Floodplain Forests Drying as Water Levels Decline in the Apalachicola River

Forests of the Apalachicola River floodplain have changed in recent decades to a drier mix of species. They have also lost 4.3 million trees, a decrease of 17 percent from 1976 to 2004. These are among the findings of a report released by the U.S. Geological Survey (USGS) that indicate an overall loss of swamps in the largest river floodplain in Florida.

A loss of swamps would decrease floodplain forest productivity and reduce the contribution of organic matter that sustains food webs in the river and Apalachicola Bay. The changes are the result of flood durations decreasing since river levels began to decline in 1954.

"We compared floodplain forests in 2004 to forests described in studies conducted in the late 1970s," said USGS botanist Melanie Darst, the lead author of the report. "The greatest changes were in tupelo-cypress swamps where tree species composition was 8.8 percent drier in 2004 than it was in 1976."

Tree density declined in swamps, according to Darst, overall by 37 percent. Water tupelo, the most dominant tree in swamps, decreased in density by 20 percent. Ogeechee tupelo, the primary source of tupelo honey in north Florida, decreased in density by 44 percent.

"We also compared different size-classes of trees to each other on the same plots," said Darst. "Smaller and usually younger trees were a significantly drier mix of species than the larger, older canopy trees on the same plots in 2004. As the younger trees replace the older canopy trees in coming decades, the shift toward drier species composition will continue. We expect that floodplain forests will be at least 38 percent drier in composition before the end of this century."

The declines in river levels over the past 50 years were caused by erosion of the river channel after 1954 and decreased flows in spring and summer months since the 1970s. Water-level declines were documented in a report released by the USGS in 2006 (http://pubs.water.usgs.gov/sir2006-5173).

Spring and summer are the most critical time of the year for wetland tree growth, fish reproduction, and many other important biological processes, said retired USGS scientist Helen Light, the second author of the report.

Unfortunately, spring and summer are also the time when cities and farms in the upstream basin have the greatest water demands.

This study included the non-tidal portion of the Apalachicola River from the Georgia-Florida line downstream to the beginning of the tidal reach about 20 miles upstream from Apalachicola Bay. The 82,000-acre study area is part of the Apalachicola-Chattahoochee-Flint River Basin, which covers one-quarter of the state of Georgia, extends from headwaters in northern Georgia through southwestern Georgia and eastern Alabama to the Florida panhandle, discharging into the Gulf of Mexico at the town of Apalachicola.

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