Identifying canopy-scale adjustments to the extreme climate in a semi-arid pine forest using eddy covariance and close-range sensing data

Semi-arid forests represent some of the most sensitive ecosystems to climate change. Identifying adjustments to extreme conditions can indicate their resilience, and that of forests undergoing increasing aridity trends. We used eddy covariance and close-range sensing measurements over four years in a semi-arid pine forest to identify canopy-scale adjustments to the short active season and long seasonal drought. Peaks in light use efficiency (LUE), leaf chlorophyll content (LCC), and increasing absorbed photosynthetic active radiation (APAR; based on canopy absorption coefficient in the green range), all converged to support an early peak (March) in gross primary productivity (GPP), exploiting the narrow optimum between PAR_in, temperature and the rapidly decreasing soil moisture in spring. In contrast, during the long dry period (>200 days), while PAR_in increased, LCC and LUE decreased, offering physiological photoprotection as GPP sharply declined under the stressful conditions. The strong negative correlation between ρ_NIR and PAR_in indicated canopy biophysical adjustments that enhance light absorption under low radiation and eliminate photodamage under excessive radiation.  The results provide clear indications of canopy-scale adjustments underlying the high productivity of the forest and its resistance to the harsh conditions, which may soon apply to forests in currently milder climatic regions.