Explaining abrupt spatial transitions in agro-ecosystem responses to periods of extended drought

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During the 1930’s, the North American central grassland region (CGR) experienced an extreme multi-year drought that resulted in broad scale plant mortality, massive dust storms and losses of soil and nutrients. Southern mixed grasslands were among the worst affected and experienced severe broad scale mortality. However, the severity of drought effects were varied in other grassland types of the CGR and in some instances responses formed spatially abrupt transitions between severe and minor drought impacts over short distances. This is exemplified in native grassland sites in southeastern Nebraska which suffered severe plant mortality while neighboring and ecologically similar sites in southwestern Iowa experienced reductions in cover. Additionally, a similar abrupt spatial transition was observed for crop production for the same area. Despite the environmental and economic consequences of the 1930’s drought our understanding of the drivers of these abrupt spatial transitions remains limited. Our objectives were to use available long term data to: 1) characterize the spatial and temporal patterns in responses of native plants and crops before, during, and after the 1930s drought, and 2) to identify the climatic, soil, and land use drivers for the abrupt spatial transition in plant responses. We used corn production from the USDA Census of Agriculture for cropland as a proxy for grassland response. Published data and maps from the 1920’s to 1940’s along this transition zone were used to identify the climatic, soil and land use drivers contributing to abrupt transitions zones. Our results suggest the transition from reduction in grass cover in Iowa to severe grass mortality in Nebraska during the 1930’s were the result of small increases in temperature and subtle decreases in precipitation along this spatial gradient. Additionally, long-term plant available water decreased from east to west corresponding to increased susceptibility to drought and ultimately increased grass mortality. These abrupt transitions in vegetation response would have resulted in an increase in ecosystem susceptibility to multi-year drought and, in grasslands where grass mortality occurred, increases in the rate of soil erosion by wind and water. Without long term data on these types of events cumulative effects of multiyear drought will continue to challenge our ability to predict ecosystem responses to catastrophic events.