Vegetation Responses to 35 and 55 Years of Native Ungulate Grazing in Shrubsteppe Communities

Abstract

Ungulate populations are managed in shrubsteppe ecosystems around the world, but relatively few long-term datasets are available that test the impacts of these grazers on shrubsteppe structure and function. This study evaluated 8 exclosures in 4 shrubsteppe communities to determine the effects of deer and elk over 35 or 55 years on (1) plant biomass and species composition, (2) soil nitrogen (N) mineralization and net nitrification rates, Olsen extractable phosphorus (P), and C:N ratios, and (3) arthropod diversity and abundance. The site with deer, the highest ungulate densities, and coldest climate (Sinlahekin Wildlife Area, WA) had greater vegetative biomass and cover inside the exclosures than outside. The sites with elk, moderate ungulate densities, and intermediate climatic conditions (Oak Creek and Wenas Wildlife Areas, WA) had no effects of exclosures on vegetative biomass or cover. The site with elk, the lowest ungulate densities, and driest climatic conditions (L.T. Murray Wildlife Area, WA) had greater shrub cover outside of the exclosures. Ungulates significantly increased net N mineralization and nitrification rates, providing a potential explanation for compensatory growth by grazed plants. Our data suggest that arthropod herbivory does not substitute for ungulate herbivory. To the contrary, arthropod diversity reflected ungulate-induced changes in plant biomass. In addition to effects on standing biomass, ungulates were associated with increased exotic plant species richness at all 4 sites. Our results suggest that there may be a balance between indirect positive and direct negative effects of herbivory on plants that varies with ungulate densities and site characteristics, and that arthropod herbivory does not functionally replace ungulate herbivory.

Impacts from Winter-Early Spring Elk Grazing in Foothills Rough Fescue Grassland

“…We conclude that long-term elk grazing during winter-early spring degraded the vegetation, ground cover, and soils of this foothills rough fescue grassland. Our results add to the small but growing body of literature that documents wild ungulate overgrazing effects on soils and herbaceous vegetation in western North America (Zeigenfuss et al 2002, Best and Bork 2003, Binkley et al 2003, Rexroad et al 2007, Gass and Binkley 2011.…”

An integrated perspective to explain nitrogen mineralization in grazed ecosystems

“…Peco et al, 2006). Moreover, in a number of studies, it is difficult to disentangle the impact of herbivores on N mineralization via changes in soil moisture content and via changes in vegetation quality, because soil moisture, plant quality and N mineralization were all reduced under grazing (Rexroad et al, 2007;Pei et al, 2008). Consequently, on coarse-textured soil, both classical effects of herbivores through altering resource quality (e.g.…”

Chronosequence and direct observation approaches reveal complementary community dynamics in a novel ecosystem

“…Unless otherwise noted species naming follows that of Hitchcock and Cronquist [ 29 ]. Cattle grazing is limited in the study area but mule deer grazing is common and likely to suppress woody plant growth [ 30 ]. Permission to work at the study site was provided by the local land manager for the Washington Department of Fish and Wildlife.…”