It’s Complicated: The Relationship between Climate and Seed Production in Longleaf Pine

A longleaf pine stand in the Carolina sandhills, near one of the study sites. Longleaf pine in this site produced seed on a 4-year cycle, which is a slightly longer cycle than in the other sites. Photo by Jack Culpepper, U.S. Fish and Wildlife Service, courtesy of Wikimedia Commons.
A longleaf pine stand in the Carolina sandhills, near one of the study sites. Longleaf pine in this site produced seed on a 4-year cycle, which is a slightly longer cycle than in the other sites. Photo by Jack Culpepper, U.S. Fish and Wildlife Service, courtesy of Wikimedia Commons.

The longleaf pine tree is a finicky and slow seed producer, and scientists have long suspected that fluctuations in seed production are related to climate. U.S. Forest Service research ecologist Qinfeng Guo and colleagues recently found evidence of a complicated relationship between seed production and climate.

A long-term dataset that spans 10 sites and six states revealed that longleaf pine operates on a seed production cycle that takes roughly 3 years – sometimes 4 – to complete. Guo and his colleagues examined the cone production cycle in an earlier study, and their new study, which was recently published in Ecosystem Health and Sustainability, shows that climate influences the cycle, although its effects are quite complex.

Longleaf pine is critical to the function of ecosystems, and in areas where it persists, it is still a keystone species. The health and abundance of animals such as the endangered red-cockaded woodpecker and many plants such as wiregrass, pitcher plant, and other species are tied to the health of longleaf pine. Longleaf pine ecosystems once covered up to 90 million acres across the southeastern U.S., but over the past century the species has declined – both in terms of how many trees are present and which areas they grow in.

Managers are trying to restore longleaf pine, but efforts to naturally regenerate the species are hampered by variation in its annual seed production. However, the study reveals that longleaf pine trees have begun producing more cones – of the eight sites with data from before and after the mid-1980s, six of them showed slight to moderate increases in cone production in more recent years.

The increase in cone production coincides with greater climatic variability, especially temperature fluctuations. However, understanding the connections between cone production and climate will require further investigation.

“We did not detect a clear and consistent correlation between climate events and cone production at all the sites,” says Guo. “However, our study shows that life-history cycles and climate change can jointly affect seed production.”

Because it takes several years for longleaf pine cones to mature, significant events in the process of seed development do not neatly match with seasonal conditions. “Detecting climate’s influence on seed development is challenging,” says Guo. “There is a temporal mismatch between seasonal climate cycles and life-history events.”

Some sites in the dataset have been measured for as long as 56 years, and the study illustrates the value of long-term data from multiple sites. Studies based on shorter periods of observations have suggested that longleaf pine produces cones in either a 5 to 7 year cycle or an 8 to 10 year cycle, but this study found no evidence of either pattern.

The study sites included Alabama, Florida, Georgia, Louisiana, and South Carolina. Longleaf pine has a relatively large native range, and grows in a wide variety of elevations, soil types, and climates across the Southeast. Across all sites, moderate climate conditions appear to promote cone production, and although cone production varied tremendously across the study sites, those close to each other showed similar trends in cone production.

“Viewing the entire longleaf pine native range, it is clear that the dynamics of cone production are very complex in both space and time,” says Guo. “Many interactive factors are involved in controlling cone production.”

Read the full text of the article.

For more information, email Qinfeng Guo at qguo@fs.fed.us

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