Study site
Of the total land area of Iran, about 52.4% are rangelands; 8.6% are forests and 19.5% are deserts including bare salty lands. There are three main climatic zones in Iran, including arid and semi-arid regions of the interior and far south, Mediterranean climate (mainly in the western Zagros Mountains, the high plateau of Azerbaijan, and the Alborz mountains) and humid and semi-humid regions (mainly in the Caspian, but also in west Azerbaijan and the southwest Zagros). The study area is located in plant geography zone of Europe - Siberia (the Hyrcanian area). The study area, the southern hillsides of Damavand Mountain, is located in the North part of Iran (52°11′–52°5′ N; 35°46′–35°49′ E). It covers ca. 6000 ha and has an elevation stretching from 2500 m asl to 34600 m asl. The climate is semi-arid cold. Minimum monthly mean − 0.3°C and Maximum monthly mean 19.2°C. The mean annual temperature of 12.7°C; and the mean annual precipitation of 652 mm. The climatic varibales were computed from the mean annual precipitation, mean annual moisture and mean annual temperature data recorded for the period 1985–2010 by the Rineh network of meteorological stations (35°54′; 52°04′E) (Fig. 1). The soil type of the study area is sandy-loamy and silty soil. The bedrock is primarily calcareous. The soils are variable, ranging from deep brown soils to shallow lithological soils, the latter exerting a greater influence on vegetation. Basic cations and carbonates promote an alkaline pedogenesis that can be followed by an acidic pedogenesis after carbonate lixiviation.
Experimental design
The experiment was carried out in three different habitats: grassland, dominated by Agropyron repens, Festuca ovina, Bromus tectorum and Bromus tomentellus; shrubland, dominated by Artemisia aucheri, Astragalous aegobromus, Eringium billardeieri and Thymus kotschyanus; and a transitional fringe between both habitats, which we called “mixed area”, dominated by Astragalus ochrodeucus, Festuca ovina, Onobrychis cornuta and Bromus tomentellus. These habitats are adjacent to each other in our study area and they have different community compositions, structures, and dominant species. From mid spring and summer of 2016, sampling was down in a 1×2 m plots within each habitat. In each habitat, in total 150 sampling plots were selected randomly with the constraint that it was at least 1 m from the margin to avoid edge effects. On the field, each plot was separated at least by 100 m from the others, to avoid spatial autocorrelation. We estimated the cover of each species per plot.
Plant functional trait measurements
We select the 44 most abundant species. Between four and 20 individuals per species were measured in sites. One well-developed entire leaf was collected per individual for trait measurement. The leaves were scanned and analyzed in the lab using the Leaf area matter software to measure their surface. We also measured the leaf dry mass after drying and weighting the leaves. The vegetation height (VH, m) was measured in the field as the distance between the top of the photosynthetic tissue and the ground. Specific leaf area (SLA; mm2 mg-1) was calculated as the ratio of the leaf surface to its dry mass. Seed mass (SM; mg) were compiled from seed databases (Goethe University Frankfurt, http://www.seed-dispersal. info, and the Royal Botanic Gardens, http://data.kew.org/sid/). For the few species that we did not find seed mass values, we estimated the seed mass of each of these species from the average value of the species of the same genus that were registered (de la Riva et al. 2019), because seed mass is strongly conserved through the phylogeny (Lord et al. 1995). Given the multidimensionality of plant functions, we chose these three traits because of their importance in providing information about different independent axes of ecological strategies (see Díaz et al. 2016; Bruelheide et al. 2018). Specifically, we selected SLA, VH and SM as a subrogate of the plant strategies related with resource acquisition, competition for light and reproduction, respectively (Laughlin et al., 2010).
Soil collection and processing
Soil samples of the top 30 cm of depth (where nutrient uptake mostly occurs; Jobbágy and Jackson 2001) were collected from each sampling plot. Soil samples were randomly collected from three points using a bucket auger and mixed into a single soil sample and were brought into the laboratory in airtight plastic bags. All of the soil samples were air-dried and then filtered through a 0.2 mm sieve, discarding the visible roots and other plant debris. Soil pH was measured using a pH meter with a glass electrode (soil / KCl ratio is 1 V 2:5). Soil organic carbon was determined using the Walkley–Black method and a factor of 1.3 was applied to adjust the organic C recovery (Nelson and Sommers, 1982). The soil available nitrogen, phosphorus and potassium were measured using the methods of Miller and Keeney (1982).
Statistical analysis
Following Garnier et al. (2004), the traits were weighted by the relative abundance of their constitutive species to calculate the community weighted mean (CWM) in each plot. The phylogenetic tree was obtained with the comprehensive Angiosperm species-level phylogeny from Zanne et al. (2014), as updated by Qian and Yin (2015), which is included in the R package ‘S. PhyloMaker’ (Qian and Yin.2015). The distance of the few species that were not found in the PhytoPhylo were supplanted by the distance of the closest species of the same genus found in the mega-phylogeny tree (de la Riva et al., 2019). For each plot we calculated the species richness (Pielou, 1969), represented by the total number of species per plot, the Faith’s phylogenetic diversity index (PD), the functional richness (Frich) and the mean pairwise dissimilarity (MPD) index for both functional and phylogenetic diversity. PD was calculated with the R package 'PICANTE'; Kembel et al. (2010). To quantify the Frich index, we built a functional space through a Principal Coordinate Analysis (PCoA), using a Euclidean trait matrix after standardizing traits (mean = 0, SD = 1) (Maire et al. 2015). We calculated the functional and phylogenetic MPD indices (FMPD and PMPD, respectively) based on existing algorithms (de Bello et al. 2016). These four indices integrate the main components of community structure enclosed by other related indices.
In order to account for trait coordination and to identify the major axes of trait variation, we run a Principal Component Analysis with the three CWM traits values (VH, SM and SLA) (Biotic_PCA). In addition, to reduce the number of variables characterizing the soil environment and their collinearity, other PCA was performed with the soil variables measured in this study (Soil_PCA). When necessary, variables were previously standardized and log-transformed to fulfil assumptions of normality (based on Kolmogorov-Smirnov) and homoscedasticity. We used an ANOVA analysis to check for differences on the distribution of the three types of habitats (Grassland, Shrubland, and Mixed) along the principal components analysis (PCA) in both biotic and soil PCAs. Post-hoc Tukey tests were performed to check the significance of differences between the least square means of each type of habitats. We also performed a one-way ANOVA to check for the variation of the diversity indices among habitats.
In order to evaluate shifts in the functional and phylogenetic structure distributions in response to edaphic factors, we used the generalized least-square model (GLS) controlling for the potential spatial correlation between soil variables associated with the type of habitat. We included a spatial autocorrelation structure using the cor-CAR1 function. We built one generalized least square regression model per community property; for each CWM trait and diversity index (respond variable) with the first and second Soil_PCA axes as predictors, using the gls function of the nlme package (Pinheiro et al. 2016). The variance explained by the GLS model was estimated by extracted the marginal R2 value, which was calculated with the rsqueared function in the ‘piecewise SEM’ package (Lefcheck 2016).
In addition, we used null models to check if the functional and phylogenetic diversity patterns were a trivial consequence of species richness variation. We randomized the traits combinations across species, while fixing the number of species of each plot (999 runs) and estimating the same functional and phylogenetic indices. For each index, we estimated the Standardized Effect Size (SES, = (observed difference - mean null difference) / SD null differences), in order to assess if the relationship observed between the functional and phylogenetic indices and the soil variables were significantly different from those obtained between randomized communities (null differences).
All the statistical analyses were performed using the R 3.1.3 software (R Development Core Team, 2014).