Impact of exotic Ponderosa pine (Pinus ponderosa Doug. ex Laws.) plantations on water resources in northwestern Patagonia, Argentina

Abstract

Exotic ponderosa pine (Pinus ponderosa Doug. ex Lawson) plantations are being planted within the natural distribution area of cordilleran cypress (Austrocedrus chilensis (D.Don) Pic. Ser. et Bizzarri) in Patagonia, Argentina. The productivity of these exotic plantations is much greater than that of native forests, suggesting greater water use. Before these plantations become widespread, it is crucial to determine what effects ponderosa pine plantations may have on scarce water resources in the region. In the first study, I examined the seasonal variation in transpiration and soil water depletion for four forest stands: high and low density ponderosa pine plantations, and high and low density cordilleran cypress stands. Measurements were taken during two growing seasons with contrasting amounts of precipitation. Transpiration was greater in ponderosa pine than in cordilleran cypress stands of similar density. The high density ponderosa pine plot used a greater amount of water from deeper soil layers compared to all the other plots. In the second study, I conducted measurements of above- and below-canopy rainfall, and water storage capacity of branches to estimate the proportion of water that it is intercepted by the canopy and evaporates back into the atmosphere without reaching the soil. A simple analytical model was used to estimate annual interception losses by the four forest stands. Cordilleran cypress stands showed much larger interception losses than ponderosa pine stands of similar density. Thus, the negative effects of increased transpiration in ponderosa pine may be offset by their smaller interception losses when compared to cordilleran cypress. In the third study, I used a detailed process-based model to close the annual water budget of the stands, and evaluate the balance between ET and precipitation under several climate change scenarios, based on the projections made in the IPCC’s fourth assessment report. Model predictions suggested that high density ponderosa pine plantations are not hydrologically sustainable (ET > precipitation) under current conditions, and that none of the high density stands would be hydrologically sustainable under future scenarios. Management of stand density in both native cordilleran cypress and ponderosa pine plantations would be necessary to reduce the impacts of future climate change on water resources.