Wissahickon Valley Park's Forbidden Drive was recently named trail of the year by Pennsylvania officials for its unique five miles of twists and turns that wind through the city but that can feel as if they are miles from anywhere.

Now Mother Nature is doing her best to wreck trees along the trail — and experts fear trees already stressed by climate and other environmental factors will take another hit with the next nor'easter forecast to start shortly. That means damage to the forest, and a lot of  heavy, dangerous work for volunteers as well as professionals.

"In this past storm we have received reports of 10 or 20 trees down across the trails," said Peg Shaw, director of land management for Friends of the Wissahickon, a 2,000-member nonprofit that works with Philadelphia's Parks and Recreation Department. That estimate doesn't include all the trees that have fallen in the woods away from trails.

But the worst might be yet to come for the trees, with up to six inches of wet, heavy snow set to fall on Philadelphia — accompanied by high winds — starting Tuesday and continuing into Wednesday.  Soil around the trees is already sodden everywhere in the region, including the 1,800 acres of park running from the city's northwestern corner down to East Falls.

Shaw said the park was already under tremendous stress due to climate change.  More frequent, powerful storms are taking their toll.  Invasive species make the trees more vulnerable. In addition, development around the park slopes is eroded by storm-water runoff from impervious areas, such as roofs, parking lots, and streets.

"The Wissahickon is under a lot of environmental stressors," Shaw said. "That means the forest overall is not at its strongest place.  Our canopy trees are not as strong as they could be.  So anytime we get a storm like the one we just had, or the one we are expected to get, the trees are susceptible to snapping or falling."

Forbidden Drive, also called Wissahickon Valley Park Trail, was designated a National Recreation Trail in 1975 and remains a popular route for runners, walkers, and cyclists.  It runs five miles along Wissahickon Creek, passing Thomas Mill Covered Bridge and Valley Green Inn, and connects to 50 miles of dirt or rock trails.  The park is populated by tulip poplars, American beech, oaks, white pine, sycamores, box elders, and red maples, among other species.

Downed trees can close off wide sections of the park for users. The Friends of the Wissahickon ask volunteers to take GPS coordinates if they spot a fallen tree that blocks a trail.  That way, the organization can map the locations and dispatch crews.

The volunteer crews will scramble to cut and haul away the wood in time for heavy spring use of the trails.  Last year, they cleared 52 trees from just the upper part of the trail system. The volunteers often have to use chain saws and maneuver logs along steep, slippery slopes, which can be hazardous.

Shaw said erosion from storm water has helped weaken the grip that roots have on the soil. Runoff has created corrosive gullies — harsh environments for trees. To help prevent erosion, her group has launched a project aimed at mitigating the impact of surface water by enlarging a basin at the upper parking lot in the Northwest section.  It is installing a series of vegetated rock cascades leading from the basin down a hill and through forest.  A rain garden and swale are also being installed at the Wissahickon Environmental Center off Northwestern Avenue.