A Report on the City of Pittsburgh's Existing and Possible Tree Canopy
Total Page:16
File Type:pdf, Size:1020Kb
DRAFT A Report on the City of Pittsburgh’s Existing and Possible Tree Canopy Why is Tree Canopy Important? Project Background Tree canopy (TC) is the layer of leaves, branches, and stems of trees that The goal of the project was to apply the USDA Forest Service’s cover the ground when viewed from above. Tree canopy provides many Tree Canopy (TC) Assessment Protocols to the City of Pitts- benefits to communities, improving water quality, saving energy, lowering burgh. The analysis was conducted based on 2010 data. This city temperatures, reducing air pollution, enhancing property values, project was made possible by funding from Tree Pittsburgh. providing wildlife habitat, facilitating social and educational opportunities, This analysis of Pittsburgh’s tree canopy (TC) was conducted in and providing aesthetic benefits. Establishing a tree canopy goal is crucial collaboration with the Tree Pittsburgh, Allegheny County, and for communities seeking to improve their green infrastructure. A tree can- the USDA Forest Service’s Northern Research Station. The Spa- opy assessment is the first step in this goal-setting process, providing esti- tial Analysis Laboratory (SAL) at the University of Vermont’s mates for the amount of tree canopy currently present in a city as well as Rubenstein School of the Environment and Natural Resources the amount of tree canopy that could theoretically be established. carried out the assessment. How Much Tree Canopy Does Pittsburgh Have? An analysis of Pittsburgh’s tree canopy (TC) based on land cover data de- rived from high-resolution aerial imagery and LiDAR (Figure 1) found that 14,883 acres of the city were covered by tree canopy (termed Existing TC), representing 42% of all land in the city. An additional 33% (11,914 acres) of the city could theoretically be modified (termed Possible TC) to accom- modate tree canopy (Figure 2). In the Possible TC category, 9% (3,436 acres) of the city was classified as Impervious Possible TC and another 24% was Vegetated Possible TC (8,406 acres). Vegetated Possible TC, or grass and shrubs, is more conducive to establishing new tree canopy, but estab- lishing tree canopy on areas classified as Impervious Possible TC will have a greater impact on water quality and summer temperatures. Figure 2: TC metrics for Pittsburgh based on % of land area cov- ered by each TC type. Key Terms TC: Tree canopy (TC) is the layer of leaves, branches, and stems of trees that cover the ground when viewed from above. Land Cover: Physical features on the earth mapped from aerial or satellite imagery, such as trees, grass, water, and impervious surfac- es. Existing TC: The amount of urban tree canopy present when viewed from above using aerial or satellite imagery. Impervious Possible TC: Asphalt or concrete surfaces, excluding roads and buildings, that are theoretically available for the establish- ment of tree canopy. Vegetated Possible TC: Grass or shrub area that is theoretically Figure 1: Land cover derived from high-resolution aerial imagery for the City available for the establishment of tree canopy. of Pittsburgh. 09/08/11 1 DRAFT Mapping Pittsburgh’s Trees Parcel Summary Prior to this study, the only comprehensive remotely-sensed esti- After land cover was mapped city-wide, Tree Canopy (TC) metrics mates of tree canopy for Pittsburgh was from the 2001 National were summarized for each property in the city’s parcel database Land Cover Database (NLCD 2001). While NLCD 2001 is valuable for (Figure 4). Existing TC and Possible TC metrics were calculated for analyzing land cover at the regional level, it is derived from relative- each parcel, both in terms of total area and as a percentage of the ly coarse, 30-meter resolution satellite imagery (Figure 3a). Using land area within each parcel (TC area ÷ land area of the parcel). high-resolution aerial imagery and LiDAR acquired in 2010 and 2006 respectively (Figure 3b), in combination with advanced auto- mated processing techniques, land cover for the city was mapped Parcels in much greater detail (Figure 3c). NLCD 2001 substantially under estimated Pittsburgh’s tree canopy, at 24%, largely because it failed to capture smaller patches. a. NLCD 2001 Percent Tree Canopy (30 meter) Existing Tree Canopy (TC) b. 2008 Aerial Imagery (1 meter) Possible Tree Canopy (TC) c. Land Cover Derived from 2010 Aerial Imagery Tree Canopy Grass/Shrub Bare Soil Water Buildings Roads/Railroads Other Paved Figure 3a, 3b, 3c: Comparison of NLCD 2001 to high-resolution land Figure 4a, 4b, 4c: Parcel-based TC metrics. TC metrics are generat- cover. ed at the parcel level, allowing