Several different disturbances affect forests dominated by coast redwood (Sequoia sempervirens). Among the most important disturbances are fire, which has occurred historically, and sudden oak death (SOD), an emerging disease caused by the exotic pathogen Phytophthora ramorum. Both of these disturbances affect redwood forest ecosystems in critical ways; some effects are similar across these two disturbance types, while others are dramatically different. In this dissertation, I present three original field studies designed to further scientific understanding of the effects of fire and sudden oak death in isolation. In addition, I integrate these findings to develop comprehensive insights into the disturbance ecology of redwood forests.
Fire was frequent and ubiquitous in redwood forests prior to European settlement, but fires have been exceedingly small and rare over the last 70-80 years because of aggressive fire prevention and suppression policies. As a result, many aspects of redwood fire ecology remain poorly understood. Sudden oak death, a novel disturbance that was first discovered in the 1990s, is currently affecting redwood forests throughout coastal California. The most severely affected species, tanoak (Notholithocarpus densiflorus syn. Lithocarpus densiflorus), is still widespread and abundant in the redwood ecosystem, but diseased areas have begun to experience considerable mortality. Tanoak is extremely valuable as a food source to numerous wildlife species and thus its decline could have major impacts on redwood forest communities. Despite substantial public concern and a great deal of research attention, many effects of this devastating disease have yet to be studied.
Chapter 1 presents an investigation of tree regeneration in second-growth redwood forests experiencing sudden oak death-induced tanoak mortality, including a discussion of the factors that may be limiting seedling recruitment. I studied heavily impacted stands in Marin County (CA) and found the following: (1) despite reductions in canopy cover, there is no evidence that any species other than tanoak has exhibited a regenerative response to tanoak mortality, (2) the regeneration stratum was dominated by redwood and tanoak (other tree species were patchy and/or scarce), and (3) some severely affected areas lacked sufficient regeneration to fully re-occupy available growing space. These results indicate that redwood is likely to initially re-occupy the majority of the ground relinquished by tanoak, but also provide evidence that longer-term trajectories have yet to be determined and may be highly responsive to management interventions.
Chapter 2 presents an assessment of how sudden oak death has affected - and may eventually affect - stand structure in redwood forests. I utilized a stratified plot design and a stand reconstruction technique to assess structural impacts, at present and in the future, of this emerging disease. I found that residual trees in diseased plots were more aggregated than trees in unaffected plots, and my models predicted that the loss of tanoak will lead to the following short-term changes: (1) greater average diameter, height, height-to-live-crown, and crown length, (2) higher standard deviations of diameter, height, and crown length, and (3) larger average nearest neighbor differences for diameter, height, and crown length. In addition, plots lacking tanoak (living or dead) - as compared to plots with tanoak - exhibited (1) greater average diameter, (2) higher standard deviations of diameter and crown length, and (3) increased nearest neighbor differences with regard to diameter, height, and crown length. This chapter also includes preliminary explorations of how sudden oak death-induced structural changes compare with typical old-growth characteristics and how this disease may affect the structure of old-growth forests.
Chapter 3 presents an examination of the role of fire in the stand-level competitive dynamics of forests dominated by coast redwood, with a particular focus on post-fire survival rates and basal sprouting responses of redwood and tanoak. This study was initiated in response to a storm event in 2008 that ignited numerous fires throughout the redwood region and provided a rare opportunity to conduct replicated fire effects research. One year post-fire, bole survival and basal sprouting were quantified, for redwood and associated species, at four field sites that spanned much of the latitudinal range of redwood and encompassed second-growth and old-growth stands, burned and unburned areas, and a wide range of fire severities. I employed a mixed effects analytical framework and found that: (1) the probability of bole survival was greater for redwood than for tanoak, (2) this divergence was much more pronounced at higher fire severities, and (3) tanoak exhibited a slight advantage in terms of post-fire basal sprouting, but the dominance of tanoak basal sprouts in burned areas was reduced relative to unburned areas. In summary, fires of all severities increased the abundance of redwood relative to tanoak, but higher severity fires more strongly favored redwood.
In chapter 4, findings from all three chapters are integrated to facilitate a detailed comparison of SOD and fire, as well as a discussion of several other aspects of redwood ecology and management. Major points include the following: (1) the effects of SOD and fire are similar in some ways (both favor redwood and remove understory/subcanopy trees), but very different in others (e.g. SOD may lead to the complete extirpation of tanoak from redwood forests), (2) the low levels of regeneration in many SOD-impacted areas may be a result, at least in part, of key differences between SOD and historical disturbances, and (3) interactions between SOD and fire may have more profound effects than either of these disturbances in isolation. Redwood forests are currently undergoing dramatic changes, many of which represent challenges to forest health and ecological integrity; some of these problems are effectively insurmountable, but others may be amenable to management interventions. Researchers and land managers must acknowledge that redwood forests are transitioning to a novel state, and recognize that successful stewardship of the redwood forest ecosystem will require sustained inquiry and considerable experimentation.