The Chester County Food Bank offers a large social network of volunteers and affiliated institutions, such as schools. Many of the farms participating in food bank programs offer on-site visits for educational groups. Indeed, no single network typology dominates the center of Fig. 5, indicating that some farms which engage in wholesale practices may be no more removed from what Hinrichs calls “the aura of personal relations and social connection” than those which market through farmers’ markets. On the other hand, the most socially intertwined networks are farm-to-farm sharing of product and farm-to-restaurant . Farm-to-farm networks may be important, and often overlooked, threads in the social fabric of food policy. Indeed, these networks are overlooked in the County Comprehensive Plan. Embeddedness theory is further complicated by the realization that many farms engage in multiple networks. Every farm engaged in CSA marketing also attends a farmers’ market, 30 litres plant pots but the reverse is not true . Many farms that specialize in wholesale marketing also market through farmers’ markets. As the interviewees note, networks are in flux. Farmers may attend a farmers’ market to build a clientele list for CSA sales.
In turn, the ability of the farm to host pick-up and farm visits is regulated through zoning, such that some farms which participate in farmers’ markets and which may be inclined to host CSA sales are unable to do so. In this sense the embeddedness of local food may be constrained as much by price and values as land-use regulations. In summary, the Chester County networks grew out of proximal, historic relationships between farms and urban areas. County experts agree that there is more capacity to grow these networks, particularly if already existing networks are leveraged to create more synergies. Gleaning farms already participate in a variety of CSAs, farmers markets, school education outreach and host school field trips. To allow these farm networks to flourish, zoning codes could better accommodate farm visits with parking, signage, and the non-traditional farm uses associated with multifunctional farming . Interviews suggest that zoning reform to allow or promote urban gardening, raised beds, or high-tunnels may also help indirectly stimulate agricultural education programs, fresh food production, and nutritional meal plans for the county’s under-served through work with the food bank.There is now strong evidence that the Earth’s climate is changing due to human activities . According to the most conservative estimates, global mean annual temperatures are now outside the historic range of the last 1,300 years . Simultaneously, mean annual precipitation has declined in many parts of the Northern Hemisphere, resulting in increased drought events .
Extreme climatic shifts are predicted to affect, both directly and indirectly, biogeochemical cycling, energy fluxes, wildlife habitat, and ecosystem goods and services on a global scale . An important component in preparing for the effects of these events is to understand how communities will change in response to them, making this a critical topic for ecological research . For species to survive in dry climates, they must have evolved drought tolerance mechanisms . However, extreme climate events can expose species that are typically capable of withstanding regular drought stress to conditions outside of their normal range. Furthermore, physiological responses to extreme drought can also have a negative feedback on plants’ defensive abilities, rendering them susceptible to biotic attack including by insects or disease agents . Consequently, synergies between extreme climatic events and biotic attack will likely lead to more dramatic changes than would otherwise occur in historically “drought tolerant” plant communities . Future climate change is expected to exacerbate these interactions worldwide. . Widespread tree mortality from drought has been documented in forested systems around the world , and biotic attack has been associated with many of these events . However, much less focus has been given historically to understanding the consequences of extreme drought on shrubland communities like chaparral , particularly in conjunction with biotic influences. Therefore, as we face predictions of hotter, longer, and more frequent drought , it is becoming increasingly critical to hone in on the mechanisms, tipping points, and ecosystem impacts of these events. Furthermore, identifying plant mortality thresholds is of upmost importance for predicting susceptibility to extreme drought events of the future . California recently experienced a record-breaking, multi-year drought from 2012- 2018, estimated to be the most severe event in the last 1,000 years , with the 2013-2014 winter season being one of the driest on record .
