Physiological and growth responses to experimental warming in first-year seedlings of deciduous tree species

Abstract

Increasing temperature might affect physiological and growth traits of seedlings, which are particularly important for tree survival. This study was conducted to investigate the physiological and growth responses of first-year seedlings to open-field experimental warming during one growing season. Seedlings of three deciduous tree species (Fraxinus rhynchophylla Hance, Zelkova serrata (Thunb.) Makino, and Quercus variabilis Blume) were warmed with infrared heaters with a mean air temperature difference of 3.07 °C between the treatments. Physiological traits (net photosynthetic rate, transpiration rate, stomatal conductance, and total chlorophyll content) were measured in July, September, and October 2014, and growth traits (root collar diameter (RCD), shoot length, component biomass, and root mass to stem mass ratio (RSR)) were measured in June, August, and October 2014 for harvested seedlings. Net photosynthetic rate, transpiration rate, and stomatal conductance were not affected by the warming treatment, whereas total chlorophyll content increased. Shoot length, leaf biomass, and stem biomass were enhanced under the warming treatment, whereas RCD and root biomass did not differ between the treatments. Thus, relative root growth declined under the warming treatment. It is likely that the elevated temperature provides optimal conditions for the biosynthesis of chlorophyll. Moreover, seedlings allocated more carbon to aboveground growth than to belowground growth when temperatures were elevated. In contrast, net photosynthetic rate, transpiration rate, and stomatal conductance were hindered, failing to increase as an adaptive mechanism to warming-induced water stress. Further studies are needed to elucidate (1) the direct effect of a decline in soil moisture, (2) why RSR declines to different extents in different species, and (3) the relationship between decreased root growth and seedling survival under the warming treatment.