Controls over the strength and timing of fire-grazer interactions in semiarid rangeland

Authors: Augustine, David; Derner, Justin

Submitted to: Journal of Applied Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/18/2013
Publication Date: 1/21/2014
Publication URL: http://handle.nal.usda.gov/10113/58405
Citation: Augustine, D.J., Derner, J.D. 2014. Controls over the strength and timing of fire-grazer interactions in semiarid rangeland. Journal of Applied Ecology. 51:242-250.

Interpretive Summary: We studied whether prescribed burning in shortgrass rangeland of northeastern Colorado influences the foraging distribution of free-ranging cattle. We conducted a patch-burning experiment where 25% of the pastures available to cattle were burned each year. Burned patches were moved each year over four years. We used GPS collars with activity sensors to measure locations of cattle every five minutes during 28-day tracking sessions during the May – September growing season each year. Activity sensors mounted in the GPS collars were used to determine whether cattle were grazing or not grazing for each location that the collars recorded. We found that cattle spent 31% of grazing time on recent burns made up 25% of the landscape, which represented significant positive selection of the burned patches for foraging. This selection strength was only half as strong as that measured in tallgrass prairie of Oklahoma. Strong selection for burns occurred when vegetation was growing rapidly, regardless of when during the summer the greenup occurred. Outside such intervals, cattle grazing distribution was primarily influenced by topography. Thus, the relative importance of fire and topography in controlling cattle grazing distribution was dependent upon when rainfall occurred. Our results show that interactions between fire and herbivores are a consistent feature of both shortgrass and tallgrass rangelands, but the strength of the fire-grazing interaction is greater in tallgrass rangeland. Management of shortgrass rangeland to sustain ecological processes and landscape heterogeneity should include strategies that allow cattle to shift their grazing distribution seasonally in response to fire, topography, and precipitation patterns. Combined management of fire and grazing for conservation objectives can be consistent with and even complementary to livestock production goals.

Technical Abstract: The degree to which large herbivores select and forage within recently burned areas is a key driver of vegetation heterogeneity in rangeland ecosystems. Yet few studies have carefully quantified the strength and timing of herbivore selection for burns or how selection strength varies among ecosystems. We conducted a patch-burning experiment in semiarid rangeland of Colorado, USA, where 25% of the area available to cattle was burned each year. Burned patches were shifted annually over four years. We used GPS collars with activity sensors to quantify the distribution of free-ranging cattle at a high temporal resolution (5-minute intervals) during the May – September growing season each year. We used a classification tree model to discriminate between cattle grazing versus non-grazing locations, which significantly increased precision in quantifying burn selection strength. We fit generalized linear models predicting the frequency of cattle use of a given location within for each study area and month, enabling comparisons between the relative influence of burns and topography on grazing distribution. Across multiple growing seasons, cattle selectively spent 31% of grazing time on recent burns which comprised 25% of the landscape; this selection strength was half as strong as that documented in mesic rangeland. At a monthly temporal scale, strong selection for burns occurred during periods of rapid vegetation growth regardless of when during the growing season such greenup occurred. Outside such intervals, burn selection strength was inconsistent and cattle grazing distribution was primarily influenced by topography. Thus, the relative importance of fire and topography in controlling grazer distribution was temporally contingent upon the timing and size of precipitation pulses. Synthesis and Applications: Spatiotemporal interactions between fire and herbivores are a consistent feature of both semiarid and mesic rangelands, with interaction strength varying across gradients of precipitation and primary productivity. Management of semiarid ecosystems to sustain ecological processes and landscape heterogeneity should include strategies that allow ungulate herbivores to shift their grazing distribution seasonally in response to fire, topoedaphic variation, and precipitation patterns. Combined management of fire and grazing for conservation objectives can be consistent with and even complementary to livestock production goals.