Water and tree-understory interactions: A natural experiment in a savanna with oak wilt
Savanna trees influence water, light, and nutrient availability under their canopies, but the relative importance of these resources to understory plants is not well understood. Ina three-year study in a Texas savanna, trees infected with the disease oak wilt were used in a natural experiment to isolate the effects of light and soil resources, particularly water, in oak-understory interactions. Herbaceous biomass and survival of transplanted Prosopis glandulosa (mesquite) seedlings were monitored in plots under healthy and symptomatic Quercus fusiformis (live oak) trees, and in open sites. Shade cloth maintained similar midday light levels in plots under symptomatic and healthy trees. Plant physiological attributes, soil parameters, and woody plant densities were also compared across habitats. Water availability was significantly lower near healthy trees than near symptomatic trees or in the open. Shade-cloth plots under symptomatic trees had over twice the herbaceous biomass of ambient-light plots under healthy trees. As shade-symptomatic plots had similar light and lower nutrient levels than ambient-healthy plots, greater water availability under symptomatic trees was probably a major factor increasing herbaceous productivity. Shade also affected herb growth, and its importance varied seasonally and annually with water availability. Woody seedling densities and 1996 mesquite transplant survival were significantly higher under trees than in the open, indicating facilitation of young woody plants by oaks. However, lower water potentials in larger shrubs near healthy trees and similar shrub densities across habitats (in contrast to seedling densities) suggested that oaks may compete with other woody species as the latter plants age. Our data indicate that both facilitation and competition have important roles in this savanna community, and competition for water may be a key mechanism in oak-understory interactions.
Anderson, LJ; Brumbaugh, MS; Jackson, RB
Volume / Issue
Start / End Page
International Standard Serial Number (ISSN)
Digital Object Identifier (DOI)