Allegheny County Urban Tree Canopy Assessment

TREND 4: MUNICIPAL CANOPY CHANGE

Municipalities with larger land areas are experiencing the largest gains and the biggest losses.

Of Allegheny County’s 130 municipalities, 101 experienced a net loss of tree canopy and 29 a net gain of tree canopy between 2015 and 2020. Net change acres ranged from a 453 acre loss in tree canopy in South Fayette to a gain of 94 acres in Fawn Township. Larger land area municipalities on the outer ring of the county experienced the largest gains and losses in terms of acreage in the County with a few exceptions. The City of Pittsburgh, the largest land area municipality as well as the largest at 14,029 acres of canopy in the County had a net gain of tree canopy of nearly 4 acres.

The larger municipalities outside of the urban inner core of the county tend to have less dense land development patterns as well as more rural land uses. These factors, along with others, contribute to the significant losses and gains of acreage. 100 acres or more of tree canopy were lost in 14 municipalities during the study period, which amounts to 68% (-2,803 acres) of the net tree canopy loss of the county between 2015 and 2020. Interestingly, these 14 municipalities represent only 30% of the county's land area and tree cover. In contrast, seven (7) municipalities experienced a net gain of 20 acres or more during this period, accounting for 17% of the county's land area and 20% of its canopy cover. Together, these municipalities added 338 acres of tree canopy cover.

Change Factors: Pine Twp. as seen in Map 4 has seen significant residential development,similar developments can be seen in Ohio Twp., Franklin Twp., South Fayette Twp, Bethel Park, Moon Twp. and Findlay Twp. Several municipalities saw land clearing for commercial development including Findlay Twp. and Ross Twp. Another contributor to canopy change in the County was work done by the Pennsylvania Turnpike Commission. Construction of the Southern Beltway in South Fayette Twp. and lane expansion of the PA Turnpike in Indiana Twp. and Hampton Twp. have cleared considerable tree canopy.

ALLEGHENY COUNTY URBAN TREE CANOPY CHANGE ASSESSMENT UPDATE: COUNTYWIDE TRENDS

LIDAR data and high-resolution aerial imagery of Allegheny County from 2015 and 2020 was analyzed to benchmark tree canopy coverage and to compare changes in tree canopy over time. As of 2020, Allegheny County has a considerable canopy cover of 51.7%. The comparison between 2015 and 2020 revealed that 224,212 acres of tree canopy experienced no change, 16,931 acres of tree canopy was added, and 21,047 acres of tree canopy was lost. The net change in tree canopy coverage for the 5-year timeframe was a decrease of 1.7% (4,115 acres) in Allegheny County. The reasons for the changes in tree canopy coverage are numerous and could be influenced by factors such as urban development, climate change, and tree management practices. For this report, the analysis focused on four trends that have contributed to the changes in tree canopy coverage in Allegheny County.

Prepared by Davey Resource Group for Tree Pittsburgh and the Allegheny County Canopy Alliance, with funding provided by the Richard King Mellon Foundation

Map 4. Large areas of tree canopy in Pine Twp. primarily caused by new housing developments.

TREND 1: RESIDENTIAL CANOPY LOSS

Residential parcels which make up 46% of the County's land area saw the largest net loss in tree canopy (-3,781 acres) between 2015 and 2020.

Among the eight (8) Land Use Classes, residential is the biggest in terms of land area and tree canopy, with 50% of the County's tree canopy found on residential land. Although residential land experienced a significant gain of 8,697 acres of tree canopy, it also saw a loss of 12,478 acres, resulting in a net change of -3,781 acres lost, equivalent to a percent change of -3.1% as compared to the countywide percent change of -1.7%. The majority of tree canopy change, 92%, occurred on residential property, with all other land uses having minimal impact on the County’s tree canopy change (Table 1).

Residential land use classes in Allegheny County were categorized into several groups for this analysis, including single-family, multi-family, high density, and vacant (which applies to several other class descriptions and is not limited to residential).

