Climate influences trigger an east-west division in forest seed production

Mature western forests, such as this mixed coniferous stand in California’s Sequoia National Park, may be less capable than younger forests in the east of being replanted and regenerated after major climate change-related extinctions, a new study conducted by Duke University has found. Credit: USGS

Younger, smaller trees that make up most of North America’s eastern forests have increased seed production under climate change, but older, larger trees that dominate forests in much of the West have reacted less, a new study led by Duke University reveals.

Declining seed production or fertility of these trees could limit the ability of western forests to regenerate after the large extinctions associated with rising temperatures and intensified droughts that are now occurring in many states and provinces.

This continental division, first published in a new study, “could dramatically change the composition and structure of 21st century North American forests,” said James S. Clark, a distinguished professor of environmental science at Nicholas in Duke, who led the research.

Knowing the adverse reactions – and understanding why they occur – will help scientists more accurately predict future changes in North American forests and develop conservation and management strategies to mitigate the changes, he said.

Researchers from 48 institutions collaborated with Clark on a peer-reviewed study that appears Feb. 23 Nature Communications.

Fertility is a measure of the ability of trees to regenerate after extinction and other major disturbances by spreading seeds to habitats where their chances of future survival are more favorable. This is an important factor in determining future forest responses to climate change, but like many environmental processes it is noisy, highly variable, and incredibly difficult to estimate.

Fertility changes over time based on changes in tree size, growth rate or access to light, water and other resources, and is driven by two indirect climate impacts – climate-dependent growth effects and tree-dependent climate impacts – which are not currently taken taken into account in the models used to predict future changes.

“It was the only major demographic process that triggered a forest response to climate change and for which we did not lack field assessments,” Clark said.

To address this problem, he devised new statistical software that allowed him to synthesize decades of raw data on size, growth, canopy distribution, and resource access for nearly 100,000 individual trees at long-term research sites and experimental forests across North America. Unfiltered raw data revealed what previous meta-analyzes based on average measurements missed: On the continental scale, fertility increases as the tree grows, to the point. And then it starts to decline.

“This explains the East-West division. Most trees in the East are young, growing fast and entering a size class where fertility increases, so any indirect climate impact that encourages their growth also increases their seed production,” Clark said. “We see the opposite happening with older, larger trees in the West. Of course, there are small and large trees in both regions, but the regions differ enough in size structure to be able to respond in different ways.

“Now that we understand together how it all works, the next step is to apply it to individual species or stands and incorporate it into the models we use to predict future forest changes,” he said.

The data used in the study came from trees in the Mast Monitoring and Forecasting Network (MASTIF), which includes more than 500 sites of long-term field research nationwide, including plots that are also part of the National Environmental Observation Network (NEON).

An increasing drought threatens almost all American forests

More information:
“Fertility of trees across the continent driven by indirect climatic effects,” Nature Communications (2021). DOI: 10.1038 / s41467-020-20836-3

Provided by Duke University School of Nursing

Citation: Climate impacts trigger east-west division in forest seed production (2021, 23 February) retrieved 23 February 2021 from .html

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