The forests that cloak mountainsides are one of the defining ecosystems of the western US. Recent big bark beetle outbreaks and wildfires raise questions about how forests are changing and how we should respond.
Forests rely on disturbances to open the canopy and clear the way for new, young trees. Over millennia, forests evolved with insects and disease, drought and wildfire, temperature swings and water availability. Today, human activities like logging, road building, and fire suppression, as well as natural disturbances like insects, pathogens, and storms, constantly shape forests.
Temperature is a major driver behind three big natural forest disturbances: drought, bark beetles, and wildfire. When a region warms up, even if precipitation stays the same, more water evaporates from soil and transpires from plants, drying out trees. Warmer spring weather, earlier snowmelt, and longer summers also dry out vegetation and create conditions conducive to wildfires. And warmer winters allow insects, fungi, and pathogens to survive better, reproduce faster, and more easily kill dry, weak trees. A spell of warm years has driven the recent big wildfire seasons and beetle outbreaks. No one expects temperatures in the western US to cool off anytime soon, so what might forests look like 50 or 100 years from now?
Making such predictions requires understanding how current disturbances compare to those of the past, how forests respond to disturbances, and what disturbances might be like in the future. Ecologists map historic forest responses to shifts in temperature and build models that take into account factors such as soil and vegetation types, moisture, temperature, and more. They test these models by measuring whether they can accurately demonstrate known conditions, over, for example, the last century, and then use the models to simulate future forest behavior.
Several models that map forests in the coming century show continued warm temps exacerbating disturbances, leading to declining tree cover, especially in the southwest and northern Rocky Mountains. One model estimates that shrub and grassland ecosystems will replace forests over about 15% of the West, while another study predicts that the suitable climate range for 130 tree species throughout North America on average will decrease by 12% and shift north by 435 miles by 2100. Still another found that, at least in some regions, tree populations are not yet migrating northward, but rather, are growing and dying faster at the southern ends of their ranges where climates are warmer and wetter.
Forests are changing. They will never again look to us the way they did in, say, 1980. These changes have implications for anyone who relies on forests for timber or rangelands, drinking water or carbon sequestration, recreation or scenic vistas. How we adapt as our forests transform will be one of the great challenges of the coming decades. The articles in this issue of Western Confluence illustrate some possible responses and hopefully will trigger new thinking about ways we might adjust to our changing world.
By Emilene Ostlind
 Phillip Mantgem et al., “Widespread Increase of Tree Mortality Rates in the Western United States,” Science Magazine 323, no. 5913 (January 2009): 521-23, doi:10.1126/science.1165000.
 A. L. Westerling, H. G. Hidalgo, D. R. Cayan, and T. W. Swetnam, “Warming and Earlier Spring Increase Western U.S. Forest Wildfire Activity,” Science 313, no. 5789 (August 2006): 940-943, doi:10.1126/science.1128834.
 Barbara Bentz et al., “Climate Change and Bark Beetles of the Western United States and Canada: Direct and Indirect Effects,” BioScience 60, no. 8 (September 2010): 602-613, doi:10.1525/bio.2010.60.8.6.
 Xiaoyan Jiang et al., “Projected Future Changes in Vegetation in Western North America in the Twenty-First Century,” Journal of Climate 26, no. 11 (June 2013): 3671-3687, doi:10.1175/JCLI-D-12-00430.1.
 A. Park Williams et al., “Temperature as a potent driver of regional forest drought stress and tree mortality,” Nature Climate Change 3 (March 2013): 292-297, doi:10.1038/nclimate1693.
 Daniel McKenney et al., “Potential Impacts of Climate Change on the Distribution of North American Trees,” BioScience 57, no. 11 (December 2007): 939-948, doi:10.1641/B571106.
 Kai Zhu, Christopher Woodall, Souparno Ghosh, Alan Gelfand, and James Clark, “Dual impacts of climate change: forest migration and turnover through life history,” Global Change Biology 20 (2014): 251-264, doi:10.1111/gcb.12382.