Drought and Ecological Site Interaction on Plant Composition of a Semi-Arid Rangeland

Abstract

Fluctuating climatic patterns are increasing the frequency and severity of drought, a concern for native plant communities on grazed semi-arid rangelands. Vegetation successional models have focused on the impact of management and have failed to quantify the effects of extreme drought. From 2001 to 2011, plant community composition was sampled on ecological sites in a semi-arid rangeland managed with conservative grazing and frequent fire since 1937. Ordination and classification were used to assess the interactive effects of ecological site and extreme drought on plant species composition, holding all other external drivers constant. Deeper soil clay loam sites had 4x greater beta diversity than shallower and rockier low stony hill sites, an indication of greater species turnover and instability in response to extreme drought. Cumulative effects of drought years explained similarity between sites and species composition. Response to extreme drought varied by species; no response (Bouteloua curtipendula), decreased (Nassella leucotricha), and increased (Bouteloua rigidiseta and Eriochloa sericea). Annual C3 plant responses were explained by short-term drought and perennial C3 and C4 plant responses were explained by long-term drought. Clay loam sites had maximum species richness and diversity values during neutral periods with quadratic declines associated with climatic extremes (dry or wet) compared to the more xeric sites which had minimum species richness and diversity during neutral periods with quadratic increases during climatic extremes. The interaction between site and drought, holding all other external drivers constant, can enhance our understanding of plant community dynamics and secondary plant succession of degraded semi-arid rangelands.