Introduction
Species interactions are widely acknowledged as one of the most
important drivers of the plant community structure, biodiversity,
ecosystem function and dynamics (Callaway et al., 2005; Brooker et al.,
2008; Jankju, 2013). However, the outcome of plant-plant interactions
may vary, ranging from competition to facilitation, depending on
environmental severity (Grime, 1977;
Brooker
& Callaghan, 1998) and on the character of coexisting species (Armas et
al., 2011; Pugnaire et al., 2011). Interactions among plants may reduce
the extreme abiotic and biotic stresses such as aridity (López et al.,
2016) and intensive grazing (Smit et al., 2007 & 2009; Holmgren &
Scheffer, 2010; Soliveres et al., 2011) by creating suitable
micro-habitats for drought- or grazing-intolerant species (Bruno,
Stachowicz & Bertness, 2003; Farzam & Ejtehadi, 2017).
Grazing is essential biotic stress in dry rangelands due to its
extensive application and potential contribution to variation of
community structure, species composition, and degradation of ecosystem
services (Li et al., 2013, Díaz et al., 2007; Kéfi et al., 2007).
Research in the ecosystems driven by large herbivores shows that
unattractive, toxic or thorny plants may induce positive indirect (i.e.,
grazer mediated) effects on palatable herbs, shrubs or trees (Bakker et
al., 2004; Callaway et al., 2005; Smit et al., 2006). Grazing may affect
the outcome of plant-plant interactions as well, depending on the
ability of the nurse plant to moderate the effects of herbivores and on
the tolerance of the present species to herbivory (Baraza et al., 2006;
Vandenberghe et al., 2009). Further, nurse plants may protect the
neighbours against herbivory and enhance their recovery by increasing
resource levels (Rand, 2004; Acuña-Rodríguez et al., 2006).
The CRS strategy, distinguishing between the competitors, ruderals and
stress tolerators, provides a detailed view on the character of species
typical for different types of vegetation, i. e. with varying intensity
of environmental stress or grazing. The competitor, stress tolerator,
and ruderal (CSR) theory (Grime 1979), based on the quantitative
functional traits and life
strategy,
significantly improved the possibilities for analyzing ecological
processes (Grime, 1977; Hodgson, Wilson, Hunt, Grime, & Thompson,
1999).
Previous researches have explored the effects of livestock grazing and
climate on the relationships between plants (Metz & Tielbörger, 2016;
Berdugo et al., 2018) but its effects on plant-plant interactions have
rarely been addressed (but see Soliveres and
others 2011,
Verwijmeren et al., 2014; Filazzola et al., 2017). Moreover, the results
of these studies are not consistent. For instance, Soliveres et al.
(2011) showed that rabbit grazing caused positive interactions between
the bunch grass
(Stipa
tenacissima ) and saplings of the shrub (Retama sphaerocarpa )
during winter and autumn. However, because of higher grazing intensity
in the summer, the interspecific interactions shifted to neutral.
Similarly, Holthuijzen and Veblen (2016) found that positive
interactions between Artemisia tridentata ssp.wyomingensis and Poa secunda decreased with increasing
grazing intensity in the arid region because grazing reduced
productivity during the drought periods more intensively. This may
result in the absence of positive interactions between plants due to
different stressors (Smit and others 2009; Verwijmeren and others 2013;
Michalet and others 2014). On the contrary, Noumi et al. (2016)
showedthat suppressive effects of shrubs on Acacia tortilisseedlings shifted to neutral with increasing grazing stress. Therefore,
an increase in facilitation due to the combination of these two
stressors can be expected. This research aimed at investigating the
intensity of herbivore effect on plant-plant interactions when it is
concurrent with drought. In addition, the CSR plant life strategies were
used to interpret the inconsistency in the literature on changes in
biotic plant interactions.
The selected dominant species, Artemisia kopetdaghensis , is an
aromatic shrub that is widely distributed, ranging from warm and arid to
cold and semi-arid steppes of northeast Iran (180-400 mm) and parts of
Turkmenistan. We used A. kopetdaghensis and its understory plants
as a model species to study the combined effects of grazing and climate
(arid region: high/low grazing, semi-arid region: high/low grazing),
aiming to answer the following questions:
(i)What
is the prevailing type of interaction between the dominant shrubA. kopetdaghensis and the surrounding understorey herbs? (ii) Are
the relations between A. kopetdaghensis and plants in its
surroundings affected by aridity and intensive grazing? 3)
Do
the effects of the shrub canopy vary according to the different plant
life strategies under the combined effects of grazing and aridity?