Canopies, the Final Frog-tier: exploring responses of a specialist treefrog to prescribed fire in a pyrogenic ecosystem

BACKGROUND

Pine flatwoods of the southeastern United States were shaped by frequent fires. Land managers use prescribed fires to control fuels but also to restore historical fire dynamics. Broad outcomes of this practice are well-understood, but impacts on many organisms are still being explored. Frogs, for example, have upland and wetland requirements, limited mobility, and skin susceptible to desiccation. Treefrogs spend most of their lives in uplands away from water. When fire approaches, animals may escape to an unburned area, shelter in place, or be killed by the fire. We examined which of these mechanisms is the prevailing short-term response for a specialist treefrog in a pyrogenic flatwood system.

RESULTS

We assessed the short-term impacts of prescribed fire on the dynamics of an upland flatwood specialist, the pinewoods treefrog Dryophytes femoralis, using a replicated before-after-control-impact field experiment. We set pipes as treefrog refugia at 3 m, 6 m, 9 m, and 9⁺ m in 12 pine trees spread evenly across two treatments: reference trees in units burned in 2020 and trees in units with 2021 prescribed fire. Prescribed fires occurred on 16 April and 21 July 2021. Every 2 weeks between 5 March and 5 September, we checked pipes for frogs and assigned them unique color marks. We observed 78 individuals with 199 additional recaptures. We modeled abundance (as raw counts), survival, and vertical movement using mark-recapture methods, multi-state, and mixed linear models with a Bayesian framework. Survival and recapture were comparable among prescribed fire treatments, but abundances and movement probability varied. Frogs in trees in areas burned during the study were more likely to stay in place and less likely to descend to lower heights. We observed more frogs in trees after a 2021 fire compared to reference trees.

CONCLUSIONS

The prevailing mechanism for resiliency to fire for pinewoods treefrogs was migration up large pines, then likely recolonization to lower vegetation layers when plants regreen post-fire. This substantiates conclusions from other works that the integrity of mature pines is key to sustaining native biodiversity. Future work and management should consider the three-dimensional structure of habitat when developing burn prescriptions and study designs.

Impacts of Fire on Butterfly Genetic Diversity and Connectivity

How do novel fire regimes and a long history of fire suppression influence species genetic diversity? Genetic diversity provides the raw materials for sustaining viable populations and for allowing adaptation to novel environmental challenges, and at present, few studies address the genetic responses of animals to fire management. Here we study the genetic responses of 2 butterfly species to a landscape gradient of fire timing and severity in Yosemite National Park using a large set of genome-wide single nucleotide polymorphisms (SNPs). Butterflies are important bio-indicators of invertebrate diversity and play important roles in both bottom-up and top-down ecosystem processes, and typically increase in abundance following wildfires, due to an increase in abundance of flowering plants. However, it is not clear how genetic diversity and genetic connectivity of butterflies respond to landscape change following fire, and whether fire management has positive or negative effects. We found evidence to suggest that fire increases genetic diversity and reduces isolation in 2 butterfly species, but that aspects of the fire regime (severity, extent, timing, and frequency) differ in importance depending on the ecology of the specific species. This research is the first study to address fire management impacts on genetic diversity in invertebrates, and the results will allow fire managers to predict that fire reintroduction in protected areas will generally benefit butterfly populations.