Effects of fire and canopy on understory hardwoods in a restored longleaf pine savanna

Pine savannas once covered much of the southeastern United States.  Historically, high savanna diversity has been maintained by frequent disturbances.  With increased fire suppression, hardwood species that normally exist in the understory can reach the canopy and block light necessary for herbaceous diversity.  In this study we investigated the effects of time since fire and canopy type on woody species in the understory.  Plots burned either in the year of, or year prior to, data collection.  Three canopy types were examined: closed canopy, gaps open 10 years, and gaps open at least 30 years.  Hardwood stem density was expected to be greater, and stems were expected to be shorter following a more recent fire; no fire effects were expected with regard to number of hardwood species.  Greater stem density, number of species, and stem height were expected under gaps; no significant differences were expected between 10-year and 30-year gaps.  As expected, stem density was greater (F_1,24=4.41, p=0.0464), and stems were shorter (F_1,8=24.01, p=0.0012) following recent fire, with no effects on number of species.  No differences between open and closed canopy were observed for stem density or number of species, but stems were significantly taller (F_1,16=28.95, p<0.0001) under open canopy.  When 10-year and 30-year gaps were compared, significantly fewer stems (F_1,24=5.67, p=0.0255) and species (F_1,24=4.54, p=0.0435) were found in the 30-year gaps; stem height was not significantly different between gap types.  Although fewer stems may be present, those that persist are able to attain similar heights.  Shrubs respond to increased light with greater heights, but after continually having to resprout following fire, the number of stems and species in a gap may decline due to competition with rapidly growing herbaceous species.  Ecological restoration may be used to control hardwood growth and maintain savanna diversity.