The Thirsty Tree
In the tropical prefecture of Xishuangbanna, a massive ecological experiment has been unfolding for decades. Since the 1950s, and accelerating rapidly since the 1990s, vast areas of biodiverse rainforest have been replaced by monoculture plantations of the rubber tree, Hevea brasiliensis. This land-use change has had deep consequences, especially than on the region's water cycle. Locals have a telling nickname for the rubber tree: the "water pump". Scientific research has shown this name is disturbingly accurate.
A long-term, 15-year study using paired catchments—one with native rainforest, one with a rubber plantation—produced results. The rubber plantation's total annual evapotranspiration (water lost to the atmosphere from soil evaporation and plant transpiration) was measured at 1,137 mm. The rainforest, by contrast, only lost 969 mm annually. This means the rubber plantation consumed 17% more water than the forest it replaced. The annual runoff from the rubber plantation catchment was only 367 mm, a massive drop from the 565 mm of runoff generated by the rainforest. In some rubber catchments, streams that once flowed year-round now run dry during the rainless season.
Scientists used tools like eddy covariance towers to directly measure water vapor fluxes and sap flow sensors on individual trees to track their water consumption in near real-time. The data confirmed that rubber trees, native to the humid Amazon, are poorly adapted to Xishuangbanna's seasonal climate and consume large amounts of water during the rainy season. Their deep root systems are adept at pulling water from the soil, a trait that can deplete soil moisture reserves needed to sustain streamflow during the dry months.
A Drier, Foggier Landscape
The hydrological impact extends beyond streamflow. The large-scale conversion to rubber plantations has altered the regional climate. An important feature of Xishuangbanna's native rainforest is its ability to trap moisture, creating a dense fog, especially in the winter dry season, which helps sustain the ecosystem. The more open canopy of rubber plantations is less effective at this, and studies have recorded a dramatic decline in the frequency and density of fog in the region. This loss of fog reduces an important source of moisture for remaining native plants and affects regional food security.
The soil itself is also transformed. Under the native forest, a thick litter layer and complex root systems create porous soil that readily absorbs monsoon rains. In rubber plantations, the soil is often more compacted and exposed. This leads to less water infiltration and more surface runoff during storms, increasing soil erosion. One study estimated that each hectare of rubber plantation loses an average of 22.5 tons of topsoil per year. Isotope tracing studies show that during storms in rubber catchments, a much higher percentage of the streamflow is composed of direct rainwater runoff (62-69%), unlike in forest catchments where older, stored groundwater is the dominant source (71-79%). This change in runoff generation fundamentally alters river behavior and water quality. The expansion has been extensive, with rubber plantations covering 6.38% of Xishuangbanna's land area in 1990, growing to 24.52% by 2017.