| Summary: | The moss Hylocomium splendens has both sympodial and monopodial forms of growth. One aim of this study was to document the growth-form of shoots from different populations. A further aim was to discover the extent to which genetic or environmental factors determine whether monopodial or sympodial growth-form is predominant in a population. Switching between growth-forms within shoots occurs in most populations. Populations in forest habitats in temperate to mid-Arctic environments have predominantly sympodial shoots whereas shoots of populations from tundra habitats, in high-Arctic environments or at high altitudes, are predominantly monopodial. Transplant experiments showed that sympodial and monopodial shoots can respond plastically, by changing growth-form to some extent in different environments, and that high nutrient availability is an important environmental factor in promoting sympodial growth-form. However, even after 14 years, transplants did not show the same variation in growth-form as shoots in natural populations at the transplant sites. This suggests that populations are also genetically differentiated with respect to growth-form.
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