Water Use Efficiency of plant communities in N.W. Patagonia Argentina: effects of land use changes and availability of water resources. Eficiencia en el Uso del Agua de comunidades vegetales en el N.O. de la Patagonia argentina: efecto del cambio en el us

RIVERO, D.; GYENGE, J.; MARIA ELENA FERNANDEZ; LICATA, J.; SCHLICHTER, T.; BOND, B.

Mérida

Congreso; Reunión de la Ecological Society of America, "Ecología en una era de globalización; 2006

In N.W. Patagonia, Argentina, the climate is semiarid, and productivity is limited by water availability. Afforestation with exotic conifers is currently encouraged by subsidies provided in the relatively new Forest Investment Law. Natural grasslands and woodlands dominated by the native cypress Austrocedrus chilensis are being replaced by Ponderosa pine plantations. Currently, there are only 70 000 ha with forest plantations, but there is potential for afforestation of about 2 million ha. Therefore, it is very important to determine the potential impact of this land use change on local water resources. Water use and productivity of forest plantations and native ecosystems are being studied at different spatial scales using sap flow measurements, water balances and remote sensing analysis. In this poster, we present results concerning water use efficiency (WUE; (the ratio between annual biomass accumulation and transpiration) of forest plantations compared to adjacent ecosystems. At the individual tree level, if we compare pines of the same age and size, WUE is higher in those trees growing in more dense stands (e.g. DBH: 25 cm, WUE: 0,9 and 0,6 kg/m3 at 1300 and 500 pines/ha, p0.05). However, because WUE increases with pine size (this trend was not observed in the studied cypresses), at the stand level WUE is higher in open pine stands than in dense stands of the same age (9 and 7 kg/ ha year mm). This is due to the higher mean tree size in open stands. Within a stand, WUE of dominant trees is higher than in suppressed pines. This suggests that WUE increases when relative availability of water resources also increases. Analysis at the landscape level was carried out considering a precipitation gradient (from 400 to 1200 mm of mean annual precipitation). In all sites, WUE of forest plantations was higher than that of native grasslands or shrublands (mean: 7.7 and 3.9 kg/ha year mm). We also observed a trend towards a higher WUE in sites with higher water availability in the precipitation gradient. This was observed in forest plantations as well in native ecosystems. These results are important for forest management and to quantify the impact of land use changes, as well they provide valuable information for the current debate about changes in resource use efficiency as a function of resource availability.