Prolonged flooding decreased stem density, tree size and shifted composition towards clonal species in a central Florida hardwood swamp

Abstract

Human modifications of the landscape often cause changes in regional surface and subsurface water flow that in turn can result in alterations in the natural cycle of flooding and drawdown in wetland ecosystems. In the present study, excessive flooding resulting from altered land-use activities increased mortality in wetland tree communities. We investigated tree decline status, tree size, species composition, and capacity for clonal propagation along an elevation/hydrologic gradient to understand community structure and composition dynamics resulting from excessive flooding. Prior to disturbance, the density of small stems (2 cm≤ DBH<10 cm) doubled with a decrease of <1 m elevation along the transect while density of other tree sizes ( DBH≥10 cm) remained relatively constant at all elevations. This increase in small stems corresponded with an increase in the number of individuals growing clonally. Tree communities at the highest elevations were dominated by less flood-tolerant species, such as Quercus laurifolia and Liquidambar styraciflua, whereas tree communities at lower elevations were dominated by highly flood-tolerant species such as Nyssa sylvatica var. biflora and Fraxinus caroliniana. Prolonged flooding was causing tree communities at low elevations to shift towards a more shrubby structure and composition dominated by more flood-tolerant species that could regenerate clonally. Among most species, trees greater than 10 cm DBH were generally more sensitive to flooding than smaller trees at high levels of flood disturbance and the ability to reproduce clonally was found to be an important mechanism allowing species such as F. caroliniana and Acer rubrum to persist under near permanently flooded conditions. The mechanism of clonal propagation for surviving severe flooding events may be important for wetland forest communities throughout the southeastern US.