Rangelands cover more of Earth’s land surface than any other ecosystem type, producing significant amounts of livestock, and are most commonly used and managed (Reid et al. 2014), Also are under different stress (Leadley et al. 2010). They are natural ecosystems in the world and have an important role in the potential production and supply of forage, stability, and soil fertility, including carbon sequestration, climate regulation, and ultimately the balance of ecosystems (Asadian et al. 2016). Knowing the interaction between plants and management factors like livestock grazing is crucial, given the significance of plants in maintaining the ecosystem and the numerous direct and indirect benefits that humans derive from them (Tahmasebi et al. 2011).
Livestock grazing is the most important land use sector in rangelands worldwide (Herreo and Thornton 2013). Therefore, it is one of the most influential disturbances in species composition (Huntley 1991), along with habitat destruction (Arévalo et al. 2011), and the invasion of exotic species (Pauchard et al. 2009). Several features, such as short height and the presence of ruderal species, are indicators of this disturbance (Díaz et al. 2007). The impact of livestock grazing depends on the type of vegetation, species composition, type of grazing animal, seasonality of climatic, past and current stocking rates, and grazing management (Chillo and Ojeda 2014; Eldridge et al. 2015).
Iran is one of the most important countries in the Middle East and Western Asia for the conservation of biological diversity (Noroozi et al. 2019). The complex and varied climates, topography, geological formations, and varied vegetation have led to a rich and unique biological diversity. There are approximately 8,000 species of plants in the rangeland ecosystems of Iran (Mohammadian 2015). Rangelands in Iran are about 90 million hectares, which account for 54.6% of the country's total area and 65% of natural resources, and the majority of its species are exposed to this livestock grazing disturbance (Amiri 2009, Rouchiche and Haji Mirsadeghi 2002). about 575 thousand hectares are located in Mazandaran province that have high plant diversity and many of the native species. This high plant diversity in the Mazandaran region is due to the altitudinal gradient and considerable heterogeneity in climatic and physiographic conditions. However, due to mismanagement and the high density of sheep and cattle, biodiversity has seriously decreased (Jafarian et al. 2019).
There are still significant gaps in our understanding of rangeland degradation (Ahlborn et al. 2020). Therefore, to determine the effects of perturbation caused by grazing intensity, it is necessary to use characteristics and indicators that determine the extent of the effects of various factors related to environmental disturbances (Magurran et al. 1988). Understanding how environmental factors and grazing impacts interact may assist in clarifying the characteristics that distinguish one rangeland from another and may promote more sustainable livestock management. (Addison et al. 2012; Bestelmeyer et al. 2015). In recent years, this problem has caused more motivation to search for measurable and universally usable predictions of ecosystem function and response to changes. Research conducted in different regions of the world (e.g. Díaz and Cabido 1997; Grime et al. 1997; Reich et al. 1997; Westoby and Wright 2002; Pierce et al. 2017lıç et al. 2018), shows increasing evidence that there are such predictions and they can be found as single traits or a set of plant functional traits (Violle et al. 2007). Functional traits rarely act separately, usually forming part of a set of attributes that work together to affect survival. This set of traits can be described as “ecological strategies” or “adaptive strategies”, whose nature reflects fundamental trade-offs in the allocation of resources available in the plant for various functions such as vegetative growth, regeneration or maintenance of tissues (Grime and Pierce 2012). Thus, the study of functional traits may help to understand how plants respond to new environmental conditions and resource availability with disturbance events (Villalobos et al. 2022; Hernández -Vargas et al. 2019). For this reason, approaches based on functional traits have come out as a promising way to understand plant ecological strategies, plant-herbivore interactions, and their linkages to ecosystem functioning (Rahmanian et al. 2019).
Plant functional traits can be used as effective indicators to study the responses of plant species under livestock grazing disturbances (Díaz et al. 2007; Zheng et al. 2015lıç et al. 2018; Jäschke et al. 2019). For example, plant resistance to grazing is related to both avoidance traits (such as low plant height and leaf area) and tolerance traits such as high specific leaf area (SLA) and leaf nitrogen concentration (LNC) (Díaz et al. 2007; Zheng et al. 2010). These traits can reflect plant survival strategies and their ability to acquire resources and compete for nutrients under different grazing intensities. Therefore, livestock prefers plants with specific functional traits, which significantly reduces the population of these plants under intensive grazing (Wang et al. 2011). For instance, Díaz et al. (2007) and Niu et al. (2009) have shown in previous studies that the abundance of tall herbaceous species decreases after grazing.
