Yosemite's giant trees disappear

The oldest and largest trees within California's world famous Yosemite National Park are disappearing.
Climate change appears to be a major cause of the loss.

The revelation comes from an analysis of data collected over 60 years by forest ecologists. James Lutz and Jerry Franklin of the University of Washington, Seattle, US and Jan van Wagtendonk of the Yosemite Field Station of the US Geological Survey, based in El Portal, California collated data on tree growth within the park gathered from the 1930s onwards.

Their key finding is that the density of large diameter trees has fallen by 24% between the 1930s and 1990s, within all types of forest. "These large, old trees have lived centuries and experienced many dry and wet periods," says Lutz. "So it is quite a surprise that recent conditions are such that these long-term survivors have been affected."

Large trees are not only older, but they play a distinct and important role within forest ecosystems.

"One of the most shocking aspects of these findings is that they apply to Yosemite National Park," says Lutz. "Yosemite is one of the most protected places in the US. If the declines are occurring here, the situation is unlikely to be better in less protected forests."

The cause is difficult to pin down, but "we certainly think that climate is an important driver," says Lutz.

Higher temperatures decrease the amount of water available to the trees. The suppression of natural wildfires in the park also allows younger trees and shrubs to grow, increasing the competition for the water that is around.

"The decline in large-diameter trees could accelerate as climate in California becomes warmer by mid-century," the researchers warn in the conclusions to their study.

The impact of that is unclear.

"We know that large trees disproportionately affect the ecosystem," says Lutz. "But what the consequences could be of a decline in average large tree diameter, no-one really knows."