Densification and State Transition Across the Missouri Ozarks Landscape

Abstract

World-wide, some biomes are densifying, or increasing in dense woody vegetation, and shifting to alternative stable states. We quantified densification and state transition between forests ecosystems in historical (ca. 1815‐1850) and current (2004‐2008) surveys of the Missouri Ozark Highlands, a 5-million ha landscape in southern Missouri, USA. To estimate density of historical forests, we used the Morisita plotless density estimator and applied corrections for surveyor bias. For contemporary forests, we used known densities at plots to predict continuous densities with random forests, an ensemble regression tree method. We also calculated basal area and percent stocking to determine changes in wood volume. Historical forests had densities ranging from about 75 to 320 trees/ ha. Current forest densities were about 2.3 times greater and more uniform, at about 300‐400 trees/ ha (DBH ‡ 12.7 cm). Not all forests have increased in basal area and percent stocking because trees in contemporary forests are smaller in diameter than historical forests. Although oak species still are dominant (as defined by ‡10% composition), oak dominance is being replaced by many fire-sensitive species, of which only eastern redcedar and maples have become dominant. Densification and community changes in functional traits have produced a state transition from open oak forest ecosystems to predominantly closed eastern broadleaf forests in the Missouri Ozarks. This shift is not at equilibrium, as fire-sensitive species are continuing to increase and turnover in long-lived oaks is slow.