Diurnal fluctuations of gas exchange and water potential in different stand structures of Pinus ponderosa

Leaf-level gas exchange and leaf water potential (Ψ_leaf) measurements were made over a diurnal time-course in multi-aged and even-aged stand structures of ponderosa pine (Pinus ponderosa Dougl. ex Laws.) in central Oregon (June) and western Montana (July) to test for differences in physiological performance due to stand structure. Total site occupancy was similar between the geographic regions as measured by basal area, leaf area index, and stand density index. No differences in net photosynthesis (A_net), stomatal conductance (g_s), transpiration (E), instantaneous water use efficiency (WUE) or Ψ_leaf were observed in June in Oregon. As a whole, even-aged plots appeared to be more water-stressed than multi-aged plots that were able to maintain higher rates of E in July in Montana. There were no differences in WUE between multi-aged and even-aged stand structures in Montana, but because both A_net and E tended to be less in even-aged trees, overall productivity and efficiency of foliage may be less than in multi-aged stand structures. It is concluded that under environmental conditions that are not limited by water, patterns in gas exchange and water use are unaffected by stand structure. The data from this study further suggest that water-limiting conditions found later in the growing season may influence diurnal gas exchange patterns in a way that could result in lower productivity in even-aged stand structures.