Patch-specific woody plant mortality following severe drought in a semi-arid Texas savanna

Background/Question/Methods

Recent severe droughts in Texas have resulted in high levels of woody plant dieback in areas that have experienced decades of woody encroachment.  These were the most severe droughts on record since the 1950s.  Following an exceptional drought from 1951-1957, a study was conducted to quantify rates of dieback for various woody plant species.  In 2011, we repeated this study within three long-term grazing treatments that were established in 1948 in order to answer the following questions: How do recent patterns of drought-induced woody plant dieback in Texas 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 dieback or are shifts dependent on differences among species, soils, land use, and plant demography?  Ten transects were established in each grazing treatment to quantify the frequency and cover of live and dead 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.  Our findings were compared to those found in the 1951-1957 study.

Results/Conclusions

Flash droughts from 2000-2011 were the second most severe since 1919 (low PDSI = -4) and were only surpassed by the prolonged drought from 1951-1957 (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 Ashe juniper trees in deep soils (67.3%, 33 of 49 trees), combined with the recruitment of oak species where grazing had been excluded for the last 60 years, were the only patches to shift from a Juniperus-Quercus woodland to a Quercus virginiana dominated overstory with a diverse mixture of short growing shrubs dominating the understory.  Flash droughts since 2000 resulted in significant dieback, but dieback in cover was 28% greater following the more severe drought of the 1950s.  Neither resulted in widespread shifts in woody vegetation, and species-specific patterns of dieback were contingent on localized interactions between topoedaphic factors and long-term land use.  Specialized species-specific responses to drought occurred as a result of differences in ecohydrology among soil types, species-specific water use strategies, density dependent mortality relationships, and legacy effects resulting from long-term livestock management practices.