How will native southern Appalachian conifers respond to winter warming?

Background/Question/Methods

Winters have been warming in the southern Appalachians over the last two decades, and more frequent freeze-thaw cycles combined with longer midwinter thaw periods may intensify physiological stress in native conifers, potentially shifting distributional boundaries. However, there has been minimal exploration of the ways in which native southern Appalachian conifers tolerate winter and repeated freeze/thaw cycling. Given the high ecological, economic, and recreational value of conifers in this region, a deeper understanding of the possible physiological changes in store for southern Appalachian conifers allows for more informed decisions in conservation, farming, and land acquisition in the future.This study aims to characterize the specific overwintering strategies of three southern Appalachian conifers (Abies fraseri, Picea rubens, and Tsuga canadensis), the physiological tradeoffs associated with those strategies, and their potential implications for survival in warming winters. Sampling has occurred every 2-6 weeks since September 2021 at four sites across western NC ranging from 1200-2037 m in elevation. Foliar and woody samples are taken from one sun-exposed distal branch per individual (n=5 per species, per site) and used to assess chlorophyll fluorescence (Fv/Fm), cold hardiness (°C ), water potential (ψ), percent loss of hydraulic conductivity (PLC), and xanthophyll cycle state.

Results/Conclusions

Since September 2021, minimum air temperatures were lowest at Mount Mitchell, the highest elevation site (2037 m), and PLC in all species was greater at lower minimum temperatures (p=0.006). A. fraseri exhibited the greatest PLC at all sites during the fall to winter transition period (Sept-Dec 2021) except Mount Mitchell, where losses were greater in P. rubens (p=0.03). Photosystem II (PSII) efficiency was similar among species but was significantly reduced with decreasing minimum temperatures across all sites (p< < 0.001). Midday water potentials followed a similar trend, with no species-level differences and becoming significantly more negative with decreasing minimum temperatures across sites (p=0.005). Both trends were most pronounced on Mount Mitchell (p< 0.001 for PSII efficiency, and p=0.003 for water potentials). Maximum recorded daily temperatures did not have a significant effect on either parameter. Analyses of xanthophyll cycle and cold hardiness (derived from chlorophyll fluorescence values) are ongoing and will be reported at the meeting. The preliminary data suggests that these species may have divergent plasticity in their site-specific temperature response. Next steps in data analysis include assessing the species-level relationships between unit change in PSII efficiency, xanthophyll cycle state, cold hardiness, and PLC with changes in minimum air temperature.