In a matter of hours, Don Outen can evaluate the extent and pattern of forest coverage in every watershed in Baltimore County.

And he never has to leave his office.

Instead, he can scan a series of color-coded maps that illustrate forest and woodland patch sizes. The maps show there is not much dark green “interior forest” in the county. But there is a lot of purple “edge forest” around the perimeters of the county’s numerous isolated forest patches. A palette of other colors illustrate forest fragments of different shapes and sizes — most are smaller than Outen would like to see.

“The scientific literature all says the larger it is, the better it is,” said Outen, chief of policy, planning, research and development of the county’s Department of Environmental Protection and Resource Management.

The maps are produced by a computer-assisted, forest evaluation methodology developed by Outen’s staff with assistance from the environmental consulting firm Biohabitats Inc. It allows Outen, or anyone else, to apply a ranking methodology to forests within any of the county’s 189 watersheds. Using information about water quality, habitat and the impacts of human activities, the methodology can “score” each forest patch for its environmental values, conservation potential and restoration opportunities.

The ranking parameters were chosen from an extensive review of scientific literature on forest ecological functions. Other information in the methodology comes from satellite imagery and digital land use data. The computer-assisted methodology was developed under a grant from the Maryland Department of Natural Resources to evaluate greenways and riparian corridors for multiple ecological values.

While the program doesn’t plant trees, it does something that may be even more important — it shows Outen, or other resource managers, where to plant to accomplish specific objectives.

In Baltimore county, one of those objectives is restoring large forest blocks.

Scientific literature indicates a strong link between forest size and water quality, and that headwater streams are the most important components of stream systems with respect to stream dynamics and ecology. Indeed, the county’s own monitoring system, which uses volunteers to check on more than 100 stream sites, has shown a relationship between water quality and forest size: the more forest, the greater the water quality and the diversity of aquatic life in the streams.

Literature also shows that many birds use forest interiors for breeding and that larger, contiguous blocks of forest cover provide more usable habitat for these species than do forest fragments. Scientific literature also shows that biodiversity tends to be greater in older forest ecosystems that experience natural disturbance events than in those harvested or otherwise artificially disturbed at more frequent intervals. Also, upland and riparian forest ecosystems have historically been the most important habitat types for Maryland’s native plant and animal communities, according to the scientific review used in developing methodology.

The methodology uses that kind of research to help rate individual patches for conservation and restoration. Only about 30 percent of Baltimore County is forested, and the maps reveal that the remaining woodlands tend to be highly fragmented. The methodology, though, quickly shows places where isolated forest patches — given zoning, land ownership and other variables — could be linked to create continuity among forest patches.

It enables users to evaluate individual forest patches within subwatersheds for the amount of interior forest habitat — forest at least 500 feet from the edge — and the presence of “gaps” either within or along the perimeters of forest patches. The next step in the methodology is to lay forest patch maps on tax maps to determine public or private ownership patterns, as well as the number of land owners for each patch. Sometimes, what appears to be forest is actually a large-lot subdivision. Areas under multiple ownership may be more difficult targets for conservation or restoration than sites with only a few individual landowners.

“We’re looking at a lot of things at the same time,” Outen said.

When it’s all put together, managers can target forest conservation and restoration efforts where they will achieve the most benefits and are most practical. Gaps within forests, for example, get high rankings for restoration potential unless ownership is badly fragmented.

It also allows planners and managers to be more pro-active. Maryland has a variety of programs that can help preserve forest land, but the state doesn’t have the manpower to go out and tell landowners about them. As a result, they wait for landowners to come to them.

But with this ranking system, it’s easy to see where restoration will do the most good and to steer the programs toward landowners in those areas instead of restoring forest patches here and there.

Already, the program is being put to use. Many groups interested in restoration projects are seeking advice about potential sites. “We’re already finding that we’re rolling out these maps and looking at them,” Outen said.

Others are beginning to use the evaluation system.

The Maryland Forest Service, under contract to the City of Baltimore, will soon be using it as part of an analysis of forest lands that surround the city’s reservoirs. The goal is to see how the forest around the reservoirs can be managed, and restored, to protect water quality.

“It’s a great step forward in helping us understand the most important places for restoration to occur, and it serves as an excellent communication tool,” said Rob Northrop, a watershed forester with the Maryland Forest Service. “Particularly in Maryland, where the forest is fragmented, prioritizing forest restoration work is often difficult. We have a limited number of technical people and a limited budget. The program assists us in focusing our work on those areas where we can gain the greatest conservation values.”

Northrop has been using the system in the DNR’s 5,500-acre Fair Hill Natural Resource Management Area in the northeast corner of the state. About a third of the area is forested, and the ability to plant more forest is limited by the fact that the area is an important habitat for a number of grassland-nesting birds found in the state.

Nonetheless, Northrop has adapted the system to identify areas within the natural area that are ripe for reforestation — such as long slivers of grasslands cutting into forests, creating a lot of unwanted “edge habitat” that is too narrow to serve as a good nesting site. By filling in those gaps, large amounts of interior forest will be gained.

Ultimately, the system will allow him to identify sites outside the area that are important to forests and water quality, and the forest service will be able to work with neighboring landowners on restoration efforts. The system makes working with landowners easier, too, Northrup said, because it is logical and easy for people to understand.

“You can see the way that forest patches connect and don’t connect with one another, both on the property, and how they function within the Elk Creek watershed,” he said.