The northern red-legged frog (Rana aurora) is a sensitive species facing challenges to its persistence due to climate change and habitat fragmentation. As a lentic breeding species — one that relies on still-water habitats like ponds and lakes for breeding — the red-legged frog is primarily terrestrial, traveling to ponds in the late winter for a short period to breed, before returning to its upland feeding grounds. While little is known about its terrestrial microhabitat usage, it is understood that the forest understory plays a critical role in helping the red-legged frog meet its moisture, body temperature, and feeding requirements. In the Pacific Northwest, climate change is expected to bring warmer, drier conditions in summer which will increase rates of evapotranspiration, decrease soil moisture percentage, and lower plant-available water – water the plants can access from the soil – which can limit existing understory habitat. Western sword-fern (Polysticum munitum) is a dominant understory plant in forests that supports the red-legged frog and is drought tolerant and capable of creating moist environments. An association between the red-legged frog and the western sword fern has been cited observationally, but is still not understood and requires further study.

The goal of this project is to understand the extent and drivers of the possible association between red-legged frog and any microhabitat use, the relative value of western sword fern microhabitat, and how that changes with rainfall patterns. Katie will be working with Oregon Department of Fish and Wildlife (ODFW) and Metro, a regional governance agency of the Portland area, which owns the Burlington Creek Forest (BCF) where the study will take place. To determine if there is a microhabitat association with the red-legged frog, Katie will radio-track frogs using custom-fit transmitter belts with little antennas to note their habitat usage upon each relocation and the habitat characteristics of the areas where they are found. To assess the relative value of western sword fern microhabitat, Katie will characterize the forested upland habitat of BCF broadly, as well as the fern microhabitat, by measuring temperature, humidity, soil condition, and food resources relative to the ferns and overall canopy cover, slope, aspect, and understory composition. Lastly, to understand how western sword fern will be impacted by projected precipitation changes as a result of climate change, Katie will create a hydrologic model of the catchment area, the land area where water collects and flows into streams and wetlands, relative to where the western sword fern has been recorded and estimated from the habitat surveys, to determine subsurface flow. Once the model is developed, a manager could use it to test different climate scenarios to see how flows change and western sword fern will be affected.

Metro is considering how best to manage the uphill portion of BCF for multiple uses. Given the concentration of western sword fern habitat and known red-legged frog population, this study will be critical to inform their management decisions. Not only will this project produce valuable information on habitat use for ODFW, it will also create a blueprint for the impacts of multi-use development for Metro. Western sword fern understory microhabitat, believed to support the sensitive red-legged frog, may be at risk from climate change, further complicating the feasibility of conservation management for the frog. Through the development of a fern hydrological model, a key deliverable of this research project, altered precipitation patterns can be evaluated to best manage and enhance the resilience of this at-risk yet vital habitat. Based on the findings of this study, Katie can make suggestions for red-legged frog conservation and management based on habitat use and climate change vulnerability. Additionally, through a complete assessment of western sword fern microhabitat features, microclimate existing below fern leaves will be quantified and evaluated as cool-air refugia for wildlife.