When it comes to safeguarding the ecological health of the Chesapeake Bay and the rivers and streams that feed it, little is more pernicious than development and nothing more beneficial than trees.

Yet despite long-running, wide-ranging efforts to restore the Bay, high-resolution aerial survey data show that an area larger than the District of Columbia is being covered by pavement and buildings every five years. Over the same time period, an area the size of Arlington County, Virginia, loses tree cover, dwarfing watershed-wide tree planting efforts aimed at replacing cover already lost.

That data, recently released by the Chesapeake Bay Program, highlight the as-yet unmet challenge of reversing the harm that development is causing to the Bay and its tributaries. The 2014 Chesapeake Bay Watershed Agreement signed by all six Bay watershed states and the District of Columbia pledges only to “evaluate policy options, incentives and planning tools” that local officials might use to curb forest loss and reduce the spread of runoff-inducing paved surfaces.

Comparing aerial imagery and other data gathered between 2013 and 2018, the nonprofit Chesapeake Conservancy, the U.S. Geological Survey and a University of Vermont laboratory tallied 3,012 square miles of the Bay watershed covered by buildings and pavement. Such impervious surfaces keep rainfall from soaking into the ground; instead, the rain picks up pollutants as it washes toward local waterways. Stormwater runoff is a significant and, according to Bay Program computer models, growing source of pollution degrading the Bay.

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Analysis of high-resolution imagery has revealed that pavement and buildings cover about 45% more of the Chesapeake Bay watershed than had been previously identified. (Dave Harp, Bay Journal)

While impervious surfaces currently cover less than 5% of the bay watershed’s 64,000 square miles, they are spreading at the rate of 50,651 acres or 79 square miles every five years, the groups’ analysis found. The District of Columbia encompasses 68.3 square miles, by comparison.

The analysis found that buildings accounted for a little less than a third of the increase in impervious surfaces, while roads added 4%. Nearly two-thirds of the spread represented the cumulative impact of new driveways, parking lots, runways, rail lines and the like.

The aerial surveys found that 8,307 acres of trees had been planted across the Bay watershed from 2013 to 2018, with efforts in Maryland accounting for more than 80% of that. Yet communities throughout the watershed lost more than 25,000 acres of canopy, three times what was planted, for a net loss of about 16,000 acres, or 25 square miles. Arlington County, Virginia, covers 25.8 square miles, as a comparison. While Maryland had the largest acreage in tree plantings, it also had the greatest net loss of trees in that period, the groups found.

Trees provide a panoply of ecological and health benefits. They soak up rainfall, stabilizing soil and preventing runoff of nutrient and sediment pollution that harms water quality. They also reduce air pollution and provide shade that mitigates summer heat.

Bay Program participants called the data sobering, but said they believed it could spur local officials to do more to curb the impacts of development.

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“Data and technology can inform and empower the Chesapeake conservation movement like never before,” said Joel Dunn, president of the Chesapeake Conservancy. “In this case, land use decisions in the watershed will finally be informed by both the amount and the value of tree canopy status in every county, one of the most significant factors for water quality.”

Matt Stegman, a lawyer in the Maryland office of the Chesapeake Bay Foundation, called the data “a wake-up call for local jurisdictions to target reforestation projects and policy solutions in places most rapidly losing canopy.”

The data released in August by the Bay Program largely tracks with preliminary analysis of the aerial surveys first reported in 2022 by the Bay Journal. At that time, the groups said the watershed was adding more than 12,000 acres annually of runoff-inducing pavement and buildings.

They also reported in 2022 that communities in the Bay watershed cumulatively suffered a net loss of more than 29,000 acres in urban tree canopy. That’s higher than the current net loss tally of about 16,000 acres, but further analysis found that some of those losses were offset by tree cover forming on otherwise developed lands, according to Bay Program geographer Sarah McDonald.

The latest analysis doesn’t mention another significant trend. In 2022, the groups’ preliminary analysis found the watershed was losing more than 20,000 acres of forest a year. McDonald said the overall forest loss number remains the same, but the groups chose not to report that to the public again because they are “working on better understanding land conversion,” particularly the generally permanent loss of forest to development versus the short-term but potentially replaceable loss of forest to timbering or farming.

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Examples by state of net loss of tree cover, 2013-14 to 2017-18

Source: Chesapeake Bay Program, 2023

Timothy B. Wheeler is a reporter for Bay Journal, a media partner of The Baltimore Banner.

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