ACIDIFICATION, NITROGEN DEPOSITION AND RAPID VEGETATIONAL CHANGE IN A SMALL VALLEY MIRE IN YORKSHIRE

Changes in vegetation and environment at Askham Bog, a small valley mire near York, UK, have been followed over a period of 13 years by resurveying sites recorded in 1978 and by studying permanent quadrats. The site has a concentrically zoned series of vegetation types, with base-rich for vegetation around the margins and acidophilous, species-poor vegetation on acid peat in the centre. It has become more acid during this period: average peat pH has declined by 0.5 units, and the least acid peats by more. The composition of the tree flora has changed extensively, with birch declining in favour of oak, alder and ash. Alder regenerates episodically at the site, and there has been a major recruitment event since 1978. Permanent quadrats showed that the ground flora has undergone rapid change; numerous species have declined in base-rich vegetation zones, but only the moss Sphagnum fimbriatum in the acid vegetation zone. Few species increased in abundance, but the nitrophilous Urtica dioica was one. Mapping the distribution of Sphagnum spp. showed that S. squarrosum as well as S. fimbriatum had become less abundant, and even the more acid-tolerant S. palustre had declined where the grass Molinia caerulea had expanded. Experiments involving adding nitrogen and cutting M. caerulea suggested that vigorous Sphagnum was inhibited by low levels of N addition (1.2 g N m2 year1), considerably below those now entering the site as atmospheric deposition. Where Sphagnum was growing poorly and Molinia was abundant, adding N had no effect but cutting Molinia in summer was beneficial. These data are discussed in relation to atmospheric inputs of S and N. It appears that heavy N deposition may be responsible for rapid vegetation change at the site. The implications for conservation of such sites are worrying.