Radial Growth Patterns in Trees Surviving Canopy Gap Disturbances

Forest disturbance regimes in eastern North America have long been conceptualized as localized gap disturbances, but recent studies suggest that more widespread, infrequent disturbance events may also be important. However, uncertainties in tree-ring methods used to detect disturbance, and in the factors that drive growth release of trees, limit the conclusions that can be drawn from regional disturbance analyses. I designed a study to further elucidate forest disturbance dynamics in eastern North America by using two approaches: 1) a case study examining the sensitivity of the radial growth averaging method of detecting disturbance and the factors influencing detection of a known disturbance event, and 2) a regional-scale study using publicly available tree-ring data to explore disturbance regimes of eastern North America and the possibility of spatial and temporal synchrony in disturbance across the region. The case study revealed strong variation in the number and timing of disturbances detected with different thresholds and window criteria required by radial growth averaging. A threshold of 50% growth change, with symmetrical 15 year windows is suggested for future tree-ring reconstructions of disturbance history. Furthermore, intermediate and suppressed individuals may be more likely to respond strongly to canopy disturbance than overstory trees. Regional disturbance analysis suggested periods of elevated disturbance across eastern North America, including 1690-1710, 1774-1780, 1880-1900, and 1920-1936. Anthropogenic influence in the region was highlighted in the past 140 years, likely caused by widespread logging, damage from introduced pathogenic organisms, and finally forest protection efforts post-1930. These findings suggest the importance of further study regarding the interaction between anthropogenic disturbance and disturbance events driven by stochastic climate anomalies.