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Changes in Tree Species Composition and Stand Structure in a Mature Upland Oak-Dominated Forest Reflect Differences in Recruitment, Survival, and Longevity

ABSTRACT: 
 Poor regeneration of some species of oaks (Quercus spp.) within undisturbed closedcanopy forests is common throughout much of the eastern United States. Oak regeneration failure is often inferred from negatively skewed size-class distributions, but evidence of replacement by non-oaks requires investigation of changes in composition and size distributions. I examined how tree species composition and the log-transformed diameter distributions of oaks and non-oaks changed between 1999 and 2013 in an old-growth forest on the University of Mississippi campus. There was a general weak trend towards replacement of oaks by non-oaks. Oaks showed nonskewed or negatively skewed diameter distributions in 1999 and 2013. Diameter distributions of southern red oak (Quercus falcata) became significantly more negatively skewed, and the abundance of this species declined. However, there was no significant change in skewness of diameter distributions of the more fire-tolerant but longer-lived post oak (Q. stellata) and no significant decline in its abundance. Diameter distributions of trees of mockernut hickory (Carya tomentosa) became significantly more positively skewed, as it increased in abundance due to high recruitment and survival. In contrast, there was no significant change in diameter distributions in sweetgum (Liquidambar styraciflua) despite high recruitment, due to significant mortality of medium-sized trees of sweetgum. In addition to differences in fire tolerance between oaks and non-oaks, this study shows that differences in survival and longevity are important predictors of future species composition and that fire tolerance is not synonymous with early-successional status.