OOS 41-2 - Fine-scale spatial heterogeneity, not fire frequency, drives soil microbiome impacts on conifer seedling re-establishment after wildfire

Forest ecosystem health is intimately linked to belowground microorganisms that drive biogeochemical cycling and mediate successional dynamics through plant-soil feedbacks. However, increasingly frequent wildfires and concomitant drought may impact successional trajectories, raising concerns of potentially widespread forest regeneration failures in the near future. While fire is critical to sustaining ecosystem dynamics for fire-adapted forests, little is known about the cumulative effects of frequent contemporary wildfires on soil microbial communities and forest regeneration. We tested how frequency and fine-scale variation in wildfires and drier post-fire climates affect the soil microbiome’s influence on conifer and shrub seedling 1) growth and 2) drought tolerance to better understand the influence of soil microorganisms on post-fire revegetation dynamics. We measured how soil inocula from six north central Oregon field sites with varying degrees of burn frequency (0, 1, and 3-burn) impacted tree (grand fir, Abies grandis) versus shrub (snowbrush, Ceanothus velutinus) seedlings in a 9-month, 420-pot greenhouse experiment. We then measured the impact of 3 months of simulated drought on 364 of the remaining live seedlings. Although we did not detect a consistent impact of fire frequency on early seedling growth or drought tolerance, we identified strong spatial heterogeneity in the impact of soil microorganisms on seedling performance (shoot height, leaf count, and leaf size) and mycorrhizal fungal colonization of roots. While soil inocula from a single once-burned site increased seedling size four-fold, soil inocula from another once-burned site decreased their size relative to a sterile control. Variation in the effect of soil inocula was greatest for conifer seedlings. Inocula that increased seedling growth also tended to leave seedlings susceptible to drought stress. These results show that the impacts of soil microbiota collected from distinct locations differ strongly in their impacts on conifer seedling performance. Informed use of microbial inocula could be key to improving revegetation strategies post-wildfire.