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1 July 2016 Contingency in Defoliation and Moisture Effects on Northern Mixedgrass Prairie Composition and Diversity
Tanner S. Broadbent, Edward W. Bork, Walter D. Willms
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Little is known about how defoliation intensity and frequency alter plant community composition and diversity in northern Great Plains mixedgrass communities. We evaluated defoliation effects in combination with watering on vascular plant composition and diversity in two contrasting ecological sites, a drier upland and more mesic lowland, in the Dry Mixedgrass natural subregion of Alberta, Canada. Treatments were applied for three growing seasons (2010 through 2012, inclusive) and included defoliation regimes of high intensity at high frequency, high intensity at low frequency, low intensity at high frequency, and defoliation deferred until the end of the growing season. Moisture regimes were ambient and elevated. Defoliation rather than moisture was the primary determinant of plant composition after 3 yr, particularly in the lowland site. Watering effects on composition were more apparent in the drier upland. All growing season defoliation regimes markedly altered composition relative to the deferred control, with relatively minor differences in composition among growing season defoliation treatments, particularly in the mesic lowland site. We conclude that growing season defoliation alters mixedgrass composition by reducing canopy dominant grasses (Pascopyrum smithii and Hesperostipa comata) and releasing shorter-statured grasses and forbs, which can either increase or decrease diversity depending on site (edaphic) conditions and the relative dominance of midgrasses and shortgrasses (Koeleria macrantha and Bouteloua gracilis). Finally, increased moisture did not ameliorate defoliation effects during the growing season, suggesting compositional responses were predictable and independent of growing season defoliation regime.

© 2016 The Society for Range Management. Published by Elsevier Inc. All rights reserved.
Tanner S. Broadbent, Edward W. Bork, and Walter D. Willms "Contingency in Defoliation and Moisture Effects on Northern Mixedgrass Prairie Composition and Diversity," Rangeland Ecology and Management 69(4), 292-299, (1 July 2016).
Received: 13 January 2015; Accepted: 29 February 2016; Published: 1 July 2016

defoliation frequency
defoliation intensity
ecological site
soil moisture
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