No abstract

No 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.

Most North American oaks (Quercus spp.) are adapted to drought-prone sites by an ability either to avoid, or to tolerate, water stress, or both. Generally, they have deep-penetrating root systems, enabling them to maintain relatively high predawn water potentials during drought. Oaks have thick leaves and some have relatively small stomata, both characteristics that favor high water use efficiency. However, some species, from warm regions, have large stomatal pores. The rapid evaporative cooling made possible by large stomata, may be an adaptation to high temperature. Some southeastern species display leaf curling during drought, and Q. douglasii a native of California is drought deciduous. Oaks have a ring-porous xylem anatomy, allowing rapid sap movement in large diameter, early-wood vessels when soil water is plentiful, and slower, but sustained, water movement in narrower, late-wood vessels, which are more resistant to cavitation, during drought. Oaks frequently maintain a higher rate of photosynthesis at low leaf water potentials and high vapor pressure deficits than co-occurring species of other genera. An exception is Quercus rubra, which is generally restricted to relatively mesic sites. During drought, many oak species, especially those native to arid regions, undergo changes in tissue osmotic potential. However, it remains to be shown whether such changes are phenological or drought induced. Reported values for bulk modulus of elasticity vary widely among species and studies, and have been observed to both increase and decrease during drought in a way that is unrelated to region or to changes in predawn water potential or osmotic potential. Diurnal leaf water potential during drought is probably a poor indicator of differences among oak species in gas exchange rate, because of interspecific variation in desiccation avoidance and tolerance.

No abstract