Spatial and temporal variability in seed dynamics of machair sand dune plant communities, the Outer Hebrides, Scotland

Aim The subjects of seed banks and seed rain represent comparatively neglec ted areas of biogeography, yet at the community scale, exhibit interesting patterns in both space and time. This paper describes the seed bank and see d rain characteristics of the machair sand dune communities of the Outer He brides. As well as looking at individual species distributions and variabil ity, the seed banks and seed rain are examined in terms of their detailed s ubcommunity composition and its local spatial and temporal variation. The m achair plant (sub)communities show extensive degrees of anthropogenic modif ication because of past and present agricultural management, including cult ivation for cereals over wide areas and for potatoes in large numbers of 'l azy beds' or small patches. Thus over the historical period, large areas of machair have undergone regular ploughing and cultivation, which have provi ded the opportunity for a patchwork of secondary succession to occur. This pattern continues to the present day. Furthermore, most other non-cultivate d plant (sub)communities are intensively grazed, primarily by cattle and al so by sheep and rabbits. Location South Uist, the Outer Hebrides, north-west Scotland. Methods At two carefully selected locations, a range of these various succe ssional subcommunities have been sampled for their seed banks, by taking co res and for their seed rain, by using specially designed traps located wher e each seed bank sample was removed. This paired sampling strategy allowed direct comparison of the seed bank and the seed rain. Both individual speci es distributions and the community assemblages of seed bank/seed rain speci es are examined in space and time using techniques of numerical classificat ion [two-way indicator species analysis (TWINSPAN)] and ordination [detrend ed correspondence analysis (DCA)]. Results and conclusions There is considerable heterogeneity within and betw een machair subcommunities in terms of seed bank and seed rain characterist ics. The soil seed banks and seed rain of the agriculturally disturbed mach air subcommunities are consistently more dense and more species rich than n on-cultivated areas of the machair. Overall, machair seed banks are small a nd stable with no discernible seasonal trends in either size or species com position. In contrast, seed rain on the machair is characterized by a disti nct temporal trend. Both seed banks and seed rain are potentially very poor sources of propagules for recolonization following disturbance, indicating that the majority of revegetation following anthropogenic and/or environme ntal interference is through vegetative reproduction.