Dearth under earth: Understudied plant-soil-fire feedback as drivers of forest mesophication and oak regeneration failures

Abstract

Regeneration of drought-tolerant and fire-adapted (pyrophytic) trees like oaks (Quercus spp.) is broadly limited by mesophication – a positive feedback where darker, wetter, and cooler microsites promoted by fire suppression favor drought-intolerant and fire-sensitive (pyrophobic) trees like maples (Acer spp.). Given projected increases in fire and drought, mesophication could reduce forest resilience to future stressors. Although the issue is widely recognized, corrective management has almost exclusively focused on aboveground drivers (i.e., fire suppression effects on fuel and microclimatic properties). We propose a complex feedback framework involving mature and immature trees, fire, and abiotic and biotic soil properties (i.e., “plant-soil-fire feedback”) that may provide a more complete understanding of mesophication. Focusing on the eastern US, we: 1) review the current mechanistic understanding of mesophication, 2) identify overlooked belowground drivers (i.e., plant-soil-fire feedback), 3) explore future research needs, and 4) derive forest management implications. We argue that fire suppression directly and indirectly increases soil moisture and nutrient availability and alters soil microbial communities in ways that favor pyrophobic tree species. Such trees then outcompete pyrophytic trees by further promoting such belowground conditions that reinforce their dominance and further exclude fire. We conclude that mesophication cannot be fully understood – or reversed – without considering plant-soil-fire feedback. Such perspective can inform forest management that ensures resilience by promoting drought-tolerant and pyrophytic trees like oaks.