The myth of the biological threshold: A review of biological responses to soil heating associated with wildland fire

Soil heating caused by prescribed or wildland fire commonly focuses on a single biological thermal threshold of 60 degrees C for the duration of one minute to represent organism death. This metric severely misrepresents the heterogeneity of the soil environment, the physiological attributes and tolerances of organisms, and the complexity of heat transfer through soils. Measurements of biotic death in simulated laboratory experiments render research findings difficult to extrapolate to forest and grassland soils. The disparity between assumed biological responses and the diversity of biological responses after wildland fire events calls for a thorough review of soil biological heating thresholds. In this review, we synthesize and compare research that directly relates soil heating temperature and duration to biological responses, provide relevant models for temperature-duration responses of soil organisms in lieu of a strict threshold, and recommend applications of soil heating data for wildland fire and ecosystem management. For no single study or group of organisms was a threshold of 60 degrees C for one-minute duration evidenced. All soil organisms reviewed, which included roots, mesofauna, bacteria, fungi, microbial biomass, and soil respiration, displayed both positive and negative responses to soil heating across temperature and duration gradients. We, therefore, discourage the use of the traditionally accepted metric of 60 degrees C for the duration of one minute. Instead, we present models of duration-temperature relationships of soil biota and invite interdisciplinary efforts from researchers and managers to directly measure biological responses on a case-by-case basis.