Endozoochorous dispersal, seedling emergence and fruiting success in disturbed and undisturbed successional stages of sheep-grazed inland sand ecosystems

This study examines the seedling emergence and fruiting success of plant species endozoochorously dispersed by sheep in a field experiment in a Koelerion glaucae area. We designed a factorial experiment (faeces/soil disturbance/successional stage/year) that mimics sheep-mediated microsites and analysed all occurring vascular plant species from September 2002-December 2004. The faeces samples were collected in the same ecosystem and additionally analysed in a common-garden experiment (seedling emergence potential). According to the latter, 28 vascular plant taxa were identified (on average 124 seedlings per 100g air-dry sheep faeces). In the field, 15 species emerged directly out of faeces, only five of which were able to set seeds (the threatened species Medicago minima, Phleum arenaritan, Silene conica, Vicia lathyroides and the non-threatened Vulpia mytiros). Graminoid competitors were not able to set seedlings in the field but did so in the common-garden experiment (mainly Carex hirta). The total numbers of seedlings emerging out of faeces (5% of the potential) and reaching fruit ripeness (0.4%) were very low. In the stage of seedling emergence, ten of 42 species were significantly influenced by faeces, 11 species by disturbance and two species by an interaction of the two treatment factors (Bromus tectorum, Saxifraga tridactylites). In the fruiting stage, two species profited by faeces (Bromus tectorum, Vulpia myuros) and two species by disturbance (Erophila verna, Corynephorus canescens). Overall, the density and diversity of fruiting individuals were significantly influenced by both treatment factors. The faeces factor showed a strong dependence on year (increasing effect on both dependent variables in the first year, decreasing effect in the second year). The disturbance factor is dependent particularly on successional stage (increasing effect especially in the later successional stage). Our study revealed the paradox that threatened species with low nutrient demands are the most successful direct colonisers of sheep-faeces deposits. For them to reach the fruiting stage is rare but is of importance if new habitats are to be made accessible. (c) 2006 Elsevier GmbH. All rights reserved.