A new collaborative study conducted by researchers from UC Berkeley, UC Davis and the U.S. Geological Survey shows that large trees in California’s forests are on the decline, while the prevalence of smaller trees is increasing.

Led by former UC Berkeley postdoctoral fellow Patrick McIntyre, the research reveals that the decrease in large trees is due to water stress that has been happening since the 1930s. Increasing temperatures, changes in water availability and loss of carbon remain concerns for the issue of forest composition throughout California.

“There has been a shift in California in the fundamental structure of forests,” McIntyre said.

Large trees — those with diameters of more than 2 feet — store more carbon than small trees, which have diameters of fewer than 6 inches. According to McIntyre, the loss of a large tree would need 10 smaller trees to compensate.

“We are concerned because forests are such an important ecosystem for us,” said Maggi Kelly, a campus professor and cooperative extension specialist of environmental science, policy and management who also contributed to the research. “They help provide clean water.”

Kelly, whose contribution in the research included digitizing the historical data, said that in addition to looking at changes in California’s landscape over the last 70 to 80 years, the goal was also to work successfully with historical data — an opportunity that allowed the team to work with photographs, hand-drawn maps and information collected by Albert Wieslander, who graduated from UC Berkeley in 1914.

Barbara Allen-Diaz, a campus professor in the division of agriculture and natural resources, rescued and sheltered the data at her lab on campus before the research team received enough funding to digitize the data. Kelly and her team digitized the data plots by transferring them from handwritten cards into a database and inputting the information into a geographical mapping system that would show where all the plots were on a computer.

“This collection is in particular a really rich one, and there’s a lot of work in ecology that uses similar kinds of historical data when we can find them,” Kelly said, adding that her team has been working with the same historical data for more than a decade. “This collection was so detailed and so comprehensive; that was the exciting thing.”

Researchers will continue to work with the historical data set — which they have made available for download — to find out what might be in store for climate change. They also encourage interested students and researchers to download the data and apply their particular questions to further investigation.

“This is the first real comprehensive comparison between the old data and the new data, but there’s way more to be done,” Kelly said. “We want to help validate some of things we’re seeing here, to look at how California has changed.”