Climate change is fueling an east-west divide in forest seed production

Feb. 23 (UPI) -- Across North America, seed production patterns in eastern and western forests are responding differently to climate change, new research suggests.

According to a new study, published Tuesday in the journal Nature Communications, climate change has fueled an increase in seed production across eastern forests, while seed production has steadily declined in western forests.

The climate-driven divide can be explained by the composition of forests east and west of the Mississippi. Trees in eastern forests are relatively young, whereas trees in western forests tend to be older.

"[The divide] could dramatically alter the composition and structure of 21st century North American forests," lead study author James S. Clark, professor of environmental science at Duke University, said in a news release.

By better understanding how different types of forests are affected by climate change, authors of the new study say forest managers and conservationists will be able to better protect North America's forests.

Scientists were able to identify the east-west divide by surveying changes in fecundity, a measure of trees' ability to regenerate at scale -- specifically, by distributing seeds to places where the trees are likely to find a competitive advantage.

Measuring fecundity is difficult, because the ecological process is highly variable. Age, size and growth rate, as well as access to light, water and nutrients, can all influence fecundity.

Climate trends also affect fecundity, as climate change influences tree growth rates, but the climate's impact on tree growth depends on the size of the tree.

The mess of confounding and interrelated variables made modeling climate change's affect on fecundity difficult.

"It was the only major demographic process driving forest response to climate change that we lacked field-based estimates on," Clark said.

Clark and his colleagues tackled the problem by amalgamating raw data on size, growth, canopy spread and access to resources for some 100,000 individual trees growing in experimental forest plots across the continent.

The raw data revealed what the averaged data from various meta-analyses had missed: Fecundity increases as trees get bigger, up to a point, and then decreases.

"This explains the East-West divide. Most trees in the East are young, growing fast and entering a size class where fecundity increases, so any indirect impact from climate that spurs their growth also increases their seed production," Clark said.

"We see the opposite happening with the older, larger trees in the West. There are small and large trees in both regions, of course, but the regions differ enough in their size structure to respond in different ways," Clark said.

Clark and his colleagues are now working on taking what they've learned and applying it to models designed to predict how climate change will affect the growth and health of different forests.