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Open Access Publications from the University of California

Scholarly Works (12 results)

  • Thesis
  • Peer Reviewed
UC Davis Electronic Theses and Dissertations (2021)

Semi-arid cities must weigh the cooling effects of urban vegetation against water conservation needs. Many cities have adopted residential outdoor water use reduction policies, resulting in an increase in drought-tolerant yard landscaping. Questions remain about whether drought-tolerant landscaping affects microclimates at the scale at which residents differentially experience heat: within their own yards. I investigated the effects of landscaping choices on temperature and thermal comfort in yards across Sacramento, CA, asking, 1) Are yards landscaped for drought-tolerance hotter than those landscaped with conventional turfgrass? and 2) Does the tree canopy interact with landscaping to affect yard-scale heat? To answer these questions, I sampled drought-tolerant and conventional turfgrass yards that had a range of tree canopy cover, at both yard- and patch-scales. I measured air temperature, relative humidity, incoming solar radiation, and wind speed and direction within the yards, and estimated thermal comfort using the NOAA Heat Index (HI), Universal Thermal Comfort Index (UTCI), and Physiological Equivalent Temperature (PET).

When considered independently from the effects of tree canopy, air temperature anomalies were +0.30°C greater in drought-tolerant yards. Canopy cover within the yard offered a cooling effect across both landscaping types. The effect of high amounts of tree canopy on improving thermal comfort was more dramatic in drought-tolerant yards. In yards with low canopy cover, drought-tolerant landscaped yards exhibited an average UTCI anomaly of +3.32°C, while conventional yards were cooler at just +1.47°C. In yards with high canopy cover, however, drought-tolerant yards had an average UTCI anomaly of -3.69°C and conventional yards had an average anomaly of -2.66°C. Drought-tolerant yards were extremely heterogeneous and included a wide range of landscaping materials. Regardless of canopy cover, drought-tolerant yards with predominantly living landscaping (e.g., succulents, perennial grasses) had cooler air temperatures than those with predominantly non-living landscaping (e.g., hardscape, gravel).

This study demonstrates the importance of quantifying how changes in urban landscaping at the scale of an individual yard can affect the severity of heat experienced by residents. Advancing our understanding of localized heat and vegetation dynamics allows residents and municipal policymakers to make informed decisions at the intersection of water conservation and heat risk.

