Photosynthetically distinct responses of an early-successional tree, Betula ermanii, following a defoliating disturbance: observational results of a manipulated typhoon-mimic experiment

The recovery of leaf photosynthetic rate of an early successional tree, Betula ermanii, to its pre-disturbance state following a defoliating disturbance was brought about by its fast leaf morphological response (LMA, leaf mass per area) but not physiological one (leaf N) as a resilience mechanism. Climate extremes possibly impair terrestrial vegetation and alter carbon cycles. Meanwhile, large uncertainty remains for evaluating the climatic impact on the vegetational function and structure and ecosystem CO2 exchange. We manipulated a defoliating disturbance experimentally in early September 2011 to simulate an extreme typhoon that actually hit a cool-temperate forest in Japan and ripped away a tremendous amount of foliage of early-successional B. ermanii trees in early September 2004, which corresponded to a decrease in plant area index (PAI) by 1.7 m2 m−2 from 3.3 m2 m−2 of the maximum PAI during the plant growth period. Over the 4-year observations in 2011–2016, we measured leaf-level photosynthesis following the manipulated disturbance. The highest maximum photosynthetic rate of leaves (Amax) occurred in 2014 among the observational period, which was the third year after the disturbance, and the subsequent Amax returned to the pre-disturbance state. We examined what biotic factors regulated Amax following the disturbance and found that the increase in the morphological attribute of leaf mass per area (LMA) enhanced Amax with a significant positive slope between LMA and Amax (R2 = 0.14 and p < 0.1). Meanwhile, Amax was hardly affected by the physiological attribute of leaf nitrogen (N) as indicated by a nonsignificant slope of the leaf N–Amax relationship (R2 = 0.04 and p = 0.281) throughout the observational period. This study would be a first challenge of a stand-scale defoliation experiment and provides a possible ecological inference regarding the functional and structural responses of early-successional trees following such disturbances.