Structure and composition of an oak-hickory forest after over 60 years of repeated prescribed burning in Missouri, U.S.A

Abstract

Prescribed fire is often used in upland oak forests to achieve objectives related to woodland restoration or oak regeneration. While not uncommon for prescriptions to include repeated burning at short fire return intervals, long-term frequent fire effects in hardwood forests have rarely been quantified. We describe the effects of over 60years of repeated prescribed burning on stand structure and composition in a Missouri Ozark oak-hickory forest. Data were collected in 2010 and 2013 from 40×40m plots arranged in a randomized complete block design with two blocks and three treatments: an unburned control (Control), annual prescribed fire (Annual; one-year fire return interval), and periodic prescribed fire (Periodic; four-year fire return interval). In 2013, the diameter distribution of the Control was an exponential decay, whereas those in both burn treatments were unimodal. Repeated prescribed burning, especially at four-year fire return intervals, reduced overstory density and canopy cover and shifted overstory composition from a mixture of white oak species, red oak species, and hickories to primarily white oak species. Although small-diameter stems were top-killed on Periodic plots, resprouting created a dense mid-story of small stature saplings. In contrast, Annual plots were virtually void of woody seedlings and saplings. There was no apparent tree recruitment on either Annual or Periodic plots. The abundance, richness, diversity, and evenness of the understory plant community were greater on the burned plots than on the Control plots, with forbs dominating the Annual plots and woody species dominating the Control plots. Periodic plots included a mixture of forbs and woody species, and graminoids were not common in any plots. Understory species richness per square meter was nearly three times greater on both burn treatments relative to the Control. Our results indicate that long-term, repeated prescribed burning can approximate woodland conditions. However, the lack of tree recruitment suggests that periods without fire are critical for replacing overstory trees. Moreover, additional treatments such as mechanical thinning may be important for modifying the understory environment to benefit graminoids or for reaching management objectives on desirable timescales.