Spatial Correlates of Lower Treeline Position in the Western US

Abstract:

Lower and upper treelines jointly determine the distribution of forests in many mountainous regions. Although upper treelines across the world have received extensive scientific attention, generalizable studies of the climate controls of lower treelines are largely absent from ecological literature. Lower treelines are thought to be ultimately limited by plant water balance, and are expected to contract with predicted increases in water deficits. However, where the position of lower treeline is constrained by land use and disturbance rather than by water balance, the distribution of forests will likely be less sensitive to climate changes.

In this study, we investigated the relative importance of climate, land use, and disturbance for determining the position of lower treeline in the western US. We developed a moving window method to automate the mapping of lower treelines in the Intermountain West, an arid region encompassing gradients of precipitation (both magnitude and seasonality), growing season length, geology, disturbance history, and land use. We used classification and regression trees to identify climatic thresholds most related to lower treeline position and important effects of land use and disturbance.

Preliminary results show that lower treeline is associated with the interaction of soils and seasonal water balance, although the strength of the relationship varies by forest type and region. Furthermore, land use (wood harvest, grazing, and infrastructure development) and disturbance (fire) often coincide with the lower treeline boundary, suggesting that the actual position of lower treeline is partially controlled by secondary constraints. Future work will compare the modeled climate potential of lower treeline to the actual distribution, distinguishing between treelines that are likely at their ecophysiological limit (and thus susceptible to contraction in a warming or drying climate) from those contrained above their climate potential (areas of potential down-slope expansion).