YOU’RE WALKING ALONG in the vast Harvard Forest in Petersham, the air sharp and redolent of fallen leaves, when suddenly you reach a brightly painted wooden barrier. “Trail closed,” it reads. “Safety hazard.” It’s the first stop of the Hemlock Hospice, an art installation and interpretive trail by designer David Buckley Borden and his team of forest ecologists. As its name suggests, the mile-long project is an elegy for the New England hemlock forest, which is dying. “People who walk these trails expect a certain experience and then they run into this,” said Borden. “It’s meant to jar them, to say, ‘You have to think about your woods in a whole new way.’” …These tiny beetles suck all the nutrients out of the hemlock’s needles and leave ghost trees, bare and vulnerable to toppling over in the wind.