High species richness of sheep-grazed sand pastures is driven by disturbance tolerant and weedy short-lived species

Abstract

We selected 15 sheep-grazed sand pastures along increasing grazing intensity to study fine scale biomass patterns of main fractions (green biomass, litter) and that of plant species and functional groups (life forms and social behaviour types). We classified them into five grazing intensity levels based on stocking rate, proximity to drinking and resting places, and the number of droppings and other tracks of grazing animals. We formulated three study questions: i) How does increasing intensity of sheep grazing affect the amount of green biomass, species richness and their relationship in sand pastures? ii) How does increasing intensity of sheep grazing affect the biomass of perennial and short-lived graminoids and forbs? iii) How does disturbance value (expressed in the biomass ratio of disturbance tolerant and ruderal species) change along the gradient of grazing intensity? A unimodal relationship between green biomass and species richness was detected. Along the grazing intensity gradient, we found an increasing trend for species richness; significant differences for green biomass (decreasing trend) and litter (decreasing trend), moreover for graminoids (decreasing trend), and short-lived forbs (increasing trend). We found an increasing amount of disturbance tolerant and ruderal species with increasing grazing intensity. We concluded that stocking rate and proximity to drinking and resting places jointly affected vegetation and created an uneven pattern for composition and amount of biomass in all grazing intensity levels. Our findings might be instructive for pastures in densely populated regions which are prone to the encroachment of disturbance-tolerant and ruderal species.