Management of Urban Trees to Meet Canopy Cover Goals: Falls Church Residential Landscape Plan Comparison

2013 | David C. Chojnacky, Virginia Tech, Department of Forest Resources and Environmental Conservation
This study takes a step toward a long-term goal of effective use of tree canopy requirements during redevelopment to positively influence urban tree canopy preservation and regrowth on private lands. Requirements for minimum tree canopy cover when lots are developed or redeveloped in the City of Falls Church, Virginia—20% canopy cover within 10 years—are typical of most municipalities in the Chesapeake Bay watershed. In this project, scientists from Virginia Tech are collaborating with the Falls Church City Arborist, who is responsible for implementation of the canopy cover requirement, to develop science-based tools to help determine the effectiveness of redevelopment plans.

The primary objective is to develop methods for verifying if plan specifications are achieving their goals; little scientific information is available on how long it takes for various tree arrangements to reach 20% cover. Results from this small study also will be used to stimulate awareness and support for study and implementation of sound scientific methodologies. Field measurements will determine current inventory and status of trees on a sample of 5–10 properties in Falls Church City that have been redeveloped within the past 14 years. These data will be compared to information from the same lots at the time of development. Analyses will use data in a Geographic Information System (GIS) environment to build survival and canopy cover projection models, focusing on crown growth modeling to meet arborists’ management needs for green urban redevelopment. Use of remote sensing will also be evaluated as a means for reducing costly field work.

Final Project Narrative

Because of the important roles trees play in Chesapeake Bay restoration, federal and state mandates to improve Bay health have prompted local government standards to maintain and increase the tree canopy cover in cities. For example, in the City of Falls Church, within the greater Washington DC area in the Chesapeake Bay watershed, each residential redevelopment is required to retain or plant enough trees for 20% canopy cover within 10 years. This ordinance is administered by the City Arborist, who has two years during site redevelopment to influence tree canopy cover on this private land through the city-approved redevelopment plan. Solid, science-based tools are needed to determine if tree conservation-planting requirements in redevelopment plans are achieving their goals. Do the mandated plans truly result in the specified canopy cover, and if not, how can specifications be changed to be sure that they do?

Virginia Tech and the City of Falls Church collaborated to compare tree inventory on urban sites prior to their redevelopment (where the original house was torn down and often canopy removed to allow for a larger house) with current tree inventory to see if canopy regrowth goals were met. Field sampling, examination of redevelopment plans, extensive calculations in a GIS environment, and statistical modeling were used to analyze changes in preserved and planted trees on 21 lots 1–18 years after redevelopment.

Our assessment of non-overlapping tree cover showed that Falls Church redevelopment is generally meeting the city code requirement of 20% canopy cover 10 years after redevelopment. However, the data also showed that only on 6 lots did the cover 1–18 years after redevelopment exceed the initial cover (before any trees were removed for construction). If cover is reduced dramatically in redevelopment, particularly if big trees are cut, it takes a long time to return to its initial value.

A model was produced through this study that allows growth in canopy cover to be estimated from basal area growth (a standard metric in forestry) and the number of trees planted. Growth modeling included a 2-step process where average basal area growth was first predicted for 4 categories of trees—deciduous preserved, deciduous planted, evergreen preserved, and evergreen planted. Second, basal area growth was converted to canopy cover growth through the basal area-to-cover relationship. Individual tree and species details were not used in modeling because measurements taken for the tree inventories recorded prior to redevelopment did not meet conventional forestry inventory standards; instead, lot-scale deciduous and evergreen categories were used. The resulting growth model nevertheless appears reasonable for at least 10-yr projections. The goal of our model is to improve urban forest management of residential redevelopment by helping urban forest managers better understand how redevelopment plan practices impact the tree canopy.