In the early 1900s, an average forested acre in California supported fewer than 50 or so trees. After a century of efforts to fight wildfires, the average has risen to more than 300 (albeit mostly smaller) trees. Some might reckon such growth wonderful, but it is a problem far more serious than, say, the fact that horses can no longer trot through areas where they once could. The extra fuel turns today’s wildfires into infernos hot enough to devastate the landscape, torching even the big older trees that typically survived fires in the old days. Beyond this, the extra trees are worsening California’s driest ever drought.
“Like too many straws in a drink,” trees suck up groundwater before it can seep into streams that feed reservoirs, says David Edelson of The Nature Conservancy. The project director for the Sierra Nevada range, source of 60% of California’s consumed water, notes that as a warmer climate lengthens the growing season, trees’ thirst will only increase. This has led to a push for large numbers of trees to be cut or burned down. Overgrown forests catch more snow and rain on leaves and needles, where wind and sunlight increase the amount of moisture lost to evaporation.
To reduce what Tim Murphy, a Forest Service ecologist, considers an excessive number of trees in forests, the service thinned 600 square miles of California’s watershed in the year to October, up from 367 the previous year. By burning or removing about 40% of tree and plant-life in these areas, the Forest Service wants to do more than put extra water in reservoirs. The goal is also to reduce the severity of wildfires and to get water into the bigger trees left standing—more than five years of drought have killed more than 66m trees in California, aerial surveys show.
Thinning efforts are off to a great start but must accelerate, says Timothy Quinn, head of the Association of California Water Agencies. Five times as much forest should be thinned every year, estimates Roger Bales, a hydrologist at the University of California, Merced. To find out how much extra water a thinned watershed produces, the university has placed sensors in thinned and control plots in the Stanislaus-Tuolumne Experimental Forest north of Yosemite National Park. Depending on landscape and precipitation, thinned areas shed 10-40% more water into streams, Mr Bales estimates.
From The Economist: https://www.economist.com/news/united-states/21711930-cutting-often-preferable-burning-greater-cost-can-be-offset-payments