Long-term forest study in Yosemite

Scientists in California have set up a unique long-term experiment to track the life histories of some of the world's oldest and tallest trees. The project is designed to follow up research, in the Yosemite National Park, which suggests that giant trees are perishing as a result of climate change.

An analysis of data collected over 60 years has led scientists from the University of Washington and the Yosemite Field Station of the US Geological Survey, to conclude that the density of large diameter trees fell by 24% between the 1930s and 1990s.

"We want to identify the reasons for tree mortality and if those are changing," says Dr James Lutz, a research associate at the university's College of Forest Resources.

Little research has been done on a long-term basis to monitor the lives of large trees. Unlike studies with smaller plants and almost all animals, no individual scientist is able to track a forest giant for its entire lifespan - from germination to death. They live for hundreds of years and play a vital role in the ecosystem long after they have died.

Yosemite National Park is a vast area of wilderness covering 3,027 sq km (1,169 square miles), 321km (200 miles) from San Francisco. Large trees like the giant sequoias play a crucial role in the forest ecosystem. They provide a habitat for birds and insects while they are alive and also when they are dead. Crucially, they are resistant to fire and are seen as pivotal to a forest's ability to recover from a major blaze. The forest also provides people with a range of ecosystem services such as water supplies.

Dr Lutz and his team have set up the Yosemite Forest Dynamics Plot to monitor the forest over a period of decades, and possibly centuries. It is a 25-hectare plot of dense woodland, comprising mainly Sugar Pine and White Fir trees. The area has not burned for at least 70 years. The plan is to measure and map almost all of the trees, which are estimated to number about 30,000. The cut-off point is woody stems that are less than 1cm in diameter at chest height. "We plan to come back every year to do a mortality assessment to evaluate all of the trees that have died and hopefully the reason they die," says Dr Lutz. Hopefully the study will reveal subtle changes in the composition or the structure of the forest.

Traditionally, the funding of long-term experiments that involve monitoring nature has been difficult to secure. The Yosemite project received about $15,000 (£9,000) from the Smithsonian Institution, although the grant funded only the supplies needed to set up the project. "A typical funding cycle might run two or three years and the sponsoring agency would expect the experiment to be concluded then," explains Dr Lutz.

But this project is open-ended and has been made possible only through the co-operation and enthusiasm of unpaid researchers and land surveyors.

"I plan on monitoring this plot for the next 25 or 30 years after which I will turn the plot over to someone in the next generation of forest ecology. The value in these long term projects is only realized after 50 or even 100 years" says Dr Lutz.

Finding answers to why giant trees are dying early will be a slow process. But preserving the forest for centuries to come may be impossible without long-term projects like this.