Degree

Doctor of Philosophy (PhD)

Department

Renewable Natural Resources

Document Type

Dissertation

Abstract

Floodplain forest species diversity is driven, in part, by variation in disturbance regime. Flood patterns create heterogeneity in microsite quality from small differences in elevation across a floodplain which, in turn, influence flood timing and duration. Differences in species’ regeneration niches in relation to hydrologic patterns can account for long-term coexistence of various species. In the past century floodplain forests have exhibited a wide range of changes in stand development and species composition as a result of altered hydrology in rivers and floodplains. I evaluated the role of regeneration in floodplain forest systems of the Lower Mississippi River Alluvial Valley to gain insight into the mechanisms behind compositional transitions. Specifically, I focused on how flood timing related to species-specific germination and first-year seedling survival patterns and processes.

Through a controlled greenhouse experiment, I tested the effects of flood timing and duration on first-year seedlings of floodplain forest species. Results showed the effects of complete submergence on first-year seedling survival is affected by the age of the seedling at the time of flooding, and the duration of flooding. Moreover, species displayed specific responses to time of flood onset and flood duration that varied by their post-germination age. The onset of a spring flood event post-germination is therefore likely to have variable effects on woody seedling composition depending on its timing relative to germination. Through examining seedling dynamics at four floodplain locations in the Lower Mississippi River Alluvial Valley (LMAV), I found first-year seedling species composition was significantly influenced by floodplain hydrology through relationships between the timing of flooding and species-specific germination and first-year seedling survival processes. Species generally considered flood-tolerant at maturity tended to demonstrate later germination windows (May and beyond) while many (not all) species considered less-flood tolerant at maturity emerged early in the spring (March-May). Flood duration preceding a seed’s germination window inhibited germination when it fully overlapped with a species’ germination window. Flooding also benefited germination when it receded prior to the close of species’ germination windows, likely through improved soil moisture availability, but possibly through other mechanisms such as nutrient supplementation. Additionally, post-germination flood timing significantly influenced seedling composition through the event of a second flood which induced seedling mortality. Spatial and temporal variation in the timing of flood recession would diversify species’ opportunities to benefit from flooding in the LMAV because of the distinct emergence windows demonstrated by species in this study. This would ultimately lead to diversification in seedling species composition in both space and time. A temporally and spatially dynamic annual flood regime would likely promote greater diversity in seedling species composition through differentiation in species’ germination and seedling survival processes.

Date

7-8-2020

Committee Chair

King, Sammy

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