UC Merced researchers have evidence that California’s forests are especially vulnerable to multi-year droughts because their health depends on water stored several feet below ground.
Trees typically need about the same amount of water every year — more in hotter years.
If the drought is long enough and especially hot, as was the case from fall 2011 through 2015, large numbers of trees run out of water and die.
The researchers delved into data from sites across the semi-arid West to determine the amount of water storage available in the root zones of different areas.
Bales, Rungee and their colleagues wanted to know how dependent different areas in the West are on that subsurface water storage and how many dry years the plants can survive.
Our wet seasons are getting shorter, which means that the dry seasons are getting longer and drier — we’re getting less snow than rain — and the wet seasons are warmer, meaning the stored water gets used earlier in the year, leaving less for the hottest parts of the year Bales explained that areas with Mediterranean climate, such as California, are especially vulnerable because of their climate variability, even though they have considerable subsurface storage.
Areas with year-round precipitation, such as Colorado, are vulnerable, too, because they have less subsurface storage, even though their year-to-year precipitation is less variable than California’s.
“Usually the year-round precipitation would see those areas through drought years, but longer, hotter droughts are going to pose more problems,” Bales said.
“In California, more than half of the water use by forests, grasslands and shrublands comes during the dry season, and it comes from water stored in the root zone after rainfall and snowmelt end in spring.” In contrast, less than one-third of water used by vegetation at sites they studied in Colorado, New Mexico and Arizona was drawn from subsurface storage during dry periods.
“In some areas, this means thinning overstocked forests to prevent further tree mortality,” Bales said.