One of the most prominent schemes of evaluating the life history strategies of plants with respect to plants’ response to environmental factors is the CSR (C: competitive; S: stress-tolerance; R: ruderal) ecological strategy theory (Grime 1974, 1977), That enables the understanding of the functional characteristics of species (Pierce et al. 2017). Functional diversity (FD) can be seen as a component of diversity (Tilman 2001) and it is a key driver of ecosystem processes, ecosystem resilience to environmental impacts, and ecosystem services (Díaz et al. 2007; Laliberté and Legendre 2010, Laliberté et al. 2013; Schellenberger Costa 2017; Mammola et al. 2021). However, FD can be estimated in distinct ways that reflect different dimensions of FD, leading to the development of several indices for measuring FD (e.g., Petchey and Gaston 2007; Mason et al. 2013; Schellenberger Costa et al. 2017).
According to studies by Mason et al. (2013)d schke et al. (2019), FD can be separated into three main components: functional richness (FRic), functional evenness (FEve), and functional divergence (FDiv). The FRic is positively associated with vegetation resilience, high FEve indicates effective utilization of the available resources, and FDiv is positively related to the degree of niche differentiation among species (Jäschke et al. 2019). The FEve measures the regularity of the distance between the species in the trait space (e.g. the single functional regularity index) and also the evenness of the distribution of the abundance of the species. The value of the index decreases when the relative abundance of species is less evenly distributed and the functional distance between species is irregular (Villéger et al. 2008; Mouillot et al. 2005). The FDiv quantitatively expresses how trait values are distributed along the range of trait space. This index, for a single-trait approach, shows how abundance is distributed along a functional trait axis, within the range occupied by the community (Mason et al. 2005; Villéger et al. 2008). But when there is more than one attribute, the linear domain is replaced by a multi-dimensional domain such as a convex hull (Pla et al. 2011). Rao’s quadratic entropy index of diversity (RaoQ) has been proposed by Rao (1982) and follows the theory of entropy. The RaoQ may be considered as the amount of expected contrast between species. This index is calculated by dividing the Euclidean distance by the number of traits used (Botta-Dukát 2005). The RaoQ index can provide a general approach to partitioning biodiversity into different spatial components to understand the underlying processes of species distribution and estimator of diversity. Indeed, the RaoQ is currently the only diversity estimator that combines various measures of species dissimilarity (e.g., phylogenetic or functional) with relative species abundance, and provides a standard workable method for comparing the α, β, and γ components between It offers different aspects of diversity e.g. taxonomic, phylogenetic, and functional diversity. These salient features of the RaoQ index can reveal new perspectives for understanding the mechanisms that drive diversity efficiency at environmental and temporal scales (De Bello et al. 2010).
For many ecological theories and analyses, the concept of niche is essential, especially the ecological concepts of niche differences among species. Niche differences are key to understanding the distribution and structure of biodiversity. To examine niche differences, we must first characterize how species occupy niche space, and two approaches are commonly used in the ecological literature. The first uses species’ traits to estimate multivariate trait space (FD); the second quantifies the amount of time or evolutionary history captured by a group of species (phylogenetic diversity, PD) (Tucker et al. 2018). Plants have been particularly well-studied in terms of functional traits, many of which turn out to have significant phylogenetic signals, such as a wide variety of leaf and root traits (Kraft and Ackerly 2010; Kembel and Cahill 2011). By PD we mean the total amount of phylogenetic distance among species in a community, which is influenced both by how related species are to each other on average and by how many species are present (Srivastava et al. 2012).
In this context, the present study aimed to assess the effect of grazing intensity on plant community traits and CSR strategies, and in species richness, PD, and different aspects of FD. Therefore, we try to test the following hypotheses: I) Higher grazing intensity led to a decline in plant taxonomic, functional, and phylogenetic diversity; II) Plant strategy change as a function of grazing intensity in which rare and ruderal strategies changed into stress-tolerant under low and high grazing intensity.