Biological Flora of the British Isles: Pseudorchis albida (L.) A. & D. Love

1. This account presents information on all aspects of the biology of Pseudorchis albida (Small White Orchid) that are relevant to understanding its ecological characteristics and behaviour. The main topics are presented within the standard framework of the Biological Flora of the British Isles: distribution, habitat, communities, responses to abiotic and biotic factors, structure and physiology, phenology, floral and seed characters, herbivores and disease, history, and conservation. 2. Pseudorchis albida is a native British herb growing in several plant communities of semi-natural habitats, typically in recently burned heather moorlands, upland well-drained pastures and meadows, and cliff ledges. Although the species is able to tolerate a wide range of ecological conditions from mildly acid to base-rich soils, it is limited mainly to open, short and nutrient-poor stands. Typical British plant communities include Calluna vulgaris heathlands, Anthoxanthum odoratum - Geranium sylvaticum hay meadows, and Nardus grasslands. The species often grows in transitions from heath to grass-dominated communities. The ecology of P. albida in the northern acidic environments is poorly understood. 3. Pseudorchis albida is a perennial geophyte whose populations are maintained predominantly by sexual reproduction, as vegetative spread is limited. The main perennating organ is a highly divided root tuber, which is completely replaced every year. 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 and typically lasts for one, sometimes 2 years. The adventitious roots and distal parts of the root tuber are colonized by basidiomycetes from the family Tulasnellaceae. 4. The species is pollinated by crepuscular pyralid and pterophorid moths, most likely attracted by the sweet scent. During the day, pollinia can be also transferred by dance flies (Empididae). The breeding system of P. albida is poorly known but high fruit set, often over 90%, raises the possibility of spontaneous autogamy, particularly in arctic-alpine areas. The probability of flowering in consecutive years in P. albida is strongly influenced by cost of flowering and fruiting. 5. Pseudorchis albida has suffered a significant decline in distribution in Britain and other European countries. In Britain, it has been lost from more than 65% of its historical sites, particularly in lowland areas. The main causal factors are the cessation of traditional agricultural practices, habitat fragmentation and disturbance associated with housing and road construction, and agricultural improvement, including reclamation, fertilization and overgrazing. Conservation of remnant populations primarily depends on the maintenance of a short turf by extensive grazing, controlled burning and shrub cutting to promote flowering of adult plants and seedling recruitment.