Effects of grazing season and stocking rate on seed bank in sheep dung on the semiarid Loess Plateau

Abstract

Endozoochorous dispersal of seeds by livestock has long attracted the attention of grassland scientists, but little is known about seed dispersal via Tan sheep dung on the dry grasslands of the Loess Plateau. We investigated the composition of dung seed bank of Tan sheep under summer and winter rotational grazing regimes at different stocking rates (2.7, 5.3 and 8.7 sheep ha–1), and assessed the relationships between seed mass and shape, egested seedling density and species richness and diversity. We also assessed the effects of stocking rate and grazing season on seedling density and species richness and diversity of the dung seed bank. Seeds of 10 pasture species germinated from Tan sheep dung with only two being annual species (Eragrostis pilosa and Chenopodium glaucum). The mean seed mass (±s.e.) was 2.59 ± 0.23 mg, ranging from 0.10 mg (Cleistogenes songorica) to 10.59 mg (Thermopsis lanceolata), and the mean seed shape index (±s.e.) was 0.10 ± 0.02, ranging from 0.03 (Lespedeza bicolor) to 0.19 (Stipa bungeana). Species richness and diversity and seedling density decreased with increasing seed mass and shape index, suggesting that small, round seeds are most suitable for endozoochorous dispersal. Stocking rate had no effect on the number of germinated seeds that collected from summer or winter grazing pastures. Species richness and diversity and seedling density were greater in winter grazing pastures than in summer grazing pastures. Jaccard coefficients of similarity between the Tan sheep dung bank and aboveground vegetation were <0.5 for all stocking rates, indicating their weak relationship, but were significantly higher for winter than summer grazing. Plant seeds on the Loess Plateau could disperse through Tan sheep grazing, endozoochory can increase the heterogeneity of rotationally grazed plant communities, and this dispersal mode is an adaptation of plants to the harsh environment of the semiarid areas.