Introduction
It is a truism in ecology that grazing has a crucial role in maintaining grassland biodiversity (Briske, 1996; Metera et al., 2010); although it is a disturbance that affects both morphological characteristics and functional trait composition of plant communities (WallisDeVries et al., 2002; Díaz et al., 2007). Effects of grazing on species composition, vertical and horizontal structure, regeneration capacity, and functional composition are necessary to study to avoid overgrazing (Dong et al., 2012; Hao & He, 2019), which is one of the most serious problems in sustainable management of pastures in many regions of the world (Gao & Li, 2016; Li et al., 2018a; Török & Dengler, 2018). Former research found that plant community responses along a gradient of grazing intensity showed marked changes, which leaded to altered stability and ecosystem functioning (Li et al., 2018b). When studying the effect of grazing, grazing intensity, livestock and habitat types can be regarded as sort of main approaches, and various combinations of these approaches can be found in the scientific literature.
Sheep grazing has some specific characteristics, for example i) there is a higher selectivity for forbs compared to cattle grazing; in addition, ii) sheep can consume plant parts closer to the ground, iii) and sheep rather prefers vegetative plant parts (Metera et al., 2010; Jerrentrup et al., 2015; Tóth et al., 2018). Sheep grazing likely supports seedling establishment on bare soil surfaces: (i) seeds lying on the soil surface can be buried to an optimal depth for germination by sheep trampling (Eichberg et al., 2005); (ii) flocks usually consist of up to a few hundred head of sheep, and are frequently herded by shepherds for relatively long distances; consequently, they can contribute to high dispersal distance for certain seeds (Rosenthal et al., 2012), (iii) moreover, sheep trampling opens dense vegetation cover, and it can create safe sites for seedling emergence and establishment (Faust et al., 2011; Freund et al., 2014).
Species richness often shows a humped-back curve along a gradient of increasing disturbance or along an increasing amount of biomass; these patterns can be explained by the intermediate disturbance hypothesis (Connell, 1978; Gao & Carmel, 2020). These findings are also supported by studies dealing with sheep grazing (del Pozo et al., 2006; Süss et al., 2007; Lázaro et al., 2016). However, the spatial (Süss et al., 2007) and temporal scale of the study (del Pozo et al., 2006) can influence the detection of a humped-back relationship and monotonously decreasing species richness can be also detected, which was the case for example in sheep-grazed desert steppes (Zhang et al., 2018).
Beside of the nature and patterns of biodiversity, a further important question arises is how changes in species richness are reflected in the abundance of plant life- and growth forms. In their meta-analysis, Díaz et al. (2007) analysed plant trait responses to grazing. They found that increasing grazing intensity favoured stoloniferous plants, rosette formation likeliness, short height, and increased the abundance of short-lived and fast-growing species when climatic conditions and grazing history were both taken into consideration. Some findings of the above meta-analysis were partly confirmed for sheep-grazed pastures by Pettit et al. (1995), Yang et al. (2022) and Farmilo et al. (2023). However, none of the above papers studied the biomass of life- and growth form groups along a gradient of grazing intensity in sheep grazed pastures.
Studying sheep-grazed pastures can lead to a better understanding of the functioning of low productivity ecosystems (Süss et al., 2007) like the less-studied sand grasslands classified in the EU Habitat Directive as Pannonian and Pontic sandy steppes (E 1.1a). Sandy steppes are situated in Central and Southeast Europe and are critically endangered according to EC Directorate-General for Environment et al. (2017). With the study of sand pastures maintained by an increasing intensity of sheep grazing we aimed to address the following 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?