Experimental response of understory plants to mechanized disturbance in an oak-pine forest

Resource managers at Fort Benning, Georgia, must maintain environmental conditions necessary for military training as well as promote longleaf pine (Pinus palustris) habitat. Understory vegetation controls the eventual species composition of these forests but is also sensitive to military activity. Past research from an observational study at this site suggests that impacts from military activity are best indicated by understory plant families and Raunkiaer life forms – specifically an increase in cryptophytes (plants with underground buds) and therophytes (annual plants). Our study tests that conclusion experimentally using a tracked vehicle to manipulate an oak-pine forest occurring on an upland-riparian gradient. In May 2003, a D7 bulldozer removed extant vegetation and surface soil organic matter along three treatment transects. Braun-Blanquet vegetation surveys were conducted within plots in mid and late summer during 2003 and 2004. The response of total understory cover, bare ground cover, litter cover, species richness, family richness, Fabaceae cover and life-form cover was analyzed using repeated measures analyses of variance. Total understory cover, bare ground cover, species richness, family richness and cryptophyte cover showed a significant treatment × time interaction reflecting the transient response of these metrics to this isolated disturbance as most metrics returned to control values within two growing seasons. Although therophyte cover did not display significant interactions in this experiment, the increase in cryptophyte cover supports the use of life forms as indicators of mechanized disturbance to understory structure. While further experimental testing is needed across ecosystems and training activities, Raunkiaer life forms show potential to serve as part of a suite of indicators of understory structure and diversity in forest environments subject to military training.