DENSITY-DEPENDENT REGULATION OF RAMET RECRUITMENT BY THE RED-FAR-RED RATIO OF SOLAR-RADIATION - A FIELD-EVALUATION WITH THE BUNCHGRASS SCHIZACHYRIUM-SCOPARIUM
Js. Murphy et Dd. Briske, DENSITY-DEPENDENT REGULATION OF RAMET RECRUITMENT BY THE RED-FAR-RED RATIO OF SOLAR-RADIATION - A FIELD-EVALUATION WITH THE BUNCHGRASS SCHIZACHYRIUM-SCOPARIUM, Oecologia, 97(4), 1994, pp. 462-469
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.