Discussion
Species richness and green biomass
We found a humped-back relationship between species richness and green
biomass that supports the intermediate disturbance hypothesis (Connell,
1978; Gao & Carmel, 2020). Moderate levels of disturbance provide
favourable conditions for a wider range of species (Metera et al.,
2010). The highest species richness were detected at the fourth and
fifth grazing intensity levels. In accordance with our results, several
studies detected that species richness is higher under moderate grazing
pressure than, for example, in ungrazed pastures (Sasaki et al., 2009;
Fensham et al., 2011; Deng et al., 2013). According to the results of
Deng et al. (2013), the density, the height, and the cover of vegetation
were the highest in the ungrazed parcel. Furthermore, the density of
dominant, good competitor species had the highest scores in the ungrazed
parcel. When grazing intensity is low, better competitor plants can
reach greater height as they acquire more resource, and can overcome
resource limitation more effectively in dense, ungrazed vegetation and
grow efficiently (Westoby et al., 1999; He et al., 2021). Livestock
grazing decreases green biomass and litter (Magnano et al., 2019) which
is obviously more pronounced under more intensive grazing. These
findings can explain the higher proportion of biomass samples belonging
to the first and second level of intensity at higher biomass values
(Figure 2). Although we detected a humped-back relationship between
species richness and green biomass in the studied pastures, the
possibility of a decreasing trend should not be excluded in unusually
arid years (Milchunas et al., 1988; Gao & Carmel, 2020).
One would expect that the samples originating from the same grazing
intensity levels would cluster in the ordination, but this was not
validated by the results (see Figure 5). The likely reason for this is
that there might be a fine-scale heterogeneity of vegetation. Fine-scale
heterogeneity may have several reasons we wish to detail on below. Godó
et al. (2017), studying the vegetation composition of alkaline and sand
pastures at multiple-scales, observed that there is a significant
relationship between the plot size and grazing effects: with increasing
plot size they found decreasing levels of differences in species
composition, i.e. the small-sized plots showed higher beta diversity
than larger ones. This is likely the reason why our fine-scale samples
(harvested in 20 cm × 20 cm plots) were not clearly separated along the
gradient of green biomass and species richness. Beside of the selective
defoliation effects of grazing, fine-scale heterogeneity of vegetation
is also supported by multiple fine-scale side effects provided by
grazing: uneven patterns of trampling, defecation, and seed input with
dung or fur (Ruifrok et al., 2014). Grazing also supports the formation
of bare soil patches (Eichberg & Donath, 2018) and provides higher
availability of light (Ruifrok et al., 2014). But the bare soil surfaces
can also act as safe sites for weedy species to colonize. In pastures
intensively grazed by sheep, we can expect higher amounts of bare soil
surfaces than under lower grazing intensity based on the results of
former research (Watt & Gibson, 1988; Süss & Schwabe, 2007; Teuber et
al., 2013). However, the seeds of several species cannot successfully
germinate if they fall on bare soil surface, unless they get buried to
an optimal depth subsequently, which might be ensured by the trampling
of sheep (Eichberg et al., 2005).
Plant species with variable characteristics are also able to diversify
the fine-scale species richness and biomass: in grazed pastures,
coexistence between plant species occurs more frequently if the grazing
intensity is suitably low (Vázquez-Ribera & Martorell, 2022), which
contributes to a vegetation consisted of more variable plants. Briske
(1996) explains in detail how the different plant characteristics
contribute to a better grazing resistance, for example herbivore
accessibility is limited if plant height is lower. Based on this
argument, it is reasonable that plant height is more varied at moderate
levels of grazing where short and tall plants are alike occurred. This
idea, the distribution of our samples, and the humped-back relationship
are confirmed also by Oksanen (1996) who argued and visualized that
humped-back relationship can even be artificially produced if following
the rules that the number of plant individuals increases below a
”crowding point” (where grazing resistant species are more frequent),
but above it (where better competitor species are more frequent), plant
size is what rather increases, therefore (in the sense of intermediate
disturbance) the highest species richness is at intermediate biomass, if
using small, fixed plot size. Beyond these assumptions, it should be
added that the spatiotemporal patterns of grazed and ungrazed fine-scale
patches is another important variable that reasonably becomes less
marked with the extremely increased or decreased grazing intensity.
Main biomass fractions and the biomass of life forms
We found significant differences along the grazing intensity gradient
for main biomass fractions and biomass of life forms as well. Both the
green biomass + litter fraction and the litter fraction were
significantly lower at higher grazing intensities. Consumption of plants
by livestock contributes to a decreased green biomass and litter
(Magnano et al., 2019). According to Kemp et al. (2000), perennial
graminoids are the most sensitive to grazing, and with increasing
grazing disturbance, subordinated species are enabled to spread (Grime
& Mackey, 2002) which might explain why we detected significantly lower
amounts of perennial graminoids at higher grazing intensities. Green
biomass showed significant differences and though the biomass of
short-lived forbs was significantly higher at the highest grazing
intensity, total green biomass fraction decreased with increasing
grazing intensity. Three explanations may be behind this pattern. First,
grazing may be less selective at higher intensities (Golodets et al.,
2009; Tóth et al., 2018), which causes a net loss of total green
biomass, but at the same time it favours short-lived forbs, due to their
fast regrowth rate and colonization ability in gaps (Westoby et al.,
1999; Hofmann & Isselstein, 2004). Second, the mean height of species
is typically lower at higher grazing intensities (Deng et al., 2013;
Török et al., 2016), therefore, they may be represented by less biomass.
