For nearly 30 years, Maryland developers, home builders and governments have been required to replace some of the trees they cut down for subdivisions, shopping centers, schools and roads. One option has allowed them to buy forest “credits” from the “banks” of other property owners, even miles away, who agree to preserve acreage on their own land.

The system, builders and some county officials say, has helped to offset losses in tree canopy while allowing for growth in built-up areas that have little space to replant.

However, an October opinion by the state’s attorney general threw the decades-long practice into disarray and has renewed debate in the Maryland General Assembly about whether the state is doing enough to protect trees from development.

County officials implementing the state’s Forest Conservation Act, the legal opinion said, had misinterpreted it for decades. The law, the opinion said, intended for lost forest to be replaced by planting new trees, not by preserving existing ones.

The decision prompted planning departments in Montgomery, Prince George’s and other counties to stop allowing the credits for existing forest. Without that option, planners say, some development proposals have come to a halt.

“People looked at [the legal opinion] and said, ‘This is a problem,’ ” said Adrian Gardner, general counsel for the Maryland-National Capital Park and Planning Commission, which reviews forest conservation proposals in Montgomery and Prince George’s. “This is something we’ve been doing for decades without ever thinking there was a problem with the statute. There are projects that are going to be on hold until they figure out a viable substitute for their mitigation obligations.”

On Thursday, the state Senate gave preliminary approval to a bill that would explicitly allow cleared trees to be replaced with credits for existing forest. The House approved another version of the bill last month.

If differences between the two are resolved and the legislation is signed by Gov. Larry Hogan (R), counties could resume the use of credits for existing forest. The Senate version would allow it only until mid-2024, when some lawmakers say they would like to further scrutinize the law’s effectiveness after a study of the state’s tree canopy.

The legislation has ignited questions over whether Maryland sufficiently protects forests for wildlife and water and air quality or allows too much tree loss by giving short shrift to new planting. Those who support the banking system for existing forest say it has protected the state’s most mature and valuable forest — 5,365 acres, according to the Maryland Department of Natural Resources. Critics say forest preservation is laudable but, without planting more trees, doesn’t help meet the state’s expressed goal of “no net loss” to the overall canopy.

“We’re losing about a dozen acres of forest every day, on average, in Maryland,” said Ben Alexandro, water program director for the Maryland League of Conservation Voters. “We need a big, comprehensive fix to this crazy-complicated law that still allows thousands of acres of forest to come down every year.”

Developers, he said, should be encouraged to preserve trees on-site as much as possible, particularly the large, contiguous swaths that provide the best wildlife habitat.

Del. James W. Gilchrist (D-Montgomery), the bill’s sponsor, said counties need the ability to keep development moving until a “full, comprehensive” review of the state’s forest conservation law can be done.

“It’s to protect what we had in place,” he said.

Sen. Paul G. Pinsky (D-Prince George’s), chairman of the Education, Health and Environmental Affairs Committee, said the panel strengthened protections by requiring that credits for existing forest be purchased at a minimum 5-to-1 ratio, up from a minimum 2-to-1 ratio required in many counties. Under that plan, for every acre of trees cut down, the developer would have to buy at least five acres’ worth of credits.

Even so, Pinsky said, “You’re still knocking down trees and not planting new ones.”

The question of how and where cleared trees should be replaced also has surfaced as part of building the 16-mile Purple Line through Montgomery and Prince George’s. The light-rail line, which will run through densely populated inner suburbs, required clearing about 42 acres of trees.

While some will be replanted along the alignment, the state replaced the cleared acreage by buying bank credits for existing forest in Accokeek and Dickerson, both 25 or more miles from the Purple Line alignment.

Under a bill sponsored by Del. Lorig Charkoudian (D-Montgomery) and recently passed by both chambers, a new “urban tree program” would plant trees in communities where a transportation project, including the Purple Line, requires cutting.

The Forest Conservation Act applies to parcels of 40,000 square feet — about an acre — or more. Developers and builders first are required to preserve or replant trees on-site or nearby. If those options aren’t possible, they can replace trees off-site, usually by buying credits.

Local planners determine how much lost acreage must be offset based on formulas depending on the type of development proposed and its location, Gardner said. In many cases, farmers are creating banks by preserving their own land.

Matt Johnston, Anne Arundel County’s environmental policy director, said the county sought the attorney general’s opinion because local officials thought they didn’t have the authority to allow mitigation banks created from existing forest.

The county tightened its reforestation requirements in early 2020 and, since the attorney general’s opinion, has favored mitigation plans based on new trees. The change has had little effect on pending development proposals, he said.

In Montgomery and Prince George’s, however, the planning commission’s lawyers said the attorney general’s opinion could bring development to a “screeching halt.”

Developers in the densely developed suburbs need off-site bank options, Gardner said, because replacing trees on-site can be difficult, especially around transit stations, where both counties are trying to focus growth. Conservation banks based on newly planted trees also are in short supply, he said.

After the legal opinion put “hundreds” of acres of existing forest off-limits for credits, Gardner said, there was a “run” on the remaining banks created from newly planted trees. Those credits have been bought up in both counties, Gardner said.

Without access to the conservation banks, Gardner said, developers who need to replace trees off-site will be left paying a fee to the county, which uses the money to plant trees. But builders and county officials say that process is more expensive and cumbersome for everyone, and environmental advocates say it’s difficult to track how the money is spent.

Such fees also are the least efficient way to save forests, Gardner said.

“It’s more advantageous to have conserved, mature, quality forest under perpetual preservation as opposed to money,” Gardner said. “It takes time to create new forests.”