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?