SREL Reprint #2157

Aboveground production in southeastern floodplain forests: a test of the subsidy-stress hypothesis

J. Patrick Megonigal1, William H. Conner2, Steven Kroeger3, and Rebecca R. Sharitz4

1Botany Department, Duke University, Durham, North Carolina 27708 USA
2Baruch Forest Science Institute, Box 596, Georgetown, South Carolina 29442 USA
33105 Ruffin Street, Raleigh, North Carolina 27607 USA
4Savannah River Ecology Laboratory, Drawer E, Aiken, South Carolina 29802 USA

Abstract: It has been hypothesized that periodically flooded forests have higher rates of aboveground net primary production than upland forests and near-continuously flooded forests, but a competing hypothesis holds that the benefits of periodic inputs of nutrients and water may be diminished by stresses associated with anaerobic soils or drought. To test these hypotheses, we measured groundwater table depths and aboveground productivity in floodplain forests of South Carolina and Louisiana. We established paired plots on locally dry, intermediate, and wet topographic positions across three hydrologic transacts in each state. These plots encompassed upland hardwood, bottomland hardwood, and cypress swamp forests. Measurements of leaf litterfall, wood production, and groundwater table depth were made in 1987 and 1988. We then used mean growing-season water depth (MWD) to group the plots into three classes: wet (>0 cm), intermediate (0 to -60 cm), and dry (<-60 cm).
Aboveground net primary production (NPP) on wet plots (2-yr mean ± 1 SD 675 ± 271 g·m-2·yr-1) was significantly lower than on intermediate and dry plots (P ≤ 0.02). There was no significant difference between intermediate and dry plots (107 ± 189 and 1038 ± 91) g·m-2·yr-1, respectively). In addition, aboveground NPP on intermediate plots was not significantly different from 22 temperate upland forests in the literature.
Combining our data with data from the literature, we found that aboveground NPP on wet plots was negatively related to MWD with a slope of -5 g·m-2·yr-1·cm -1. On sites with evidence of hydrologic disturbance (>25% dead stems) the slope of this line was 5 times greater (-24 g·m-2·yr-1·cm -1).
We conclude that the subsidy-stress hypothesis does not adequately describe patterns of NPP across Southeastern U.S. floodplain forests. Conditions of periodic flooding and flowing water do not often lead to high rates of productivity compared with upland forests. However, extensive flooding is nearly always a significant stress on forest productivity, particularly when the flooding regime has been recently perturbed through levee construction or impoundment. Our data support a more complex interaction between subsidy and stress factors.

Key words: flooding regime and net primary production; flooding stress and floodplain forest production; floodplain forests, southeastern United States; hydrologic disturbance and net primary production; hydrologic disturbance in floodplain forests; Louisiana, USA; net primary production in bottomland hardwood forest; South Carolina, USA

SREL Reprint #2157 

Megonigal, J.P., W.H. Conner, S. Kroeger, and R.R. Sharitz. 1997. Aboveground production in southeastern floodplain forests: A test of the subsidy-stress hypothesis. Ecology 78:370-384.

This information was provided by the University of Georgia's Savannah River Ecology Laboratory (srel.uga.edu).