Trailing edge contractions common in interior western US trees under varying disturbances

As climate warms, trees are expected to track their ideal climate, referred to as ‘range shifts’; however, lags in tree range shifts are currently common. Disturbance events that kill trees may help catalyse tree migrations by removing biotic competition, but can also limit regeneration by eliminating seed sources, and it is unknown whether disturbance will facilitate or inhibit tree migrations in the face of climate change. Here we use national forest inventory data to show that seedlings of 15 dominant tree species in the interior western United States occupy historically cooler areas than mature trees, as expected with climate warming. However, the climatic differences between adults and seedlings are the result of widespread regeneration failures in the hottest portions of species’ ranges. Disturbances did not uniformly catalyse climatic range shifts; differences were species- and disturbance-specific. Assisted migration programmes may thus be needed to help trees adapt their ranges to climate change.