Urban Forest Health

Alaska cedar is declining and experiencing elevated mortality in large areas. Alaska cedar decline has been attributed to earlier snowmelt, which exposes roots to damage from lower temperatures. Projected future warming is expected to cause additional mortality from freezing-induced root damage, because more snow will be delivered as rain and winter snowpacks will be thinner and more variable. Additionally, warmer winters may allow pests and diseases to expand into previously unavailable territory in Alaska.

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. Increases of 15-25 days are projected for southwestern and south-central parts of the state, with the growing season extending to more than 200 days in a large portion of southwestern Alaska.

There is general agreement between different climate scenarios for future precipitiation projections in Alaska. Generally, the largest increase is projected for the far northwest of the state and the smallest changes in the Inside Passage region. By the end of the century, precipitation increases range from 10% to 35% in different parts of the state. Spring precipitation is actually projected to decrease in some scenarios, but there is generally high uncertainty between future projections.

Alaska has warmed twice as fast as the rest of the U.S. over the past several decades. All climate models agree that temperatures are projected to increase over the 21st century across Alaska, with larger increases projected for later in the century. The greatest warming is expected in northern Alaska, with less warming projected for the southeast part of the state. The greatest warming is expected during winter months, with less dramatic increases expected during the summer.

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. Drought and increased temperatures due to climate change have caused extensive tree death across the Southwest. In addition, winter warming due to climate change has exacerbated bark beetle outbreaks by allowing more beetles, which normally die in cold weather, to survive and reproduce. Wildfire and bark beetles killed trees across 20% of Arizona and New Mexico forests from 1984 to 2008.

Water is central to the region’s productivity. Winter snowpack, which slowly melts and releases water in spring and summer, when both natural ecosystems and people have the greatest needs for water, is key to the Southwest’s hydrology and water supplies. Over the past 50 years across most of the Southwest, there has been less late-winter precipitation falling as snow, earlier snowmelt, and earlier arrival of most of the year’s streamflow.

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 interior of California.

The Southwest has already experienced a modest increase in some measures of heavy precipitation, thought not to the extent of other regions. For example, storms in the 99th-percentile category increased by 10% from 1958 to 2016 across the region. Intense rainfall is expected to continue to occur more frequently in the future, even though overall precipitation is expected to decline. For example, large rain events that historically had an expected return of 5 years are expected to occur twice as often by the end of the century, under a high emission scenario (RCP 8.5).

There is uncertainty between different climate scenarios for future precipitation projections in the Southwest. Generally, there is a south-to-north gradient in annual precipitation projections, with decreases projected from central California, Nevada, Utah, and Colorado to the south. Decreases are largest in southern California, Arizona, and New Mexico (3-12%). The northern half of the region is projected to experience no change or a slight (0-3%) increase in annual precipitation.