Search

Moisture availability is the main limiting factor of plant growth and biomass production in arid and semi-arid grasslands. The question whether plant responses to changing precipitation are species-specific, or change over entire plant communities is still controversial. Our study focussed on intraspecific changes in the plant traits canopy height, plant width, specific leaf area, chlorophyll fluorescence, performance index, and individual biomass of three congeneric species pairs with changing precipitation in Mongolian rangelands, covering a gradient from the desert to the forest steppes. Using this trait data set, we focussed on three questions: (i) Is the replacement of congeneric species along an environmental gradient also reflected in their trait values? (ii) Can intraspecific trait variation patterns be derived from patterns in species abundances, i.e., are trait values optimal where species are most abundant? (iii) Is the within-population trait variability lowest in populations growing under very dry conditions, i.e., under highest environmental stress, caused by stronger filtering? We tested the responses of the six traits to changing precipitation according to species’ identity and abundance. We found unimodal relationships between most of the species’ traits and precipitation, and strong associations between species abundances and trait values, but not for all investigated species. Trait variability did not significantly differ between populations from different positions along the precipitation gradient. Our results highlight that species show multiple or even opposite trait responses along the precipitation gradient. It thus remains challenging to predict how plant distributions will shift under changing environmental conditions based on their trait composition.

There are still major gaps in our understanding of rangeland degradation. Assessing the interactions between climate and grazing effects could help to explain what unifies and separates rangelands, and may therefore promote a more sustainable management of livestock. We studied 15 local land-use transects along a 600 km long climatic gradient in Central Asia to test the hypothesis that grazing effects differ between relatively moist equilibrium (EQ) and dry non-equilibrium (NEQ) rangeland systems. We analysed plant community composition, species diversity and indicator species for different grazing intensities. We found pronounced differences in community composition along our climate gradient, revealed climate-related grazing effects on richness, responses of Simpson's diversity, and also found different grazing indicator species along the larger transect. We conclude that in NEQ rangelands, grazing effects are limited to sacrifice zones and environmental filtering dominates vegetation composition. With increasing precipitation, resource availability gains in importance leading to more complex communities dominated by grazing-tolerant species under EQ dynamics. Hints for xerophytization in the transition zone between EQ and NEQ highlight the vulnerability of rangelands that temporally shift from one state to the other. This calls for extra care in the management of livestock numbers in these transition areas.

In dry steppes, strong climatic constraints, especially highly variable precipitation, and grazing are the most important factors controlling plant life. Growth is strongly limited by water availability, while grazing may affect species presence and performance. However, there is a lack of studies on population genetics of dryland plants in general, and of those addressing grazing effects in particular. To determine the landscape-scale genetic structure of dryland species, and if grazing has an impact on that, we chose the Eurasian steppe grass Stipa glareosa for a population genetic study employing nine polymorphic Simple Sequence Repeat (SSR) markers. We assessed genetic fingerprints of 200 individuals from six populations in Mongolia, which were sampled along a large-scale precipitation and altitudinal gradient. Nested within this gradient, sub-populations were sampled along short local transects representing different grazing intensities. Overall, S. glareosa populations showed rather low levels of genetic diversity at a mean Bruvo distance among individuals within a given population of 0.494 (mean expected heterozygosity He = 0.053). Linear mixed model analysis implied that genetic diversity was affected by both climatic constraints and local grazing conditions. We found a moderate isolation-by-distance pattern across all populations; grazing additionally influenced the genetic structure at local scale. Analysis of Molecular Variance revealed a modest genetic differentiation between populations (9 % of variation) and among sub-populations representing different grazing levels (11 %). Moreover, we detected indicator alleles that were exclusive for populations along the precipitation gradient; other alleles were associated with certain grazing levels across all populations. Thus, our data suggest that climatic constraints affect the genetic structure of S. glareosa populations, while at local scales differences in grazing disturbance may also matter.

The phreatophytic tree species Populus euphratica Oliv. (Euphrates poplar) forms riparian (tugai) forests along the Tarim River at the northern and eastern fringe of the hyper-arid Taklamakan Desert (Xinjiang province, NW China). These forests are threatened by decreasing groundwater levels due to excessive withdrawal of water for agricultural irrigation. At three plots each at the middle (Yingbazar) and the lower reaches (Arghan) of the Tarim River, we investigated the long-term effects of different water supply due to different distances to the groundwater (from 2 to 12 m) or to additional supply with water from reservoirs, which is being conveyed to the Tarim River's lower reaches since 2000 to conserve and restore the poplar forests. We conducted tree-ring analyses for the time period 1946–2011 and quantified the water use at the tree (Etree) and the stand level (Estand) using the Granier method. For the last four decades of investigation, the average tree-ring width and basal area increment (BAI), the cumulative Etree (May–August) and Estand were significantly higher on the plots with the largest water supply than on the plots with the largest distance to the groundwater. However, the maximum values of xylem sap flow density and Etree values averaged for the vegetation period did not differ significantly among plots and between sites. These transpiration values and the relatively large BAI even at a large distance to the groundwater indicate that—in contrast to previous studies—Euphrates poplars can grow vigorously even at a groundwater depth of up to 12 m. Supply with additional water resulted in a marked increase in BAI. Lower Estand at lower water supply can be explained with a sparser stand density. Interannual fluctuations in the water supply resulted in distinct alterations in BAI in trees that grow at a short distance to the water table or are supplied with additional water. Against the background of limited water availability in the region, we recommend concentrating the conveyance of additional water to forest stands with a short distance to groundwater to keep those forests fully functional and to maintain their ability to regenerate.