Spruce-fir forests occupy the highest peaks in the southern Appalachian mountains of the eastern USA, where mesoclimate is characterized by frequent cloud immersion and cool temperatures. Understanding the processes that maintain the lower elevational limit of these forests is critical for predicting their future response to climate change, especially due to rising cloud ceilings that are predicted for this region. In particular, the mechanisms affecting the high mortality of first-year seedlings (FYS) at the lower elevational boundary of these spruce-fir forests remain unclear.
We studied the effects of cloud immersion on FYS-level microclimate, ecophysiology, and population demography across the spruce-fir – broadleaf ecotone (~1750 to 1900 m) at Roan Mountain, TN. It was hypothesized that cloud immersion would generate favorable seedling microclimate, improve plant water status, and reduce mortality of this bottleneck life stage. Microclimate measurements included leaf wetness and soil water potential (ψsoil) near the soil surface. We measured FYS stomatal conductance (gs), stem water potential (ψstem), and declines in hydraulic conductivity (KFYS) throughout the growing season. Germination and survival of this earliest life stage were recorded across three growing seasons (2016 – 2018) to evaluate population-level sensitivity to FYS annual survivorship.
Results/Conclusions
At the highest elevations, cotyledons were wet 39.5% of the time during the growing season, but much less frequently at the lower end of the ecotone. ψsoil remained above -100 kPa and cloud events increased ψsoil by ~5 kPa relative to periods with clear skies. Early-season KFYS, ψstem, and gs did not vary along the elevational gradient, although mid-summer KFYS declined more rapidly at lower elevations, which significantly reduced ψstem and gs here during mid-summer droughts. Both the maximum number of germinants and the probability of survival into the 2nd growing season increased exponentially as a function of elevation during the 2016 growing season, with germination reaching 792 seedlings/m2 at the highest elevations, but no recruitment at the lower end of the ecotone. This relationship showed high inter-annual variability, and no new germinants were found in 2017.
Our results indicate that the elevational limits of these forests are highly sensitive to FYS survival, and that this process is at least partly dependent upon cloud immersion. The effect of rising cloud ceilings associated with climate warming on the survival of FYS’s may lead to an upward migration and diminution of these Southern Appalachian spruce-fir forests.