Woody Seedling Dynamics in an East Texas Floodplain Forest

Abstract

We monitored woody plant seed deposition, seedling emergence, and the survival and growth of seedlings (i.e., plants ≤0.5 m tall regardless of age) in an East Texas river floodplain forest from 1979 through 1984. In addition, we estimated the relative importance of flooding, drought, fungal attack, herbivory, proximity to a conspecific adult, and shade in causing seedling mortality. Tree species fell into two major groups on the basis of their demographic characteristics and responses to unfavorable conditions. The first group was composed of heavy—seeded species, of which water oak (Quercus nigra) was the primary example. They produced few seeds, but had high seedling survival. Seedlings of these species emerged late in the summer, thereby avoiding peak periods of flooding and damping—off mortality. Seedling survival was little affected by drought, herbivory, or proximity to a conspecific adult. The second group included most of the common tree species (e.g., ironwood, Carpinus caroliniana; sweetgum, Liquidambar styraciflua; red maple, Acer rubrum; American elm, Ulmus americana). They produced large crops of light seeds which dispersed throughout the study area. In these species, seedling survival was low for the 1st yr, but increased substantially thereafter. Flooding, drought, damping—off, proximity to a conspecific adult, and herbivory were important causes of 1st—yr seedling mortality. for these species was not constant over the growing season, but was concentrated in peaks associated with particular events (e.g., a drought in 1980, and flooding and damping—off in 1981 and 1982). Seedlings emerging earlier in the spring were usually better able to survive these periods of environmental stress. During the course of the study, extensive flooding in 1979 and during 1983—1984 resulted in increases in the proportion of water oak in the seedling layer, while periods of reduced flooding during 1980—1982 allowed several of the more prolific, lighter seeded species (especially ironwood, sweetgum, and deciduous holly, Ilex decidua) to increase in importance. The two spring—dispersing, light—seeded species, red maple and elm, emerged late in the spring and therefore increased in relative abundance during 1984 when early floods killed seedlings of earlier emerging, autumn—dispersing species. Thus, spatial and temporal variation in understory seedling composition in our study was due largely to (1) species differences in the ability to increase rapidly in numbers during favorable periods as compared to the ability to survive stressful periods, and (2) the interaction of emergence phenology with the occurrence of environmental stress. These results demonstrate that variation in flooding or other environmental stresses can alter seedling layer species composition and may thereby influence opportunities for canopy replacement, thus helping to maintain species diversity in southern floodplain forests.