The Influence of Elevated Air Temperatures on the Physiology and Morphology of Seedlings from Four Southern Appalachian Red Spruce (Picea rubens Sarg.) Seed Sources

Abstract

Red spruce (Picea rubens Sarg.) is a relic of the Pleistocene epoch that has historically been declining in the southern Appalachians. Red spruce in this region remain as spatially fragmented populations confined to cool, moist high-elevation mountaintops. These populations are thought to exhibit limited genetic variability and a restricted ecological niche, which may be threatened by the anticipated rise in average global air temperature. Currently, there is a limited understanding of the thermal acclimation capacity or physiological and morphological comparisons between the populations of red spruce in the southern Appalachians. The present study aims to 1) identify the effect of increased air temperatures on the gas exchange and growth of four southern Appalachian seed sources occurring at varying latitudes and elevations, Kuwohi (1,750m), Roan Mtn. (1,700m), Massie Gap (1,450m), and War Spur Ridge (1,100m), and 2) to compare the physiology and morphology of these seed sources, determining if there are differing plasticity among the sources. Seedlings were grown in greenhouses, and during their second growing season, placed into to either a cold treatment (20.8℃/16.6℃, Day/Night averages) or warm treatment (26.8℃/23.0℃, Day/Night averages) temperature chamber in the greenhouse. We found that the seed sources did not respond consistently to the treatments, with Roan Mtn. and Massie Gap displaying significantly different average photosynthetic rates between the two treatments, while statistically significant differences in Kuwohi and War Spur Ridge seedlings were not found. Thermal optimum temperatures (Topt) for photosynthesis varied between seed sources, with Kuwohi and War Spur Ridge exhibiting the lowest and highest, respectively. However, all the seed sources shifted their Topt between 0.09℃ and 0.15℃ per 1℃ of warming. Some growth characteristics varied by seed source, and warm-grown seedlings were, on average, smaller and exhibited lower total seedling weights and needle surface areas. The results of this study indicate seed sources vary in their capacity to respond to warmer growth temperatures. Nonetheless, the shifts in Topt appear insufficient considering the anticipated rise in average global air temperatures although it remains promising the capacity of some sources to increase photosynthetic rates in warmer growing conditions.