Evidence of synchronous extreme disturbance at regional to subcontinental scales in a largely asynchronous syste
Predicting forest response to climate change requires recognition of complex interactions between top-down and bottom-up processes. The most frequent disturbances in many temperate broadleaf forests operate from stand to landscape scales and are often asynchronous through time. When viewed over broad spatial scales over the last 50-100 years, the sum of these disturbances suggests a dynamic equilibrium. In contrast, records covering wider spatial scales and longer time periods in these same forests indicate synchronous, but infrequent transitions in forest type at broader scales that are partly driven by climate. These disparate scales of forest dynamics lead to questions of how today’s temperate broadleaf forests will respond (or not) to projected climatic change. Tree-ring records collected at the stand-scale that also span broad spatial scales can provide insight into: (1) disparate scales of forest processes and (2) complex interactions between top-down and bottom-up processes. We used a network of stand-level tree-ring collections from broadleaf-dominated forests covering much of the subcontinental eastern US to reconstruct growth and canopy disturbance history over the past 400 years. We investigated these records for the range of temporal and spatial scales of disturbance and the possibility of synchronicity in a region where asynchronous events are common.
Among a long-time series of many asynchronous disturbance events, we found evidence of two synchronous regional to extra-regional events that occurred within the maximum age trees in today’s forest. An event in the 1770s, covering a large portion of the Southeastern US, followed repeated, severe drought and a significant frost event to indicate that extreme climatic events can synchronize canopy disturbance at large scales. An event in the late 1600s was resulted in a spike in tree recruitment across much of the eastern US. The trigger(s) of this event are still less understood. New recruitment data from old trees gathered over a three-state region during the first half of the 20th century supports the initial finding of a synchronous recruitment at regional scales. Taken together, these events indicate the potential of a rapid change in complex forests following extreme events at broad spatial scales. It appears that these synchronous, extra-regional events (top down) can override local ecological factors (bottom up). However, we also find evidence that local ecological conditions can ameliorate the impact of these severe disturbances. Because of complex interactions between bottom-up and top-down processes, stand-level impacts will vary.