Long-term changes in an oak forest's woody understory and herb layer with repeated burning1

Abstract

Although fire exclusion is thought to be linked with declining plant diversity in oak forests, few studies have examined long-term changes in their shrub and ground layers resulting from repeated burning. In this study, we compare the composition and structure of woody understory and ground layer vegetation in burned and unburned oak forest after 17 years of annual dormant season low-intensity burns. Over time, burned forest had 97% reduction of shrubs and small saplings, but only 38% loss of stems in the > 5–10 cm size class. Canopy openness was similar in burned and unburned forest plots prior to the onset of burning, but it was significantly greater in burned forest after 17 years of fire. Ground layer vegetation structure also changed significantly, with responses differing by guilds. Spring herbs were the dominant guild before burning and did not change over time. However, cover and abundance of summer herbs increased over time in burned forest, probably in response to greater light assimilation under the more open canopy. This resulted in greater overall species richness in burned plots without loss of the spring herbs. Burning eliminated most alien shrubs, although common buckthorn persisted in small numbers. The alien herb garlic mustard also persisted and had greater abundance in burned plots, apparently by re-colonizing from unburned micro-habitats and adjacent forest. These results indicate that long-term burning can eliminate shrub and small sapling canopy cover, thereby increasing canopy openness and promoting greater richness and cover of summer forbs. Fire also probably had a positive effect on seedling establishment through removal of litter. Resulting tradeoffs to this gain in diversity include loss of native vines, shrubs, understory trees and forest interior bird habitat, as well as persistence of alien plants.