A new dendroecological method to differentiate growth responses to fine-scale disturbance from regional-scale environmental variation

A new dendroecological method is developed to differentiate growth responses to fine-scale disturbance from regional-scale environmental variation. In spruce-fir forests of central British Columbia, release from suppression in response to overhead canopy tree mortality was calibrated as > 60% change in radial growth (%CRG, adjacent 15 year periods compared) using gap-maker-gap-filler pairs with known years of mortality and response. Many release events, attributed to regional-scale environmental variation (e.g., bark beetle outbreaks), were counted. Species-specific regional-scale chronologies were subtracted from standardized gap-filler series producing residuals and 1 was added to all residual indices. Percent divergence (%DIV) values were calculated as the percent change in residuals (adjacent 15 year periods compared). A %DIV criterion was set at > 15% increase in the residual series. The %CRG and %DIV criteria were applied to an independent data set of ring-width series, determining the date(s) of release for each tree. %CRG and %DIV criteria were used in a complementary approach to differentiate (i) release due to fine-scale canopy gaps, (ii) no response to a gap and regional-scale environmental variation, (iii) release due to regional-scale environmental variation, and (iv) response to a fine-scale canopy gap but not detected by the %CRG criterion.