Résumé
Plants are expected to respond to drought by maximizing the efficiency of the most limiting resource, the water use efficiency (WUE), at the expense of nitrogen and carbon use efficiencies (NUE and CUE). Therefore, plants resource use efficiencies are viewed as indicators of species drought tolerance. We tested these predictions by measuring leaf-level intrinsic WUE (WUEi, the ratio of net assimilation to stomatal conductance), photosynthetic NUE (PNUE, the ratio of daily maximum net assimilation to leaf nitrogen content) and leaf-scale CUE (approached by the ratio of night-time respiration to daytime net assimilation, R-d/A(n)) in pinon pine and juniper, two tree species that differ in drought tolerance and vulnerability to drought-induced mortality. Variations in resource use efficiency in the two species were measured in response to seasonal drought and in response to an ecosystem-scale precipitation manipulation experiment comprising three precipitation treatments: ambient, irrigation (+30%) and partial rainfall exclusion (-45%). Increasing water limitation, either seasonally or across treatments, resulted in increased WUE and decreased PNUE and CUE in both species. WUE, PNUE and CUE varied more strongly in response to water limitation than across species and converged to the same relationships against precipitation for pinon and juniper. Plasticity in WUE, PNUE and CUE in response to water limitation was associated, in both species, with low carbon acquisition during drought. Our results exhibited a convergence in resource use efficiency across pinon and juniper which contradicts the paradigm that resource use efficiencies are indicators of species drought tolerance and ecological strategy.