Effects on site water balance of conversion from native mixed forest to Douglas-fir plantation in N.W. Patagonia

Abstract

One of the main concerns of afforestation with exotic fast-growing species is their higher soil water consumption compared with the native vegetation they replace. In this regard, sites downstream of the plantations may suffer temporal or permanent droughts. Under this hypotheses, soil water use of Douglas-fir (Pseudotsuga menziesii) plantations and natural woodlands or shrublands (“nirantales”) was evaluated in two sites during two growing seasons in El Foyel Valley (Rio Negro province), Patagonia Argentina. Two methodological approaches were applied: water balances and sapflow measurements. Douglas-fir forests doubled the leaf area index (LAI) of the native forests (12.6 and 5.1 m2 m−2, respectively). Methodological problems were found while constructing water balances because a non-quantified influx of water to the soil was detected in all systems at the beginning and end of the growing season. This flux was not in correlation with precipitation input demonstrating the geological complexity of the valley. For this reason, caution has to be paid to water balance results. However, we have confidence on sapflow measurements, which indicated, as suggested by the water balances, that there are no differences in transpiration between systems (average of 4.8–3.6 mm day−1 for the exotic and native forests, respectively) in spite of Douglas-fir plantations having twofold LAI than native nirantales. However, a different response of transpiration to atmospheric demand was found between the exotic and native species, suggesting differences in their ecophysiological characteristics.