Drought could render the U.S. Northeast's mixed forests unsustainable after 2050 while Washington's Cascade Mountains may require tropical and subtropical forest species, according to researchers using a new type of mathematical model at Washington State University. The Tolerance Distribution Model (TDM) is the first to use the tolerances of different types of forests to drought, flood and shade to determine how the forests may respond to future climate change. In contrast to existing methods, the new approach can be applied at a continental scale while maintaining a direct link to ecologically relevant stressors.

WSU Vancouver mathematicians Jean Liénard and Nikolay Strigul, and ecologist John Harrison, predict the Pacific Northwest's climate may be warmer and wetter, requiring the establishment of forest types seen in places like southeastern China, southern Brazil or sub-Saharan Africa.

In the northeastern U.S., the model projects forests of maple/beech/birch, spruce/fir and white/red/jack pine combinations will be ill-suited to withstand predicted drought conditions by the latter half of the 21st century. Other forested areas that were identified as being at risk from drought included the northern Great Plains and the higher elevations of the Rocky Mountains.

Meanwhile, low altitude areas of Texas may eventually host tropical dry forests similar to regions of eastern Mexico. Moist, deciduous forests found in locations like Cuba could one day thrive along the U.S. Gulf Coast.

"Until now, our ability to predict exactly how and where forest characteristics and distributions are likely to be altered as a result of climate change has been rather limited," said Liénard, a postdoctoral researcher and the paper's first author. "With our model, it is possible to identify which forests are at the greatest risk from future environmental stressors. Forest managers and private landowners could then take steps like planting drought tolerant seedlings and saplings to prepare."