Tree Plantation in South America and The Water Cycle: Impacts and Emergent Opportunities

Abstract

South American tree plantations expand at a rate of 5,000 km²/year favored by increasingly globalized markets and local economic conditions. The main hydrological impacts of these plantations involve shifts in (a) the partition of precipitation inputs between vapour vs. liquid fluxes (associated to transpiration and canopy interception shifts) and (b) the partition of liquid fluxes between run-off and fast flow vs. deep drainage and base flow (associated to infiltration and surface water routing shifts). In sloped terrains global stream flow measurements in paired watersheds indicate declining water yields (40% less on average) under plantations vs. native vegetation. These effects are stronger under drier climates, where host vegetation is herbaceous, and where planted trees are eucalypts. In flat landscapes with native grassland vegetation, tree plantations switch the water balance from positive (net recharge) to negative (net discharge) triggering local salinization. Contrastingly, where native vegetation has been a woodland tree plantation can remediate the undesirable recharge and water table rise/salinization problems brought by agriculture. In degraded rolling (sub)tropical landscapes with intense rainfall inputs and high run-off, tree plantations can increase infiltration rates, reducing erosion, stabilizing flow, but cutting total water yield. As a result of these shifts, erosion can be reduced and the stability and quality of water provision improved, yet these benefits can be erased by large scale clear cutting practices. Context (climate, current vegetation and topography/geology) and design (species, densities, harvesting methods, and scale/pattern) can decide the magnitude and sign of tree plantations effects and need to be carefully considered to get the best ecological outcome of afforestation in the continent.