PS 20-197 - Relations among tree demography, microclimate, and soil conditions at upper treelines in the Adirondack Mountains, northeastern United States

Background/Question/Methods

In the Adirondack Mountains of New York, and across the northeastern United States, unique alpine plant communities residing on mountain summits may be threatened by upslope movement of high-elevation spruce-fir forests due to climate-induced warming. Upper forest treelines have been shown to be temperature-sensitive on a global scale, but it is unclear how their structure varies with climate and other factors on a regional scale, or to what level treeline canopies can buffer climate warming. To gain further insight into the variables affecting upper treeline structure and function, we established ten survey sites at the treelines of seven different Adirondack mountains to characterize local environmental conditions and vegetation structure. Our objectives were to (1) quantify the buffering effect of treeline canopy cover on temperatures near the ground, (2) characterize treeline structure and composition, and (3) investigate the influence of environmental factors (e.g., microclimate, soil depth) on treeline location. Temperature sensors were placed 150cm and 25cm above ground to record microclimate above and below canopy cover. At each survey site, we counted all tree stems within six circular subplots (r = 2m) and classified them by size (< 1m, 1-2m, and >2m); soil depth and tree cover composition were also recorded.

Results/Conclusions

Tree canopies had statistically and ecologically significant effects on microclimate, as temperature below canopy was lower than above it by 1.96°C for daily maximum temperature and 0.72°C for daily mean temperature, on average. Although minimum temperatures were not significantly different above and below canopy, daily temperature range (amplitude) was lower on average by 1.78°C below-canopy. Mean lapse rate, (rate of temperature decrease with increasing elevation), was higher above the canopy (0.52°C/100m) than below it (0.32°C/100m). Below canopy lapse rate was roughly half of the global average (0.6°C/100m). Treelines were comprised of balsam fir (Abies balsamea), paper birch (Betula cordifolia), red spruce (Picea rubens), and mountain ash (Sorbus americana). Total tree cover was unrelated to elevation or temperature, but was positively correlated with soil depth. Paper birch sapling abundance was positively related to below-canopy temperature, but not above canopy temperature. Thus, our results suggest that both temperature and soil depth are important drivers of different aspects of treeline structure. Looking towards the future as temperatures continue to warm, the shade provided by tree canopies at high-elevation treelines can create a significant buffer against higher temperatures, potentially providing a more favorable growth environment for cold-tolerant plants that can also withstand canopy shade.