Many forests in dry mountain regions are characterized by a lower elevational treeline, below which trees are unable to grow. This lower treeline is often the edge beyond which conditions are too dry for trees, and as such is likely to shift in response to global climate changes that result in increased aridity.
Extensive drought-induced tree mortality has been observed at dry forest range edges in the Intermountain West, facilitating the invasion of annual grasses and altering fire regimes. Lower treelines that are currently at their climate limit are expected to be more sensitive to changing climate. However, physical barriers, land use, and fire may also constrain the distribution of trees, preventing lower treelines from reaching what would otherwise be their climate-driven potential. Lower treelines constrained by non-climatic factors are less likely to respond directly to climate change but may be sensitive to other processes, such as changing disturbance regimes or land use practices. Understanding the controls on the position of lower treeline can help managers anticipate forest shifts in response to climate change and prioritize vegetation treatments.
In this study, we used existing vegetation classifications to map lower treelines for a 1.7 million km2 region in the U.S. Intermountain West. We randomly generated 7,000 sampling points within a 1-km buffer distance of the lower treeline boundary for each of three major forest types. We modeled the topoclimatic drivers of lower treeline position for each forest type to identify topoclimatically limited treelines. We then used spatial data of soil properties, recent fire, and land use to identify lower treelines potentially constrained above their ecophysiological limits by non-climatic processes.