Prescribed fire effects on understory vegetation across a topographic moisture gradient in oak forests

This study examined the effects of repeated prescribed fires on understory plant communities, tree seedlings populations, and a rare woodland grass across a topographic gradient of soil moisture and nutrient availability. This research is an integral component of a larger project examining the response of mixed-oak forest ecosystems to the long-term application of prescribed fire treatments. The study was conducted at four sites in the unglaciated Allegheny Plateau region of southeastern Ohio. Each study site was comprised of an unburned treatment unit (no fire), a periodic burn unit (prescribed fires applied in 1996 and 1999) and an annual burn unit (fires applied 1996-99). The study sites were stratified by an integrated moisture index (IMI) that predicted relative soil moisture based on topographic position and mapped soil water-holding capacity. The stratification delineated areas of predicted soil moisture into three general classes: xeric, intermediate, and mesic. Within each treatment unit, permanent vegetation plots were placed in each IMI class for a total of 108 plots across the four sites. Understory vegetation was monitored on a 25 X 25 m portion of each plot. Sixteen 2 m quadrats, placed in a stratified random design, were sampled in the spring and late-summer each year 1995-99. Each year, twelve of the sixteen quadrats were placed in different random locations. In each quadrat the presence of all vascular ii plant species, including tree seedlings, was recorded so that the basic unit of analysis for species was the frequency of occurrence per plot. Prior to fire treatments (1995), variation in understory species composition was most strongly related to topographic variations in predicted soil moisture (IMI), N mineralization rate, nitrification rate, and soil pH. In addition floristic variation at the regional scale was correlated with variations in soil texture, nitrification, pH, and PO4 resulting from differences in soil parent material complexes among sites. Species richness was positively correlated with N mineralization rate and nitrification rate and was inversely correlated with tree basal area. Strong species richness-N mineralization correlations indicate an important link between below-ground processes and plant diversity. A total of 452 taxa were recorded from 1995-99, 74% of which were herbaceous perennials. Only 15 species were exotics, none of which were abundant either before or after fire treatments. Understory vegetation responded similarly on both the periodic and annual burn treatments, among the four study sites, and across the three IMI classes. Temporal change in community composition was evaluated by indirect ordination (nonmetric multidimensional scaling). Post-burn compositional shifts resulted primarily from shade-intolerant species and summergreen herbs that increased in frequency from seed bank germination and also from tree seedlings and several spring-season herbs that decreased in frequency. Species richness (per 2 m) became significantly greater on burned units as forb and graminoid richness increased. At the plot-scale, species evenness became greater on periodic burn units but plot-