Invasive alien species (IAS) are considered one of the greatest threats to biodiversity, particularly through their interactions with other drivers of change. Horizon scanning, the systematic examination of future potential threats and opportunities, leading to prioritization of IAS threats is seen as an essential component of IAS management. Our aim was to consider IAS that were likely to impact on native biodiversity but were not yet established in the wild in Great Britain. To achieve this, we developed an approach which coupled consensus methods (which have previously been used for collaboratively identifying priorities in other contexts) with rapid risk assessment. The process involved two distinct phases: Preliminary consultation with experts within five groups (plants, terrestrial invertebrates, freshwater invertebrates, vertebrates and marine species) to derive ranked lists of potential IAS.Consensus-building across expert groups to compile and rank the entire list of potential IAS.Five hundred and ninety-one species not native to Great Britain were considered. Ninety-three of these species were agreed to constitute at least a medium risk (based on score and consensus) with respect to them arriving, establishing and posing a threat to native biodiversity. The quagga mussel, Dreissena rostriformis bugensis, received maximum scores for risk of arrival, establishment and impact; following discussions the unanimous consensus was to rank it in the top position. A further 29 species were considered to constitute a high risk and were grouped according to their ranked risk. The remaining 63 species were considered as medium risk, and included in an unranked long list. The information collated through this novel extension of the consensus method for horizon scanning provides evidence for underpinning and prioritizing management both for the species and, perhaps more importantly, their pathways of arrival. Although our study focused on Great Britain, we suggest that the methods adopted are applicable globally.

The hectad (10 × 10 km square) distributions of the 1405 native British and Irish vascular plants were classified by the SPHERIKM cluster analysis program into 20 clusters, each of which is characterised by the key species used to initiate the cluster. The clusters reflect the influence of climate, altitude, geology and habitat on distribution patterns at this scale. Clusters with restricted distributions have high concentrations of threatened species, particularly the Medicago sativa cluster, centred on Breckland (55% of the species are threatened in Britain, although only 29% are regarded as priorities for conservation), and the Carex atrata cluster of montane species (45% threatened, and 49% conservation priority species). Some clusters are composed predominantly of species with similar European distributions whereas others are much more phytogeographically heterogeneous. A comparison with a similar analysis of the distribution of British and Irish mosses and liverworts reveals many similarities, especially between the vascular plants and the mosses, although there are many more common vascular plants than bryophytes and many more coastal species.Keywords: Climate, Cluster analysis, Coast, Geology, Phytogeography, SPHERIKM, Threat 2 IntroductionThe systematic collection of information on the distribution of British and Irish vascular plants has been a major preoccupation of botanists for over 150 years. Initially, records were summarised for groups of counties, called 'provinces' in Britain (Watson 1847-59) and 'districts' in Ireland (Moore & More 1866), then for sub-provinces (Watson 1872), vice-counties (Watson 1873-74;Praeger 1901) and latterly for 10-km grid squares (Perring & Walters 1962). However, there have been relatively few attempts to analyse patterns of distribution revealed by the data which have been compiled so meticulously. Watson (1847-59) did use the data he had collected on the occurrence of species in provinces to identify the 'types of distribution' shown by British species. His 'types' were widely used in the 19th century by the authors of county floras to analyse the phytogeographical make-up of the plants in their areas (Preston & Hill 1997), but they were replaced in the 20th century by classifications based on the wider distribution of species, following E.J. Salisbury (1932) and J.R. Matthews (1937Matthews ( , 1955. Praeger (1902) devised a similar classification of Irish plants into distributional types but these were never widely used.The Atlas of the British Flora (Perring & Walters 1962) provided distribution maps which, because of their finer scale, revealed much closer correlations with ecological factors such as altitude, climate and geology than had been apparent from previous maps. At the same time methods of numerical classification were being devised which provided more objective ways of classifying distribution patterns than earlier classifications by eye and expert judgement. Proctor (1967), for example, analysed the vice-comital distribution of British liverworts by...

Branch specimens were collected from the very tops of tropical tree crowns in southern Cameroon, West Africa. An analysis of branching patterns revealed a consistency of form across unrelated taxa. All specimens showed evidence of rhythmic growth cither due to the regular occurrence of dormant terminal buds or due to sympodial growth with loss or flowering of terminal buds. Study of bud anatomy revealed an extensive array of protective devices associated with drought tolerance and herbivore defence. Normally a considerable excess of dormant, well protected axillary buds were present which (almost without exception) existed in a viable state. In very many instances the large dormant bud population was due to the presence of accessory buds, i.e. > 1 bud in the axil of each leaf. The microclimate at the outer surface of a tropical rainforest may experience a daily increase in temperature and associated depression in humidity; the canopy surface characteristics are more akin to chaparral shrub vegetation than to familiar rainforest understorey vegetation.

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