DENSITY-DEPENDENT REGULATION OF RAMET RECRUITMENT BY THE RED-FAR-RED RATIO OF SOLAR-RADIATION - A FIELD-EVALUATION WITH THE BUNCHGRASS SCHIZACHYRIUM-SCOPARIUM

Depressions in the red to far-red ratio (R:FR) of solar radiation aris ing from the selective absorption of R (600-700 nm) and scattering of FR (700-800 nm) by chlorophyll within plant canopies may function as a n environmental signal directly regulating axillary bud growth and sub sequent ramet recruitment in clonal plants. We tested this hypothesis in the field within a single cohort of parental ramets in established clones of the perennial bunchgrass, Schizachyrium scoparium. The R:FR was modified near leaf sheaths and axillary buds at the bases of indiv idual ramets throughout the photoperiod without increasing photosynthe tic photon flux density (PPFD) by either (1) supplementing R beneath c anopies to raise the naturally low R:FR or (2) supplementing FR beneat h partially defoliated canopies to suppress the natural R:FR increase following defoliation. Treatment responses were assessed by simultaneo usly monitoring ramet recruitment, PPFD and the R:FR beneath individua l clone canopies at biweekly intervals over a 12-week period. Neither supplemental R nor FR influenced the rate or magnitude of ramet recrui tment despite the occurrence of ramet recruitment in all experimental clones. In contrast, defoliation with or without supplemental FR benea th clone canopies reduced ramet recruitment 88% by the end of the expe riment. The hypothesis stating that the R:FR signal directly regulates ramet recruitment is further weakened by evidence demonstrating that (1) the low R:FR-induced suppression of ramet recruitment is only one component of several architectural modifications exhibited by ramets i n response to the R:FR signal (2) immature leaf blades, rather than le af sheaths or buds, function as sites of R:FR perception on individual ramets, and (3) increases in the R:FR at clone bases following partia l canopy removal are relatively transient and do not override the asso ciated constraints on ramet recruitment resulting from defoliation. A depressed R:FR is probably of greater ecological significance as a sig nal of competition for light in vegetation canopies than as a density- dependent signal which directly regulates bud growth and ramet recruit ment.