Sapling growth and recruitment as affected by flooding and canopy gap formation in a river floodplain forest in southeast Texas
Hall, Rosine Blount Wilson
Harcombe, Paul A.
Doctor of Philosophy thesis
I used sapling population data collected since 1980 to investigate how canopy disturbance interacts with chronic understory disturbance to determine regeneration patterns. I reconstructed flooding history using a combination of river gauging, a local water-level recorder, and contour maps. 1979 was an extreme flood year for this site, as was 1989. Reduction in flooding frequency since dam construction in 1965 was significant for all elevations, while reduction in flooding duration was significant only for the upper half of the site. Using constrained ordination, I showed that sapling occurrence varies primarily along a flooding/soil moisture gradient, and secondarily along a canopy-openness gradient. This confirms that both flooding and light influence local variation in species composition. Small sapling density increased by more than five times during the decade, while large sapling density increased over 70%. I suggest that this increase is related to the decline in frequency and duration of flooding, and specifically, to the pattern of flooding from 1979-1989, characterized by severe flooding in 1979 and 1989. Sapling survivorship also responded to temporal variation in flooding over the decade. A damage survey revealed a size component to flooding damage, with small individuals making up a disproportionate share of those in the highest damage classes. In addition, there is a relationship between river flow and the elevation of saplings that died, suggesting that the effects of elevation are not simply related to flooding, but perhaps to soil moisture as well. Analyses of sapling growth suggest that there is a trade-off between tolerating flooding in wet years and being subject to moisture stress in dry years. I presented evidence that canopy gaps play an important role in determining sapling growth. Gaps appear to be crucial to the continued success of large saplings, and the fastest-growing individuals are found in them. This may also confer a measure of flood-tolerance related to sapling size.
Biology, Ecology; Agriculture, Forestry and Wildlife; Agriculture; Biology