An Investigation of Character Variation in Chaetobromus Nees (Danthonieae: Poaceae) in Relation to Taxonomic and Ecological Pattern
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AN INVESTIGATION OF CHARACTER VARIATION IN CHAETOBROMUS NEES (DANTHONIEAE: POACEAE) IN RELATION TO TAXONOMIC AND ECOLOGICAL PATTERN by George Anthony Verboom Town Cape of Submitted in fulfilment of the requirements of the Master of Science degree UnivesityUniversity of Cape Town March 1995 The copyright of this thesis vests in the author. No quotation from it or information derived from it is to be published without full acknowledgementTown of the source. The thesis is to be used for private study or non- commercial research purposes only. Cape Published by the University ofof Cape Town (UCT) in terms of the non-exclusive license granted to UCT by the author. University CONTENTS Abstract ............................................ 3 Acknowledgements .................................... 5 Chapter 1. Introduction . 7 Chapter 2. Materials and methods ......................... 11 Chapter 3. Results .................................... 27 Chapter 4. Character variation and analysis ................... 49 · Chapter 5. Systematic pattern and taxonomy ................. 63 Chapter 6. Variation and sampling ......................... 91 Chapter 7. Niche characteristics and regenerative biology ......... 99 Chapter 8. Conclusions .............................. " 117 Literature cited ..................................... 119 Appendices . 133 1 2 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. 3 4 ACKNOWLEDGEMENTS This study was carried out under the supervision of Peter Linder, to whom I am thankful for his support and infectious enthusiasm through both thick and thin. Many of the ideas presented here are the fruits of discussion both with him and other members of the plant systematics group at the University of Cape Town, most of whom also offered technical advice at one time or another. Nigel Barker, Sioban Munro and Wendy Hitchcock were foremost among these. Thanks are also due to my father, Nico Verboom, for his general support and advice on artistic issues. Finally, I am indebted to Sioban Munro, Wendy . Hitchcock and Casper de Villiers for agreeing to proof-read the final draft of this work. The study was funded by the Foundation for Research Development, Pretoria, the University of Cape Town and the Flora Conservation Committee of the Botanical Society of South Africa. 5 6 CHAPTER 1. INTRODUCTION Chaetobromus Nees, a genus of perennial CJ grasses endemic to the semi-arid and arid west Cape region of South Africa and the extreme south western corner of Namibia, is thought to have considerable potential as a pasture crop. However, various factors negate the successful exploitation of the grass, including an unstable taxonomy, and a poor understanding of its biological requirements. The pasture potential of Chaetobromus is widely acknowledged (Acocks 1971, 1990; Ellis . 1988; Van Breda and Barnard 1991). Most recently, a manual of west Cape grasses issued by the South African Department of Agricultural Development (Van Breda and Barnard 1991) listed the grass as'a highly palatable species (sic) - the first used by animals'. In addition this text notes that historically'... hartebeest had a great predilection for this grass species (sic) and would travel great distances to fields of the grass'. Similarly, Ellis (1988) noted that ' ...Chaetobromus appears to possess excellent potential as a fodder grass', introducing the possibility of its use in commercial agriculture and restoration of overgrazed vegetation. The potential utility of Chaetobromus as a pasture grass serves to amplify the need for a reliable and practical taxonomy. Hitherto, complex patterns of character variation paired with inadequate past sampling have yielded a series of alternative classifications, in which the number of species formally included in the genus has fluctuated from one initially (Schrader 1821) through three (Nees 1841), four (Stapf 1899; Chippendall 1955) and two (Barker 1990). Character intergradation, even in diagnostic characters, has always threatened the integrity of interspecific boundaries, and ultimately led Ellis (1988) to favour the concept of a single highly variable species. The central aim of the current study is the resolution of these conflicts through an extensive analysis of variation in anatomical, morphological, and cytological characters. Although not included here, a parallel investigation of DNA-based variation has a similar goal. 7 The current study A good taxonomic treatment should accurately reflect biological variation (including character, geographical and ecological variation) in the taxon under study (Stebbins 1950; Davis and Heywood 1963; Hawkes 1986; Stuessey 1990)' ultimately representing this in a formal classification. An additional but much underemphasised aim of biosystematic research is the interpretation of empirical patterns to provide baseline hypotheses for further biological investigation (Davis and Heywood 1963; Stuessey 1990). A taxonomy which achieves these basic goals is both a valuable baseline for further research, and essential to ensuring successful commercial exploitation of plant species (Davis and Heywood 1963; Hawkes 1986; Heywood 1986; Stuessey 1990), and is the broad goal of the present study. Detailed work leading to a sound taxonomy that accurately describes variation in Chaetobromus is adequately justified by its potential utility (Ellis 1988). Equally important is an understanding of factors limiting the distribution of the grass, specifically in relation to the possibility of historical overgrazing (Acocks 1990; Vogel et al. 1978; Ellis 1988). Although each is introduced individually, I present here, a brief outline of the issues considered in the discursive chapters. Chapter 4. As the basis of taxonomic pattern, character variation is of primary importance in taxonomy, and is therefore discussed in some detail. Given a large character set, this chapter evaluates the taxonomic utility of competing characters using different criteria, and tests whether the same set of characters are useful in defining cladistic and phenetic taxa. Finally, a method for deriving cladistically useful characters from 'noisy' quantitative data is explored. Chapter 5. This chapter focusses on pattern in character variation, using this to derive a formal taxonomy of Chaetobromus, including complete descriptions, distribution maps and diagnoses. Both phenetic and cladistic pattern are used, in concert, to delimit taxa and to establish taxon ranks. The establishment of a phylogenetic context for the genus early in the chapter, validates the exploration of character evolution in the group. Chapter 6. The effect of sampling on taxonomic pattern remains poorly explored, although inadequate sampling is almost certainly responsible, in part, for the historical instability in Chaetobromus taxonomy. This chapter explores the effect of alternative sampling strategies on observed character variation and group detection in Chaetobromus, with the goal of establishing the minimum sampling effort necessary to capture a majority of the variation present and to permit taxonomic group detection. In so doing, this study tests 8 the validity of its own sampling procedure, making it unique in this regard. The results provide insight into the distribution of character variation in Chaetobromus and this is discussed here. Chapter 7. An ecological understanding of Chaetobromus is essential to its commercial exploitation, but has hitherto