Triggers, thresholds, disturbance regime shifts, and state changes in forest ecosystems with climate warming
Climate warming is increasingly linked to substantial and historically novel changes in forest disturbance regimes, driving a range of forest ecosystem responses from incremental small adjustments to abrupt fundamental changes in ecosystem patterns and processes. This synthesis addresses the interactive climate and disturbance drivers of major state changes in forest ecosystems, focusing on relationships among drought, warm temperatures, and tree mortality through combinations of forest dieback and die-offs, forest fires, and insect outbreaks. Recent trends of more extreme forest disturbances and associated ecosystem transitions from the Southwestern US will be highlighted (including relationships among warming climate, regional forest productivity, tree mortality, bark-beetle outbreaks, wildfire, and extensive conversion of forests to shrublands and grasslands), building to broader trends extending from western North America (e.g., sub-continental patterns of diverse bark beetle outbreaks and stand-replacing forest fires) to emerging global-scale forest risks (e.g., extensive climate-induced forest die-offs and high-severity “mega-fires”).
Key topics regarding climate:disturbance drivers of forest ecosystem state change addressed in this synthesis include: climate triggers of forest stress and associated disturbances; extreme climate events and altered disturbance regimes; climate synchronization of multiple and interactive disturbances; the synergistic, threshold, and cumulative effects of disturbance interactions as drivers of forest state change; and processes that stabilize ecosystem state changes. If current mainstream climate projections of substantial global warming this century emerge as modeled, major re-organizations of forest ecosystems (including ecosystem state changes and type conversions) can be expected through the effects of novel climate-modulated disturbance processes.