Effects of Disturbance in Grassland Plant Communities
Limb, Ryan Frank
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In the north central Oklahoma study, plant species richness declined with increased eastern redcedar canopy cover. However, the rate of decline in species richness closely followed that which was predicted by a species-area relationship. Furthermore, the decline was uniform among C3, C4 and forb species groups. Annual herbaceous above-ground production declined with increasing eastern redcedar canopy cover, but was most variable at intermediate canopy cover. These results indicate that eastern redcedar reduces species richness. However, canopy cover up to 80% does not impose an ecological threshold. In the central Kansas study, we used tracked vehicles to impose anthropogenic focal soil disturbance within a mixed-grass landscape extensively disturbed with livestock grazing or hay harvest. In both landscapes focal soil disturbance had a larger influence on plant community composition than either livestock grazing or hay harvest. Moreover, combined focal and extensive disturbance did not have a greater effect than focal disturbance alone. Despite differences in initial plant species composition, successional trajectories following focal soil disturbance were similar between grazed and hayed communities, and both plant communities recovered from focal soil disturbance within two growing seasons. Tracked vehicle disturbance attracted preferential livestock grazing, which promoted structural heterogeneity within the plant community. However, the effect was ephemeral and only lasted one growing season.
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