each property to be evaluated ac- cording to its Existing TC and Possible TC. 09/08/11 2 DRAFT Land Use Allegheny County maintains a comprehensive land use layer for the region. Existing and Possible Tree Canopy (TC) were summarized for the twelve aggregated land use classes (Figure 5, Table 1). For each land use class, Tree Canopy (TC) metrics were calculated as a percentage of all land in the city (% Land), as a percentage of land area in the specified land use category (% Category), and as a percentage of the area for a given TC type (% TC Type). Residential land is the largest land use type within the city. The fact that 43% (% Category) of residential land is covered by tree canopy results in residential land being the largest contributor to the city’s overall tree canopy (35%). With 98% of its land covered by tree canopy, agricultural land it is the clear leader in the % Category field for Existing TC, but the amount of agricultural land in the city is so small that the overall contribution is insignificant (% TC Type). Government land use also had a high percentage of land covered by Existing TC (65%). Residential land also accounts for the largest single amount of Possible TC in the city, at 38% (% TC Type), but industrial land and areas where the land use is either not available (N/A) or classified as other all have nearly 50% of their land area available for tree canopy (% Category). The challenge in establishing tree canopy on industrial land is that much of the available space for planting trees falls into the Possible TC Impervious category (paved surfaces that are not buildings or roads). Establishing new tree canopy on lands considered to be resi- dential or other should theoretically be easier as they have relatively high percentages of land in the Possible TC Vegetation category. Figure 5: Tree Canopy (TC) metrics summarized by land use. Existing TC Possible TC Vegetation Possible TC Impervious Land Use % Land % Category % TC Type % Land % Category % TC Type % Land % Category % TC Type Agricultural 0% 98% 0% 0% 2% 0% 0% 0% 0% Commercial 6% 36% 14% 3% 19% 13% 3% 21% 35% Government 11% 65% 26% 3% 19% 13% 1% 8% 13% Industrial 0% 17% 1% 0% 16% 2% 1% 33% 9% N/A 0% 33% 0% 0% 22% 1% 0% 26% 2% Other 2% 41% 6% 2% 37% 9% 1% 11% 6% Residential 15% 43% 35% 11% 32% 47% 2% 5% 17% Rights of Way 6% 31% 15% 3% 14% 12% 1% 6% 12% Utilities 1% 37% 3% 1% 23% 3% 1% 17% 5% Area of TC type for zoning district Area of TC type for zoning district Area of TC type for zoning district % Land = % Category = % TC Type = Area of all land Area of all land for specified land use Area of all TC type The % Land Area value of 15% indicates that 15% of Pitts- The % Land value of 43% indicates that 43% of land in the The % TC Type value of 35% indicates that 35% of all tree burgh’s land area is covered by tree canopy in the Residen- Residential land use class is covered by tree canopy. canopy is in Residential land use. tial land use class. Table 1: Tree Canopy (TC) metrics were summarized by land use. For each land use class, TC metrics were computed as a percentage of all land in the city (% Land), as a percentage of land in the specified land use class (% Category), and as a percentage of the area for a given type (% TC Type). 09/08/11 3 DRAFT Parks Analysis The Existing and Possible Tree Canopy were analyzed for all parks within the Pittsburgh boundary. There are many outliers on both ends of the spectrum with some parks in forested areas having 100% tree canopy and others having no tree canopy. The parks with 0% tree canopy tend to be less than an acre and are playgrounds or sports fields. As a result, it is not socially desirable to plant trees on these parks regardless of the Possible TC percentages. Pittsburgh’s four largest parks in the area range from 63% to 90% Existing Tree Canopy. The same four largest parks ranged from 8% to 32% Possible Tree Canopy. Existing Tree Canopy Possible Tree Canopy Figure 6 Existing TC (left) and Possible TC (right) as a percentage of land area by park. Decision Support Parcel-based Tree Canopy (TC) metrics were integrated into the city’s existing GIS database (Figure 7). Decision makers can use GIS to query specific tree canopy and land cover metrics for a parcel or set of parcels. This information can be used to estimate the amount of tree loss in a planned development or set tree canopy improvement goals for an individual property. GIS Database Figure 7: GIS analysis of parcel-based TC metrics for decision support. In this example, GIS is used to select an individual parcel. The attributes for that parcel, including the parcel-based TC and land cover metrics, are displayed in tabular form providing instant access to relevant information.