First-year tree seedling dynamics in floodplain forests of the Lower Mississippi Alluvial Valley

Complex dynamics within the regeneration process limits floodplain forest tree species composition. These complexities include how the stages of regeneration (i.e., seed production, dispersal, germination, establishment, and survival) interact with components of the floodplain hydroperiod (e.g., flood duration, water level, timing, frequency), which historically created both regeneration opportunities and limitations through time and space, resulting in a wide array of species composition patterns in early regeneration stages. We examined the relationship between first-year tree seedling emergence and flood patterns across four floodplain sites within the Lower Mississippi Alluvial Valley (LMAV). We established eight water-level monitoring stations to measure flood depth and duration at each site, and surveyed seedlings every two weeks across 21 plots (20 m x 20 m). We documented a total of 15,832 first-year seedlings, representing 17 species and two genera across the three growing seasons. We found that both the time of year and the timing of flood recession were important predictors of species-specific emergence. Lighter-seeded species’ (e.g., Ulmus) probability of emergence was higher immediately following a flood event (0–50 d) as compared to later dates (>50 d). Only one of the three heavier-seeded Quercus spp. showed a significant positive response to flood recession. Water level also affected emergence probability with lighter-seeded species, such as Ulmus spp. and Fraxinus pennsylvanica, showing a more immediate positive response to water at or near the soil’s surface. Overall, this study demonstrates that flooding can obstruct the germination window of a given species by reducing or eliminating its emergence probability, or it can improve emergence probability through properly timed flood recessions. Dynamic flood conditions across space and time likely contribute to increased diversity of first-year seedling composition in the LMAV. However, disruption of historic flood patterns and drying of floodplains may trigger significant changes in the seedling layer that have long-lasting effects on the floodplain forest species composition.