Rubber plantations in Southeast Asia have high water use rates despite long dry season and annual defoliation
Ecosystem services in Mainland Southeast Asia (MSEA) could be strongly affected by ongoing land cover change, especially the expansion of tree-covered landscapes, including rubber (Hevea brasiliensis) plantations. Rubber cultivation is leading the explosive growth of tree plantations in MSEA. A major concern is the extent to which the expansion of these managed ecosystems is affecting water flows. Some prior research suggests that rubber might use water at higher rates than the ecosystems it replaces, including natural forest. We installed and operated eddy covariance flux towers to measure evapotranspiration (ET) over a two-year period at two rubber plantations, one each in northeastern Thailand and central Cambodia.
At both sites, ET is strongly controlled by available energy and leaf area, and moderately controlled by soil moisture. Rubber is deciduous and drops all or most of its leaves early in the dry season. Despite the annual defoliation and the six-month dry season in MSEA, ET rates for the rubber plantations were very high. Mean annual ET after adjustment for energy closure was 1211 and 1459 mm yr-1 at the Thailand and Cambodia sites, respectively. These ET rates for rubber are significantly higher than those of other tree-dominated land covers in the region, including forest. The rate for the Cambodian site approaches the highest ET rates estimated for non-irrigated land surfaces globally. The tree species used in plantations are selected in part for their fast growth traits. Rubber is commercially viable because of its high latex production, supported by rapid carbon uptake. Linked by leaf-level controls on gas exchange, carbon uptake and water use are strongly correlated. Thus, the current rapid expansion of plantations of fast growing trees is likely to cause an increase in regional ET, which might have negative consequences for water and food security in the region.