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Changes in a Population of Spiranthes Spiralis (L.) Chevall. at Knocking Hoe National Nature Reserve, Bedfordshire, 1962-65

T. C. E. Wells
Journal of Ecology
Vol. 55, No. 1 (Mar., 1967), pp. 83-99
DOI: 10.2307/2257718
Stable URL: http://www.jstor.org/stable/2257718
Page Count: 17
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Changes in a Population of Spiranthes Spiralis (L.) Chevall. at Knocking Hoe National Nature Reserve, Bedfordshire, 1962-65
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Abstract

1. Observations were made from 1962 to 1965 on the reproductive behaviour of a population of Spiranthes spiralis growing in chalk downland at Knocking Hoe, Bedfordshire. Individual plants were recorded each year by reference to fixed markers. The condition of each plant, whether vegetative or flowering, was ascertained and the height of each inflorescence and the number of flowers per plant measured; their variation is described in relation to environmental factors. 2. There was no evidence to support the view that large fluctuations in numbers of plants or inflorescence production occurred from year to year. 3. The diverse reproductive behaviour in 1964 and 1965 of the 463 plants which flowered in 1963 suggested that flowering was not controlled by a single climatic factor. On the contrary, the data suggested that flower production was controlled by a complex of factors, in which the activities of mycorrhiza were of importance. 4. The orchid was able to pass at least a year and possibly more as an underground tuber with no leaves. It was capable of flowering the following year, which suggested that mycorrhiza played an important part in the nutrition of the plant. The ability to remain as an underground organ for at least a year may be significant for survival of the plant in unfavourable seasons. 5. Increase in the population by vegetative reproduction was slow, about 5% of the plants producing a new plant in this way in one year. 6. The number of flowers per inflorescence varied from three to twenty-one with mean values of 10.9, 10.1 and 11.3 for the whole population in 1963, 1964 and 1965 respectively. These differences were not statistically significant. 7. The length of the inflorescence varied from 4.5 to 19 cm but plants at the two extremes were rare. The mean height of the inflorescences in 1965 was 10.50 cm, compared with a mean height of 8.86 cm in 1964. This difference was significant at the 5% level of probability and the increase in height may be associated with the higher summer rainfall of 1965. 8. The basal rosette of leaves was not eaten by herbivores. Grazing by sheep at a density of 10 per hectare resulted in 30% of the inflorescences being damaged but at lower stocking densities grazing by herbivores, including rabbits, caused less damage.

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