PATTERNS OF SEEDFALL AND SEEDLING ESTABLISHMENT IN AN EAST TEXAS RIVER FLOODPLAIN FOREST: THE CONTRIBUTION OF SEEDLING DYNAMICS TO SPECIES COEXISTENCE (PLANT DEMOGRAPHY, DISPERSAL, TREE REGENERATION, SURVIVORSHIP, MORTALITY)
STRENG, DONNA RAE
Doctor of Philosophy
I investigated spatial and temporal patterns of seed deposition, seedling appearance, survivorship, and growth in an East Texas river floodplain forest from 1980 through 1984, and estimated the importance of several factors including flooding, drought, shading, insect herbivory, and fungal attack in causing seedling mortality. Tree species fell into two groups on the basis of their demographic characteristics and responses to unfavorable conditions. For heavy-seeded species, of which water oak was the primary example, few seeds were produced, but seedling survival was high. Seedlings appeared late in the summer, thereby avoiding peak periods of flooding and damping-off mortality. Seedling survival was little affected by drought, insect herbivory, or proximity to a conspecific adult. Most of the common tree species were light-seeded, produced large seed crops, and saturated the study site with seeds due either to long dispersal distances (e.g. red maple, sweetgum, and elm) or high adult abundance (e.g. ironwood). However, these species often survived poorly as seedlings. Flooding, drought, damping-off, proximity to a conspecific adult, insect herbivory, and shade were important causes of mortality. Mortality for the light-seeded species was not constant, 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 germinating earlier in the spring were better able to survive these periods of stress. Conditions unfavorable to the survival of the light-seeded species resulted in increases in the proportion of water oak in the seedling layer, while favorable conditions allowed the more prolific, light-seeded species to increase in importance. I hypothesized that spatial and temporal variation in seedling composition leads to differential species success in canopy openings by favoring species currently important as seedlings. As a test of this hypothesis, seedling survival and abundance in the understory were compared to sapling abundance in nearby canopy gaps. Results for both sweetgum and water oak support the hypothesis that seedling abundance prior to gap formation is an important determinant of successful canopy recruitment. However, for red maple, success in gaps appeared more closely related to proximity of a seed source rather than advance regeneration of seedlings.