Intraclonal regulation in a perennial caespitose grass: a field evaluation of above- and below-ground resource availability

1. Intraclonal regulation of the C4 caespitose perennial grass Schizachyrium scoparium was evaluated in response to various levels of above-ground (radiation intensity) and below-ground (soil volume) resource availability in the field for three successive growing seasons. We reasoned that the relative plasticity of clonal growth in response to various levels of resource availability may provide insight into the mechanism of intraclonal regulation. 2. Six naturally occurring clones were randomly assigned to each of five treatments: (i) unrestricted soil volume (control); (ii) large soil volume in which roots were confined to a cylinder of soil with a radius four times that of the clone; (iii) small soil volume, twice the clone radius; (iv) large soil volume with radiation intensity reduced to 55% of ambient; and (v) small soil volume with reduced radiation intensity. 3. Ramet recruitment increased within the first season following root confinement in large soil volumes, while ramet and leaf growth exhibited less plasticity. The effects of a reduction in radiation intensity were smaller, and, in contrast, increased ramet and leaf growth, but did not modify ramet recruitment. 4. The pattern and magnitude of ramet mortality in clones confined to the large soil volume were similar to those of clones in unrestricted soil volume, indicating that intraclonal regulation was sufficient to reduce ramet recruitment and thus density to within the carrying capacity established by resource availability. 5. Similar growth and ramet demography of clones in the unrestricted and small soil volumes suggest that unrestricted clones normally access resources from a soil volume similar to that of the small soil volumes. 6. Rapid clonal growth in response to root confinement to large soil volumes, resource acquisition from a relatively small soil volume that contains the zone of resource accumulation, and the capacity for rapid expression of ramet plasticity provide circumstantial evidence that individual caespitose clones may respond to resource accumulation in soils beneath them.