Studies And Hybridization Of Suppressus Species Grown For Timber In Kenya

Abstract

Cupressus lusitanica Mill. is grown as an exotic forest crop in Kenya for softwood timber production in compensatory plantations. In the Kenya Highlands it is the most successful exotic softwood, ana timber produced annually from thirty years old plantations already exceeds that produced from the dwindling reserves of indigenous softwood forest which they were planted to replace. It provides a valuable export product. In cultivation a preferred form of Cupressus lusitanica has arisen which shows superior growth, straight lightly fluted boles, simple fine branching and low susceptibility to Monochaetia canker. It has been suggested that this form has arisen by hybridization of Cupressus lusitanica with C. macrocarpa Hartw. and, perhaps, with other species. The history of the introduction of Cypresses is outlined and it is shown that, if Kenya Cupressus lusitanica iso hybridized, possible male parents are New World strains of C. lusitanica from Central America, Cupressus macrocarpa Hartw. from Monterey. and, less probably, Cupressus torulosa Con or ~ arizonica Greene. The formal taxonomy of the genus Cupressus L. is briefly reviewed and it is concluded that eleven species of Cypresses may be recognised, four in the Old World and seven from the New World. Delimitation of species in the genus is extremely difficult because cypresses are very variable trees showing a wide range of vegetative form within one species. Examples of extreme variation in vegetative form in Kenya . . and in Costa Rica are given, emphasising the difficulty of taxonomic discrimination in this genus. The specific status of Cupresstls benthamii Endl. from Mexico and Cupressus lindleyi Klotsch (now considered to be respectively a variety of, and synonomous with, C. lusitanica ~ill.) from Northern Central America has long been disputed. The historical and herbarium evidence for their being conspecific with Cupressus lusitanica Mill. is reviewed. , Biometric evidence from cones collected from native stands in Mexico, and studies of the progeny of Bentham's cypress, both in Mexico and in Kenya, established that this tree can only be regarded as a variety of lusitanica. The American origin of Cupressus lusitanica (which name has priority) and its conspecificity with Mexican trees, formerly named Cupressu~ lindleyi, are established by re-examination of herbarium material in both British and Mexican herbaria and by physiological evidence obtained by Gas Chromatography of heartwood infiltrates and of leaf waxes. The identity of Kenya Cupressus lusitanica with European strains of the species is established by comparison of herbarium material, by biometric comparisons of four distinguishing characters in Kenya crops and by analysis data of leaf

waxes. Hybridization The production of hybrids of lusitanica with macrocarpa and with C. arizonica by"controlled pollination is described. The hybrids are shown to be intermediate between the parents in respect of vigour of growth, colour of foliage and leaf wax constitution. Putative hybrids, occurring naturally in Kenya, have been detected by their intermediate appearance and confirmed as intermediate by biometric.comparison of distinguishing features and by analysis of their leaf waxes. Natural hybrids of Cupressus macrocarpa with lusitanica, and with torulosa were found. Such hybrids are nowadays rare in managed Cupressus

lusitanica crops, but are not uncommon in natural regeneration clumps and, in the past, were more frequently found in plantations established from carelessly collected seed. In a final discussion, it is argued that hybridization has not played an important part in the development of the Kenya preferred form of Cupressus lusitanica. This is derived by selection from Cupressus lusitanica of European origin, but with some interbreeding with American strains. widespread hybridization of the crop, such as that described by Cousens amongst Oak crop in Scotland, has not yet occurred. With Cypresses in Kenya, hybridization is undesirable and must be avoided. It can be avoided by careful selection of seed sources, and this study provides strong arguments in favour of continued work to improve the quality of East African cypress seed.