Effect of Climate Change and Grazing on Populations of Cleistogenes squarrosa in Inner Mongolia Steppe

Abstract

Most rangelands in northern China are water_limited ecosystems. Even small changes in precipitation amount or season can affect the biological components that maintain nutrient, water and energy movement within and through these ecosystems. Inner Mongolia steppe is generally semi_arid with most of the precipitation coming during the summer from June to August. Land_management practices, such as livestock grazing, are affecting ecosystem structure and function and these activities may accelerate impacts of climate changes. Grazing_related changes to ecosystems become more pronounced as humans shift from nomadic or unrestricted practices of livestock management to practices that focus on human settlement. At the scale of a management unit (e.g., watershed, allotment or paddock), precipitation amounts do not always correlate positively with vegetation productivity. Nutrient availability may strongly influence botanical composition and productivity. In general, spring and summer precipitation favors herbaceous vegetation that is more efficient in extracting water and will favor plants with C 4 (warm_season) vs. C 3 (cool_season) photosynthesis. The northern China steppes have few C 4 plants. Little experimental work is being done in the field to examine the ecological impacts of climate change and grazing and their interactions on C 4 plants in this region. Cleistogenes squarrosa is a C 4 plant. It is a short, perennial bunchgrass with drought tolerance, and tends to occupy medium degradation grassland in Inner Mongolia steppe. Therefore, it is a good indicator to study effect of climate change and grazing on C 4 plants. In order to understand the responses of Cleistogenes squarrosa populations and ecological strategies to climate change and grazing, we monitored the data of climate (precipitation and temperature), C. squarrosa population and its community for 18 years under non_grazing conditions. The grazing (free grazing outside of fence) and non_grazing (fenced) experiment was conducted for 6 years. The nutrient efficiency ratios of the main herbages were evaluated in this study. The experiment was conducted in the Leymus chinensis grassland of the Inner Mongolia Grassland Ecosystem Research Station located at 43°26′_44°08′ N, 116°04′_117°05′ E with elevation about 1 000 m above sea level. The results showed that aboveground biomass and its proportion in the community increased with annual mean temperature, and the regression equation between its aboveground biomass and mean temperature ( t ) and total rainfall ( r ) from April to August was: y=-12.451 1+0.018 7r+0.060 1t 2 ( R 2=0.533,p = 0.003). Generally, grazing significantly reduced plant height and aboveground biomass per bunchgrass, whereas its coverage, density and total aboveground biomass per square meter increased under grazing. Its nutrient efficiency ratios, such as N and S, were higher compared with other dominant species, such as L. chinensis . 100 g C produced required 3.17 g N and 0.31 g S for C. squarrosa , while L. chinensis population requires 4.24 g N and 0.41 g S under grazing. The high nutrient efficiency ratios maybe benefit C. squarrosa populations under competition with other species in the infertile soil.