Differences in photosynthetic and water use characteristics of four co-existing tree species on the treeline of the Japanese Alps

Abstract

Abstract In this study, we analyzed photosynthetic and water use characteristics of two deciduous broad-leaved (Sorbus matsumurana and Betula ermanii) and two evergreen coniferous (Abies mariesiiand Pinus pumila) tree species on the treeline ¾i.e., the line connecting the highest patches of forest composed of trees at least three meters high¾of Mt. Norikura, central Japan, to explore the physiological adaptations of trees to alpine environments. We monitored diurnal changes in leaf gas exchange rates and leaf and soil water potentials in each tree species during the growing season, as well as bulk leaf water relations based on pressure–volume curves and whole-tree hydraulic structures. The four species under study relied on different physiological characteristics at both leaf and plant levels for adaptation to their natural habitat. In S. matsumurana, photoassimilate acquisition occured during the short growing season, and reducing transpirational water loss was associated with osmotic adjustment. In contrast, B. ermanii showed higher photosynthetic rates, while maintaining higher hydraulic conductance and transpirational water loss in the absence of severe stomatal restrictions. In turn, A. mariesii showed higher leaf succulence, and a leaf water storage capacity that was favorable for growth, while reducing water loss. Meanwhile, Pinus pumila compensated for water loss through higher leaf hydraulic capacitance and cell-wall elasticity, and exhibitedthe longest daily transpiration period, while maintaining high stomatal conductance, even as soil water potential decreased. Our observations demonstrates that tree species rely on specific physiological characteristics to adapt to the alpine treeline environment.