Submitted to: Journal of Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 6, 1999
Publication Date: N/A
Interpretive Summary: Soil resources like nitrogen have been found to accumulate directly beneath plants in a wide variety of environments, but it is not clear how resource accumulation affects plant growth or response to grazing. This investigation evaluated aboveground growth of little bluestem (a perennial grass) in response to changes in soil volumes (indicative of soil resources) and light levels over three growing seasons. Plants displayed substantial changes in leaf, tiller (lateral vegetative shoot) and plant level variables to the modifications in soil volume. Changes were greatest at the plant level and smallest at the leaf level. These changes were largely completed within the first growing season. New tillers on each plant and the total number of tillers on a plant doubled for plants with large soil volumes compared to plants with small soil volumes. Belowground resources had a greater influence on growth than did light, and these resources were taken up from beneath plants. It appears that growth of this grass may be influenced as much by the amount of soil water as by the level of nitrogen that is beneath plants in the soil. These results indicate that belowground resources play a pivotal role in determining growth responses of individual plants and that proper grazing may be as much a function of belowground as aboveground resource management.
Morphological plasticity of the caespitose, perennial grass, Schizachyrium scoparium, was evaluated in response to soil volumes and radiation intensity for three successive growing seasons. Growth responses were largely completed within the first growing season following root confinement. Morphological plasticity rankings were clone canopy > ramet recruitment > leaf variables. Cumulative ramet recruitment and ramet number per clone approximately doubled for clones with large compared to small soil volumes. These demographic responses provide indirect evidence for the existence of an intraclonal ramet regulation mechanism that is mediated by resource availability. Belowground resource availability had a much greater influence on clonal growth, morphological plasticity and ramet demography than did radiation intensity. Comparable growth and ramet demography of clones in the unrestricted and small soil volumes indicate that clones access resources from a soil volume approximated by a cylinder one radius greater than that of the clonal circumference. Results from this field investigation provide circumstantial evidence to support the ecological significance of soil resource accumulation to clonal growth in the caespitose growth form. However, within the context of this experimental protocol, it appears that soil water may be of equal or greater importance than soil nitrogen in determining clonal growth.