Although seedling survival has an important influence on forest dynamics, little is known about how temperate tree species differ in their tolerance of novel cold stress. Extreme climatic events, such as severe frosts, disproportionately affect tree seedling survival and growth because seedlings have limited resource reserves compared to later life stages; moreover, these events are predicted to increase in future climate scenarios. In order to quantify interspecific variation in cold-stress tolerance, we grew seedlings of five common northern temperate tree species (Abies balsamea, Acer saccharum, Pinus resinosa, Pinus strobus, Quercus rubra), and transferred five-month old seedlings to a climate-controlled facility where seedlings were gradually acclimated to simulated fall, followed by winter temperatures and photoperiod. Seedlings were then briefly exposed to spring temperatures (7°C) and photoperiod before experiencing one of three cold stress scenarios (7°C control, -8°C, -16°C). In a subset of individuals, we harvested root tissue, and in the case of conifer species, leaf tissue, to determine percent electrolyte leakage, which provides an indicator of plant stress. Following the cold-stress treatments, we monitored seedling survival and growth for two months.
Results/Conclusions
Quercus rubra survival was reduced by the -16°C treatment relative to the -8°C and control treatments, but did not differ among treatments in the remaining four species. In all species except Acer saccharum, stem growth was lower in the -16°C and -8°C treatments relative to controls. Pinus strobus and Pinus resinosa growth exhibited the strongest response to cold temperatures. Abies balsamea and Acer saccharum seedlings exposed to -16°C had elevated root damage; Pinus resinosa and Quercus rubra seedlings showed similar, but weaker trends. Abies balsamea leaves were more damaged by -16°C than -8°C or 7°C, but Pinus spp. leaves were not damaged by cold. We conclude that the effects of cold stress on seedling survival and growth vary substantially among species. Moreover, our data suggest that seedling responses to cold stress can exhibit threshold dynamics. The species examined here are projected to decrease in abundance over the next century due to warming growing-season temperatures, and our findings suggest that increased exposure to frost events in early spring could exacerbate these declines by preventing seedling establishment. Ultimately, conservation and management efforts may need to account for extreme spring climate events in order to understand intraspecific or spatial variation in recruitment dynamics.