May 22, 2009

Mysterious Disappearance Of Yosemite’s Giant Trees

Experts say climate change appears to be a major cause of the disappearance of the oldest and largest trees within California's world famous Yosemite National Park, BBC News reported.

An analysis of data collected over 60 years by forest ecologists has raised cause for concern over the majestic trees.

Some fear that even more large trees may be dying off elsewhere, considering the current decline is happening within one of most protected forests within the US.

Data collected on tree growth within the park gathered from the 1930s onward was compiled by researchers 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.

They found that between the 1930s and 1990s the density of large diameter trees within all types of forest had fallen by 24 percent.

Lutz explained that these large, old trees have lived centuries and experienced many dry and wet periods.

"So it is quite a surprise that recent conditions are such that these long-term survivors have been affected," he added.

The large trees also play a distinct and important role within forest ecosystems, as their canopies help moderate the local forest environment and their understory creates a unique habitat for other plants and animals.

The older trees tend to seed the surrounding area and are often able to withstand fires, short-term climatic changes and outbreaks of insect pests that can kill or weaken smaller trees.

Lutz's team suggests, however, that these large specimens are no longer faring so well.

The researchers collated all the data that existed on tree growth within the Yosemite National Park that 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," Lutz said of the report published in Forest Ecology and Management.

Both surveys recorded data from 21 species of trees, in which the density of large diameter trees fell from 45 trees per square hectare to 34 trees, a decline of 24 percent in just over 60 years.

The trees that were affected the most included White Firs (Abies concolor), Lodgepole Pines (Pinus contorta) and Jeffrey Pines (Pinus jeffreyi), while smaller size trees were mostly unaffected.

Lutz said one of the most shocking aspects of these findings is that they apply to Yosemite National Park, which 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," he added.

He also pointed out that while the cause is difficult to pin down, they certainly suspect climate is an important driver, as higher temperatures decrease the amount of water available to the trees.

Because of the lack of natural wildfires in the park, younger trees and shrubs grow more rapidly, increasing the competition for the available water.

The researchers warned that the decline in large-diameter trees could accelerate as climate in California becomes warmer by mid-century.

Lutz said they know that large trees disproportionately affect the ecosystem, but what the consequences could be of a decline in average large tree diameter is still unclear.


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