Radial-growth averaging criteria for reconstructing disturbance histories from presettlement-origin oaks

A novel dendroecological procedure was developed to elucidate canopy disturbances spanning a >300-yr period for oak (Quercus) forests of central Pennsylvania. Running comparisons of sequential 10-yr ring-width averages may effectively neutralize both short-term (i.e., drought) and long-term growth trends associated with climate while enhancing detection of abrupt and sustained radial-growth increases characteristic of canopy disturbance. Thinning-response studies revealed the conservative tendencies of overstory oak, with substantial basal area reductions (> 1/3) required to attain moderate and consistently detectable growth increases. Based on empirical evidence, a minimum growth-response threshold of 25% was established to depict Canopy disturbances. This is in contrast to the 50-100% sustained radial-growth release often used to detect disturbance using understory trees in closed forests. Our default threshold was adjusted higher as necessary for those trees highly correlated to climatic trends (as represented by the Palmer drought severity index). Canopy disturbances detected with this dendroecological approach were further substantiated using tree-recruitment data (age cohorting). By coupling these data sets, we estimated return intervals of standwide disturbance from 21 yr in presettlement times (prior to 1775) and during heavy Euro-American exploitation (1775-1900) to 31 yr in modern times (after 1900). Although disturbance periodicity remained stable between presettlement and early post-settlement (exploitation) eras, the mode of disturbance shifted from mainly natural (wind and fire) to anthropogenic forces (intense harvesting for Charcoal production), based on the historical record. In the process, presettlement oak-pine (Pinus)chestnut (Castanea) forests on ridges were rapidly converted to young coppice stands of oak and chestnut. The reduction of harvesting and fire events COupled with the eradication of chestnut by blight this century have allowed these coppice stands to mature into oak-dominated forests that exist today. This analytical technique for ascertaining disturbance histories holds much potential and should be considered for use with mature, overstory trees in other forest types with appropriate modifications.