Drought tolerance has long been considered a common trait of shrub species in California chaparral communities where hot, rainless summers are the norm . However, in the Santa Ynez mountain range in Santa Barbara County, the dominant and widespread big berry manzanita exhibited dramatic die back related to multi multi-year drought along with infection by opportunistic fungal pathogens in the Botryosphaeriaceae . These observations indicate that this species may be reaching a threshold in its drought resistance capabilities. Studies have reported Arctostaphylos spp. to exhibit unusual scales of dieback during periods of extreme drought stress, , however this could be the most severe dieback event in recent history, both in terms of scale and severity. Manzanita are important members of the chaparral ecosystem, providing habitat for wildlife and food through their nectar and berries . Additionally, their structure makes them important components of historical chaparral fire regimes, and their fire-induced germination strategies contribute to post-fire successional trajectories . Large-scale mortality of this species could reduce resource availability for wildlife, as well as alter fuel composition and structure in the region, resulting in an increased risk of more intense, faster burning fires. Therefore, the potential continued dieback of A. glauca is of great concern for both ecosystem functions and human populations alike. Significant dieback of A. glauca in Santa Barbara county, California, was first observed in winter, 2014 . Preliminary observations revealed patterns of dieback occurring along an elevational gradient, with effects being most pronounced at lower elevations than at higher elevations. It was also observed that dieback was most prevalent in stands located on steep, exposed southerly-facing slopes. These observations are consistent with findings by previous studies , Since A. glauca is classically drought-tolerant and able to function at very low water potentials , it raises the question of what is driving this extreme dieback event? Could A. glauca be reaching a tipping point as a result of extreme drought stress, presence of a fungal pathogen, or both?My dissertation research focuses broadly on the influence of drought and fungal pathogens on this classic, drought tolerant chaparral shrub species. Through a combination of methods, I explore the individual and interacting roles of water stress and opportunistic fungal pathogens in A. glauca in a major dieback event, and track the fate of individual shrubs through the progression of an historic drought. My findings are organized into three chapters based on the following questions: What are the identities and distribution of fungal pathogens associated with A. glauca dieback ; How do drought stress and fungal infection interact to promote dieback and mortality in A. glauca ; and How does A. glauca dieback progress over time during drought, and how do landscape variables and drought stress correlate with dieback ? In Chapter 2, I identify fungal pathogens in A. glauca, and discuss their distribution across the landscape in the Santa Barbara county front country region. Based on preliminary findings showing significant levels of N. australe in the field, I expected to find high incidence of this opportunistic pathogen in A. glauca across the landscape, 25 liter pot plastic suggesting their role in drought-related dieback. The data support this prediction, as over half of the pathogens isolated were members of the Bot. family, and the majority of these were identified as N. australe, a novel pathogen in the region. Furthermore, Bot. infection was highly correlated with dieback severity, which was greatest at lower elevations. Taken together, the results show that opportunistic Bot. pathogens, particularly N. australe, are highly associated with A. glauca dieback across the landscape, and that lower elevations may be particularly vulnerable. In Chapter 3, I address the hypothesis that extreme drought and N. australe function synergistically to promote faster and greater mortality than either factor alone. I designed a full-factorial greenhouse experiment to identify whether A. glauca dieback is driven by extreme drought, infection by the fungal pathogens, or both. The results of this experiment support my hypothesis. Young A. glauca inoculated with N. australe while simultaneously exposed to extreme water stress exhibited faster stress symptom onset, faster mortality, and overall higher morality than those subjected to either factor alone.
These results provide strong evidence that the severe A. glauca dieback event observed during the 2012-2018 drought was the result of synergistic interactions between extreme drought and opportunistic pathogens, rather than the nature of the drought or particularly virulent pathogens. In Chapter 4, I explore factors that are associated with climatic stress in order to draw correlations between A. glauca stress and dieback severity. Identifying such relationships can be useful in making predictions on dieback and mortality across the landscape. By analyzing data on predawn xylem pressure potentials and net photosynthesis in shrubs along an elevational gradient, I found that patterns of water availability and physiological function both varied greatly across the landscape, and only weakly correlate with dieback severity, suggesting factors other than elevation and aspect must also be important in driving plant stress and dieback. Extreme heterogeneity across this landscape likely confounded my results, yet may also play an important role in supporting the resiliency of A. glauca populations as a whole. By measuring the progression of dieback in these same shrubs over time, I found that dieback severity throughout the drought increased most at lower elevations compared to high, providing evidence that shrubs at lower elevations may be particularly vulnerable. Unexpectedly, no new mortality was observed in surveyed shrubs as the drought progressed, even though many plants exhibited severe levelsof dieback throughout the study. This result shows that high levels of dieback severity do not necessarily predict morality in A. glauca. In summary, my dissertation provides strong evidence that A. glauca dieback during the recent California drought was caused by synergistic interactions between extreme drought stress and infection by widely distributed opportunistic fungal pathogen N. australe. 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, 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.