Testing Binkley's hypothesis about the interaction of individual tree water use efficiency and growth efficiency with dominance patterns in open and close canopy stands

A well-recognized phenomenon during forest development is that stand level forest growth rate begins to decrease after canopy closure. Binkley [Binkley, D., 2004. A hypothesis about the interaction of tree dominance and stand production through stand development. Forest Ecology and Management 190, 265–271] proposed an interesting hypothesis relating individual contribution of trees to forest growth patterns, considering changes in dominance levels and resource use efficiency (ReUE, biomass growth per unit of resource used) of dominant vs suppressed trees. He stated that “the decline in standlevel growth near canopy closure is driven by increasing dominance of larger trees, leading to declining efficiency of resource use by smaller trees”. This decrease in ReUE of suppressed trees once canopy closes would lead to a general decline in standlevel ReUE and thus, in stand growth. This author with other colleagues found evidence according to this hypothesis, but much more research is needed to prove its generality in forests of different species and environmental conditions. The goal of this study was to test Binkley's hypothesis using information of growth patterns, water use efficiency (WUE) and growth efficiency (GE) in different stands of Pinus ponderosa in N.W. Patagonia. According to the hypothesis, we found no dominance in open stands, and some degree of dominance in closed canopy stands. However, in contrast to predictions, WUE differed between the smallest and the largest trees of each stand both with and without dominance. No differences in GE were observed between different tree size groups, and considering individual trees, the more suppressed ones were those with the highest GE. Based on our results, we proposed another hypothesis relating individual and stand ReUE: a decrease in ReUE is not determined by the establishment of dominance and subsequent decrease in suppressed trees efficiencies, but by the availability of resources for individual trees. In this regard, we postulate that differences in ReUE between trees of a stand can appear before dominance, and that these differences in ReUE lead to differences in size of the trees and later, in its social position. When trees grow, they have access to more resources, increasing at the same time, their efficiency using the resources. This efficiency is going to decrease when availability of resources decreases (for competition or environmental stress). The stand ReUE will be maintained, decreased or increased based on the balance of available resources for the individual trees, which in turn will depend on their growth rates, competitive ability and environmental site conditions.