Variation in aster tripolium L., with particular reference to some British populations

(1) The following is a study of variation in Aster tripolium in relation to the ecology of the species and in view of its widespread occurrence on British salt marshes. The main period of the investigation was between 1965 and 1968. (2) Whilst recognisable as a taxonomically distinct species A. tripolium is highly variable at the phenotypic level, and at least 18 infraspecific taxa have been recognised in European popu1ations by traditional methods. Most, or all, or these taxa are of doubtful status. It is unimportant whether varietal names are retained for extreme climatic types such as arcticus and pannonicus. (3) In contrast to the extremely high level of phenotypic variation a sample of populations investigated were cytologically uniform. The diploid number 2n = 18 was counted in all plants from 14 British and 7 continental popu1ations (including 3 varieties) and the karyotype of all 7 British and 5 continental populations studied was found to be highly uniform. The count is the first known count of British material. The karyotype has features which are thought to be evolutionarily primitive. (4) Aster tripolium is both cross and self fertile. Three crosses between plants geographically separated by considerable distances have produced fertile fruit. There was no evidence of apomixis. (5) A total of 56 populations were sampled, all but 15 of which were from the British Isles. Two sampling programmes were carried out, 39 populations being sampled as part of a programme designed to sample a small number of plants extensively within the species' range, and 11 populations being sampled as part of a programme designed to sample larger numbers of plants (ix) at distinct points along coastal gradients in 5 areas . Descriptions of these areas, 2 in Lincolnshire, 2 in Norfolk and 1 in Cheshire, are given. Populations from an inland site in Cheshire were also sampled. (6) In order to make comparisons between populations from ecologically similar sites a subsample of 24 British salt marsh populations (11 'intensively' sampled and 1 from the 'extensive ' sampling programme ) was used to divide the salt marsh areas on which Aster tripolium occurs into three categories, high, mid, and low. Two methods were used to make this division. The first was based on a number of ecological factors related to tidal submergence studied at the 5 main coastal areas , and involved the recognition of a critical level separating two types of intertidal marsh. The second method used an association analysis of the species recorded from each sampling area. There was a good measure of agreement between the results obtained by the two methods. (7) Results obtained by comparative cultivation and by growth of selected populations in a range of culture conditions suggest that the wide ecological amplitude of Aster tripolium in salt marshes is the result of adaptive divergence involving variables of fundamental biological importance such as life cycle, fruit size, germination requirements and growth in saline soils. Laboratory and field experiments were carried out to test further hypotheses relating to specific trends of differentiation. (8) A north/south cline in year to first flowering in European populations was accompanied by similar variation in British salt marsh popu1ations across the coastal zone. Populations from high marshes contained a high percentage of first-year flowering pauciennial types in contrast to those from mid and low marshes which were perennial and flowered for the first time in the second or third year of growth. First year flowering types flowered later and less synchronously than second or third year types. Growth under a spectrum of conditions indicated that high marsh pauciennials are able to respond to cultivation, behaving either as true annuals or, in certain conditions, overwintering or flowering more than once. The possible effects of variation in year of first flowering, number of flowering times and reproductive output was examined by a simple mathematical model. (9) Populations sampled from points across the coastal zone in three areas showed a cline of variation in disc fruit size (accompanied in two areas by an inverse cline in fruit number per head) - the high marsh populations having significantly smaller fruits. Fruits formed by ray florets were significantly smaller than those formed by disc florets. The wide variation in total fruit production per plant was largely a function of variation in the amount of axillary branching. The small within-population variation in fruit size under a range of conditions was interpreted as indicating that both fruit size and stability of fruit size were ecologically important characters. In particular the relationship between initial fruit size and seedling growth rate was examined and the hypothesis is advanced that the elimination of seedlings on unstable substrates by the mechanical effect of wave action may be an important factor selecting large fruited types on low marshes. This was supported by the variation in lateral root development and by studies of marked clumps of seedlings during the establishment phase in one low marsh area, (10) Studies of germination in selected populations revealed variation from high to low marshes in the percentage of fruits requiring prechilling to break dormancy, the germination rate in dilutions of sea water and the germination rate of large and small fruits. The ecological significance of this variation is discussed. Germination rates were slightly increased in all populations when the fruits were germinated in the dark. (11) The growth of seedlings in nutrient culture solutions to which different amounts of sodium chloride had been added was studied. Within-population effects of increasing NaCl were generally greater than between-population ones in short-term nutrient solution experiments. In particular growth depression was accompanied by reduced lateral root production. Survival and growth in soils given a range of simulated tidal treatments was relatable to the origins of the populations tested. Analyses of the ash of a low and a high marsh population grown in increasingly saline solutions suggested that the preferential absorption of potassium in high sodium environments may be an important feature of adaptation to growth in saline soils. Survival rates of high marsh populations in various conditions of cultivation in ordinary soils were consistently higher than those of mid marsh populations, which in turn were higher than those of low marsh populations. In one experiment larger numbers of low marsh than mid marsh plants were eliminated by competition with garden weeds. (12) Variation in a number of morphological characters between high, mid, and low marsh populations was investigated by collateral cultivation. A number of these characters (branching woodiness, sub-basal leaf development, and number of heads per plant) were correlated with life cycle variation, whilst others (inflorescence height, height of perennating bud, basal leaf shape, rosette habit and leaf colour) revealed consistent patterns of variation from high to low marshes. Cultivation in different conditions suggests that these characters vary in their plasticity. (13) The recent spread of the rayless form in south-east England is described and the results of controlled crossing experiments to determine the mode of inheritance of the ray floret character are discussed. The differential ecological distribution of full rayed, partly rayed and non rayed plants is described. The hypothesis is advanced that the spread of the rayless form may be explained by the selection of characters for which it is a pleiotropic marker enabling its enhanced survival in low marshes, and by the increase in such marshes created by the parallel spread of Spartina anglica. Adaptive divergence of the rayless form may be aided by the isolating factor of non-coincident flowering time, thus reducing gene flow between adjacent populations. (14) Much of the variation in Aster tripolium from high to low salt marshes, and especially that between closely adjacent populations, was clinal, and discontinuities were blurred by numbers of intermediate types. The low incidence of clear-cut patterns of variation is thought to be due to a number of factors. These include (a) the graded, as opposed to sharply marked or stepped, variation in the environment , (b) the existence of different environmental gradients related to salinity , exposure and so on, (c) the strong possibility of gene flow between adjacent populat10ns as a result of widespread outbreeding and effective fruit dispersal mechanisms, (d) the important role of plasticity in adaptation to fluctuating , heterogeneous environments (e) the possibility that many of the characters being considered are under polygenic control, (f) the use in some experiments of seed, rather than mature plant, population samples, and (g) the stretched nature of the response of maritime plants to ordinary soils. (15) Adaptation to maritime environments is believed to be a complex process involving adjustment to environmental factors at all stages of the p1ant'-s life cycle. In Aster tripolium such adaptation has involved a mixture of genetic and plastic responses which is likely to have included undetected physiological variation. The features which characterise the high marsh populations are commonly regarded as those exhibited by weedy aggressive species, a fact which is understandable in view of the increased competition with which such popu1ations are faced. The increasing effects of competition factors towards the upper zones and the increasing effects of factors related to tidal submergence towards the lower zones are thought to be the principal agents of disruptive selection, populations showing marked responses to them as the ecological limits of the species is approached. Although the intensity at which they do so may vary from one area to another these factors eventually become limiting, restricting the species to suitable maritime and paramaritime habitats