Trees on the Move: Blackgum Migrates North

Blackgum is a medium-sized, deciduous tree native to the eastern U.S. Its range has shifted northward over the past few decades.  Photo courtesy of the Kentucky Division of Forestry.
Blackgum is a medium-sized, deciduous tree native to the eastern U.S. Its range has shifted northward over the past few decades. Photo courtesy of the Kentucky Division of Forestry.

As the climate warms, animals and plants alike are expected to migrate northward. Forest trees that cannot disperse their seeds over long distances may not be able to keep up with the changes in climate, but some trees such as blackgum have a ready means of migration. In lieu of feet, blackgum migrates by wing: birds eat and disperse the seeds. “This is one of the reasons we studied blackgum, rather than a different tree species,” says U.S. Forest Service Southern Research Station (SRS) scientist Christopher Oswalt.

Oswalt, a research forester at the SRS Forest Inventory and Analysis (FIA) unit, and his colleagues are among the few researchers to empirically study regional shifts in forest tree distribution. Johanna Desprez, a graduate research assistant at Purdue University, led the study, which was recently published in Climatic Change.

The researchers looked for changes in the number, size, and age of blackgum trees growing in forests across 37 states. The study area spanned four ecoregions: Central Hardwood, Forest-Prairie Transition, Northern Hardwood, and the Southern Pine-Hardwood Regions, and scientists used county-level FIA data to compare blackgum populations from the 1980s to those in 2007.

“Our analyses revealed that the native range of blackgum appears to be shifting northward,” says Oswalt. Along the northern edge of its range, younger blackgum trees have become far more numerous, and mature blackgum trees have become more dominant. The researchers also detected northward expansion by comparing the historic blackgum range to the areas where it was found in the more recent time periods. Conversely, blackgum populations in the South had declined significantly. “Overall, we found that in the South, blackgum density is dropping and recruitment continues to decline,” says Oswalt.

Changes in blackgum’s range and health are probably not related to disturbance, since blackgum is resilient to short-term disturbances such as drought, fire, flood, wind, and hurricanes. And because blackgum is not a valuable timber species, and has been a stable (although minor) member of the forest community for at least 300 to 400 years, timber management practices and competition with other trees do not explain changes in the distribution of the species.

“We expected to see a response to the changes in climate that have occurred over the past decades,” says Oswalt. The northward shift in blackgum’s range is consistent with what scientists expect from the changes in temperature and precipitation over the past several decades.

“Some research suggests that some tree species will not be able to keep up with future changes in temperature and precipitation,” says Oswalt. “Our study suggests forest trees that thrive under a variety of environmental conditions and also have a way to disperse seeds over long distances may be able to migrate fast enough to keep up.”

Read the full text of the article.

For more information, email Christopher Oswalt at coswalt@fs.fed.us.

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