Distribution of Oaks in the Eastern United States: Troubling Changes in Abundance and Species Composition

Oaks (Quercus spp.) are one of the most widely distributed tree genera in North America and prevalent in most of the major forest type groups in the eastern United States. In spite of this prevalence, concern exists that oak forests are aging with an insufficient base of young stands to replace them. Using data from the USFS Forest Inventory and Analysis (FIA) program collected in 1989 and 2000, I estimated the current distribution and abundance of select oak forest types and oak species within the red and white oak subgenera in 29 eastern states. I also examined trends in the distribution and abundance of the species and oak forest types over the past decade. In 1989, oak stems represented 32% of the intermediate forest canopy and 52% of dominant and co-dominant classes. These numbers declined to 21% and 47%, respectively, by 2000. Also, the average density of maples (Acer spp.) in the forest understory increased three-fold within oak stands between 1989 and 2000 from 625 to 2144 stems/ha, and currently have a 35% greater stem density than that of oak species in the understory. These data expose two trends detrimental to the long-term conservation and persistence of oak forests in the eastern United States. First, oaks are declining in prevalence in stands where they tended to be the most dominant, and there appears to be a poor reserve of intermediate stems available to replace them. Second, the density of maple stems is increasing rapidly in oak stands, especially within the understory, and this may interfere with oak seedling recruitment. Maples and other species [e.g., beech (Fagus grandifolia) and poplar (Lirodendroan tulipifera)] are either more shade tolerant or grow faster than oaks and thus are capable of out-competing oaks following canopy disturbance, a situation which ultimately can result in a change in forest composition. These changes in forest composition are likely the result of a combination of factors including disruption of historic disturbance regimes (e.g., fire suppression), poor silvicultural practices (especially on private lands), impacts of white-tailed deer (Odocoileus virginianus) on forest regeneration, and introduced insects and pathogens that reduce oak stand health and recruitment. The continued loss of oaks from eastern forests will negatively impact numerous wildlife species that use acorns as a primary or secondary food source and have unknown effects on the overall forest ecosystem. Such a loss would be similar to that of the American chestnut (Castanea dentata), except that oaks served to fill their niche and mitigated the detrimental impacts of their loss. No comparable replacement species exists for oak trees.

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