How Large, Decayed, and Moist Must Rotting Logs Be to Act as Thermally Buffered Microhabitats in Temperate Eastern United States Forests?

Deadwood is an essential habitat for forest biodiversity, yet coarse woody debris (CWD) is declining owing to silvicultural practices that impede recruitment (e.g., clearcutting, short logging rotations), and/or forest sanitization that prevents retention (e.g., removal of "waste wood"). Furthermore, connections among studies examining the ecological roles of CWD as potential refugia for saproxylic organisms susceptible to heat and desiccation, or cold, have been hampered by inconsistent thresholds used to define "large-diameter" logs. Here we measured temperature inside and outside rotting logs over four seasons in temperate montane Southern Appalachian forests, and assessed relationships between several metrics of thermal buffering with log diameter, decay stage, and moisture content. Our analyses showed that the microclimates of larger diameter logs exhibit significantly decreased temperature ranges in spring and fall, and increased minimum temperatures in winter and on extreme cold days. Qualitatively, logs with diameters of at least 25-30 cm provided modest to considerable thermal buffering (depending on the metric), and this may represent a useful threshold for targeted enhancement of CWD habitats. However, there was no detectable reduction of maximum temperatures in summer or on extreme warm days, suggesting that, in natural temperate mixed pine-oak forests in the eastern United States, a key benefit that saproxylic organisms gain from living in rotting logs may be avoidance of cold stress. Neither decay stage nor moisture content were significant predictors of thermal buffering in any season. Ultimately, larger diameter logs provide important microhabitats, and management strategies that prioritize their creation and retention should have far-reaching benefits for biodiversity in forests.