Infection and dieback severity varied considerably across the landscape, however, there is some evidence to suggest that populations at lower elevations may be at highest risk for severe dieback, either due to increased water stress, close proximity to fungal inoculum sources, or both. Additionally, shrubs located on southwest-facing slopes may also be more vulnerable due to increased sun exposure and thus environmental stress. Management efforts may want to focus on these areas when this region experiences future drought. Finally, although extreme dieback was recorded throughout the study, none of the observed shrubs succumbed to mortality. This may be the result of overall physiological resiliency, and the ability of adult shrubs to allocate resources to keep portions of the canopy alive. It could also be that the region’s slightly more mesic climate offers a climatic buffer that prevented shrubs from reaching their mortality thresholds. More research is needed to identify these exact mechanisms and thresholds in A. glauca. Collectively, the results of this dissertation work provide valuable knowledge on the severe dieback of an important chaparral shrub during an historic drought, with the potential for ecologically and economically costly consequences. Additionally, the data I present provide insight into the scale and progression of A. glauca dieback in a chaparral system, and potential patterns of future dieback in the face of predicted climate change. Future research that seeks to further resolve landscape and environmental variables contributing to plant stress would help in identifying these patterns.
Heterogeneity and rugged topography across the landscape, round garden pot while likely beneficial for the resilience of regional A. glauca populations during drought, presents significant challenges for on-the-ground monitoring. Out of necessity for safe access , many of the plants surveyed were located on the outer boundaries of stands, where edge effects may have been a factor. Monitoring intact, undisturbed stands using drones would yield valuable additional insight into the extent of disease deeper into stands and in stands on steep terrain or that are outside of normal visual range. The challenges of working in rugged landscapes covered in impenetrable vegetation highlight the need for using and refining remote sensing technologies, such as drone imaging, Light Detection and Ranging , and hyperspectral imaging as monitoring tools. Large-scale, long-term monitoring using these tools would allow researchers to retrieve data in areas that have previously been inaccessible, while also gaining a larger scale understanding of drought impacts. They ultimately will enable future studies to reveal more nuanced patterns across the landscape and between years of varying climatic conditions.Plant pest and disease outbreaks play a major role in shaping ecosystems around the world. Outbreaks can alter ecosystem structure and function, often with substantial consequences . Over the past 200 years, pest/disease outbreaks have increased due to mass exchange of biological materials from global trade and a rise in unusual climate events resulting from global climate change .
Prolonged climate irregularities can subject plants to environmental stress outside of their normal resistance thresholds and make them susceptible to pests and pathogens . For example, the increase in extreme droughts, defined here as greater in intensity and duration than historical drought regimes, has been directly linked to enhanced mortality in woody plant systems worldwide, often in association with pest/pathogen outbreaks . Plant disease outbreaks are often economically costly , and can result in loss of ecosystem services in natural ecosystems. With global trade continuing to spread pests and pathogens, and global change-type drought events predicted to increase , incidences of plant disease outbreaks are expected to increase. Understanding the role of drought and pathogens in plant dieback and mortality is therefore of critical importance. Latent fungal pathogens are of particular concern for natural ecosystems yet their ecological roles remain poorly understood. These pathogens can live as asymptomatic endophytes within their hosts and remain undetected for long periods of time . The Botryosphaeriaceae fungi, a groupthat causes considerable damage to hundreds of agricultural, ornamental, and naturally occurring host species around the world , includes many latent fungal pathogens that are difficult to detect in wild plant populations. Members of this diverse family can occur as endophytes, pathogens, and saprophytes on diverse woody hosts . They are best known as pathogens that cause leaf spots, cankers, severe branch dieback, and death in economically important hosts such as grapevines , avocado , and eucalyptus . While Bot. fungi are rapidly becoming one of the most important agents of disease in agricultural plant hosts , relatively few studies have been conducted on these pathogens in natural systems .
The Bot. fungi have a long history of taxonomic confusion, in part due to indistinctive morphological characteristics among species and from other fungal taxa, as well as historically poor and inconsistent descriptions early on in their discovery . Furthermore, Bot. host specificity and pathogenicity can vary widely among species and across geographical regions, complicating our understanding of their influence in various host species and across systems . While advances in molecular sequencing and data basing have added clarity in this area , challenges remain in understanding the diversity and pathogenicity of Bot. species among hosts and across regions. As a result, there is a dearth of knowledge on their ecological roles, particularly in native ecosystems.One consistent finding is that disease outbreaks from Bot. fungi in agriculture are often associated with environmental stress, such as extreme heat fluctuations and drought . Furthermore, studies have shown latent pathogens like Bots cause more damage to water-stressed hosts , and some Bot. species have been shown to grow well in water potentials much lower than what their plant hosts can tolerate , suggesting drought conditions increase virulence by these pathogens. Therefore, regions that have historically dry climates or experience periodic extreme drought may be especially vulnerable to disease outbreaks from latent pathogens as they are predicted to experience an increase in drought events due to climate change . Mediterranean-type climate areas are projected to be global change “hot spots” , and dry shrublands are predicted to experience some of the most rapid increases in mean temperatures . Indeed, recent drought-related morality in California’s semi-arid Mediterranean climate shrublands has provided support for these predictions . Furthermore, the combination of dense human settlement and agricultural lands in close proximity to many natural shrubland habitats in southern California creates a likely pathway for exotic pathogen introductions and movement of pathogens from agricultural settings into wildland species. Not surprisingly, Bot. species have been retrieved on a variety of native chaparral shrub species in California, including Ceanothus spp. , Malosma laurina , and other species of Arctostaphylos . Understanding the response of native species and these pathogens to extreme weather conditions will help to predict future vegetation change and potential species losses . From 2011-2018, southern California experienced one of the most severe droughts in recorded history, with 2014 being the driest in the past 1,200 years . Field observations in winter 2014 identified high levels of branch dieback, and in some cases mortality, large round plant pots in a common ecologically important shrub, Arctostaphylos glauca in coastal California. Two well-known Bot. species were isolated from the symptomatic shrubs . Like other members of the Bot. family, both N. australe and B. dothidea infect a broad range of hosts, and are known to be responsible for disease outbreaks associated environmental stress in agricultural species . While B. dothidea is well established in California, with over 35 different host species having been identified , phylogenic evidence suggests N. australe may be more recently introduced . Its impact on shrublands of California has not been quantified.