    Cover page: Balancing water conservation against heat mitigation: The effect of residential landscaping on thermal comfort in a semi-arid city
    • Article
    • Peer Reviewed
    UC Davis Previously Published Works (2021)
    Cities and towns are complex ecosystems with features that can vary dramatically in space and time. Our knowledge of the spatial structure of urban land and ecological systems is expanding. These systems have been investigated across spatial scales, urban to rural gradients, networks of urban macrosystems, and global megalopolises. However, the temporal dimensions of urban ecosystems – such as those related to ecological cycles and historical legacies – are far less understood and investigated. Here, we outline the main dimensions of time that can shape how events in urban ecosystems unfold, which we categorize as: (i) time flows and duration, (ii) synchrony, lags, and delays, (iii) trends and transitions, (iv) cycles and hysteresis, (v) legacies and priming, (vi) temporal hotspots and hot moments, and (vii) stochastic vs. deterministic processes affecting our ability to forecast the future of cities and the species that live in them. First, we demonstrate the roles of these understudied dimensions by discussing exemplary studies. We then propose key future research directions for investigating how processes over time may regulate the structure and functioning of urban land and biodiversity, as well as its effects on and implications for urban ecology. Our analysis and conceptual framework highlights that several temporal dimensions of urban ecosystems – like those related to temporal hotspots/moments and stochastic vs. deterministic processes – are understudied. This offers important research opportunities to further urban ecology and a comprehensive research agenda valuing the “Urban Chronos” – the change of urban ecosystems through time.
      Cover page: Valuing the Role of Time in Urban Ecology
      • Article
      • Peer Reviewed
      UC Davis Previously Published Works (2016)
      Nitrogen (N) deposition is a key global change factor that is increasing and affecting the structure and function of many ecosystems. To determine the influence of N deposition on specific systems, however, it is crucial to understand the temporal and spatial patterns of deposition as well as the response to that deposition. Response of the receiving plant communities may depend on the life stage-specific performance of individual species. We focus on the California oak savanna because N deposition to this system is complex-characterized by hotspots on the landscape and seasonal pulses. In a greenhouse experiment, we investigated the relative influence of N deposition on plant performance during early growth, peak biomass, and senescent life stages across different soil types, light, and community compositions. To represent the community we used three grass species-a native, naturalized exotic, and invasive exotic. At early growth and peak biomass stages performance was measured as height, and shoot and root biomass, and at the senescent stage as seed production. Simulated N deposition 1) increased shoot biomass and height of the native and, even more so, the naturalized exotic during early growth, 2) positively affected root biomass in all species during peak biomass, and 3) had no influence on seed production at the senescent stage. Alone, N deposition was not a strong driver of plant performance; however, small differences in performance among species in response to N deposition could affect community composition in future years. In particular, if there is a pulse of N deposition during the early growth stage, the naturalized exotic may have a competitive advantage that could result in its spread. Including spatial and temporal heterogeneity in a complex, manipulative experiment provides a clearer picture of not only where N management efforts should be targeted on the landscape, but also when.
        Cover page: The Effect of Nitrogen Deposition on Plant Performance and Community Structure: Is It Life Stage Specific?
        • Article
        • Peer Reviewed
        UC Davis Previously Published Works (2022)
        Ecologists who study human-dominated places have adopted a social-ecological systems framework to recognize the coproduced links between ecological and social processes. However, many social scientists are wary of the way ecologists use the systems concept to represent such links. This wariness is sometimes due to a misunderstanding of the contemporary use of the systems concept in ecology. We aim to overcome this misunderstanding by discussing the contemporary systems concept using refinements from biophysical ecology. These refinements allow the systems concept to be used as a bridge rather than a barrier to social-ecological interaction. We then use recent examples of extraordinary fire to illustrate the usefulness and flexibility of the concept for understanding the dynamism of fire as a social-ecological interaction. The systems idea is a useful interdisciplinary abstraction that can be contextualized to account for societally important problems and dynamics.
          Cover page: Systems in Flames: Dynamic Coproduction of Social–Ecological Processes
          • Article
          • Peer Reviewed
          UC Davis Previously Published Works (2018)
          Urbanization is thought to decrease riparian tree canopy cover, but this outcome may vary depending on geographic context. We examined changes in land cover and riparian tree canopy near Sacramento, California, a Mediterranean climate region impacted by agriculture before urbanization. We used aerial imagery to quantify changes in land cover and the area, width, and density of riparian tree canopy between 1937 and 2014 for the 85-km2 Arcade Creek watershed. While watershed land cover shifted almost entirely from pasture and crops to urban development, riparian forested area increased by 38 percent. The median width of riparian tree canopy more than doubled, and the density along smaller streams increased significantly (p < 0.0001). We used multiple data sources to examine potential drivers of these changes. Stream gage data and field observations indicated that the formerly intermittent Arcade Creek stream network receives spatially and temporally variable dry-season flow subsidies from urban runoff, but widespread channel incision may limit the impact of subsidies on riparian vegetation. Accordingly, sampling of riparian woody plants showed few hydrophilic species growing along these streams. Instead we found evidence of native oak regeneration and a high density of escaped horticultural species. The increase in riparian forest cover was thus likely due to changes in vegetation management and species composition from land use conversion, possibly augmented by increased dry-season water availability. The changes in canopy cover we quantified stand in contrast to the accepted pattern of riparian forest decline with urbanization, and are expected to affect ecosystem services.
            • Article
            • Peer Reviewed
            UC Davis Previously Published Works (2010)
