Drought-induced woody plant mortality in an encroached semi-arid savanna depends on topoedaphic factors and land management
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Questions: How do recent patterns of drought-induced woody plant mortality in Texas semi-arid savanna compare to the extended drought of the 1950s? Does the relative composition of the woody plant community shift ubiquitously across the landscape following woody plant mortality and dieback or are shifts dependent on differences among species, soils, land use and plant demography? Location: Texas Agrilife Research Station, Sonora, Texas, USA (30.1 N 100.3 W). Methods: Following an exceptional drought from 1951 to 1957, a study was conducted to quantify rates of mortality for various woody plant species. In 2011, we repeated this study within three long-term grazing treatments that were established in 1948. Ten transects were established in each treatment to quantify the frequency and cover of live and dead woody individuals for all woody species. Rates of woody plant dieback were determined for each species and tested for differences among height classes, soil categories, total woody densities and pastoral treatments. Results: Flash droughts (defined as rapidly intensifying droughts characterized by moisture deficits and high temperatures) from 2000 to 2011 were the second most severe since 1919 (low PDSI = -4) and were only surpassed by the prolonged drought from 1951 to 1957 (low PDSI = -5.17). Drought-induced shifts from one woody plant community to another did not occur uniformly across the landscape. Instead, high mortality rates of mature Juniperus ashei trees in deep soils (67.3%, 33 of 49 trees), combined with the recruitment of Quercus species where grazing had been excluded for the last 60 yr, were the only patch types to shift from a Juniperus-Quercus woodland to an alternate state. Conclusions: Flash droughts since 2000 resulted in significant mortality and dieback, but dieback in cover was 28% higher following the more severe drought of the 1950s. Legacies from long-term land management practices interacted with localized differences in topoedaphic factors to drive patch-level shifts in woody vegetation following drought. We repeated a historical field experiment to compare rates of woody mortality resulting from recent droughts to the exceptional drought of 1951-1957. Mortality was greater following the more severe drought of the 1950s than recent flash droughts from 2000 to 2011. Long-term land management legacies interacted with localized differences in topoedaphic factors to drive patch-level shifts in woody vegetation following drought. 2013 International Association for Vegetation Science.
