What drives lower treeline, and how will it change in the future?
The most familiar definition of “treeline” refers to a zone near the mountaintops – where forests eventually fade into shrubs, cushion plants, and lichen-covered rocks. The environmental conditions that prevent trees from growing above this zone are intuitive – it’s cold, snowy, and windy. But what drives lower treeline, where forests fade into desert, grass or shrublands?
New research suggests that the temperature of soil at the ground surface limits the extent of low-elevation forests in the western United States. Researchers used a combination of experimental and observational data to show that soil surface temperatures of 50-60 degrees Celsius are lethal to ponderosa pine and Douglas fir seedlings, which commonly grow at low elevations. To explore the potential impacts of climate change and disturbances such as high-severity fire in these areas, they used modeling to predict and map “potential” soil surface temperature (PSST). In other words, what is the maximum temperature the ground surface would reach without overstory trees to provide shade? Maps of PSST are well-aligned with current low-elevation forest boundaries across the western U.S, demonstrating that PSST is a good predictor of forest cover.
When researchers modeled changes in PSST into the future based on predicted changes in climate, they found that the areas that are too hot for tree seedlings to grow will nearly double by the year 2050. As a result, lower treeline will creep upwards, leading to a loss of low-elevation forested area across the west. Read more about these impacts in this publication.