Subalpine forests in the Colorado Front Range are critical providers of habitat, water, carbon storage, and recreation - but they may be in trouble. Recent decades have seen accelerating rates of tree death, labeled "forest decline" in cases when the exact cause is not known. In our research, we are forest detectives, testing hypotheses about what killed trees in the past, what is killing them now, which trees are surviving, and what it all means for the future of Colorado's forests.