Studies on Poa colensoi Hook. f. populations in the South Island, New Zealand

Abstract

After but a century of influence by European man and his introduced game and domestic animals in indigenous grasslands of New Zealand, widespread deterioration in plant and soil covers on mountainous terrain has become a problem of national concern (Zotov, 1938; Gibbs at al, 1945; Tussock Grassland Research Committee, 1954; Holloway, 1959, 1960; McKelvey, 1959; Raeside, 1960). General awareness of this retrogressive condition in watersheds of many main rivers has recently been responsible for a more unified front in research designed to measure present conditions and future trends, and concurrently evolve remedial methods. (Holloway, 1960; O'Connor, 1960; McKelvey, 1960; Raeside, 1960.) The present situation is however considerably more complex than surface appearance indicates, as it is possible that large areas of mountainous grassland, scrubland and forest are at present inherently unstable due to the effects of recent deterioration from a previous climatic optimum (Holloway, 1954: Molloy, 1960; Raeside, 1948, 1953). In addition, appreciable replacement of forest associations by induced tussock grass and scrub communities has occurred in pre-European times; the destructive agent of these forests being uncontrolled fire. (Molloy, 1960; Holloway, 1954.) Finally, mountain ranges in New Zealand are geologically immature, with steep slopes, therefore soils formed on them have generally high erosion potentials, irrespective of stability or instability in their present vegetative cover. (Speight, 1940, Molloy, 1960, Holloway, 1960). Among the most significant research programmes being conducted on complex problems of the lands described are the ecological surveys of complete watersheds of main rivers being carried out by the Forest and Range Experiment Station (Wraight, 1960; Wardle, 1960; Holloway, et al. 1959); the Soil Conservation and Rivers Control Council’s land capability classification, alpine climatological recording (Mark, 1960; Morris, 1960); genetic soil classification and detailed physical, chemical and biological analyses of regional soils, by Soil Bureau, Department of Scientific and Industrial Research (Taylor et al, 1960; Thornton et al, 1958, 1959); survey and analysis of specific soil mineral nutrient limitations to high pastoral production in the 6 million acres of montane tussock grassland (Walker, et al, 1955, 1956, 1958; Lobb, 1959; O’Connor, 1959, 1960; Ludecke, 1960); and experimental study in taxonomy, ecology and physiology of plants endemic to the rigorous environments (Fisher, Connor et al, 1960; Burrows, 1960; Scott, 1960a). But in some 14 million acres, detailed knowledge of climates, soils, botany and zoology must of necessity be exceedingly fragmentary at this time. Although the floristics of main plant Associations have received much attention since early botanical exploration, little accurate data are available concerning taxonomic and physiologic relationships of even the principal plant species. Need for detailed experimental research on New Zealand plants – especially those of indigenous communities in montane, subalpine and alpine regions – has long been emphasised (A. H. Cockayne, 1916; L. Coclayne, 1918; Oliver, 1926; Tussock Grassland Research Committe, 1954; Hercus, 1954; Moore et al. 1955; Crocker, 1956; Molloy, 1960). All advance the thesis that taxonomy, physiology and autecology of the many important plants in native grasslands must complement field measurements of synecology and sociology, if the precise ecological status of each species is to be determined. Biosystematic studies of Agropyron (Connor, 1949, 1954) and of Ranunculus (Fisher, 1954, 1960); determination of breeding systems in several grasses by Connor (1957, 1960) and for Pimelea by Burrows (1960); analysis of synecology in definite habitats (Fisher, 1952; Barker, 1953; Molloy, 1960; Wraight, 1960); and the autecology of single species by Nurse (1940), and Calder (1958), provide excellent examples of information required to begin our understanding of the nature of this extensive and complex natural community. To add to this developing knowledge of individual species and the associations they form, is the general object of this study of Poa colensoi (blue tussock), a perennial bunch grass of low stature. Choice of this species was influenced firstly by its wide distribution throughout the environmental range in indigenous grasslands (Figure 1), from lowlands to alpine barrens, and from regions of high to extremely low annual precipitation (Cheeseman, 1925). Secondly by its relative importance as a constituent of tussock swards, for both grazing and soil conservation interests (L. Cockayne, 1919a-1922c; Sewell, 1952; Chapman, 1956; Gregor, 1957). In addition, P. colensoi has several attributes of considerable physiologic and biosystematic interest. A number of references, by both graziers and research workers, to the relative palatability of P. colensoi to grazing animals, especially sheep, have given this tussock a reputation as an economically important species. Dr. L. Cockayne (1920b), while testing relative palatability of several indigenous and exotic grass species, commented upon the perplexing array of 'forms' or subspecies in P. colensoi throughout its ecological range. At this early stage, Cockyane(1919a) with remarkable perception wrote of plans for experimental comparison of these apparent subspecies in the near uniform environments of transplant gardens. This idea was being used by his contemporaries, Turreson (1922a, 1922b, 1925) in Sweden and Hall (1926) in the U. S. A., and has since become the basis of all experimental taxonomic research (Blosystematics) into variations which exist in natural populations of single species (Clausen, Keck and Hiesey, 1940, 1948; Lawrence, 1945; Gregor, 1956; McMillan, 1959b, Clausen, 1960). These genetic variations are manifested as distinct physiological and morphological differences between individuals of a population and between populations; the distribution of variation being governed largely by breeding system of the species and its ecological range. (Clausen, 1951.) The present study then, is an attempt to demonstrate some physiological and morphological variations between populations of P. Colensoi, and to correlate these with environmental differences particularly those of climate. It is hoped that the results may prove of use in furthering our understanding of indigenous tussock grassland, and the nature of its community type.