Seasonal Variation in Chlorophyll Fluorescence and Photosynthetic Light Response Among Evergreen Species in a Madrean Sky Island Mixed Conifer Forest

Unlike the snowmelt-dominated hydroclimate of more northern mountainous regions, the hydroclimate of the Madrean sky islands is characterized by snowmelt and convective storms associated with the North American Monsoon. These mid-summer storms trigger biological activity and are important drivers of primary productivity. For example, at the highest elevations where mixed conifer forests occur, ecosystem carbon balance is influenced by monsoon rains. Whereas these storms’ significance is increasingly recognized at the ecosystem scale, species-specific physiological responses to the monsoon are poorly known. Prior to and following monsoon onset, we measured pre-dawn and light-adapted chlorophyll fluorescence as well as photosynthetic light response in southwestern white pine (Pinus strobiformis), ponderosa pine (Pinus ponderosa), and Douglas fir (Pseudotsuga menziesii) in a Madrean sky island mixed conifer forest near Tucson, Arizona. Across seasons, photochemical quenching (qp), an indicator of the proportion of open PSII reaction centers, was greatest in P. strobiformis and least in P. menziesii however, qp increased in response to monsoon rains. In contrast, non-photochemical quenching (qN), an indicator of heat dissipation ability, was greatest in P. ponderosa and least in P. menziesii, but did not change in response to monsoon onset. Estimated from leaf area-adjusted photosynthetic light response curves, maximum photosynthetic rate (Amax) was greatest in P. ponderosa and least in P. menziesii. Surprisingly, while the monsoon positively influenced Amax among P. ponderosa and P. strobiformis, Amax among P. menziesii declined with monsoon onset. Calculated as the initial slope of the light response curve, light-use efficiency (AQE) was similar among P. strobiformis and P. ponderosa and least in P. menziesii and across all three species, monsoon rains increased AQE. Likewise, P. strobiformis and P. ponderosa shared a similar, greater light compensation point than P. menziesii. However, across species, the monsoon did not influence light compensation points. These results support the hypothesis that the monsoon has species-specific effects on evergreen physiological performance and are broadly consistent with predictions of stress tolerance based on latitudinal and elevational range distributions of these species. Moreover, with year-to-year rainfall variability predicted to increase under future climate scenarios, species-specific functional traits related to stress tolerance and photosynthesis may promote ecosystem functional resilience in Madrean sky island mixed conifer forests.