Beautiful Days in the Neighborhood: Land-Atmosphere Interactions as Drivers of Forest Expansion

At the end of glacial periods, warmer temperatures induce ice sheet retreat, exposing the land surface for forest establishment. Forest emergence decreases land surface albedo, leading to additional local warming, and modifies the exchange of water and energy between the land and atmosphere. These responses induce changes in the local atmosphere that have the potential to drive plant-atmosphere warming feedbacks and forest expansion beyond what is orbitally driven. Pollen records show rapid expansion of needleleaf evergreen forest in Alaska following North American deglaciation around 6,000 years ago. Using this period as a case study, we explore the possible role of plant-atmosphere feedbacks in accelerating forest expansion using a simplified reconstruction of this period. We simulate the climate response to initial forest establishment and then apply the climate modified by the forest to nearby vegetation to determine if initial forest expansion leads to more favorable growing conditions in the region. We find that the existence of the forest produces favorable conditions during the growing season nearby that would promote forest expansion. The forest acts as a source of heat and moisture for plants west of the forest, leading them to experience earlier springtime temperatures and snowmelt, and further growth. Summertime cooling and cloud formation over the forest also drives a circulation change that reduces summertime cloud cover south of the forest, increasing solar radiation reaching the plants, and driving warming. By isolating these vegetation-atmosphere interactions as the mechanism of increased growth, we demonstrate the potential for forest expansion to be accelerated in a way that has not been highlighted before. Our results can be used to improve our interpretation of biophysical impacts from climate change, afforestation projects, in addition to known past forest establishment. By examining these feedbacks, we seek to gain a more comprehensive understanding of past and potential land- atmosphere interactions