Results and Discussion - The Effect of Drought on Carolina Bay Plant Communities: Implications for Bay Vegetation Dynamics

The descriptive model portrays four vegetation types that the bays contain based on hydrology (Fig. 4 can be viewed by going following this link.). These vegetation types can be related to specific periods within a bay's hydrograph (Fig. 5), though an individual bay need not (and likely won't) exhibit all composition types as climate cycles. Given ample water, bay vegetation may be comprised of an open water zone dominated by floating and submersed aquatic species, an emergent zone of sedges and rushes, and an upland margin zone of grasses and forbs. Bays that do not hold water for any appreciable time or those impacted by drought lose the distinct zones of open water and emergent vegetation, yet the wettest area may retain numerous sedges and rushes. Further, tree and shrub seedlings become established in the drier upland margin, in addition to the grasses and forbs. Cycles of precipitation and drought may result in repeated shifting between these two states. Following extended drought or hydrologic disturbance, a bay may lose essentially all hydrophytic herbaceous vegetation and become dominated by trees and shrubs and a variety of grasses. After sufficient time, these bays may retain their woody character even once a wetter hydrologic regime returns. However, fire, by removing woody species, may allow the reappearance of a more herbaceous composition. Drainage may result in a totally forested basin, but such a complete loss of wetland hydrology and vegetation should not occur under natural circumstances and such former bays were not included in the sampling.

Fig. 5. A characteristic bay hydrograph and the regimes that are associated with specific vegetation types. Colored segments correspond to the bay vegetation types in the diagram above. The line at 0.5 m represents the level at which ponding beyond the deepest point of the bay begins. Thus calculations were based on this level. The period just prior to the greatest water depth was influenced by an El niño event. Drought conditions began in June 1998.

During the 45 months prior to the 1998 drought, four bays held >0.5 m of water at their deepest point over 85% of the period, six bays held >0.5 m of water 44-64% of the period and three bays held >0.5 m of water 33% or less of the period (Table 1). Thus there were three distinct hydrologic groups for these bays. These three groups remained essentially intact throughout the drought, with the wettest group holding >0.5 m of water for 41-63% of the 32 months of drought, intermediate bays flooded 15-30% of that time, and four bays ponded less than 3% of that time (Table 1). The wettest bays, even during the period of most severe drought in the summer of 2002, retained hydrophytic sedges and rushes, although submersed and floating aquatics had disappeared (Table 1). By 2002, bays with an intermediate hydrology had accumulated a variety of grasses, as well as seedlings of a few tree and shrub species; the driest bays contained fewer grasses yet more tree and shrub species (Table 1). Based on its hydrology, one bay changed from an intermediate to dry designation during the drought; the vegetation of this bay reflected the change and it became dominated by grasses and relatively large pines.

Table 1. Three hydrologic groupings of study bays based on the percent of pre-drought and drought months during which the deepest point in a basin held >0.5 m of water. General vegetation types associated with those groupings by 2002 and examples of specific species are also presented.