Testing a hypothesis of the relationship between productivity and water use efficiency in Patagonian forests with native and exotic species

The resource use efficiency, the relationship between productivity and the amount of used resources, have been proposed to increase as forests increase their productivity and rate of resource use (Binkley, D., Stape, J.L., Ryan, M.G., 2004. Thinking about efficiency of resource use in forests. For. Ecol. Manage. 1-2, 5-16). To test the generality of this hypothesis, we estimated annual productivity and water use efficiency (WUE) of an exotic Douglas-fir (Pseudotsuga menziesii (Mirb) Franco) plantation and a multi-species native forest in N.W. Patagonia, both growing on the same site. We hypothetized that, within a site, the more productive system (a priori the Douglas-fir plantation) has a higher WUE than the less productive system (the native forest), and that within the native forest, the more productive species have higher WUE than the less productive ones. Five taxa were the most conspicuous woody species in the native forest, which was codominated by Nothofagus antarctica and Lomatia hirsuta. Both studied systems had reached their maximum leaf area index (approximately, 5 and 11 for the native and exotic forests, respectively). Annual productivity was 2.3 times higher in the Douglas-fir plantation than in the native forest, however two native species showed similar values of biomass productivity than the exotic species. The amount of water used by the native system was lower than that used by the Douglas-fir plantation, but not as low as the observed differences in productivity, thus resulting in a higher WUE in the Douglas-fir plantation than in the native forest (1.8 times higher). In addition, WUE was also different among the native species. The lower WUE at the stand level in the case of the native forest probably is due to the presence of retamos and lauras. We concluded that: (1) the more productive system (the Douglas-fir plantation) had a higher WUE than the less productive system (the native mixed forest); (2) within the native mixed forest, the most productive species (L. hirsuta and N. antarctica) had the highest WUE of all native species. Future research is needed to explain the physiological causes (daily patterns of transpiration and carbon fixation and allocation) of the observed differences in WUE between species and whole systems.