02365nas a2200169   4500008004100000245010600041210006900147300001300216490000800229520174200237653002201979653001602001100001902017700002002036700001402056856012502070       2002                        eng d00aExpansion of woody plants in tallgrass prairie: a 15 year study of fire and fire-grazing interactions0 aExpansion of woody plants in tallgrass prairie a 15 year study o  a287 -2940 v1473 aTemporal 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 once in 15 y). Moreover, a severe wildfire in 1991, which affected all watersheds, did not markedly reverse this pattern of increase in abundance in most tree species. Four years after the addition of native herbivores (Bos bison) to three of the long-term experimental watersheds (infrequently and annually burned) woody plant abundance increased by four- and 40-fold, respectively, compared to corresponding ungrazed watersheds. Thus, the presence of large ungulate grazers in tallgrass prairie resulted in a significant increase in woody plant abundance. The most parsimonious explanation for this phenomenon is that fire intensity and extent was reduced in grazed grasslands allowing greater success of woody species.10atallgrass prairie10awoody plant1 aBriggs, J., M.1 aKnapp, Alan, K.1 aBrock, B.  uhttp://knz.lternet.edu/content/expansion-woody-plants-tallgrass-prairie-15-year-study-fire-and-fire-grazing-interactions