Increased water use by ponderosa pine plantations in northwestern Patagonia, Argentina compared with native forest vegetation

Ponderosa pine (Pinus ponderosa Doug. ex. Laws) plantations represent more than three quarters of afforestation projects in the Northwestern part of Argentinean Patagonia. Most plantations are located in the forest-steppe ecotone within the natural distribution area of cordilleran cypress (Austrocedrus chilensis (D. Don) Pic. Serm. et Bizzarri). The productivity of ponderosa pine plantations is much greater than that of the native cordilleran cypress forests, which would suggest a greater utilization of water resources. Our primary hypothesis was that ponderosa pine plantations have higher total annual transpiration per hectare than native cordilleran cypress stands of similar density. As secondary hypotheses we tested whether ponderosa pine stands: (1) have higher maximum transpiration rates, (2) utilize water over a longer season, (3) deplete soil water to a lower moisture content level, and (4) extract water from deeper soil layers than native cypress stands. Through continuous measurement of sap flow and of soil water content at various depths (from surface to 1.8m), we examined the total amount of water used by four forest stands: a high density ponderosa pine timber plantation (HDPP), a low density silvopastoral ponderosa pine plantation (Silvo), a low density cordilleran cypress forest stand (LDCip) and a high density cordilleran cypress forest stand (HDCip). Measurements were carried out during 2 years that had contrasting amounts of summer precipitation. HDPP had the highest water use at the stand level, averaging 64% and 33% greater than HDCip in the wet and dry years, respectively. Higher transpiration rate but not length of the transpiration season explained the difference in total annual transpiration between species. Water depletion occurred simultaneously at all soil depths for all the plots even in the wet year. However, the HDPP plot used a greater amount of water from deeper soil layers compared to all the other plots. Our results emphasize the need for caution when planning afforestation projects at large scales, as the increase in transpiration due to conversion of native forests to high density ponderosa pine plantations could have a large impact on water resources.