Lowland savanna of southeastern Iowa: historical and modern vegetation patterns and implications for management
Lowland savannas are a rare variant of savanna, occurring on alluvial soils, which lack quantitative data on their structure, composition, and restoration. The purpose of this thesis was to assess both historical and modern vegetation composition and structure of lowland savannas and to determine the success of recent management of a lowland savanna remnant. Lowland savanna distribution, composition, structure, and possible flood driven disturbance regime were compared to those of upland savanna and floodplain forest in the Lower Cedar River Valley of southeast Iowa, U.S.A. Furthermore, the effects of fire and abiotic variables on the flora of a degraded lowland savanna remnant were evaluated to assess restoration efforts. The General Land Office survey was used to reconstruct the pre-Euro-American settlement vegetation of the study area and both surveyors' descriptions and tree density were used to determine the distribution, composition, and structure of upland savanna, lowland savanna, and floodplain forest. County GIS layers and soil surveys were used to determine disturbance regimes for these communities. In the savanna remnant, burned and unburned treatments were compared across woodland areas, as well as those dominated by invasive Phalaris arundinacea (reed canary grass). Overstory and understory vegetation composition and abundance, elevation, ground level light, depth of inundation, soil texture, soil carbon, and soil nitrogen were measured in study plots and ordinated. Overall, our data suggested that lowland savanna occurred in the study area, but was compositionally similar to both upland savanna and floodplain forest. Large shade-tolerant tree species occurred historically in lowland savanna. This study did not provide direct evidence that flooding or fire maintained lowland savannas historically, however indirect evidence suggests fire was present on the landscape. In the savanna remnant, burned areas had lowered tree density, but not increased mean basal area. Plots dominated by P. arundinacea had nearly a 50% reduction in P. arundinacea cover. High abundance of P. arundinacea was associated with high soil carbon and nitrogen, but it remains unknown if regular flooding is driving this correlation. Techniques commonly used in upland savanna restoration may not be useful in restoring lowland savannas other than for invasive species control.