Silvopastoral systems in Northwestern Patagonia II: water balance and water potential in a stand of Pinus ponderosa and native grassland

The climate of the ecotone between forest and steppe in North-Western Patagonia, is of Mediterranean type, with rainy winters and dry summers. As a consequence, most of the region presents water deficits during part of the spring and all the summer. Water interactions between Pinus ponderosa Doug. (Laws) and Stipa speciosa (Trin. et Rupr.), a native grass species, were studied. The experimental site includes ten silvopastoral plots (five with 350 pruned pines/ha and five with 500 pruned pines/ha), one plot with 500 unpruned pines/ha, one plot with 1000 pines/ha and an open pasture. Pre-dawn water potential of pines and grasses, and soil water moisture under and between tree crowns, and in the open pasture, were measured during October to April. Also, local water balance of these situations was studied over the same period. Evapotranspiration was higher in silvopastoral systems than in the open pasture (average difference of 95 mm). Greatest soil water consumption was observed in silvopastoral systems, mainly at the deepest layers(80–140 cm). Rain interception by pines was important only at the end of the dry season. Pre-dawn water potential of trees was stable during the study period. Better water status was found in grasses growing under high tree canopy cover compared to those in the open treatments, before and after the driest month (January). Silvopastoral systems use water resources that otherwise are lost from the system. Therefore the higher productivity of these systems may partly reflect this more exhaustive use of resources.