Updated high-resolution land use and land cover data show that between 2013/14 and 2017/18, the Chesapeake Bay watershed lost approximately 25,832 net acres of tree canopy. And with updated figures from 2021/22 due to be released next year, it is all but certain that we’ll continue to see a drop in tree cover across the region.
This was enough information for the Chesapeake Bay Program’s Stakeholders’ Advisory Committee to devote their May 2023 quarterly meeting to discuss this issue and the implications that will—and currently are—arising from it.
The Stakeholders Advisory Committee consists of volunteers from across the Chesapeake Bay watershed who advise the Chesapeake Executive Council on the interests of communities and stakeholders. The committee learns and discusses state and local priorities related to water quality, living resources, wildlife habitats, community engagement and other priorities outlined in the Chesapeake Bay Watershed Agreement.
But in Harrisburg, Pennsylvania on May 24-25, 2023, it was all about trees. The Chesapeake Bay Watershed Agreement includes Tree Canopy as one of its 31 outcomes. The partnership is striving to expand the net urban tree canopy by 2,400 acres in 2025. With only 8,300 acres of trees planted in 2013/14-2017/18 compared to the loss of 25,832, the Tree Canopy Outcome is off course and will not be achieved by 2025.
Due to concerns over this loss, the Stakeholders Advisory Committee convened an expert panel to speak at their recent meeting to educate members about where and why undeveloped, or natural land, is being lost. The goal was to draft a recommendation that will be presented to the leadership of the Chesapeake Bay Program, incentivizing the protection of tree canopy.
Experts weigh in
The panel consisted of Karen Firehock of the Green Infrastructure Center (GIC), Peter Claggett of the U.S. Geological Survey at the Chesapeake Bay Program, and Julie Mawhorter of the U.S. Forest Service.
Karen Firehock, executive directive of the Green Infrastructure Center (GIC), stressed the many benefits that trees in urban settings can provide, from their ability to improve our mental and physical health to how they can help increase property values.
Firehock knows these benefits, and the consequences, inside and out. The GIC helps communities evaluate their green assets and make plans to conserve them, considering ecological, economic and cultural impacts. They do this by mapping land cover, modeling the ecosystem benefits of trees and creating strategic green infrastructure plans.
Urban trees act as a cost-effective stormwater management tool. The crown of a tree can intercept 20% of rainfall and the roots increase the infiltration capacity of soils, slowing and decreasing the amount of stormwater runoff. When trees become replaced with impervious surfaces, such as roads, roofs and cement, runoff is exacerbated, often resulting in flooding.
The current increase in impervious surfaces across the watershed, when combined with climate change, also creates hotter cities, a phenomenon referred to as urban heat islands. Areas without trees are on average 12 degrees hotter than in areas with trees in the same city.
Additionally, increases in impervious surface areas also create what is referred to as “micro-climates.” In heavier concentrated areas, like cities who have larger amounts of impervious surfaces, experts are finding their climate differs from the surrounding area. These conditions can lead to not only more extreme heat, but “rain bombs,” or high intensity, short duration storms. From 2001-2022, there were six times the number of storms resulting in damages exceeding $1 billion dollars compared to the previous two decades. If the current trend of tree loss continues, cities will become hotter and are expected to experience more extreme weather events.
Peter Claggett, a research geographer with the U.S. Geological Survey, demonstrated the high-resolution land use and land cover data that is helping to map these areas of tree canopy loss, as well as a new tool referred to as the Hyper-Resolution Hydrography mapping data. The software is capable of mapping the entire watershed at a one-by-one meter resolution, which is so detailed that it can pick up ditches, gullies and wales, and even distinguish between perennial and intermittent streams.
The data showed that between 2013/14 and 2017/18 in the 64,000-square-mile Chesapeake Bay watershed:
- 64,000 acres of natural land was cleared for development:
- 6,000 acres of natural land transitioned into agricultural use;
- 5,000 acres of natural land was cleared for mining;
- In the same period, 38,000 acres of agricultural land was converted to development.
Overall, the Commonwealth of Virginia is experiencing the highest rate of land use change.
Your community tree canopy
As the Mid-Atlantic Community and Urban Forestry Coordinator with the U.S. Forest Service, Julie Mawhorter is responsible for tracking the overall gain and loss of tree canopy across the Chesapeake Bay watershed. This information is not only informed by the high-resolution land cover and land use data, but also through progress reported by each jurisdiction as part of their Phase III Watershed Implementation Plans.
To reach a community tree planting goal, the outcome specifically measures existing tree canopy + plantings and growth - losses. The most significant actions to progress this outcome are to ensure growth (protection and maintenance) and minimize losses (mortality and removal). Included in Julie’s presentation was an overview of resources the public can utilize to learn more about their communities tree canopy.
- The Chesapeake Tree Canopy Network has created a Tree Cover Fact Sheet Data Guide for every county in the watershed. Using this resource, residents can see what percentage of their county is covered by trees, the benefits of trees, and how and where tree canopy is changing.
- American Forests released a national tree equity score tool for census urbanized areas. This tool uses an overlay of metrics in each neighborhood - such as existing tree canopy, population density, employment, income, surface temperature, and health, among others, to create a tree equity score. This information can inform where efforts need to be focused to increase tree equity across a community.
- The Chesapeake Tree Canopy Network has also created the Local Government Guide: Capitalizing on the Benefits of Trees. The guide provides support for local officials to make informed decisions about their county's tree canopy, with information on jurisdiction-specific urban and community forestry programs.
Ann Jurczyk, who is a member of the Stakeholders’ Adivsory Committee and chair to its Conservation and Land Use Subcommittee, said, “Tree canopy – particularly over asphalt – can help cool our streets and reduce flooding in a cost-effective way. This CBP high resolution data enables localities to review and potentially change their programs and policies to preserve existing canopy and fill canopy gaps in an equitable manner.”
The Stakeholders’ Advisory Committee will meet again on September 13th and 14th, 2023 in Fredericksburg, VA. They will refine their annual recommendations to advise the Leadership of the Chesapeake Bay Program at the annual Executive Council meeting held in Washington D.C. in October.