The U.S. Forest Service is conducting an urban tree census this summer to document the types, sizes and health of trees in natural and residential areas of Philadelphia.

The information will inform the city's urban forest management policies and help determine the impact of invasive plants and insects on native flora.

Five crews of graduate students and interns will spend the summer surveying about 400 representative plots in  Philadelphia County in Pennsylvania and New Castle County in Delaware this summer. They will visit both natural and residential areas, where they will knock on doors to get permission to survey backyard trees.

The teams will document the type, size and health of the trees, and take notes on shrubs and groundcover on each tenth-of-an acre plot.

Project coordinator Phillip Rodbell, with the U.S. Forest Service’s new Philadelphia field station, said teams are also noting invasive species such as Japanese knotweed.

On a recent survey in Fairmount Park, near the Please Touch Museum, he pointed to a section of the knotweed and explained how it could expand before the next surveying visit.

"On this plot, there's just a little bit in one corner. But when we return in five years the rate of expansion of that could be double, so it might be overtaking the site,” Rodbell said. “That information will inform the city about the rate of invasion of a particular invasive and will help them set priorities for management."

According to Lara Roman, also with the U.S. Forest Service, the data also will be used to gauge the benefit of the city's trees, including predicting how much shade they provide and how much carbon and particulate matter they remove from the air.

"Part of it is used simply for advocating on behalf of the urban forest,” Roman said. “It costs quite a bit to maintain trees in public and private land throughout cities. In times of hard budget cuts, it can be difficult to justify doing all that. But, in fact, the trees are a return on the investment."

Eventually, 13 counties in the area will get the same treatment.

The city parks department hopes to use the data gathered to develop an urban forest management plan.