Urbanization has been a significant cause of deforestation throughout the latter half of the 20th century, and given global demographic trends, the conversion of forested land to urban uses will likely continue. California has had a long history of converting forests, and urbanization has been one of the principle drivers. While many studies have examined how urbanization alters forest landscape structure at landscape or regional scales, little is known about how urban development influences linear forest edge structure at local scales where individual homeowner decisions dominate. We studied how forest edges at two California coastal oak woodlands (Pacheco Valle (PV) and China Camp (CV)) in the San Francisco Bay Area in California changed in the decades following urbanization. Using remote sensing and object-based image analysis, we isolated 20 urban-forest edges per site and quantified each edge's complexity (measured by sinuosity) for three time points at each site. Edges exhibited low sinuosity immediately following development (PV = 1.584, CC = 1.5625), but grew significantly more complex (in 2003 PV = 1.8705, CC 1.906). Linear forest edge structure at both sites, despite different development dates, showed similar and statistically significant increases in sinuosity by 2003. We attribute the initial, more linear structure to mortality and trunk or canopy damage caused by construction, while ascribing the later, more complex structure to tree recruitment, canopy expansion, and homeowner actions that influence natural processes.
De Chant, Tim and Maggi Kelly. 2009. Individual Object Change Detection for Monitoring the Impact of a Forest Pathogen on a Hardwood Forest. Photogrammetric Engineering and Remote Sensing 75(8): 1005-1014.
Sudden oak death (SOD) has caused widespread mortality in a number of tree and shrub species throughout coastal California. As a result, canopy changes are directly visible from remotely sensed imagery. To quantify changes in horizontal canopy structure to the oak woodlands in China Camp State Park, California, USA, a heavily hit area, we developed a novel change detection technique that tracks changes to individual objects. Using 4-band, 1 m spatial resolution aerial photography, we classified four annual images (2000 to 2003) with object-based image analysis (OBIA) and employed a GIS for our change detection technique. We identified 352 gaps that contained SOD mortality in 2000 and persisted through 2003. Their median areas and perimeters did not change significantly in that time. However, those gaps that increased in size tended to be smaller than those that decreased, indicating increased mortality in newly infected areas. Our new change detection method allowed us to monitor these gaps one-by-one, revealing ecologically meaningful results that would otherwise be obscured in a landscape-scale analysis.
Where the Woodland Ends: How Edges Affect Landscape Structure and Physiological Responses of Quercus agrifolia
Forests and woodlands are integral parts of ecosystems across the globe, but they are threatened by a variety of factors, including urbanization and introduced forest pathogens. These two forces are fundamentally altering ecosystems, both by removing forest cover and reshaping landscapes. Comprehending how these two processes have changed forest ecosystems is an important step toward understanding how the affected systems will function in the future. I investigated the range of edge effects that result from disturbance brought about by forest pathogens and urbanization in two coastal oak woodlands in Marin County, California.
Oak woodlands are a dynamic part of California's landscape, reacting to changes in their biotic and abiotic environments across a range of spatial and temporal scales. Sudden Oak Death, caused by the introduced forest pathogen Phytophthora ramorum, has led to widespread mortality of many tree species in California's oak woodlands. I investigated how the remaining trees respond to such rapid changes in canopy structure (Chapter 2), and my results revealed a forest canopy quick to respond to the new openings. Urbanization, another disturbance regime, operates on a longer time scale. Immediately following urban development, forest edges are strikingly linear, but both forest processes and homeowner actions likely work in concert to disrupt the straight edge (Chapter 3). Forest edges grew more sinuous within 14 years of the initial disturbance, and continued to do so for the remainder of the study, another 21 years. Individual Quercus agrifolia trees also respond to urban edges decades after disturbance (Chapter 4), and their reaction is reflected in declining stable carbon isotope values (δ13C). This change suggests trees may have increased their stomatal conductance in response to greater water availability, reduced their photosynthetic rate as a result of stress, or some combination of both.
Edges have far reaching and long lasting effects on forest structure and function. Investigations of their impacts on multiple spatial and temporal scales are important in determining the range of effects they have on forest ecosystems. Studies that combine remote sensing, geographic information systems, and field studies may help us understand the ecological consequences of forest edges.
Read my entire dissertation (pdf).
Maggi Kelly was my Ph.D. advisor in the Department of Environmental Science, Policy and Management at UC Berkeley. Check out her lab for more related research.