A collaboration between researchers from the U.S. Geological Survey, University of Vermont and Chesapeake Conservancy has determined the Chesapeake Bay watershed is rapidly losing tree cover to expanding urban and suburban development.
By analyzing high-resolution satellite imagery between 2013 and 2018, the Chesapeake Bay Program Land Use and Land Cover Data Project publicly tracked for the first time how the entire 64,000 square mile watershed is changing. The project was designed as a tool to improve local decision-making by communities across the watershed working to meet goals set in the 2014 Chesapeake Bay Watershed Agreement, a landmark pledge by the federal government, six states including Virginia, and Washington, D.C. to clean up the nation’s largest estuary. Its data has an anticipated accuracy of over 90%.
The watershed is losing trees to impervious surfaces, or those hard areas like parking lots that can’t absorb water, at “a surprising rate,” said Peter Claggett, a research geographer at the U.S. Geological Survey and a leader for the project.
“As areas get more urbanized, you’re going to see more land conversion there,” said Caitlin Verdu, the Virginia Department of Forestry’s watershed program manager.
In Virginia, the project revealed significant changes in the state’s Bay-side landscape.
“The main things that are happening in Virginia that make it unique is that it does have a lot of development compared to some other jurisdictions, and it has the most timber harvest,” said Claggett.
When trees are cut down and replaced by impervious surfaces due to development, the land conversion is considered permanent. According to the data, impervious surface cover throughout the watershed increased by about 50,000 acres, or just over 79 square miles, during the four-year period of the study.
Impervious surfaces, such as roofs and parking lots, do not absorb water, and “that can cause problems downstream,” said Claggett. “Not only does it bear those pollutants and toxins that it picks up from roads and parking lots, but this runoff also moves quicker over the landscape if it just runs off the surface and doesn’t soak in.”
Fast-moving runoff can erode stream channels and alter the water flow patterns that aquatic species have adjusted to over millions of years, said Claggett. Tree stands planted along streams, a feature called a riparian buffer, can help manage excess runoff. The buffers also “stabilize the banks, provide shade and cool the stream, and leaves fall off the trees that help provide for the whole ecosystem of the stream itself,” said Claggett.
The majority of change in tree canopy throughout the watershed and especially in Virginia was associated with timber harvesting, but these rotational harvests are not a permanent loss in tree canopy, according to foresters. Instead, they are seen as a transformation in the forest from late succession — an older period of growth — to early succession, said Claggett.
Although it can take seven years after a forest is harvested for “those trees to even be visible on the analysis,” said Lara Johnson, the Virginia Department of Forestry’s urban and community forestry program manager, “I think when they do the next analysis, it will show that these forests are growing and that these harvested operations are being replanted, and those trees are coming back.”
The Virginia Department of Forestry is making use of state and federal funds to plant more trees in communities throughout the commonwealth.
“Whether it’s on public or private land, we’re seeing these same radical opportunities to make a difference,” said Verdu. “We’re here to advocate for all the wonderful benefits of trees and to help folks who want trees to make them a part of their community.”
The Virginia Trees for Clean Water Grant Program is offering $500,000 in grant funding to plant trees in community areas through the remainder of 2023. The program was established in 2013 and is supported primarily by the Virginia Water Quality Improvement Fund, a special state fund created in 1997 to assist local governments, soil and water conservation districts, state agencies and others with reducing and controlling water pollution.
An estimated 150,000 trees have been planted as part of the program to date, and nearly 50,000 of those plantings happened last year, said Johnson.
In addition to state funds, the Virginia Department of Forestry received $6.6 million in federal funding this year from the U.S. Department of Agriculture Forest Service as part of the Inflation Reduction Act. The funds are intended to increase tree canopy and access to nature in disadvantaged communities.
Research has found links between historic racial inequities and tree cover, with formerly redlined communities tending to have fewer trees and more impervious surface. In Virginia, that trend has been seen in Richmond, Norfolk, Roanoke and Lynchburg.
“I think this historic funding will really help us to do more projects and support more work on the ground from an urban community forestry perspective,” said Johnson.
The Chesapeake Bay Program’s land use and land cover data can be used to help counties, cities and communities across the watershed identify areas that should be priorities for conservation, said Claggett.
A community can inventory the landscape around all of its streams or urban areas, for example, to figure out where trees are missing and then set goals. “There’s a laundry list of benefits provided by trees in urban areas, and the lack of them, particularly in underserved communities, is an issue that our data can highlight,” said Claggett.
Despite over 8,000 acres of trees that were planted across the watershed during the course of the study, communities lost over 25,000 acres of tree canopy, according to the Chesapeake Bay Program data and local government reports on tree planting. Virginia saw a loss of over 9,500 acres of canopy.
Tree cover fact sheets crafted by the program provide “an idea of what the trends in tree cover are for every county and city, and also what are some of the ecosystem benefits that you gain in terms of flood control, reduced runoff and things like that,” said Claggett.
Researchers are working to update the data by next year to include new high-resolution imagery from 2021 and 2022 as well as lower-resolution imagery from USGS archives dating back to the 1980s. That will allow them to “look back through the record and look at all the images taken about once every two weeks and see how the landscape has changed through time,” said Claggett.
KC Filippino, a senior water resources planner for the Hampton Roads Planning District Commission said that information can then be shared with the public “so they can start seeing how their landscapes around them have actually been changing over the years in a more quantitative way.”
Once the data is updated to include 2021 and 2022 satellite imagery, researchers expect to see utility-scale solar fields emerging as a significant factor affecting land use and tree cover in Virginia.
Until then, Filippino said the focus is on getting “eyes on that data and making sure it’s accurate in the eyes of the people that actually live there.”