Evaluating conditions and trends for Sky Island forests in the Chisos Mountains, Big Bend National Park: Focused condition assessment report

Barton, Andrew; Poulos, Helen

doi:10.36967/nrr-2287094

Forests and woodlands throughout the world are threatened by environmental change, which is leading to tree die-offs, reductions in tree cover, and conversion to non-forested ecosystems. The chief interrelated stressors driving these changes are increased temperature, intensified drought, amplified pest outbreaks, and hotter and larger wildfires—all generated in part from human-induced climate change. The American Southwest is no exception to these trends. The Chisos Mountains (CM) of Big Bend National Park (BIBE) comprise an important Sky Island forest resource due to their high biological diversity and endemism. The key controls on forest structure, composition, and processes in the forests of the Chisos Mountains have changed drastically over the past century. Accordingly, four resource issues motivated the Focused Condition Assessment (FCA) study presented in this report: • What was the impact of the combined 2011 freeze and drought on tree populations, seedling establishment, size structure, wildlife snag densities, and community species composition? • What was the effect of the 2011 weather events, in particular, on species thought to be drought sensitive, especially on piñon pine? • How has the risk of severe wildfire changed as a result the passage of time (16 years) and the 2011 weather events, with a focus on live tree density and surface fuels? • Given the answers to these questions, what is the condition of forest resources in 2019, in particular tree densities, surface fuel loads, and wildlife snag densities? The results of this FCA document significant recent tree mortality, shifts in forest stand structure, and increases in dead fuel loads from 2003 to 2019 across the Chisos Mountains. The 2011 five-day February freeze and subsequent severe drought played key roles in these temporal changes. The results of this FCA and climate projections point to a very high probability of change in the forests and woodlands of the Chisos Mountains. Some of these alterations will be unavoidable. Moreover, the extensive wilderness and highly dissected landscape of the CM poses challenges and limitations on large-scale forest manipulation (Lydersen et al. 2019). Nevertheless, there are three management options that could be considered for increasing the resilience of the forests of the CM. The goal would be to reduce the probability of massive canopy die-offs from drought and of anomalous large, high-severity wildfires. Most obvious, a vigorous program of prescribed fire could go far in reducing the risk of catastrophic wildfire, leading to widespread mortality and forest type conversion—a problem well-documented across the Southwest. Our analysis suggests that dense tree stands with high fuel loads on less xeric sites, especially in strategic topographic locations, are good candidates for thinning and prescribed fire. More extreme future climate and ecosystem projections raise the possibility of significant loss of montane forests in the Sky Islands. We raise these possibilities fully aware of the challenges for implementation, the potential risks to putting fire to forests, the legal issues of forest manipulation in federal wilderness and national park land, and the likely controversy of some of these methods. Accordingly, we are careful not to advocate for any particular management strategy at this point, but instead to encourage that these be carefully considered by multiple stakeholders, with the help of experienced scientists, land managers, and conservation ethicists. Clearly, the forests and woodlands of the Chisos Mountains face pressures that will only increase in the future. Adaptive management of the forests have the potential to stave off some of the impacts of the multiple impacts of climate change.

Evaluating conditions and trends for Sky Island forests in the Chisos Mountains, Big Bend National Park: Focused condition assessment report

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