Seasonal trends in photosynthetic traits suggest temporal niche partitioning among co-dominant tree species in a Madrean sky island mixed conifer forest

Background/Question/Methods

Across much of western North America, forests are predicted to transition from snow- to rain-dominated precipitation regimes due to anthropogenic climate warming. Madrean sky island mixed conifer forests receive a large portion of their precipitation from summertime convective storms and may serve as a lens into the future for snow-dominated forests after prolonged warming. To better understand the linkage between physiological traits, climate variation, and the structure and function of mixed conifer forests, we measured leaf photosynthetic (A) responses to controlled variation in internal CO₂ concentration (C_i) to quantify interspecific phenological variation in A/C_i derived ecophysiological traits among ponderosa pine (Pinus ponderosa), southwestern white pine (Pinus strobiformis), and Douglas fir (Pseudotsuga menziesii).

Results/Conclusions

Species had similar, positive responses in net photosynthesis under ambient conditions (A_net) to the onset of summertime monsoonal precipitation, but during the cooler portions of the year P. ponderosa was able to maintain greater A_net than P. menziesii and P. strobiformis. Moreover, P. ponderosa had greater A_net in response to ephemerally favorable springtime conditions than either P. menziesii or P. strobiformis. Monsoonal precipitation was associated with a sharp rise in the maximum rates of electron transport (J_max) and carboxylation (V_Cmax) in P. menziesii in comparison with P. ponderosa and P. strobiformis. In contrast, species shared similar low values of J_max and V_Cmax in response to cool winter temperatures. Patterns of relative stomatal limitation followed predictions based on species’ elevational distributions, reinforcing the role of stomatal behavior in maintaining hydraulic conductivity and shaping bioclimatic limits. Seasonal variation in ecophysiologial traits among co-occurring tree species in a Madrean mixed conifer forest may promote temporal resource partitioning and thereby contribute to species’ coexistence. Moreover, these results provide a physiological basis for predicting the ecological implications of North American mixed conifer forests currently transitioning from snow- to rain-dominated precipitation regimes.