An Investigation Of Character Variation In Chaetobromus Nees (Danthonieae: Poaceae) In Relation To Taxonomic And Ecological Pattern.

ABSTRACT

Character variation in Chaetobromus, a genus of palatable grasses endemic to the arid

western areas of southern Africa, was used to derive a classification reflecting taxonomic

and ecological pattern. The present study differs from earlier biosystematic investigations

by its much more intensive approach to sampling, with 75 anatomical, morphological and

cytological characters and 169 individual samples being used. The use of larger population

samples permitted quantification o~ variation within populations, in addition to that among

populations and groups.

Phenetic methods revealed the existence of three groups, approximating three formerly

described taxa and reflecting divergent ecological strategies in Chaetobromus. A lack of

diagnostic field characters argues against their recognition at species level, and

Chaetobromus Nees is here described as monotypic, the type species, C. involucratus

(Schrad.) Nees, comprising three subspecies C. involucratus subsp. involucratus, C.

involucratus subsp. villosus Verboom and C. involucratus subsp. dregeanus (Nees)

Verboom.

There is overlap among subspecies in most characters although many showed significant

mean differences. Within subspecies, character variation appears to be homogeneously

distributed with respect to population boundaries suggesting that these are likely to have

little impact on sampling. A investigation testing the effect of sample strategy on variation

capture and taxonomic group detection suggests that a sample of 10-15 specimens is

likely to account for most variation present.

Phylogenetically, Chaetobromus is included in the tribe Danthonieae, and, on morphological

evidence, is probably basal to a clade containing Pentaschistis, Pentameris and

Pseudopentameris. The genus appears to occupy a niche unique among the African

danthonioids, favouring lime-rich, basic soils and a strongly-seasonal winter-rainfall regime

with arid summers. Ecological differences among the subspecies are reflected in

differences in growth form and vegetative and reproductive phenologies. The niche

requirements of Chaetobromus may be adequately specific to explain the patchy

distribution of the genus.