Search

Grazing removal is widely used in grassland management. Plant responses following grazing removal at different organizational levels, however, are not well understood. We examined plant responses at different stocking rates in an Inner Mongolia grassland ecosystem dominated by Leymus chinensis and Stipa grandis. Our results indicated that plant response patterns differed significantly among stocking rates, at different levels of organization, and between wet and dry years. Community aboveground net primary production (ANPP) recovered more quickly at low and moderate stocking rates than those at high stocking rates. Response of aboveground net primary production (R) was significantly positively correlated with both individual biomass and density responses of L. chinensis. Overcompensation of L. chinensis after grazing removal contributed greatly to positive R at the community level. Significant compensatory effects were found between the two dominant species and between dominant species and the remaining non-dominant species. Variation in precipitation significantly affected community ANPP, relationships between community and species responses, and compensatory effects between species. Our study suggests that periodic grazing removal is likely to be a useful method for grassland management and that a combination of species with compensatory effects can be advantageous for reseeding practices in grassland restoration. [ABSTRACT FROM AUTHOR]

In water-limited environments, photosynthetic carbon gain and loss of water by transpiration are in a permanent tradeoff as both are contrarily regulated by stomata conductance. In semiarid steppe grasslands water limitation may covary with nitrogen limitation. Steppe grassland species are capable of optimizing their use of limiting resources, water and nitrogen, but regulation is still poorly understood. In a two-year experiment with addition of water (irrigation simulating a wet year) and nitrogen (0, 25, and 50 kg urea-N ha) we assessed intrinsic water use efficiency (WUE), nitrogen use efficiency (NUE), and related plant functional traits (PFTs) of four dominant C species ( Leymus chinensis, Agropyron cristatum, Stipa grandis, and Artemisia frigida). Water and N fertilizer supplementation significantly increased plant primary production, and N effect was more pronounced under irrigated conditions. Parallel with the responses of plant production, a strong tradeoff between WUE and NUE was detected: water supply increased NUE but decreased WUE, whereas N addition slightly increased WUE at the expense of NUE. This tradeoff occurred at the leaf level, and involved the responses of leaf N concentration and specific leaf area. WUE of species changed among treatments in a predictable manner by the parameter of leaf N content per area. Dominant plant species commonly achieved a higher utilization efficiency of the more limiting resource via altering PFTs, which was an important mechanism of adaptation to variable resource limitation in semiarid grasslands. [ABSTRACT FROM AUTHOR]

Inner Mongolia steppe grasslands are widely used for livestock farming and the regrowth ability of grassland species is therefore strongly influenced not only by water and nutrient availability but also quite heavily by grazing. However, little is known on how grazing, water and nitrogen interactively affect the dominant C3 species ( Leymus chinensis, Stipa grandis, Agropyron cristatum) and the C4 species ( Cleistogenes squarrosa). Therefore in the 2007 and 2008 growing seasons, a field experiment was carried out to test the hypothesis that under different grazing intensities the dominant species show different short-term regrowth response to simultaneous variation in the availability of water and nitrogen. Single factor and interaction effects of the addition of water (rainfed vs. simulated wet-year) and nitrogen (0 or 25 kg N ha) were analysed along a gradient of four grazing intensities (ungrazed, lightly, moderately and heavily grazed) after one month of grazing exclusion. Water and nitrogen addition affected short-term regrowth of all species in a similar way whereas species responded differently to grazing. Simulated wet-year water availability consistently resulted in higher standing biomass, relative growth rate and cellulase digestible organic matter yield. Supplementary nitrogen promoted standing biomass and crude protein concentration. The nutritive value of all species' standing biomass showed a similar increase with more intensive grazing. However, heavy grazing led to a clear shift in the relative biomass of the species, i.e. mainly a promotion of the C4 grass, C. squarrosa. In contrast to our hypothesis, there were no differences among species in their response to water or nitrogen addition, whereas, heavy grazing induced the expected species-specific response. Our results suggest that heavy grazing rather than nitrogen or water determine short-term shifts in species composition of the investigated steppe ecosystem. Furthermore, differences in the species-specific growth response to grazing may increase the proportion of the C4 grass C. squarrosa in steppe communities, whereas higher availability of nitrogen and water may lead to higher forage biomass and nutritive value of all investigated species but in short-term cannot compensate for the grazing induced changes in species composition. [ABSTRACT FROM AUTHOR]