copyright 2009, The Washington Post WASHINGTON -- Washington, home to more than 300 species of trees, is known as the 'City of Trees.' Rock Creek Park, Capitol Hill, Roosevelt Island -- the city is full of them. Casey Trees, a nonprofit organization founded in 2001, hosts monthly tree tours around the city. With guide book in hand, a leader points out various trees, notes their distinctions and (hopefully) gives you a new appreciation of the city's arbors. 'Here's an example of bark distinctive of the ash tree,' says guide Carol Herwig, pointing to the brown, scaly bark of one tree on a bright, sunny day on Theodore Roosevelt Island. Farther down the path, she notes a maple tree. 'One of the tricks of identifying maple trees: It starts with smooth bark at the bottom,' she says. The tree gazers meander slowly, stopping frequently to look more closely at the trees and debate their identities. Be prepared for a slow-moving tour and one where everyone is learning together -- even the guide. 'I love trees,' says Roberta Gutman, 70, of Capitol Hill, who fell in love with a big, old willow oak at the U.S. National Arboretum. In 2002, after she retired, trees 'just became my thing,' she says. Now she loves learning about them, looking at them, planting them, talking about them. 'I'm attuned to the trees,' she says. Birders and gardeners were among the passionate arborists, too. 'I love being outdoors, and I think it's sort of fun to identify trees,' says Camille Martone, 50, of Washington. Nancy Burns, 46, of Centreville, Va., says trees are 'why Washington is such a pretty city.' Tours, which cost $20, are limited to 20 people and fill up fast, so register early at www.www.caseytrees.org or 202-349-1907. 