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

Abstract

Aim The subjects of seed banks and seed rain represent comparatively neglected areas of biogeography, yet at the community scale, exhibit interesting patterns in both space and time. This paper describes the seed bank and seed rain characteristics of the machair sand dune communities of the Outer Hebrides. As well as looking at individual species distributions and variability, the seed banks and seed rain are examined in terms of their detailed subcommunity composition and its local spatial and temporal variation. The machair plant (sub)communities show extensive degrees of anthropogenic modification because of past and present agricultural management, including cultivation for cereals over wide areas and for potatoes in large numbers of ‘lazy beds’ or small patches. Thus over the historical period, large areas of machair have undergone regular ploughing and cultivation, which have provided the opportunity for a patchwork of secondary succession to occur. This pattern continues to the present day. Furthermore, most other non‐cultivated plant (sub)communities are intensively grazed, primarily by cattle and also by sheep and rabbits. Location South Uist, the Outer Hebrides, north‐west Scotland. Methods At two carefully selected locations, a range of these various successional subcommunities have been sampled for their seed banks, by taking cores and for their seed rain, by using specially designed traps located where each seed bank sample was removed. This paired sampling strategy allowed direct comparison of the seed bank and the seed rain. Both individual species distributions and the community assemblages of seed bank/seed rain species are examined in space and time using techniques of numerical classification [two‐way indicator species analysis (TWINSPAN)] and ordination [detrended correspondence analysis (DCA)]. Results and conclusions There is considerable heterogeneity within and between machair subcommunities in terms of seed bank and seed rain characteristics. The soil seed banks and seed rain of the agriculturally disturbed machair subcommunities are consistently more dense and more species rich than non‐cultivated areas of the machair. Overall, machair seed banks are small and stable with no discernible seasonal trends in either size or species composition. In contrast, seed rain on the machair is characterized by a distinct 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 environmental interference is through vegetative reproduction.

The potential role of seed banks in the recovery of dune ecosystems after removal of invasive plant species

“…Species of Juncus and Carex produce very large amounts of small, long-lived seeds that are abundant in several communities (Bossuyt & Honnay 2008). The presence of these taxa may explain the higher density of seeds when compared with seed banks of other sand dunes systems (7-12 germinable seeds m À2 ) (Owen et al 2001). Bossuyt & Honnay (2008) analysed numerous community seed banks and support the notion that restoration from natural seed bank is only possible in areas degraded for o 5 yr. Our study corroborates this, at least partially, showing a decrease in the unaided recovery potential of the system as invasion age increases.…”

“…Species of Juncus and Carex produce very large amounts of small, long‐lived seeds that are abundant in several communities (Bossuyt & Honnay 2008). The presence of these taxa may explain the higher density of seeds when compared with seed banks of other sand dunes systems (7–12 germinable seeds m −2 ) (Owen et al 2001).…”

“…Although A. longifolia is an aggressive invader in several ecosystems, including dunes, around the globe (Henderson 1995; Weber 2003; Elorza et al 2004; Marchante et al 2008c), studies quantifying its impact on natural seed banks and on the resilience of dune systems are scarce. There is also a general lack of data on soil seed banks of coastal sand dunes (Owen et al 2001). In order to determine the role of seed banks in the restoration of invaded communities, and the susceptibility of such communities to reinvasion, the seed banks of areas that had been long‐invaded and recently‐invaded by A. longifolia were assessed and compared with non‐invaded areas.…”

Can soil seed banks contribute to the restoration of dune slacks under conservation management?

“…At our study site, pioneer slacks are still present, if very rare, so that seed sources are still available; however, management needs to ensure that gaps in which these species can germinate are created frequently enough to ensure long‐term survival. Many species do not disperse over great distances (Verkaar et al 1983), including species of sand dunes (Westelaken & Maun 1985; Owen et al 2001), but grazing animals can help disperse seeds to potential germination sites via endo‐ and epizoochory. The importance of large herbivores for epi‐ and endozoochorous seed dispersal on sand dunes has recently been documented (Cosyns & Hoffmann 2005; Cosyns et al 2005; Couvreur et al 2005), showing that grazing animals can connect spatially separate habitat patches and that less common and rare species represent an important part of the viable seed content of horse dung.…”

“…Pemadasa & Lovell 1974;Watkinson 1978;Westelaken & Maun 1985;Houle 1996;Rowland & Maun 2001), and while there has been a recent increase in the interest in the seed bank of dunes, and especially dune slacks (e.g. Bekker et al 1999;Owen et al 2001;Bossuyt & Hermy 2004), seed bank data are still absent for many dune species (Grootjans et al 2004;Bossuyt et al 2005).…”