Third, specific leaf area (SLA, mm2/mg) is higher, and
leaf dry matter content (LDMC, mg/g) is lower in case of annuals, thus,
their dry weight is lower than those with higher LDMC and lower SLA
(E-Vojtkó et al., 2020). In the meta-analysis of plant responses to
grazing, Díaz et al. (2007) found that the abundance of perennial plant
species decreased with increasing grazing intensity. This was also
confirmed by our results to an extent, but a striking leap can be
observed at the fourth level of intensity for perennial forbs. Among the
detected perennial forbs, Thymus glabrescens had remarkably high
values at this intensity level. Without this species, there would be a
continuous decline of perennial forb biomass along the gradient of
grazing intensity. Their extensive woody stems increase the biomass of
perennial forbs, moreover, their large amount of biomass also shapes the
differences of the total green biomass scores. Phytochemicals
(monoterpenes) produced by Thymus vulgaris means protection
against grazers (Cappuccino & Carpenter, 2005), therefore it is
reasonable to think that Thymus glabrescens in our studied
pastures is similarly not favoured by grazers. The meta-analysis
mentioned above (Díaz et al., 2007) details the response of annuals to
grazing; it was found that they increased together with the grazing
intensity which is confirmed by our results in case of forbs. Another
explanation for successfully spreading short-lived plants is their
generally higher SLA values compared to perennials. High SLA is linked
to fast re-growth ability (Helm et al., 2019) and higher SLA scores at
higher grazing intensities were confirmed by a former study addressing
livestock grazing in sand grasslands (Kovacsics-Vári et al., 2023). We
found no significant differences for short-lived graminoids which is
presumably due to their low species number and the even distribution of
the species along the gradient of grazing intensity. For example,Apera spica-venti was represented by higher amounts at lower
intensity levels, meanwhile Bromus hordeaceus had higher biomass
at higher intensities.
Impact of grazing disturbance on species composition
Significantly higher biomass scores of disturbance tolerant and ruderal
species were detected at the highest grazing intensity level. A process
leading to overgrazing is written in a simply summarized form by Schulze
et al. (2019): First, the vegetation composition changes; second,
vegetation cover decreases, third, bare soil surfaces are formed, and
fourth, soil erosion becomes more severe. Our results might support this
description, but we presume that these stages are not reached step by
step but occur in parallel. For example, when bare soil surfaces are
formed, further changes can be expected also in vegetation cover as gap
strategists take the advantage of disturbance and they establish
quickly. When grazing tolerant species with a good colonising and/or
fast-regrowth ability (Westoby et al., 1999) cannot keep up with the
severity and frequency of grazing the fourth step may take effect. This
fourth step might be in accordance with the slightly lower species
richness at the fifth level of grazing intensity compared to the fourth
level. In our study, the disturbance value did not differ strikingly
between the first and the fourth intensity level, however, at the fifth
level, scores were significantly higher compared that of other levels.
Therefore, we can conclude that the proportions of disturbance tolerant
and ruderal species were considerable at the highest grazing intensity,
and this is well in accordance with the proportion of short-lived forb
biomass as they typically represent these groups. The high disturbance
values and proportions within the simplified morpho-functional groups
indicated a stronger disturbance and a process leading to overgrazing.
Midolo et al. (2021) valued plant species by disturbance categories, and
they detected that annuals are favoured by disturbance as their ability
to grow fits to circumstances which do not provide stable biotic and
abiotic features that otherwise would be favoured by better competitors
using resources efficiently on the long-run (Salguero-Gómez, 2017;
Schulze et al., 2019). In contrast, Pettit et al. (1995) and Farmilo et
al. (2023) found no significant differences between ungrazed and grazed
samples for native annual forbs.
At the highest level of grazing intensity, the highest disturbance value
confirms also the observation of Botta-Dukát (2008) according to whom
open sand grasslands are among the most exposed habitats to disturbance
in Hungary. The Nyírség region has been densely populated for centuries,
and despite the low productivity of sand grasslands, large areas were
cultivated with high cover of disturbance tolerant and ruderal weedy
species. Thus, the vegetation composition of pastures with opening
surfaces can be likely colonised by these species having typically a
good dispersal ability (Schulze et al., 2019).
The fourth and fifth level of grazing intensity included sampling sites
with relatively high stocking rates (1.1 to 4 LU/ha), and the highest
scores of species richness were observed at these levels which is due to
the higher number of short-lived species. Increasing species richness in
more intensively grazed sites was also confirmed by Kiss et al. (2006).
In their assessment of grazing intensity, they considered the proximity
of study sites to stables, and similarly to us, they detected a higher
proportion of disturbance tolerant and weedy species contributing to the
higher species richness. We detected significantly higher species
richness at the fifth and fourth level of intensity compared to the
second level. At the fourth level, presence of grassland species is
still substantial, in addition disturbance tolerant and weedy species
appeared more frequently. One can find significantly different scores if
second and fourth intensity levels are compared to the third and fifth
intensity levels which implies the possibility that flocks of sheep
graze and trample the sites representing the third and fifth level more
frequently. These significant differences suggest that stocking rate
(LU/ha) and proximity jointly drive changes in vegetation. A higher
grazing frequency and intensity are presumed with the higher proximity
to resting and watering points. The joint impact of proximity and
stocking rate on plant characteristics can be similarly explored in the
study by Kovacsics-Vári et al. (2023) for flowering period and life
forms. In addition, the findings by Kiss et al. (2006) and Tonn et al.
(2019) suggest that the effect of livestock should be studied on finer
scales than the scale of a pasture, since LU/ha, as an important metric
in grazing regimes, is determined for the entire pasture area.