            As highly managed ecosystems, urban areas should reflect the social characteristics of their managers, who are primarily residents. Since landscape features develop over time, we hypothesize that present-day vegetation should also reflect social characteristics of past residents. Using an urban-to-suburban watershed in the Baltimore Metropolitan Region, this paper examines the relationship between demographics, housing characteristics, and lifestyle clusters from 1960 and 2000 with areas of high woody and herbaceous vegetation cover in 1999. We find that 1960 demographics and age of housing are better predictors of high woody or tree coverage in 1999 than demographics and housing characteristics from 2000. Key variables from 1960 are percent in professional occupations (+), percent of pre-WWI housing (−), percent of post-WWII housing (+), and population density (−). Past and present demographic and housing variables are poor predictors of high herbaceous cover in 1999. Lifestyle clusters for 2000 are very good predictors of high herbaceous coverage in 1999, but lifestyle clusters from 1960 and 2000 are poor predictors of high woody vegetation coverage. These findings suggest that herbaceous or grassy areas, typically lawns, are good reflections of contemporary lifestyle characteristics of residents while neighborhoods with heavy tree canopies have largely inherited the preferred landscapes of past residents and communities. Biological growth time scales of trees and woody vegetation means that such vegetation may outlast the original inhabitants who designed, purchased, and planted them. The landscapes we see today are therefore legacies of past consumption patterns.
              Cover page: Landscape, vegetation characteristics, and group identity in an urban and suburban watershed: why the 60s matter
              • Article
              • Peer Reviewed
              UC Davis Previously Published Works (2018)
              This article is a case study of one Community-University Research and Action Partnership (CURAP) focused on soil lead, urban gardening, and environmental justice in Sacramento, California. We argue that creating and sustaining CURAPs requires a process of weaving together diverse strands of knowledge, resources, and lines of accountability that connect all parties involved. Like the physical process of weaving fabric, weaving CURAPs involve creative and collaborative uses and responses to tension between all elements of a partnership. This is especially true in long-term partnerships intended to address systemic environmental injustices. This case highlights the power relationships and challenges associated with such partnerships and presents several lessons to enrich the scholarship and practices of action research.
                Cover page: Weaving Community-University Research and Action Partnerships for environmental justice
                • Article
                • Peer Reviewed
                UC San Francisco Previously Published Works (2021)
                Many of the world’s major cities have implemented tree planting programs based on assumed environmental and social benefits of urban forests. Recent studies have increasingly tested these assumptions and provide empirical evidence for the contributions of tree planting programs, as well as their feasibility and limits, for solving or mitigating urban environmental and social issues. We propose that current evidence supports local cooling, stormwater absorption, and health benefits of urban trees for local residents. However, the potential for urban trees to appreciably mitigate greenhouse gas emissions and air pollution over a wide array of sites and environmental conditions is limited. Consequently, urban trees appear to be more promising for climate and pollution adaptation strategies than mitigation strategies. In large part, this is due to space constraints limiting the extent of urban tree canopies relative to the current magnitude of emissions. The most promising environmental and health impacts of urban trees are those that can be realized with well-stewarded tree planting and localized design interventions at site to municipal scales. Tree planting at these scales has documented benefits on local climate and health, which can be maximized through targeted site design followed by monitoring, adaptive management, and studies of long-term eco-evolutionary dynamics.
                  Cover page: The Benefits and Limits of Urban Tree Planting for Environmental and Human Health
                  • Article
                  • Peer Reviewed
                  UC Davis Previously Published Works (2015)
                  This study examines the distributional equity of urban tree canopy (UTC) cover for Baltimore, MD, Los Angeles, CA, New York, NY, Philadelphia, PA, Raleigh, NC, Sacramento, CA, and Washington, D.C. using high spatial resolution land cover data and census data. Data are analyzed at the Census Block Group levels using Spearman's correlation, ordinary least squares regression (OLS), and a spatial autoregressive model (SAR). Across all cities there is a strong positive correlation between UTC cover and median household income. Negative correlations between race and UTC cover exist in bivariate models for some cities, but they are generally not observed using multivariate regressions that include additional variables on income, education, and housing age. SAR models result in higher r-square values compared to the OLS models across all cities, suggesting that spatial autocorrelation is an important feature of our data. Similarities among cities can be found based on shared characteristics of climate, race/ethnicity, and size. Our findings suggest that a suite of variables, including income, contribute to the distribution of UTC cover. These findings can help target simultaneous strategies for UTC goals and environmental justice concerns.
                    Cover page: Trees grow on money: urban tree canopy cover and environmental justice.
                    • Article
                    • Peer Reviewed
                    UC Davis Previously Published Works (2015)
                    This study examines the distributional equity of urban tree canopy (UTC) cover for Baltimore, MD, Los Angeles, CA, New York, NY, Philadelphia, PA, Raleigh, NC, Sacramento, CA, and Washington, D.C. using high spatial resolution land cover data and census data. Data are analyzed at the Census Block Group levels using Spearmans correlation, ordinary least squares regression (OLS), and a spatial autoregressive model (SAR). Across all cities there is a strong positive correlation between UTC cover and median household income. Negative correlations between race and UTC cover exist in bivariate models for some cities, but they are generally not observed using multivariate regressions that include additional variables on income, education, and housing age. SAR models result in higher r-square values compared to the OLS models across all cities, suggesting that spatial autocorrelation is an important feature of our data. Similarities among cities can be found based on shared characteristics of climate, race/ethnicity, and size. Our findings suggest that a suite of variables, including income, contribute to the distribution of UTC cover. These findings can help target simultaneous strategies for UTC goals and environmental justice concerns.
                      Cover page: Trees grow on money: urban tree canopy cover and environmental justice.
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