Replacement of a historically dominant upland oak by a lowland oak in fire-excluded forests and in a fire-restored upland woodland

Abstract

Natural oak regeneration has declined within forests in parts of North America, Europe, and Asia. The specter of oaks being replaced by non-oaks because of fire exclusion has overshadowed the fact that some species of oaks have increased in some areas despite modern fire exclusion. Displacement of upland oak species by lowland oak species in uplands remains unexplained. We document a shift in oak dominance of upland landscapes in northern Mississippi (USA) from the early to mid 1800s to present day using witness tree records, scientific eyewitness accounts, and modern sampling. We also present data from a long-term restoration experiment in an upland oak woodland on changes in densities of upland Q. stelllata, lowland Q. alba, and non-oaks (e.g., mesophytes). Results revealed a shift in oak dominance of upland landscapes from Q. stellata to Q. alba from the early to mid 1800s to the 2000s. Q. alba showed an estimated four-fold increase in density in uplands. Fire restoration treatments initiated in 2004 (i.e., canopy reduction, burning) favored sapling recruitment of Q. alba over Q. stellata in an upland woodland, despite reducing overstory densities of the former. Differences in sapling recruitment resulted from higher seedling densities and survival of Q. alba. We conclude that modern fire exclusion contributed to the ongoing displacement of Q. stellata by Q. alba in uplands. Continuation of restoration treatments in the upland woodland are likely to favor the more profitable but less drought-tolerant Q. alba over Q. stellata, with implications for economic value and climate resiliency.