Radial-Growth Averaging Criteria for Reconstructing Disturbance Histories From Presettlement-Origin Oaks

Abstract

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.

Development of Northern White-Cedar Regeneration Following Partial Cutting, with and without Deer Browsing

“…We identified the moment at which growth became significantly greater than before treatment, and how long this response persisted over a 10-year period. We analyzed 3-year periods instead of single years, to filter out the variability in diameter growth due to annual changes in temperature and precipitation [22,23]. Because the exact month of the partial harvest was unknown, we did not include the year of harvesting (year = 0) in the diameter growth analysis.…”

“…After harvesting (years = 1 to 10), we used the moving average method to find differences generated by adding and dropping only one year at a time [24]. This method is frequently used in dendrochronology to search for the moment of the release of stems following disturbance of the canopy [23,25]. We constructed models for each individual seedling and post-harvesting period to compare the 8 post-harvesting periods with the pre-harvesting period (mean diameter increment of years −3 to −1).…”

Improving tree mortality models by accounting for environmental influences

“…This would not be the case for dead trees if many of them had already died before the onset of the external influence. External factors, such as disturbance or climate, very often influence tree growth (Lorimer 1985;Nowacki and Abrams 1997), and this may lead to biased mortality models. The use of untruncated data will then confound climate or disturbance growth signals with tree vigour-related growth responses and will lead to high data specificity through the fitting of data-specific noise and, thus, to overfitted models (Harrell 2001).…”

Crecimiento radial de la especie endémica Nothomyrcia fernandeziana (Hook. & Arn.) Kausel y la invasora Aristotelia chilensis (Molina) Stuntz en la isla Robinson Crusoe, Archipiélago de Juan Fernández, Chile

“…La presencia de liberaciones abruptas del crecimiento en las series de ancho de anillos, las cuales se deben a la aparición de claros en el dosel arbóreo, se detectó mediante el filtro de porcentaje del cambio en el crecimiento (PCC) (Nowacki & Abrams 1997). Las liberaciones del crecimiento se detectaron mediante la fórmula PCC = [(M2 -M1) / M1] · 100, siendo M1 y M2 las tasas medias de crecimiento de los 10 años previos y subsiguientes a cada anillo considerado, respectivamente.…”