Page last updated at 23:30 GMT, Monday, 20 July 2009 00:30 UK

Mapping America's giant trees

By Peter Bowes
BBC News, Yosemite National Park


How to track the life of a tree

Scientists in California have set up a unique 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. The park is best known for its breathtaking waterfalls, black bears and ancient giant sequoias, which are part of the redwood family of trees.

We love the land, we love nature but we build roads that lead to developments
John Knox
Land surveyor

Large trees 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 impact of a vibrant forest is also felt much wider afield.

"Forests provide a lot of ecosystem services for us, whether we live in the city or whether we live in the forest," explains Dr Lutz.

"Certainly here in California most of the water comes from the snowpack, it comes from the mountainous forests such as the one that we're in. And were that forest to be converted to a different vegetation type, perhaps there would be less snow - perhaps it would affect the water quality."

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.

Trees in Yosemite National Park
There an estimated 30,000 trees in the area

"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.

"What we want to do is identify as soon as possible 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.

Measuring the trees in Yosemite
People involved in the project see it as a long-term commitment

"I did not want to pass up the opportunity to get involved in this," says John Knox, a land surveyor from Southern California, who volunteered his services for the project.

"It's the paradox that we live with. We love the land, we love nature but we build roads that lead to developments. We lay out the destruction of the environment," he explains.

"This is a nice opportunity to lay out something for conservation and nature studies. The long-term nature of the research means that the management of the project will change hands over the decades.

"No one researcher can see the ultimate results of the work," says Dr Lutz.

"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."

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.

"It's a sense of fulfilment," says James Freund, a researcher on the project.

"You know that there's a bigger picture and that you're starting something, you're becoming a really positive part of history. It's rewarding and fulfilling knowing that people far into the future are going to come back to what we have started here."

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