PARENT SESSION
Oral Session #99: Grazing.
Presiding: T. Arredondo
Friday, August 9. 9:00 AM to 12:00 PM. Graham Meeting Room, TCC.

Fifty years of succession in semi-arid shrub communities: the effects of grazing and ungulate exclusion.

Manier, Daniel^*,1, Hobbs, N. Thompson^1,2, Theobald, David¹, ¹ Natural Resource Ecology Lab, Fort Collins, Colorado² Colorado Division of Wildlife, Denver, Colorado

ABSTRACT- It is widely believed that the introduction of domestic herbivores has modified landscapes in western North America. Domestic and wild ungulates graze extensively across the mountains and inter-mountain steppe communities of this region influencing the structure of herbaceous and shrub vegetation, which also affects fire regimes and inter-specific competition. We investigated the role of grazing pressure in changes in the structure and composition of 16 semi-arid shrub communities in western Colorado over a period of 40-50 years. We sampled the vegetation cover within and adjacent to historic domestic and wild ungulate grazing exclosures. Distributed among several shrub-grassland steppe communities, these exclosures are located within important winter habitats for several of Colorado's mule deer populations. We compare the current vegetation data to data collected in the 1950s by Colorado Fish and Game employees. We identify significant increases in shrub cover in areas where grazing intensity was limited, but a significant decrease in shrub cover in unprotected areas. Graminoids declined, while non-graminoid, herbaceous species increased significantly irrespective of treatment. Tree invasion (Pinon pine and Juniper) was evident in all treatments. Bare ground cover increased significantly across treatments over the 40-50 year period; grazing pressure significantly increased this effect. Study site was a stronger predictor of current species richness (F =16.98, p <0.0001) than the grazing treatments, however treatment effects are also a significant predictor (F = 2.49, p = 0.078) identifying both extensive variation in composition within the sagebrush type and the effects of grazing on the composition. Mean differences suggest that ungulate use reduces the coverage of shrubs, whereas grasses and non-graminoid, herbaceous species follow a pattern described by intermediate disturbance theory. We conclude that succession (in the absence of fire) has been the strongest component of change in these communities over the past 40-50 years, however the level of domestic and wild ungulate grazing is an important determinant of current sagebrush community structure and range condition.

KEY WORDS: community dynamics, shrub lands, grazing