Drought-induced woody plant mortality in an encroached semi-arid savanna depends on topoedaphic factors and land management
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AbstractQuestionsHow do recent patterns of droughtinduced woody plant mortality inTexas semiarid 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?LocationTexasAgrilifeResearchStation,Sonora,Texas,USA(30.1N 100.3W).MethodsFollowing 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 longterm 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.ResultsFlash droughts (defined as rapidly intensifying droughts characterized by moisture deficits and high temperatures) from 2000 to 2011 were the second most severe since 1919 (lowPDSI=4) and were only surpassed by the prolonged drought from 1951 to 1957 (lowPDSI=5.17). Droughtinduced shifts from one woody plant community to another did not occur uniformly across the landscape. Instead, high mortality rates of matureJuniperus asheitrees in deep soils (67.3%, 33 of 49 trees), combined with the recruitment ofQuercusspecies where grazing had been excluded for the last 60yr, were the only patch types to shift from aJuniperusQuercuswoodland to an alternate state.ConclusionsFlash 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 longterm land management practices interacted with localized differences in topoedaphic factors to drive patchlevel shifts in woody vegetation following drought.
