Evaluating the factors that regulate woody plant invasion into grassland is important for understanding the process of succession, for predicting potential impacts of global change on grassland ecosystem structure and function and for informing the restoration and management of grasslands. In this study we investigate factors influencing seedling establishment and spatial distribution of an invading native woody plant (Ulmus rubra Muhl.) across a heterogeneous successional grassland landscape in eastern Kansas. Using data collected from a landscape-level seedling census of an abandoned hayfield undergoing succession, we relate U. rubra stem abundance to landscape gradients in topographic position, soil texture, soil fertility, plant productivity, and proximity to seed sources. We also present results from a fertilization experiment to more directly evaluate the potential effect of grassland fertility and productivity on U. rubra invasion, independent from other potential causal factors that covary with soil fertility and plant productivity across the natural topographic gradient. In the landscape census, U. rubra stem densities were greatest in low-productivity micro-sites located at higher elevations in the landscape on hill-slopes and ridges. Highly productive micro-sites at low positions in the landscape and dominated by the introduced rhizomatous hay grass, Bromus inermis, contained few if any U. rubra stems despite close proximity to a seed source. In the fertilization experiment, fertilization increased plant biomass, but reduced U. rubra stem densities, confirming the suppressive influence of high productivity on U. rubra stems in the landscape. In control plots of the fertilization experiment, U. rubra stem density was negatively correlated with distance from seed source, a pattern that was obliterated by fertilization, illustrating the interaction of habitat productivity and dispersal limitation in regulating current patterns of establishment and distribution of invading U. rubra seedlings at the site. We suggest that current patterns of woody plant invasion in this region are very different from what likely occurred historically prior to settlement, due to major anthropogenic shifts in ecological context. This altered context includes the suppression of wildfire, legacy effects of prior agricultural activity and increased availability of woody plant propagules across the region.
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Vol. 136 • No. 3