RADIAL-GROWTH AVERAGING CRITERIA FOR RECONSTRUCTING DISTURBANCE HISTORIES FROM PRESETTLEMENT-ORIGIN OAKS

A novel dendroecological procedure was developed to elucidate canopy d isturbances spanning a >300-yr period for oak (Quercus> forests of cen tral Pennsylvania. Running comparisons of sequential 10-yr ring-width averages may effectively neutralize both short-term (i.e., drought) an d long-term growth trends associated with climate while enhancing dete ction of abrupt and sustained radial-growth increases characteristic o f canopy disturbance. Thinning-response studies revealed the conservat ive tendencies of overstory oak, with substantial basal area reduction s (>1/3) required to attain moderate and consistently detectable growt h increases. Based on empirical evidence, a minimum growth-response th reshold 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 de fault threshold was adjusted higher as necessary for those trees highl y correlated to climatic trends (as represented by the Palmer drought severity index). Canopy disturbances detected with this dendroecologic al approach were further substantiated using tree-recruitment data (ag e cohorting). By coupling these data sets, we estimated return interva ls of standwide disturbance from 21 yr in presettlement times (prior t o 1775) and during heavy Euro-American exploitation (1775-1900) to 31 yr in modern times (after 1900). Although disturbance periodicity rema ined stable between presettlement and early post-settlement (exploitat ion) eras, the mode of disturbance shifted from mainly natural (wind a nd fire) to anthropogenic forces (intense harvesting for charcoal prod uction), based on the historical record. In the process, presettlement oak-pine (Pinus)-chestnut (Castanea) forests on ridges were rapidly c onverted to young coppice stands of oak and chestnut. The reduction of harvesting and fire events coupled with the eradication of chestnut b y 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.