Effects of overstory retention, herbicides, and fertilization on sub-canopy vegetation structure and functional group composition in loblolly pine forests restored to longleaf pine
The desirable structure of longleaf pine forests, which generally includes a relatively open canopy of pines, very few woody stems in the mid-story, and a well-developed, herbaceous ground layer, provides critical habitat for flora and fauna and contributes to ecosystem function. Current efforts to restore longleaf pine to upland sites dominated by second-growth loblolly pine require information about how restoration treatments affect sub-canopy vegetation. We established a field experiment at Fort Benning in Georgia and Alabama, USA to determine the effects of four levels of approximately uniform canopy density (Control [~16 m²/ha basal area], MedBA [~9m²/ha basal area], LowBA [~5m²/ha basal area], and Clearcut [0 m²/ha basal area]) and three cultural treatments (NT [untreated], H [chemical control of woody and herbaceous vegetation] and H + F [chemical control plus fertilization]) on vegetation structure and functional group composition for three growing seasons following canopy removal. In general, cover (a measure of abundance) of ground layer vegetation increased with the amount of canopy removal. The ground layer was dominated by herbaceous vegetation in each year. Canopy trees generally suppressed the cover of graminoids in the first two years after treatment but only the Control plots had lower graminoid cover than Clearcut plots after the third growing season. Forb cover was significantly lower on Control plots than on Clearcut plots after only the first growing season, and woody stems/shrubs had lower cover on Control plots than on LowBA or Clearcut plots in each year. Vegetation cover increased following the first year after canopy removal, and the relative dominance of functional groups did not change through time. Canopy retention limited the development of mid-story woody stems, with the greatest stem densities in the Clearcut plots. The herbicide treatment (on both H and H + F) significantly reduced woody stem density in the mid-story in 2009, but the effect was no longer significant in 2010. Traditional methods for converting stands of other pine species to longleaf pine commonly include clearcutting followed by planting, but our results suggest that clearcutting may release woody vegetation to increase mid-story stem densities and will reduce the amount of pine needles in the fuel bed. Retaining low to moderate levels of canopy density (5–9 m²/ha basal area) in loblolly pine stands may provide an effective balance for reaching multiple restoration objectives that include maintaining desirable vegetation structure and creating fuel conditions for a frequent fire regime.