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 C4 (warm-season) vs. C3 (cool-season)

photosynthesis. The northern China steppes have few C4 plants. Little experimental

work is being done in the field to examine the ecological impacts of climate change

and grazing and their interactions on C4 plants in this region.

Cleistogenes squarrosa is a C4 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 C4 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 1t2 (R2=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.