Disassociating tree species associations in the eastern United States

Ecologists have a long history of describing species associations including oak-hickory, one of the predominant associations in the eastern United States. But historically, oak composition did not appear particularly related to hickory composition. I assessed the relevance of the oak-hickory association and other associations using older and recent (c. 1981 and 2007) USDA Forest Service surveys. For common hickory and oak species, I determined percent composition (i.e., percent of total stems >= 12.7 cm in diameter, relative density or abundance) in ecological subsections, changes in composition throughout ranges, and compared composition of oaks and hickories and other potential associations using correlation and ordination. Oaks were among the most abundant species while hickories were minor species. Hickory composition was stable while the trajectory of oak continued to decrease during the survey intervals from presettlement dominance. Rank-order correlation between oaks and hickories throughout their ranges was about the maximum as for other species (0.55 and 0.42 during the two survey periods) and in the Oak-Hickory forest region, correlation between oaks and hickories was 0.04 (older surveys) and 0.16 (recent surveys). Oaks were not associated with hickory in the "oak-hickory" forests of Missouri during the mid-1800s, nor were oaks associated with hickory more recently beyond correlations that occur between other eastern forest species. Oak-hickory association in particular is not an informative term for either historical open oak ecosystems or current eastern broadleaf forests. Mixed mesophytic associations, perhaps not best termed as an association, are eastern broadleaf forests where many tree species dominate forested ecosystems in the absence of filtering disturbance. Associations, even if species share similar traits, generally are not strong, stable in time, or extensive in space; differences between species result in different and changing distributions in response to the environment, land use, disease, and other influential factors.