Disturbance regime characteristics can influence the relative fitness of sexual and asexual regeneration modes, and altered forms of disturbance or novel interactions may further structure these selection pressures. In the coastal redwood forests of Big Sur, CA, Sudden Oak Death (SOD) is an emerging disease causing a novel biotic disturbance of widespread tree mortality in tanoak (Notholithocarpus densiflorus). In the absence of SOD, the mixed severity fire regime of the region has favored recruitment via vegetative reproduction over successful regeneration from seed. We examined whether novel disturbance interactions between SOD and fire have altered the composition and dynamics of seedling regeneration, using a network of long-term SOD monitoring plots. After establishment of these plots, a large wildfire burned across the network, and we used the natural experiment presented by this fire to investigate impacts of compounded disturbances and altered regimes on patterns of sexual regeneration. In burned and unburned forest plots, we quantified stand structure, disease progression, disturbance impacts, and vegetative reproduction between 2006-2013, as well as seedling recruitment, growth, and mortality between 2013-2015 for three species representing differing susceptibility to SOD: tanoak (a susceptible host), bay laurel (a non-susceptible host), and redwood (which is not impacted by SOD).
In plots heavily impacted by SOD, seedling communities significantly differed from reference stands and were characterized by pulses in tanoak regeneration, likely due to stress in overstory trees. Seedling growth rates were also significantly greater for all species in SOD-impacted areas, potentially due to gaps created by disease-related mortality, which may generate atypical opportunities for sexual regeneration. In burned areas, seedling composition was distinct from unburned stands but did not differ with history of disease impacts.
Overall, likelihood of seedling mortality was negatively correlated with seedling height, but this trend varied significantly with combinations of fire or SOD-related disturbance. Between 2013-15, we observed greater bay laurel and tanoak seedling survival in areas that had previously experienced disease or fire of increasing severity. Mortality rates were lowest in stands that had experienced both disease and fire, suggesting that compounded disturbances may reduce the competition experienced by seedlings. Stand-level SOD impacts did not significantly reduce susceptible tanoak seedling survival, suggesting that smaller tanoaks may not accumulate infections at the same rate as larger individuals. In a system where seedling regeneration is comparatively rare, these alterations of sexual regeneration niches could have important future impacts for population structure, genetic diversity, and community assembly.