Preliminary observations suggested high levels of branch dieback, and in some cases mortality, at lower elevation sites and along exposed ridges compared to higher elevations in coastal montane settings. We hypothesized that identifiable patterns would exist in the distribution of B. dothidea and N. australe across these landscapes that correlate with branch dieback and environmental variables associated with drought stress. Manzanita dieback has previously been causally associated with Bot. infection . A greenhouse experiment by Drake-Schultheis et al. , revealed that drought enhances onset of stress symptoms and mortality in young A. glauca inoculated with N. australe compared to shrubs subjected to drought or inoculation alone. However, to the authors’ knowledge no previous quantitative studies exist on the distribution of Bot. species in California shrubland environments with Mediterranean climates. To better understand the occurrence, distribution, and severity of Bot. infections in chaparral shrublands, we surveyed infection in A. glauca between April and September 2019. We also collected data on site elevation, aspect, and average percent canopy dieback at each site sampled for infection. While a variety of landscape variables are likely to influence plant stress at any given site , we focused on elevation because A. glauca already tends to occur mostly on xeric and rocky soils of exposed slopes, and therefore elevation was presumed to be the most significant factor influencing precipitation and water availability in this setting. Also, other studies have used elevation as a proxy for climate variation . We also recorded aspect of each sampled shrub since it influences sun exposure, temperature, and water stress. To test our hypothesis that Bot. fungi and level of stress each played a role in extensive canopy dieback in A. glauca, the following questions were addressed: What is the distribution of Bot. infection in A. glauca stands across the chaparral landscape in coastal Santa Barbara County? How do levels of infection by the two Bot. fungi, N. australe and B. dothidea, compare across elevation? and How do stand-level infection and elevation correlate with dieback severity? We predicted N. australe and B. dothidea to be presentacross all sites and elevations, but also that N. australe, having been previously isolated with high frequency in the area , would likewise have the greatest incidence in this study. Furthermore, we expected levels of Bot. infection and dieback severity to be greater at lower elevations compared to higher elevations, because lower sites typically receive less annual rainfall, thus exacerbating drought stress. This study presents the first quantitative survey summarizing the severity and distribution of Bot. fungi in natural shrublands, and seeks to identify important patterns of infection and dieback in A. glauca to predict future vulnerabilities across the landscape.The study sites were located on the generally south-facing coastal slopes of the Santa Ynez Mountains in Santa Barbara, California, USA . The sites range from a lower elevation of ~550m to an upper elevation of ~1145m, and cover an area of ~47km2 . This region is characterized by a Mediterranean climate, with wet winters and hot, dry summers. Mean annual precipitation ranges from 68.4cm at lower elevations to 90.6cm at upper elevations . During the 2013-2014 wet season, which was two years into a multiyear drought and one of the driest years on record in California , these areas received only 24.8cm and 31.6cm precipitation, respectively .Sites were initially randomly generated from polygons drawn in the field around relatively pure stands of A. glauca , and Drake-Schultheis, unpublished data. Polygons were then categorized according to elevation , and numbered within their respective elevation categories. Ten sites per elevation zone were randomly selected using random number generator for a total of 30 sites. When necessary, some randomly generated sites were substituted with nearby stands that were more accessible. Furthermore, any randomly selected sites that were discovered to be in recent fire scars were exchanged for nearby stands that contained intact, mature A. glauca.Elevation data were collected in situ using Altimeter GPS Pro and corroborated using Google Earth . Aspect was recorded in situ in degrees, then converted to radians and transformed to linear data for analysis of “southwestness” using cos according to Beers et al. . This yielded aspect values ranging from -1 to 1 , which were then used for modeling the effects of aspect on shrub dieback and Bot. infection. Sites were demarcated by >50% A. glauca cover within a stand, as determined by visual on the-ground assessments where the tops of the canopies could be viewed. Stand dieback was then visually estimated by two-to-three people as the percent of “non-green” vegetation compared to live, green vegetation within the defined boundaries of a site . Categories of NGV included yellow, brown, and black leaves, and bare/defoliated canopy, and percentages were summed to reflect total NGV within a site.