On the strength and timing of fire-grazing interactions in grassland ecosystems
Spatiotemporal interactions between fire and grazing are known to be a key process generating habitat heterogeneity and sustaining biodiversity in mesic grasslands. However, the strength and role of fire-grazing interactions in semiarid, low-productivity grasslands are poorly understood. We studied patch burn grazing in the semiarid shortgrass steppe of northeastern Colorado, comparing unburned pastures with pastures where 25% of the area was burned in the fall of each year. For each of 4 years with varying precipitation amounts and timing, we tracked cattle responses to the patch burns using GPS collars. We used direct observations of cattle foraging bouts to develop a model predicting periods of cattle activity versus inactivity based on GPS collar data. We evaluated the degree to which grazing cattle select burned versus unburned portions of pastures, and whether such models improve predictions compared to analyses based on all animal locations.
Grazing cattle selected burned patches during periods when vegetation was growing rapidly (indexed by increasing NDVI), whereas cattle grazing distribution was more strongly controlled by topography when vegetation greenness was stable or declining. In 3 of 4 years, cattle weight gains during the May-Oct grazing season were unaffected by patch burning; in the fourth year, patch burn significantly increased the rate of cattle weight gain. Our findings show that fire does significantly influence the spatiotemporal pattern of grazing in semiarid grasslands, but this interaction is substantially weaker than in mesic grasslands and is contingent upon the timing of precipitation pulses and associated patterns of vegetation growth and senescence.