Novel disturbance interactions between fire and an emerging disease impact survival and growth of resprouting trees

Abstract

Human-altered ecological disturbances may challenge system resilience and disrupt biological legacies maintaining ecosystem recovery. Yet, the extent to which novel regimes challenge these legacies varies. This may be partially explained by differences in the vulnerability of life history strategies to disturbance characteristics. In the fire-prone, resprouter-dominated coast redwood forests of California, the introduced disease sudden oak death (SOD) alters fuel profiles, fire behavior, and aboveground tree mortality; however, this system is dominated by resprouting trees that are well-adapted to aboveground damage, and belowground survival of individuals may represent the principal biological legacy connecting pre- and post-fire communities. Much of the research exploring altered disturbances and forest recovery has focused on legacies determined by seed dispersal and aboveground survival of adults. In this work, we use pre- and post-fire data from a long-term monitoring network to assess the impacts of novel disturbance interactions between wildfire and SOD on the belowground survival and vegetative reproduction of resprouters. We found that increasing accumulation of coarse woody surface fuels from SOD-killed hosts decreased the likelihood of belowground survival for resprouting tanoak trees, but not for redwoods. Tanoaks' belowground survival was negatively related to substrate burn severity, which increased with the volume of surface fuels from hosts, suggesting heat damage as a possible mechanism influencing altered patterns of resprouter mortality. These impacts increased with decreasing tree size. By contrast, redwood and tanoak trees that survived both disturbances resprouted more vigorously, regardless of post-fire infection by P. ramorum, and generated similar recruitment at the stand level. Our results demonstrate that disease-fire interactions can narrow recruitment filters for resprouters, which could impact long-term population and demographic structure; yet, compounded disturbance may also reduce stand density and disease pressure, allowing competitive release of survivors. Resprouters displayed vulnerabilities to altered disturbance, but our research suggests that legacies maintained by resprouting may be more resilient to certain compounded disturbances, compared to seed-obligate species, because of high rates of individual survival under increasingly severe events. These trends have important implications for conservation of declining tree species in SOD-impacted forests, as well as predictions of human impacts in other disturbance-prone systems where resprouters are present.

Compound disease and wildfire disturbances alter opportunities for seedling regeneration in resprouter‐dominated forests

“…Following fire, top‐killed resprouting trees in this system typically persist at high rates (ranging from ~80% to 100% across size classes; Simler et al. ). However, SOD can cause complete belowground mortality of tanoak and oak genets, and elevated substrate fire severity in SOD‐impacted stands has been associated with increased post‐fire belowground mortality in resprouting species (Metz et al.…”

“…Each of these most common tree species can resprout, exhibit high belowground survival rates, and regrow vigorously postfire (Henson and Usner , Simler et al. ). These species also can facultatively reproduce from seed (Henson and Usner ).…”

“…However, altered disturbances can also cause increased aboveground and belowground mortality of resprouting trees, following more severe fires (Simler et al. ), more frequent disturbance (Fairman et al. ), drought (Worrall et al.…”

“…In the fire‐prone coastal forests of Big Sur, California, the foundational tree species all resprout, exhibit high levels of belowground survival following disturbance, and, more rarely, recruit new individuals from seed (Simler et al. ). However, anthropogenic activities have introduced a novel biotic disturbance to these forests (Rizzo and Garbolotto ).…”

“…) and belowground genets (Simler et al. ). It remains poorly understood whether gaps created by this novel biotic disturbance will be filled by new recruitment from seed and whether less‐susceptible species will occupy these openings, facilitating compositional shifts.…”