A rapid transition from spruce-fir to pine-broadleaf forests in response to disturbances and climate warming on the southeastern Qinghai-Tibet Plateau.

A better understanding of the structure and dynamics of disturbed forests is key for forecasting their future successional trajectories. Despite vulnerability of subalpine forests to warming climate, little is known as to how their community composition has responded to disturbances and climate warming over decades. Before the 1970s, subalpine forests on the southeastern Qinghai-Tibet Plateau mainly experienced logging and fire, but afterwards they were more impacted by climate warming. Thus, they provide an excellent setting to test whether disturbances and climate warming led to changes in forest structure. Based on the analysis of 3145 forest inventory plots at 4- to 5-year resolution, we found that spruce-fir forests shifted to pine and broadleaved forests since the early 1970s. Such a turnover in species composition mainly occurred in the 1994-1998 period. By strongly altering site conditions, disturbances in concert with climate warming reshuffle community composition to warm-adapted broadleaf-pine species. Thus, moderate disturbances shifted forest composition through a gradual loss of resilience of spruce-fir forests. Shifts in these foundation species will have profound impacts on ecosystem functions and services. In the future, broadleaved forests could expand more rapidly than evergreen needle-leaved forests under moderate warming scenarios. In addition to climate, the effects of anthropogenic disturbances on subalpine forests should be considered in adaptive forest management and in projections of future forest changes.