiotemporal changes to water-conditions in the soil profile.