气候变化和放牧活动对糙隐子草种群的影响

 糙隐子草(Cleistogenes squarrosa)是一种多年生丛生小禾草，属C4植物，在研究气候变化和放牧活动对草原C4植物的影响方面是一种很好的指示植物。本文探讨了在气候变化及放牧干扰条件下糙隐子草种群地上生物量、种群特征和资源利用方式等方面的变化，分析了C4植物对气候变化的响应和在放牧干扰下的生态对策。结果表明：在气候温暖化的情况下，糙隐子草种群生物量及其在群落中的比例都有所上升，放牧大大加快了这一过程。通过逐步回归分析得出糙隐子草种群最大地上生物量与生长季（4～8月）总降雨量(r)和平均温度(t)的关系为：y=-12.451 1+0.018 7r+0.060 1t2 (p=0.003)。总体上，放牧明显降低了其种群高度和每丛生物量，但显著提高了其密度、盖度和地上生物量；密度的变化受气候和放牧互作的影响。如果以植物光合所固定的100 g的碳（C）所需要的其它大量营养元素来衡量植物的养分利用效率，则可以发现，与其它主要植物如羊草（Leymus chinensis）相比，糙隐子草对氮(N)和硫（S）的需求量最少，利用3.17 g的N和0.31 g的S就能生产100 g的C，而羊草则需要4.24 g的N和0.41 g的S才能生产100 g的C。 说明在放牧使土壤比较贫瘠的条件下，糙隐子草的养分利用效率较高，这可能是其在土壤比较贫瘠的退化植物群落竞争中处于优势地位的又一重要原因。

EFFECT OF CLIMATE CHANGE AND GRAZING ON POPULATIONS OF CLEISTOGENES SQUARROSA IN INNER MONGOLIA STEPPE

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.