Biological Conservation 37 (1986) 301-320 Disturbance Histories of Four Snowdonian Woodlands and Their Relation to Atlantic Bryophyte Distributions Mary E. Edwards* The Botany School, Downing Street, Cambridge CB2 3EA, Great Britain A BSTRA CT Small deciduous woods in the mountains of North Wales may be relicts of ancient woodland, but centuries of intensive land use mean it is very unlikely that any are completely unmodified or undisturbed. Some woods are characterised by the presence of desiccation-sensitive atlantic bryophytes, which could possibly serve as indicators of the least disturbed sites. Studies of atlantic bryophyte distributions, and historical reconstruc- tions based on pollen analyses, stand demography, and documentary records .for four woods (two rich and two poor in atlantic bryophytes) revealed the following: (1) A t least some part of each wood had suffered disturbances during the past 300 years, including coppicing, clear felling, and grazing. (2) Richness and abundance of atlantic bryophyte communities were largely correlated with availabilio' of habitats, i.e., rocks with permanent moisture, shelter, and favourable substrate. Habitats varied in quality and quantity within and between woods. (3) Where disturbance involved an extended period with an open or absent canopy, atlantic bryophytes occur in abundance only in extremely moist and shaded locations. Where disturbance was followed by rapid canopy regrowth, their distributions are not so restricted. No clear-cut relationship exists between the occurrence of rare atlantic bryophytes and disturbance histories. The persistence of desiccation- sensitive species depends on the quality of available habitats, as well as the nature of woodland disturbances. Woods richest in rare atlantic species have supported some continuous tree canopy over the last 1200 * Present address: Marlboro College, Marlboro, Vermont 05344, USA. 301 Biol. Conserv. 0006-3207/86/$03'50 f(1)Elsevier Applied Science Publishers Ltd, England, 1986. Printed in Great Britain