Distribution, historical development and ecophysiological attributes of oak species in the eastern United States

Approximately 30 Quercus (oak) species occur in the eastern United States, of which Q alba, Q rubra, Q velutina, Q coccinea, Q stellata and Q prinus are among the most dominant. Quercus distribution greatly increased at the beginning of the Holocene epoch (10 000 years BP), but has exhibited major changes since European settlement in the 18th and 19th centuries. For example, large-scale increases in Quercus species have occurred as a result of fire exclusion in the central tallgrass prairie and savanna regions. In the northern conifer and hardwood forests of New England and the Lake States region, Q rubra exhibited a dramatic increase following early logging and fire. Quercus species have also increased in the mid-Atlantic region from land-clearing, the charcoal iron industry and the eradication of Castanea dentata following European settlement. Studies of the dendroecology and successional dynamics of several old-growth forests indicate that prior to European settlement Quercus grew and regenerated in uneven-aged conditions. At times oak growth was very slow (< 1.0 mm/year) for long periods, which is usually characteristic of highly shade-tolerant species. Quercus species exhibited continuous recruitment into the canopy during the 17th, 18th and 19th centuries, but stopped recruiting in the early 20th century. 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 oak replacement species include Acer rubrum, A saccharum, Prunus serotina and others. Logging of oak forests that have understories dominated by later successional species often accelerates the oak replacement process. Relative to other hardwood tree species, many oaks exhibit high fire and drought resistance. Adaptations of oaks to fire include thick 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 adjustment and a drought-resistant photosynthetic apparatus. However, oaks typically have low tolerance for current understory conditions, despite the fact that they produce a large seed with the potential to produce an initially large seedling. Oak seedlings in shaded understories generally grow very slowly and have recurring shoot dieback, although they 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-oak tree species, oaks should have the physiological capability for long-term survival beneath their own canopies in uneven-age tie, gap-phase) or even-age forest conditions. I argue that fire exclusion this century has facilitated the invasion of most oak understories by later successional species, which are over-topping oak seedlings. If this condition, coupled with severe predation of oak acorns and seedlings, continues into the next century, a major loss of oak dominance should be anticipated.