Effects of building modifications and municipal policies on green cover in Los Angeles County

2012-08-02Ecosystems are fundamentally important for sustaining both the quality of urban environments and healthy communities, but are threatened by urban development, land use changes, population growth, etc. This dissertation represents an effort to estimate how the magnitude and character of green cover has changed and how these changes have been affected by municipal policies and city ordinances across urbanized Los Angeles County over the past decade using a variety of geospatial techniques. ❧ Chapter 1 introduces the background and motivation for estimating green cover change and evaluating the extent to which municipal policies and city ordinances can be designed and implemented to protect green cover across large metropolitan regions. ❧ Chapter 2 focuses on how green cover (trees, shrubs, and grass) have changed on single family home lots in the 20 largest cities south of the Angeles National Forest and for the 15 Los Angeles City council districts during the past decade. Separate samples of single family home lots for which the Los Angeles County Office of the Assessor had or had not recorded changes in building inputs were used with heads-up digitizing of land cover on high-resolution color imagery for two points in time to measure several forms of land cover change and estimate the rate of green cover change. The analysis suggests that approximately 305,000 trees have been lost from single family neighborhoods in the 20 cities during the past decade. Taken as a whole, the analysis performed for this chapter pointed to a 12% loss of green cover from single family homes during the past decade. ❧ Chapter 3 examines the full suite of land uses in the same 20 cities explored in Chapter 2. The analysis in this chapter uses Los Angeles County Assessor’s Office property records and an automated, object-oriented classification approach with the same aerial imagery at two points in time that was used in Chapter 2 to measure land use changes, the numbers of parcels in each of these land use lasses for which building footprint changes were recorded by the Los Angeles County Assessor’s Office, and how land cover had changed from 2000 to 2010. Each of the 20 cities and 15 Los Angeles City council districts was assigned to one of eight qualitative classes based on the direction and magnitude of the changes in building footprints, hardscape, tree and grass cover and used to indicate the very dynamic urban landscape and tremendous variability in the changes in green cover that have occurred during the past decade. Taken as a whole, the analysis in this chapter estimated substantial green cover losses from all of the land use classes and pointed, in particular to a 12% loss of green cover from the single family neighborhoods that covered approximately 61% of the land surface in the 20 cities. This last result should be good in concordance with the cumulative results obtained in Chapter 2 notwithstanding large variations in estimates for individual cities and council districts that suggest that larger samples would have been needed to validate the methods used in Chapter 2. ❧ Chapter 4 therefore took the city-wide land cover change estimates from Chapter 3 and used them to investigate the municipal policies, city ordinances, and other factors that may influence the magnitude and character of green cover and natural values within residential neighborhoods. Three types of multiple regression models - ordinary least squares, forward stepwise, and backward stepwise - were developed and used in an attempt to explain the spatial variability of green cover across the 20 cities in 2000-2001 and the rate of change in green cover from 2000 to 2010. The first set of final regression models showed how approximately 50% of the variability in green cover in 2000-2001 can be explained by variations in lot size, the age of homes, the number of protected tree species, and the size of lot setbacks across the 20 cities. The coefficients for all of the aforementioned variables were positive; indicating more of them (i.e. larger lots, older homes, etc.) indicated larger green cover extents. The second set of final regression models explained approximately 46% of the variability in green cover loss across the 20 cities using age of homes, the number of protected tree species, the size of property setbacks and the number of years cities were designated as Tree City USA® cities (if at all) as explanatory variables. The coefficient for the first variable was positive (as above) and the remainders were negative indicating, for example, that the greater the number of protected tree species, the smaller the green cover losses over the past decade. ❧ Overall, this dissertation set out to document and to characterize the magnitude and character of green cover in 2000-2001 and how it has changed during the past decade. The individual chapters utilized a variety of spatial analysis techniques with high resolution color aerial photography and property data from the Los Angeles County Office of the Assessor to estimate green cover losses across the 20 most populous cities in Los Angeles City. Further data were later collected on the kinds of municipal policies and ordinances adopted by these cities to indicate whether or not these kinds of instruments could be used by cities to manage the magnitude and character of green cover in single family neighborhoods that dominate land use in large cities across the U.S. and many other parts of the world