Aboveground Production in Southeastern Floodplain Forests: a Test of the Subsidy-stress Hypothesis

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 transects 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 i 1 SD = 675 1 271 'g.m-2.yr-') was significantly lower than on intermediate and dry plots (P 5 0.02). There was no significant difference between intermediate and dry plots (107 i 189 and 1038 i 91) g.m-2.yr-l, 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-'. 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.cmI). 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.