The Role of Black Locust (Robinia Pseudo-Acacia) in Forest Succession

(1) Early forest regeneration in southern Appalachian hardwood forests is dominated by the woody nitrogen-fixing legume, black locust (Robinia pseudo-acacia). Although it grows most prevalently on clear-felled areas, abandoned pastures, or disturbed roadsides, it may have historically been an important colonizer of burned sites. Although it commonly reproduces from seed germination, sprouting from stumps and roots is its most prevalent means of regeneration. Early sprout growth is rapid, attaining heights up to 8 m in 3 years. (2) Except for stands on high-nutrient sites, growth decreases after 10-20 years. In less vigorous stands, stem mortality may be high due to attacks by the locust stem borer (Megacyllene robiniae). The high mortality of black locust is an early successional mechanism that releases cpdominant species such as Liriodendron tulipifera, and creates canopy gaps favourable for growth of longer-lived individuals. (3) Total biomass accretion in 4, 17 and 38-year-old black locust stands growing on fertile, mesic sites was 33, 174 and 399 t ha-', respectively, in comparison to 198 t ha-' for an older, uneven-aged mixed oak forest with a history of disturbance. (4) Biomass accumulation was the predominant fate of fixed N in all three stands, with an addition to total soil N apparent only in the 38-year-old stand. (5) Symbiotic N fixation by black locust apparently increased the concentration of NO3 in the soil. The source of elevated soil NO3 is hypothesized to be high fluxes of N from leaf and root litter mineralization and nitrification, and perhaps from canopy insect frass. (6) Total stand N increased at a net average annual rate of 48, 75 and 33 kg ha-' year-', respectively, for ages 4, 17 and 38. Nodule biomass was 8, 106 and 4 kg ha-' in the 4, 17 and 38-year-old stands, respectively. (7) These patterns of N accretion are similar to those reported for other woody nitrogen-fixing species on secondary successional sites. They indicate that peak N fixation occurred from early to intermediate stages of forest succession, and declined with later successional development.