PS 28-136 - CANCELLED - Wildfire-disease interactions impact seedling recruitment and survival in a post-burn environment

Abstract: Wildfires can increase forest disease by damaging the hosts and creating an entry point for pathogens, or it can reduce disease by removing local inoculum, directly killing pathogens. These contrasting dynamics can alter forest composition and shift host density. Surviving trees can produce seeds and vegetative sprouts that regenerate understory plants, often at high densities. Massive recruitment of seedlings following fire provides an opportunity for density-responsive pathogens to spread rapidly.



We witnessed post-fire forest disease dynamics that suggest fire-disease interactions coupled with density-dependent disease development. Following the 2020 CZU wildfire in Santa Cruz County, surviving tanoaks (Notholithocarpus densiflorus) produced an unusually large number of seeds, giving rise to high-density carpets of seedlings and vegetative sprouts reaching densities of 150/m2. A subsequent outbreak of powdery mildew, an obligate biotrophic pathogen, infected the new growth.



We asked whether the disease outbreak is the expected result of density-dependent disease development or whether there is a more direct effect of wildfire impacts. We expected that powdery mildew intensity would be greater in high-density seedling carpets due to the density-dependent spread of the pathogen more than conditions created by wildfire. We established a monitoring plot (1,250 m2) and 24 satellite plots (100 m2) across a 967 ha area that was affected by the CZU fire. We tagged, mapped, and measured 6,600 N. densiflorus seedlings and resprouts for powdery mildew in the monitoring site in 2021 and censused for mortality in 2022. Satellite plots were established in 2022 to assess infection and density across a gradient of burn intensity.



We found that areas with higher density had greater prevalence of powdery mildew. One-year post-fire, healthy and infected populations of N. densiflorus had an overall 8.6% mortality rate. Larger seedlings (more leaves) suffered the highest mortality rate when infected with the disease. Across the 24 satellite plots, areas with greater burn intensity had higher-density carpets and an increased disease prevalence, suggesting a congruent relationship between density, burn, and disease. This suggests that surviving N. densiflorus trees that produce massive amounts of acorns are more likely to become infected with density-dependent diseases, lowering the chance of survival in a post-burn landscape.