Extreme events and subtle ecological effects: lessons from a long-term sugar maple-American beech comparison

Increasing extreme events that are related to global change are expected to affect the dynamics of forest ecosystems. If disruptive stressors (e.g., insects, drought) affect tree vigor without causing mortality, the ecological effects may be subtle, making subsequent ecosystem dynamics more difficult to predict than in the case of disturbances causing death. Based on the literature and our personal observations, we expected that such a subtle change could have occurred in the dynamics between sugar maple and American beech. We implemented a targeted paired-sampling design (1) to verify whether a change occurred (gradual or abrupt, recovered or not) in the growth dynamics between the two species over a 57-yr period, (2) to identify the likely causes of this change, and (3) to investigate whether such changes could trigger other long-time ecological consequences. We found that sugar maple growth was negatively affected by an extreme event (or a few events) between 1986 and 1989, while American beech was not affected. Twenty years after the 1986-1989 abrupt growth decrease, sugar maple (1) had a slower growth than American beech, although it was previously similar, (2) did not respond to monthly climatic variations as it did prior to the abrupt growth decrease, and (3) had lower resilience when faced with a new stress event. Overall, our study, besides showing that extreme events with subtle effects may change the dynamics of an ecosystem, also illustrates that these events may accelerate ecosystem misadaptation to climate. Fine-scale targeted monitoring is essential to complement broad-scale monitoring to detect such misadaptations in a global change context.