Urban Forest Health

Forest ecosystems throughout the Great Plains are exposed to a range of natural, introduced, and anthropogenic stressors. High-elevation forests and semiarid riparian forests are faced with their own distinct kinds of stressors. Invasive plants, forest pests, diseases, droughts, and floods are expected to become more damaging under climate change, and these factors may interact in unpredictable ways.

Different modeling approaches generally conclude that future climate conditions will increase the risk of wildfire across the Great Plains. Annual fire probability, calculated solely with climate data and physical principles, is projected to increase by 20% to 1200% across the region by the end of the century, with the largest increases occuring in Wyoming and western Montana. Fire probability may decrease in Texas, however.

Water is central to the productivity of the Great Plains. Projected increases in winter and spring precipitation in the Northern Plains may benefit productivity by increasing water availability through soil moisture reserves during the early growing season. The Northern Plains will remain vulnerable to periodic drought because much of the projected increase in precipitation is expected to occur in the cooler months while increasing temperatures will result in additional evapotranspiration.

For an average of seven days per year, maximum temperatures reach more than 100 degrees F in the Southern Plains and about 95 degrees F in the Northern Plains. By the middle of the century, Central Texas north to the Dakotas may experience an increase of 10-20 more hot days per year. East Texas, Wyoming, and western Montana are projected to have 22-30 more hot days by the middle of the century, according to a range of climate scenarios. Similar increases are expected in the number of nights with minimum temperatures higher than 80 degrees F in the south and 60 degrees F in the north.

The freeze-free season is defined as the period of time between the last spring frost (daily minimum temperature below 32 degrees F) and the first fall frost. The length of the annual freeze-free season has been increasing since the 1980s, and all climate models agree that it will continue to increase in the future. The largest increases are projected for the high country of Wyoming and Montana.

Simulated changes in summer precipitation by the end of the century range from a 37-percent decrease to a 12-percent increase, with a mean around an 10-percent decrease. The strongest agreement for drier summers occurs in Oklahoma and west Texas. The means of several climate models indicate that winter and spring precipitation may increase around 3 percent by the end of the century, with larger increases expected in the Dakotas. Under mild climate scenarios, changes are generally smaller than in more extreme climate scenarios.

Most of the region is projected to experience 0 to 30% more days each year with more than an inch of precipitation by the middle of the century. The largest increases (up to 45% increases) in extreme precipitation are expected in North Dakota and Montana. Days with more than 2 inches, 3 inches, and 4 inches of precipitation are also expected to occur more regularly by the middle of the century.

There is uncertainty between different climate scenarios for future precipitiation projections in the Great Plains. Generally, there is a south-to-north gradient in annual precipitation projections, with increases projected north of the Nebraska-North Dakota border and decreases projected from Kansas to Texas.

The largest decreases are expected in the northern half of the region. The smallest decrease in cold days is expected in Oklahoma and Texas, where these kinds of cold days rarely occur. Similarly, the region is expected to have up to 32 fewer days with a minimum temperature below 32 degrees by the middle of the century, particularly in western Montana, Wyoming, and the western edges of Nebraska, Kansas, Oklahoma, and Texas.

All climate models agree that temperatures are projected to increase over the 21st century across the Great Plains, with almost uniform temperature increases across the entire region. The greatest warming is expected in the northern Plains, especially the Dakotas. In winter, the greatest warming is expected in northern states. Springtime warming is generally smaller than winter warming, with the largest increases occuring in southwest Texas. Summer shows a large amount of warming, with a localized maximum in southwest Kansas.