Climate Influences Male-Female Balance in Longleaf Pines

For many reptile and fish species, temperature during egg incubation determines whether hatchlings are male or female. In the northern part of Australia’s Great Barrier Reef, scientists have discovered that 99 percent of immature green turtles hatched in warming sands are female, raising concerns about successful reproduction in the future.

longleaf-pine-catkins
Male catkins on longleaf pine before (right) and after (left) releasing pollen. Photo by University of Georgia, Bugwood.org.

U.S. Forest Service scientists have become increasingly interested in a similar idea: the connection between climate and its effects on the male-female balance in trees.

In a recent study, Southern Research Station (SRS) and Alabama A&M University researchers found that temperature changes may be related to a shift in the density of longleaf pine pollen. Their findings have implications for cone crops, seed production, and future long-term sustainability.

“Abundant evidence demonstrates that climate change affects plants in multiple ways, but some new studies have indicated that these effects could emerge in surprising ways,” says Qinfeng Guo, a research ecologist with the SRS Eastern Forest Environmental Threat Assessment Center and the study’s lead author. Research ecologist Dale Brockway coauthored the paper, which was published in Plant Ecology & Diversity.

Longleaf pine is monoecious, meaning that individual trees have male catkins and female cones. Male catkins produce large amounts of pollen that is carried by wind to fertilize the immature female cones, or conelets. Fertilized conelets eventually produce seeds.

Researchers built the study around 56 years of data collected from longleaf pines on the Escambia Experimental Forest in Alabama. In the early part of each year between 1957 and 2014, research technicians counted pollen, unfertilized conelets, and seed-bearing cone crops.

For this study, researchers paired each year of data on pollen, conelets, and cones with weather data from each previous year. They wanted to identify possible climate effects on pollen and conelet production, as well as cone crops.

They found that warmer weather resulted in greater pollen production. However, there was no increase in female conelets. With relatively fewer conelets to be fertilized, cone and seed production became more variable. In cooler years, pollen production showed a relative decrease, limiting fertilization potential.

Natural regeneration of longleaf pine is episodic due to varying seed availability, and trees don’t produce substantial seed crops until they’re about 30 years old. Plus, seeds usually take about three years to mature, which can vary due to an already complicated relationship with climate.

Similar to sex ratio shifts in Australia’s green turtles, male pollen shifts pose challenges for natural regeneration in present and future generations of longleaf pine.

gopher-tortoise
Not to be confused with Australia’s green turtles, the gopher tortoise is native to the U.S. Southeast and a keystone species of the longleaf pine ecosystem. Sustaining these ecosystems is critical for the gopher tortoise, whose burrows also provide habitat for more than 350 other species. Photo by USDA.

“The influence of climate change on sex allocation may differ between plants and animals, but the issue is better understood in animals,” says Guo. “Sex allocation has rarely been studied in monoecious wind-pollinated trees like pines, so there has been a major and critical information gap.”

Once widespread in the Southeast, longleaf pine is making a gradual comeback, thanks to significant restoration efforts by government agencies, non-governmental organizations, and private landowners. As scientists learn more about climate effects on longleaf pine reproduction, forest managers may need to step in to ensure that populations can be sustained.

“Manipulating the sex allocation and pollen density of longleaf pine may have practical importance for achieving management objectives related to species conservation or stand regeneration — particularly under the potential threat of rising temperatures,” says Guo.

Read the full text of the article.

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

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