Author: Molly Van Appledorn

Publisher:

Published: 2016

Total Pages: 564

ISBN-13:

Flooding dynamics are expected to drive compositional patterns of woody overstory species in bottomland ecosystems, yet there is some evidence that flood variability may differentially constrain assemblage composition. Numerous studies have related distributions of plant species to floodplain landforms and geomorphic processes, documenting shifts in community composition along hydrologic and hydraulic gradients. Despite clear associations between species composition and local physical conditions, models predicting composition over broad areas are underdeveloped due to imprecise or implicit characterizations of environmental gradients, difficulty of reconciling fine-scale environmental heterogeneity with broader physical constraints, and overcoming cross-site differences in species' pools. My dissertation examines the relationships between environmental gradients and species' abiotic tolerances as characterized by functional traits in order to understand: 1) what processes constrain or support functional diversity of floodplain forests, and 2) at what scales these processes operate. First, in Chapter 2 I use a regional approach to characterize broad-scale patterns in hydrologic and functional trait diversity of floodplain forest ecosystems from Michigan's Lower Peninsula and Maryland. The analysis tests the hypothesis that if flooding dynamics impose a strong environmental filtering effect as demonstrated in the literature, regional differences in flood frequency, duration and intensity should result in different regional trait pools. I find that regional shifts in hydrologic regime are not matched by similar shifts in functional trait pools, suggesting that functional diversity is maintained through processes operating at finer scales. In Chapter 3 I evaluate the relative strength of environmental filtering, a community assembly process, at the river-valley segment scale by analyzing distributions of functional traits across implicit, regional gradients of inundation dynamics found throughout Michigan's Lower Peninsula. I find evidence of important biotic interactions in certain hydrogeomorphic settings that would indicate a highly variable role of inundation dynamics in shaping floodplain forest assemblages. In Chapter 4 I then build on the findings of the previous chapter to describe how patterns of environmental heterogeneity and flood regime interact across scales affect functional diversity. The results demonstrate the importance of hydrogeomorphic context in shaping patterns of functional composition across floodplain landforms. The final two chapters investigate how ecological strategies of woody overstory species shift under multi-scalar physical constraints within the Maryland Piedmont. In Chapter 5 I characterize flood regime of four Piedmont river-valley segments of contrasting size and morphologies using a validated 2D hydrodynamic model. I demonstrate the novel application of a 3D array of temperature sensors for validating spatially-explicit flood models, and summarize model results to show that patterns of inundation dynamics observed both within and among the four study sites are consistent with river-valley morphologic characteristics. Finally, I relate spatially-explicit flood regime quantifications to systematic surveys of floodplain forest composition to understand how functional attributes of assemblages shift along gradients of hydroperiod at and across multiple spatial scales. The results of Chapter 6 underscore the complexity of biophysical interactions in floodplain ecosystems as evidenced by shifts in functional trait distributions along gradients of aggregate flood regime at both the within- and among-site levels. Overall, my dissertation research has contributed to the advancement of floodplain forest ecology by describing trait-environment linkages at and across multiple spatial scales, demonstrated several novel methodological approaches, and offered new conceptual models of flooding and flood constraint on functional responses in bottomland ecosystems.