Responses of Plant Communities to Grazing in the Southwestern United States Department of Agriculture United States Forest Service Rocky Mountain Research Station Daniel G. Milchunas General Technical Report RMRS-GTR-169 April 2006 Milchunas, Daniel G. 2006. Responses of plant communities to grazing in the southwestern United States. Gen. Tech. Rep. RMRS-GTR-169. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. 126 p. Abstract Grazing by wild and domestic mammals can have small to large effects on plant communities, depend- ing on characteristics of the particular community and of the type and intensity of grazing. The broad objective of this report was to extensively review literature on the effects of grazing on 25 plant commu- nities of the southwestern U.S. in terms of plant species composition, aboveground primary productiv- ity, and root and soil attributes. Livestock grazing management and grazing systems are assessed, as are effects of small and large native mammals and feral species, when data are available. Emphasis is placed on the evolutionary history of grazing and productivity of the particular communities as deter- minants of response. After reviewing available studies for each community type, we compare changes in species composition with grazing among community types. Comparisons are also made between southwestern communities with a relatively short history of grazing and communities of the adjacent Great Plains with a long evolutionary history of grazing. Evidence for grazing as a factor in shifts from grasslands to shrublands is considered. An appendix outlines a new community classification system, which is followed in describing grazing impacts in prior sections. The Author Daniel G. Milchunas is Research Scientist, Forest, Range, and Watershed Stewardship Department, Colorado State University, Fort Collins. Acknowledgments Primary funding was provided by the USFS Rocky Mountain Research Station, with additional support from the Shortgrass Steppe Long-Term Ecological Research project. Curt Flather of the USDA Forest Service coordinated the overall project. Curt Flather, Pat Zwartjes, Charlie McDonald, and Deborah Finch of the Forest Service provided reviews of an early draft. Jane Bock and Rex Pieper gave construc- tive and useful review comments that improved the final version of the manuscript. I thank Lane Eskew for technical editing and Joyce Vandewater for help with graphics. Norman Ambos of the Forest Service provided unpublished data for Dutchwoman butte. Cover photo: On the 300-square-mile Jornada Experimental Range near Las Cruces, New Mexico, technicians Rob Dunlap (left) and John Smith round up cattle. Photo by Scott Bauer. You may order additional copies of this publication by sending your mail- ing information in label form through one of the following media. Please specify the publication title and series number. Fort Collins Service Center Telephone (970) 498-1392 FAX (970) 498-1122 E-mail [email protected] Web site http://www.fs.fed.us/rm Mailing address Publications Distribution Rocky Mountain Research Station 240 West Prospect Road Fort Collins, CO 80526 Rocky Mountain Research Station Natural Resources Research Center 2150 Centre Avenue, Building A Fort Collins, CO 80526 Executive Summary Two primary factors determine a plant community’s relative response to livestock grazing: the evolutionary history of large-herbivore grazing and the plant community’s aboveground primary productivity. Bison were generally not part of the evolutionary history in the southwest, except for the edge of their range (less intense grazing) in far eastern New Mexico where a smaller bodied (more selective) grazer exerted possibly different pressure than that occurring in the central Great Plains. Because aridity (low productivity) and grazing are convergent selection pressures, communities of the southwestern United States may be intermediate in their response to livestock grazing. However, grazing by domestic livestock is controversial in the southwest, and grazing has been considered a reason for shrub encroachment in many communities. This report assesses the effects of grazing on 25 plant communities of the southwestern United States, in terms of plant species composition, aboveground primary productivity, and root and soil attributes. All possible studies were considered in an attempt to 1) determine the state of knowledge for each community and suggest where research is lacking, 2) assess the relative responses among communities, 3) present an unbiased, literature-based overview of grazing and the shrub encroachment issue, and 4) broadly view grazing responses in the southwest relative to responses observed in adjacent semiarid to subhumid Great Plains communities that have a long evolutionary history of grazing by bison. Responses of individ- ual communities are summarized at the end of each section. Community response to grazing increased, following theoretical predictions, as such: unproductive/long history < unproduc- tive/short history < productive/short history < productive/long history. However, variables such as precipitation, grazing intensity, and years of protection from grazing could much better predict community response on the Great Plains than the southwestern United States. This degree of unpredictability for the southwest, together with a large difference between never- grazed geologic refuges and human-made exclosure comparisons with grazed treatments in the southwest, raises questions as to whether some livestock effects may have been historic, or whether southwestern communities require temporally rare and unpredictable environmen- tal events for change to occur. Riparian communities were particularly responsive to grazing. The response of a plant species common to both Great Plains and some southwestern com- munities was not the same in the two locations. In general, southwestern communities were more sensitive to grazing than the adjacent shortgrass steppe, but were similar in response to other semiarid/arid and subhumid communities with a short evolutionary history of grazing. Further, the temporal variability in plant community species composition due to weather ap- pears to be much greater in southwestern than in Great Plains communities. The majority of evidence from experimental literature indicates that shrub encroachment occurs into ungrazed sites as well as grazed sites. There are four possibilities that could still invoke livestock grazing as a factor in this conversion of communities: 1) previous very heavy grazing during the 1800s resulted in an alternate stable state, 2) grazing interacts with fire, 3) there are highly erodible areas where long-term studies have not been in place, where current grazing could initiate alternate stable states, and 4) grazing interacts with some other factor such as climate or rodents. The evidence for these are reviewed and found to be either weak or experimentally untestable under current conditions but having factors that weaken a supporting argument. There is an increasing amount of evidence implicating changes in the seasonal proportions of precipitation as a primary cause of community change in the south- west. However, no one factor alone appears to adequately explain the complex spatial pattern of community change through parts of this region. Intense non-selective grazing is an exogenous disturbance in most southwestern commu- nities. From a conservation standpoint, there are places where grazing by livestock should be encouraged in the absence of the native grazer, and other places where livestock grazing should be discouraged or at least cautiously managed because similar grazers were not pres- ent historically. From a production standpoint, light to moderate grazing can be sustainable in the southwest in many situations. For some communities there are little data to base conclu- sions, and for other communities there are contradictory findings. However, the fate of private lands will often closely be coupled to use of public lands. Alternatives to ranching such as development and subdivision for ranchettes or agriculture may potentially be a greater threat to the integrity of southwestern ecosystems than ranching of native communities. Contents Executive Summary ............................................................................................................ i Introduction ......................................................................................................................... 1 History of Grazing by Native and Domestic Large Herbivores ...................................... 3 The Importance of Evolutionary History and Environmental Conditions in Grazing Responses .................................................................................................. 3 Evolutionary History of Grazing in Southwestern U.S. Plant Communities ................. 4 History of Grazing by Domestic, Feral, and Exotic Animals in the Southwest United States .......................................................................................... 7 Grazing Effects on Plant Communities ............................................................................ 9 Alpine Tundra ............................................................................................................... 9 Boreal Forest ............................................................................................................. 10 Mixed Conifer Forest ................................................................................................