Membership in public-sector unions is thriving in Maryland as the number of residents who belong to traditional trade unions dwindles, partly because of the poor economy and the state’s union-friendly environment, experts say. Membership in construction, manufacturing and other private unions fell by 31,396 members to 117,752 from 2006 to 2010, according to data compiled by economic professors Barry Hirsch of Georgia State University and David Macpherson of Trinity University. Their data highlight the number of union workers living in each state.