Biological Flora of the British Isles: Spiranthes spiralis (L.) Chevall.

Summary 1.  This account presents information on all aspects of the biology of Spiranthes spiralis (L.) Chevall. that are relevant to understanding its ecological characteristics and behaviour. The main topics are presented within the framework of the Biological Flora of the British Isles : distribution, habitat, communities, responses to biotic factors, responses to environment, structure and physiology, phenology, floral and seed characteristics, herbivores and disease, history and conservation. 2.   Spiranthes spiralis is a long‐lived orchid species that is widely distributed in the southern part of the British Isles, southern Europe and the Mediterranean. It is native to Britain. Its most characteristic habitats are unimproved, well‐grazed grasslands on dry calcareous soils, particularly chalk and limestone, cliff‐tops and grassy coastal dunes. It can also be found in garden lawns and, less frequently, in slightly acidic heathland habitat. It occurs on both dry and wet soils, but it does not tolerate heavy shading. 3.   Spiranthes spiralis is a non‐bulbous geophyte regenerating by sexual reproduction and by vegetative spread, although the latter is limited. Vegetative reproduction involves the growth of a lateral bud on the underground stem. The new plant forms its own tubers, and eventually the connection between the mother plant and the vegetatively produced daughter plant withers away. 4.  The main perennating organ is a tuberous root. In most years, the tuber generates a rosette of expanded leaves, and at the end of every year of the plant’s life, the tuber is completely replaced. In some cases, however, only part of the reserves is exhausted in flowering and, when this happens, a tuber may survive for several years. Vegetative adult dormancy – the failure of above‐ground parts to appear in a growing season, followed by reappearance of full‐sized photosynthetic plants in subsequent seasons – has been observed. It most commonly lasts for only 1 year, but sometimes plants can remain dormant for several years. 5.  The species is not autogamous and pollinators – mainly bumblebees and bees – are necessary for successful pollination and fruit set. Natural levels of fruit set are usually <50%. Hand‐pollination increases fruit set to c.  75%, indicating that reproduction is to some extent pollen‐limited. Sexual reproduction comes at a cost, and few individuals flower for two or more consecutive years. Within fruits many very small seeds are produced, but most are dispersed to a distance of less than a few dm from maternal plants, leading to strongly aggregated distribution patterns and significant spatial genetic structuring within natural populations. 6.   Spiranthes spiralis has suffered significant declines in distribution in Britain and other European countries. In Britain, populations were lost from many sites before 1930, when many pastures were re‐sown or converted to arable land. Losses of populations due to agricultural intensification (ploughing of old fields and fertilization) still occur and are the major causes of the decline of the species. Conservation of remnant populations primarily requires the maintenance of a short turf by grazing, most often by sheep. Ideally, grazing should be suspended during flowering and seed set, allowing new recruits to become established and adult plants to flower profusely and to produce seeds.