Matt Walker, BBC News 22 May 09;
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.
They say one worrying aspect of the decline is that it is happening within one of most protected forests within the US, suggesting that even more large trees may be dying off elsewhere.
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.
Their canopies help moderate the local forest environment while their understory creates a unique habitat for other plants and animals.
Older, larger trees also tend to seed the surrounding area and crucially are able to withstand fires, short term climatic changes and outbreaks of insect pests that can kill or weaken smaller trees.
But the study by Lutz's team suggests they are no longer faring well.
In a study published in Forest Ecology and Management, the researchers collated all the data that existed on tree growth with the Yosemite National Park. In particular, this included two comprehensive surveys: one conducted in the mid 1930s and another during the 1990s.
"Few studies like this exist elsewhere in the world because of a lack of good measurements from the early 20th Century," says Lutz.
Including 21 species of tree recorded by both surveys, the density of large diameter trees fell from 45 trees per hectare to 34 trees, a decline of 24% in just over 60 years. White Firs ( Abies concolor ), Lodgepole Pines ( Pinus contorta ) and Jeffrey Pines ( Pinus jeffreyi ) were affected the most. Smaller size trees were unaffected.
"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."
