Temporal consistency of undercanopy thermal refugia in old-growth forest

Microclimatic refugia (microrefugia) are ecologically important for the conservation of biodiversity under climate change. Year-to-year climatic consistency is an important requirement for most types of microrefugia, but refugia are often modeled using only a few years of data. Here, we used a decade-long (2009-2018) fine-scale, undercanopy microclimate temperature dataset from a landscape in the Cascades Mountains of Oregon, USA to assess the inter-annual temporal consistency of microrefugia. We used boosted regression tree microclimate models to quantify overall consistency and map locations of stable microrefugia for six biologically relevant annual temperature metrics. Microclimate temperature offsets (i.e., microclimate minus macroclimate temperatures) were remarkably stable over time, with R2 ranging from 0.69 for minimum temperature during spring to 0.90 for mean temperature during spring. We observed a high degree of coupling; that is, broad-scale climatic variation, as reflected in free-air temperature, has a major effect on microclimate temperatures – particularly in hot years – with the potential to overwhelm thermal buffering effects. In spite of this, we identified potential microrefugia locations throughout our study area, especially with respect to spring minimum and mean temperatures. To maintain microrefugia in a rapidly changing climate, conservation of old-growth and other structurally complex forest habitat is critical, especially at sites with high elevation relative to their surroundings.