Changing land use and the spread of 'winning' native or exotic plants are expected to lead to biotic homogenization (BH), in which previously distinct plant communities become progressively more similar. In parallel, many ecosystems have recently seen increases in local species (a-) diversity, yet g-diversity has continued to decline at larger scales. Using national ecological surveillance data for Great Britain, we quantify relationships between change in a-diversity and between-habitat homogenizations at two levels of organization: species composition and plant functional traits. Across Britain both increases and decreases in a-diversity were observed in small random sampling plots (10-200 m 2 ) located within a national random sample of 1 km square regions. As a-diversity declined (spatially in 1978 or temporally between 1978 and 1998), plant communities became functionally more similar, but species-compositional similarity declined. Thus, different communities converged on a narrower range of winning trait syndromes, but species identities remained historically contingent, differentiating a mosaic of residual species-poor habitat patches within each 1 km square. The reverse trends in b-diversity occurred where a-diversity increased. When impacted by the same type and intensity of environmental change, directions of change in a-diversity are likely to depend upon differences in starting productivity and disturbance. This is one reason why local diversity change and BH across habitats are not likely to be consistently coupled.

Summary1 This account reviews information on all aspects of the biology of bracken Pteridium (mainly aquilinum ssp. aquilinum) 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 biotic factors, responses to environment, structure and physiology, phenology, reproductive characters, herbivores and disease, history, and conservation. 2 Pteridium is a complex genus comprising a number of species, subspecies and varieties. The treatment here is based on a recent revision that incorporates both morphological and molecular data, and is related to its geographical distribution. 3 Pteridium is thought to be a woodland genus, but it can grow in the open. It is cosmopolitan and occurs on all continents except Antarctica. It responds to human disturbance and is often found in open spaces after forest clearance and cultivation. In some situations it can be a troublesome weed, causing problems for land managers. Moreover, its abundance and distribution in Britain are predicted to increase as a result of global climate change. 4 Pteridium aquilinum ssp. aquilinum, the most common taxon in the British Isles, occurs in many plant communities, and it is apparently limited by frost and waterlogging. Its abundance has probably increased in the relatively recent past as a result of changing land management, and this increase impinges on plant communities with a high conservation interest. The changed land management reflects changing use of agricultural land and also a reduction in the use of Pteridium as a resource. Accordingly, in many places Pteridium is viewed as a weed and management is needed to control it and restore more desirable vegetation. These management techniques are summarized.

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Summary1 Arti®cial environmental gradients were established in a series of pot experiments to investigate the eect of salinity, sediment type and waterlogging on the growth, and interactions between Spartina anglica and Puccinellia maritima. In each experiment, one environmental variable was manipulated and plants grown in pairwise combinations to examine the eect of the environmental factor on the intensity of intra-and interspeci®c interactions, quanti®ed using the Relative Neighbour Eect (RNE) index. 2 Puccinellia was found to exert an asymmetric, one-way competitive dominance above ground over Spartina in experiments where gradients of sediment type and waterlogging were established. The intensity of the competition was highest in conditions with the least abiotic stress and lower or non-existent where stress was increased. 3 The intensity of the above-ground competition was greatest in loam and least in sand sediments. Reduction in competitive intensity in sand was accompanied by an increase in below-ground Spartina biomass and it is suggested that the production of rhizomes is a potential mechanism by which this species can expand vegetatively into areas without competition. 4 Interspeci®c competition on Spartina from Puccinellia also varied in intensity in the waterlogging experiment, being more intense in non-immersed treatments, where abiotic stress was reduced. 5 The competitive dominance of Puccinellia and the competition avoidance mechanism shown by Spartina in these experiments help to explain the successional interactions between the species along environmental gradients in natural salt marsh communities.

This document is the authors' final manuscript version of the journal article, incorporating any revisions agreed during the peer review process. There may be differences between this and the publisher's version. You are advised to consult the publisher's version if you wish to cite from this article.

Summary 0 Many areas of lowland heaths are being lost due to invasion by Betula spp[\ Pinus sylvestris\ Pteridium aquilinum\ Rhododendron ponticum and Ulex europaeus[ One of the factors in~uencing the success of restoration of heathland on such sites will be the content of their viable seedbanks[ 1 Ten heathland areas in the Poole Basin area of Dorset\ where succession to one or more of the above species had occurred were studied[ The viable seedbanks of the successional sites were compared with those of nearby heathland using Canonical Discriminant Analysis[ 2 The seedbanks of all the successional stages were signi_cantly di}erent from the seedbank of the heath[ 3 The seedbanks from the Pinus sylvestris and Pteridium aquilinum successional stages contained signi_cantly lower numbers of heathland species than did the heathland seedbank\ although few non heathland species were present[ 4 The seedbanks from the Betula spp[\ Rhododendron ponticum and Ulex europaeus successional sites contained both signi_cantly lower numbers of heathland species and signi_cantly higher numbers of non heathland species than the heathland seedbank[ 5 The results are discussed in relation to the restoration of heathland on successional sites and the use of the seedbank as a source of propagales for the establishment of heathland species[

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