Giant Sequoia Seedling and Associated Microbial Community Responses To Increasing Fire Severity And Water Deficit In A Changing Climate

Abstract

Settler-colonial fire suppression, combined with accelerating climate change, has increased the size and severity of wildfires in recent decades, threatening forest persistence throughout Western United States. Among forests threatened by these novel fire regimes are old-growth Sequoiadendron giganteum (giant sequoia) groves. In 2021, the KNP complex fire in Sequoia and Kings Canyon National Parks contributed to the mortality of 13-19% of all remaining mature giant sequoia trees. While forests may eventually recover from severe wildfire through seedling recruitment, important questions remain about how higher fire severity and warmer, drier conditions shape natural regeneration. One underexplored factor influencing post-fire regeneration is the role of soil microbial communities, which are essential for forest ecosystem functioning, nutrient cycling, and may enhance seedling survival under stressful post-fire conditions. Giant sequoias form mutualistic relationships with arbuscular mycorrhizal fungi (AMF), which can promote host drought tolerance and improve biomass accumulation, and may shape seedling establishment after fire. Here, we sequenced DNA of naturally-regenerating seedling roots and associated soils across burn severity and water availability gradients in giant sequoia groves. Fire severity was a primary driver of shifts in belowground fungal and bacterial community composition, and plant communities shifted in response to both moisture availability, fire severity and their interactions. Surprisingly, ectomycorrhizal fungi (EM) fungi were detected at relatively high abundance sequoia roots despite giant sequoia being an obligate AMF host, with colonization rates highest at low severity sites where EM host competitor presence was also greatest, suggesting colonization may be mediated by proximity to actively growing EM host neighbors. Following low severity fire, EM fungi, which associate with other co-dominant conifers such as firs and pines, decreased while saprotrophic fungi increased in both soil and sequoia roots, tracking a parallel decline in EM-associated host seedlings. Sequoia seedlings were largest following high-severity fire, but this severity also corresponded to higher shrub cover and greater water stress. Seedlings in low severity and unburned sites were smallest, corresponding with higher EM host competitor presence. Together, these results suggest a Goldilocks zone for this fire-adapted species where moderate severity fire reduced EM competition and potentially shifted abiotic conditions to favor sequoia regeneration, while high-severity fire may ultimately constrain regeneration via water limitation and downstream shrub encroachment. As fires of increasing size and severity are expected in the future, understanding how these belowground communities respond to different post-fire conditions, and the consequences of these shifts for aboveground recovery, is essential for informing reforestation strategies and predicting forest resilience in a changing climate.