Metabolic acceleration quantifies biological systems' ability to up-regulate metabolism in response to episodic resource availability

Precipitation often arrives discretely in semi-arid ecosystems. Under these conditions, natural selection might favor rapid metabolic responses to the sudden availability of otherwise limiting resources. We introduce and define metabolic acceleration (α) as the first derivative of the metabolic rate of a living system with respect to time. As such, α describes the capacity of a biological system to up- and down-regulate metabolism and may be applied across scales and processes. To better understand the responses of roots and soil microbes to seasonal patterns of rainfall and plant activity, we compared soil respiratory acceleration (α_soil) derived from soil respiration time-series among three microhabitats (under mesquite, under bunchgrasses, and in intercanopy soils) in a semi-arid shrubland near Tucson, Arizona. Across microhabitats, α_soil was greatest during the warm, wet summer months and lowest during cool winter months. Throughout the year, α_soil beneath mesquite was greater than beneath bunchgrasses or in intercanopy soils. Finally, microhabitat-specific responses of α_soil to spring and monsoonal rainfall events were consistent with seasonal contrasts in the photosynthetic activity of deeply-rooted mesquite shrubs and warm-season bunchgrasses. By quantifying the capacity of living systems to respond to episodic resource availability, metabolic acceleration provides a new perspective and potentially unifying metric for biological responses to environmental heterogeneity.