Expansion of Woody Plants in Tallgrass Prairie: A Fifteen-year Study of Fire and Fire-grazing Interactions

Abstract Temporal changes in the abundance of trees and a common shrub, Cornus drummondii, were quantified for 15 y (1981–1996) in seven tallgrass prairie watersheds in Northeast Kansas. Woody plant responses to different fire frequencies and grazing were assessed with a data set that included >9000 individuals. Although 15 tree species were included in this data set, only four (Juniperus virginiana, Celtis occidentalis, Gleditsia triacanthos and Ulmus americana) were sufficiently abundant for detailed analysis. Over the 15 y study tree density increased by two- to 10-fold, except in watersheds burned annually where woody plants remained almost completely absent throughout the study. Although increased woody plant abundance was expected in watersheds protected from fire, tree and shrub density also increased substantially in watersheds burned only once in 4 y. An intermediate fire frequency (burned every 3–5 y) actually increased the abundance of C. drummondii relative to a low fire frequency (burned only...