Overgrown Arizona forests have fueled unprecedented fires, burning bigger and hotter. “Forests that once had 40 to 50 trees per acre, now have 500 to a thousand trees per acre,” said Patrick Graham, director of the Arizona Nature Conservancy, at a news conference on Thursday. Thinning trees can reduce fire danger, but such efforts have been slow to produce results so far. The Nature Conservancy and the U.S. Forest Service announced plans this week to launch a program called Future Forest to search for ways of thinning forests faster and more profitably. The program represents a new approach to an old problem, its supporters said. The Nature Conservancy, a non-profit organization, will invest in the program and work with the Forest Service to thin thousands more acres each year. Part of the goal is to become more efficient, even if it meansrewriting thinning and clearing contracts between the Forest Service and loggers.