Single-family residential, which represents 34% of the County's land area, had the greatest loss in tree canopy (-3,126 acres) between 2015 and 2020.

Change Factors: As seen in Map 1, a typical residential neighborhood in Allegheny County, much of the tree canopy loss is very fragmented. This loss can be attributed to individual tree removal as well as pruning. Trees may have been removed for any number of reasons; from managing risk and declining tree health, to infrastructure conflicts. Pruning also plays a role in tree canopy change, whether pruning for utility clearance or to remediate storm damage. However, not all canopy loss is so fragmented; some residential properties have cleared many acres of canopy for redevelopment or subdivided to build additional homes.

TREND 3: LANDSLIDE PRONE AREAS

Landslide-prone areas which represent 23% of the County land area show a net loss of tree canopy of -691 acres (17%) between 2015 and 2020.

Landslides can be attributed to more than a dozen man-made and natural factors including precipitation and vegetation removal1. Due to the climate crisis, Pittsburgh has witnessed a surge in rainfall over the last few years, with 2018 and 2019 ranking as the wettest and third-wettest years on record2, respectively, which has coincided with an increase in landslides. In 2022, the City of Pittsburgh was monitoring over 120 landslides within city boundaries3 .

Landslide-prone area maps4 were overlayed with tree canopy change data from 2015 to 2020. Of the total land area in Allegheny County, 23% (107,232 acres) is considered landslide prone. Between 2015 and 2020 landslide prone areas experienced a gain of 2,924 acres of tree canopy and a loss of 3,614 acres for a net loss of 691 acres of tree canopy (Table 3). Of the nine landslide prone area categories the "Outcrop Area of Thick 'Red Bed' and Associated Rock" is the largest with 49,547 acres of prone areas. This category had a net loss of 311 acres of tree canopy. As of 2020, the tree canopy coverage for all landslide-prone areas was 69.3%, which is significantly higher than the countywide tree cover of 51.7%. While Landslide prone areas represent 23% of the land area of the county, they contain 31% of the county’s tree canopy.

TREND 2: COMMERCIAL AND INSTITUTIONAL CANOPY GAIN

Commercial and institutional parcels which make up 11% of Allegheny County’s land experienced the greatest net gains, adding 174 acres of new tree canopy between 2015 and 2020.

Allegheny County's land use codes were combined into 12 land use categories to assess changes in tree canopy cover by land use type. The biggest gains in tree canopy cover were seen in the commercial and institutional land use categories.

» Commercial: Retail net gain of 28.72 acres (+1.57% change)

» Commercial: Office net gain 31.84 acres (+1.15% change)

» Commercial: Other net gain of 4.05 acres (+0.08% change)

» Institutional net gain of 109 acres (+1.1% change)

While commercial and institutional land use categories have lower tree canopy cover than Allegheny County as a whole, these land uses saw the greatest net gain in tree canopy cover. All other use code categories experienced a net loss of tree canopy between 2015 and 2020 (Table 2).

Change Factors: Institutional properties like cemeteries, universities, and schools generally have stable, long-term use. This allows for trees to grow with fewer impacts from development. Commercial properties that are developed, whether required by code or not, often include some component of landscaping. As can be seen in Map 2, the commercial development of the South Side Works has resulted in a canopy gain from street tree plantings and new greenspaces that were constructed in the last 20 years.

Change Factors: The significant number of landslides in recent years has caused canopy loss during the actual slide as well as additional canopy loss during remediation efforts (Map3). In addition, the tree canopy change maps show development and small clusters of tree mortality impacting landslide prone areas.

Table 3. Landslide Prone Areas and Tree Canopy Statistics

Table 1. Land Use Classes and Tree Canopy Statistics

Table 2. Use Code Categories and Tree Canopy Statistics

Map 1. Typical residential area of Allegheny County.

Map 2. Commercial tree canopy gain at the South Side works development.

Map 3. An area of tree canopy lost due to a landslide.