LB 19-192 - The influence of cold-air pooling on species composition and ecosystem function in New England forests

Background/Question/Methods:

Cold-air pooling is a global phenomenon that frequently sustains low temperatures in sheltered valleys and generates microclimates that may be buffered from macroscale climate change. Cold-air pooling events restructure the environmental gradients trees experience, creating nonlinear temperature patterns across elevation. As a result, cold-air pooling may lead to unexpected vegetation distributions, but few studies have jointly assessed cold-air pooling environmental conditions and vegetation composition patterns at fine spatial scales (< 1 km). Moreover, no studies, to our knowledge, have done so using a coordinated regional network of sites. We sought to determine the effects of temperature inversions generated by cold-air pooling on forest composition across tree life stages in New England forests. We established a network of elevation transects spanning cold-air pooling gradients across forests in Vermont, New Hampshire, and Maine. In each plot, we measured air and soil temperatures via high-frequency in-situ sensors, forest overstory and understory composition and demographics, and other key soil and ecosystem properties. We calculated a community temperature index (CTI) for each plot’s overstory and understory plant community based on historical temperature preferences of each species. This allowed us to assess patterns in the relative composition of warm- vs. cold-adapted species along slopes.

Results/Conclusions:

We observed frequent cold-air pooling across sites. Temperature inversions occurred during 20-50% of continuous sensor measurements and sometimes persisted for days. Although CTI typically decreases as elevation increases, we found inverse and nonlinear patterns in CTI with elevation at sites experiencing more frequent cold-air pooling. For instance, the Nulhegan Basin in Vermont showed a positive linear relationship between CTI and elevation (R² = 0.6, p < 0.001) such that for every 50 m increase in elevation, CTI (i.e., the average community temperature preference) increased by 1.7 °C. This was largely driven by a greater relative abundance of cold-adapted coniferous species (e.g., Picea rubens and Abies balsamea) at low compared to high elevations. Understory CTI patterns were generally similar to those of the overstory. In-situ decomposition rates also shifted nonlinearly with elevation at several transects, but generally declined with increasing elevation and soil pH when assessed across all sites jointly. Taken together, our results suggest that locations prone to frequent or persistent cold-air pooling promote cold-adapted coniferous species at low elevations and may preserve the ecosystem functions they support as climate changes.