Plant functional types and climate along a precipitation gradient in temperate grasslands, north-east China and south-east Mongolia

Using data from three field surveys along a precipitation gradient of temperate grasslands in north-east China (the Northeast China Transect, NECT) and south-east Mongolia, the spatial distribution of six plant functional types (PFTs): C₃ species, C₄ species, grasses, shrubs, forbs and succulents and their relationships with climate were analysed. The spatial distribution of different PFTs varies in different regions and in different grassland types of the study area. The species richness in each PFT also has different relationships with climate (significantly or not). Generally, the number of C₃ species, C₄ species, grasses and forbs have positive relationships with precipitation and aridity. Shrubs have negative relationship with precipitation and aridity. Succulents were found to have no relationship with precipitation and aridity. Shrubs, grasses and forbs have stronger relationships with precipitation than C₃ and C₄ species. The relationships between C₃ species, forbs and aridity are more significant than with precipitation. On a regional basis, the combined effect of precipitation and temperature, the aridity, is more significantly correlated with the distribution of C₃ species and forbs, which are more dominant in the study area, than with C₄ species, grasses and succulents.     

A shrub bending model to calculate the albedo of shrub‐tundra

At high latitudes, the albedo and energy budget of shrub‐tundra landscapes is determined by the relationship between the fractional snow cover and the fraction of vegetation protruding above the snowpack. The exposed vegetation fraction is affected by the bending and/or burial of shrubs in winter and their spring‐up during melt. Little is known about the meteorological conditions and snowpack and shrub properties required to cause bending, and few quantitative measurements of bending processes exist. Here, a model combining the few, mostly qualitative, observations available with a biomechanical model representing branches as cantilevers is proposed to provide a first approximation of bending mechanisms. The exposed vegetation fraction is then calculated using structural parameters of shrubs measured at two sites in Canada: the Granger Basin in the Yukon Territory and Trail Valley Creek in the Northwest Territories. The exposed vegetation fraction is in turn used to calculate albedo, which is evaluated against measurements at the two sites. The model considerably improves modelled albedo compared to a model which only buries but does not bend shrubs at TVC, where shrubs become completely buried. However, the model overestimates albedo at GB where only a few shrubs get buried. The bending model is then used to calculate a compression factor for use in a simple parameterization of the exposed vegetation fraction proposed by previous investigators. The parameterization, which is simpler and computationally less expensive than the full model, is evaluated and found to perform well. Despite the need for further developments, the model provides a first approximation of bending processes and contributes to the identification of measurements that are needed in order to improve the model and our understanding of the bending of shrubs. Copyright © 2012 John Wiley & Sons, Ltd.     

Patterns of shrub species richness and abundance in relation to environmental factors on the Alxa Plateau: Prerequisites for conserving shrub diversity in extreme arid desert regions

Shrub species are considered the dominant plants in arid desert ecosystems, unlike in semiarid steppe zones or in grassland ecosystems. On the Alxa Plateau, northern China, sparse vegetation with cover ranging from 15% to 30% is characterized mainly by multifarious shrubs because herbaceous species are strongly restricted by the extreme drought climate, wind erosion, overgrazing and sand burial. Patterns in shrub species richness and species abundance in relation to environmental conditions were examined by DCA (detrended correspondence analysis) and interpreted by a biplot. The relationships between species diversity and environmental factors were examined using regression analyses. Our results show that the distributions of the shrub species in response to environmental conditions can be grouped into four ecological types, corresponding with the biological traits of the shrubs and their responses to the gradients of soil texture and soil water content. Patterns in species richness and species abundance were mainly determined by the deeper soil water content, instead of the soil texture as hypothesized by numerous studies in semiarid grasslands. With exception of the deeper soil water content, soil organic matter and total N content were positively correlated with species abundance, while pH was negatively correlated with it. These findings imply that it is vital for current shrub diversity conservation to reduce agricultural water use in the middle reaches of the Heihe River, which supplies water for the lower reaches in the western parts of the plateau, and to reduce the amount of groundwater exploitation and urban and oasis water use, to increase the water supply from Helan Mountain to the eastern desert of the Alxa Plateau. Supported by National Key Technology R & D Program (Grant Nos. 2007BAD46B03, 2006BAD26B0201) and National Natural Science Foundation of China (Gant No. 40825001)     

Gender-related differences of shrubs in stands of Atriplex canescens with different histories of grazing by cattle

Atriplex canescens is a relatively common dioecious shrub in western North America. It is considered a valuable forage resource for both wild and domestic herbivores. Sex ratios and shrub dimensions were recorded in stands of tetraploid A. canescens that had been either protected from cattle grazing or summer- or winter-grazed by cattle for at least 20 years. Stem diameter and crown shape were used as surrogates for shrub age which could not be estimated by counting growth rings. Shrub sex ratios in exclosures were significantly more female biased than the empirically derived ratio for tetraploid A. canescens (55 female:35 male: 10 monecious). Conversely, shrub sex ratios in grazed pastures were not significantly different from the empirical ratio. Proportion of female shrubs in exclosures was significantly higher than in grazed pastures. Proportion of male shrubs, on the other hand, was similar in exclosures and grazed pastures. Winter-grazed shrub stands were apparently younger than both summer-grazed and protected shrubs. Protected shrubs appeared to be the oldest. Grazed female shrubs were apparently younger than grazed males, however, shrub ages of protected male and female shrubs were apparently not different. Cattle-grazing may have affected female shrubs more negatively at this site, causing gender-based differential mortality, and/or sex-shifting. Such processes could account for the differences in sex ratios, and for the apparent gender-related differences in shrub age that were observed.