DISTRIBUTION, HISTORICAL DEVELOPMENT AND ECOPHYSIOLOGICAL ATTRIBUTES OF OAK SPECIES IN THE EASTERN UNITED-STATES

Approximately 30 Quercus (oak) species occur in the eastern United Sta tes, of which Q alba, Q rubra, Q velutina, Q coccinea, Q stellata and Q prinus are among the most dominant. Quercus distribution greatly inc reased at the beginning of the Holocene epoch (10 000 years BP), but h as exhibited major changes since European settlement in the 18th and 1 9th centuries. For example, large-scale increases in Quercus species h ave occurred as a result of fire exclusion in the central tallgrass pr airie and savanna regions. In the northern conifer and hardwood forest s of New England and the Lake States region, Q rubra exhibited a drama tic increase following early logging and fire. Quercus species have al so increased in the mid-Atlantic region from land-clearing, the charco al iron industry and the eradication of Castanea dentata following Eur opean settlement. Studies of the dendroecology and successional dynami cs of several old-growth forests indicate that prior to European settl ement Quercus grew and regenerated in uneven-aged conditions. At times oak growth was very slow (< 1.0 mm/year) for long periods, which is u sually characteristic of highly shade-tolerant species. Quercus specie s exhibited continuous recruitment into the canopy during the 17th, 18 th and 19th centuries, but stopped recruiting in the early 20th centur y. Since that time, later successional, mixed-mesophytic species have dominated understory and canopy recruitment, which coincides with the period of fire exclusion throughout much of the eastern biome. Major o ak replacement species include Acer rubrum, A saccharum, Prunus seroti na and others. Logging of oak forests that have understories dominated by later successional species often accelerates the oak replacement p rocess. Relative to other hardwood tree species, many oaks exhibit hig h fire and drought resistance. Adaptations of oaks to fire include thi ck bark, vigorous sprouting and resistance to rotting after scarring, as well as benefiting from fire-created seedbeds. Their adaptations to drought include deep rooting, xeromorphic leaves, low water potential threshold for stomatal closure, high gas exchange rates, osmotic adju stment and a drought-resistant photosynthetic apparatus. However, oaks typically have low tolerance for current understory conditions, despi te the fact that they produce a large seed with the potential to produ ce an initially large seedling. Oak seedlings in shaded understories g enerally grow very slowly and have recurring shoot dieback, although t hey have relatively high net photosynthesis and low respiration rates compared to many of their understory competitors. Oak forest canopies also allow for relatively high light transmission compared with later successional forest types. Thus, without severe competition from non-o ak tree species, oaks should have the physiological capability for lon g-term survival beneath their own canopies in uneven-age tie, gap-phas e) or even-age forest conditions. I argue that fire exclusion this cen tury has facilitated the invasion of most oak understories by later su ccessional species, which are over-topping oak seedlings. If this cond ition, coupled with severe predation of oak acorns and seedlings, cont inues into the next century, a major loss of oak dominance should be a nticipated.