Size and Ecological Significance of the Physiological Individual in the Bunchgrass Schizachyrium scoparium

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Abstract
The size and ecological significance of the physiological individual were investigated in the bunchgrass Schizachyrium scoparium by experimentally manipulating the number of connected ramet generations within a hierarchy. Growth and demographic responses of juvenile ramets within the variously sized ramet hierarchies, including total hierarchy mass, were monitored at regular intervals. Growth of two-generation ramet hierarchies in a controlled environment and juvenile ramets within two-generation hierarchies in the field were suppressed by 58 and 26%, respectively, when severed from the clone. Parental ramets (secondary generation) incurred a 50% reduction in mass by sustaining growth of juvenile ramets within two-generation hierarchies. Severance of vascular connections between the primary ramet (common progenitor of the clone) and all secondary-tertiary hierarchies within the clone increased resource availability for the remaining experimental hierarchy which significantly increased juvenile ramet mass, leaf number and leaf area. Survivorship of juvenile ramets was not affected by severing ramet hierarchies from the clone, but leaf recruitment, ramet recruitment and juvenile ramet reproductive development were significantly suppressed. The ecological benefits derived from physiological integration within this species are largely confined to physiological individuals consisting of three connected ramet generations as opposed to the entire clone. Resource integration within the physiological individual enhances vegetative growth and reproductive development of juvenile ramets in the competitive environment created by the high density of established ramets characteristic of the caespitose growth form.

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