草地植物生产力主要影响因素研究综述

王常顺，孟凡栋，李新娥，姜丽丽，汪诗平,*

(1. 中国科学院青藏高原研究所，北京 100101; 2. 中国科学院大学，北京 100094)

草地是全球分布面积最大的陆地生态系统，约占陆地面积的三分之一[1]。草地具有重要的生态和社会功能[2]，为人类提供了许多产品[3]和生态服务[4]，其中，植物初级生产力是反映草地功能的重要指标[5-6]。许多研究表明，影响植物生产力的因素很多，在较大的地理尺度上气候等环境因子(如气温、降水和土壤类型等)是决定植物生产力的关键因子[7-11]，而在较小的地理单元上，生物和资源有效性等可能是植物生产力大小的主导因子[12]。随着环境条件的改善，生产力逐渐增加[8]。但是不同的生态系统在不同的时期，其生产力对于环境变化的响应程度有很大的差异[9]。这些可以归结于生物与环境相互作用模式的差异[10]。这可能是目前众多研究结论不一致甚至相左的原因[11]。特别是影响草地植物生产力的众多因素相互交织在一起，共同影响着生产力水平，而目前的大多数研究主要集中在某一个单一的因子上，因此对于这些众多因子是否存在互作效应或者是否存在可加性缺乏深入的研究。本文通过对影响草地植物生产力的主要因素或过程的有关研究进展进行扼要综述，在此基础上提出存在的可能问题和建议，希望对我国相关研究提供一些有益的借鉴和参考。

1 植物多样性对生产力的影响

自20世纪70年代以来，国内外已经开展了大量有关植物多样性与植物生产力之间关系的研究，到目前为止仍然没有得出一致性的结论[13-14]。尽管一些研究支持了高植物多样性导致了高生产力的假设[6,14-19]，但更多研究似乎发现两者呈“单峰型”关系[20]，也有研究认为没有显著关系[20-22]或者认为“单峰型”不是主要关系[13]。许多研究认为多样性提高生产力的基本假设是因为不同植物对资源利用的生态位补偿效应引起的，特别认为植物根系深度多样性对生产力的影响更大[18]。这些综述性文章对产生这些不同结果的原因进行了分析，主要包括：(1)用于研究草地植物多样性-生产力关系的许多试验都是破坏性的(如起始状态为人为翻耕后的裸地)或人为地控制试验(如人种播种控制物种数或均匀度等)，而不是在自然状态下进行的。事实上，这些实验所用物种数有限、且隐含了一种演替过程，由此得出的植物多样性-生产力间的关系会随时间而变化[23]；(2)研究的空间尺度不同造成的[24-25]，在小尺度上(如某一个地点或单个试验)的结果往往与大尺度的结果不尽相同，这也是许多发现两者间有正相关、负相关或没有关系的原因，其实决定生产力的因素很多，不完是多样性决定的[22,26]；(3)在过去的许多综述性文章中，忽略了一些具体的观测方法或指标(如用盖度、NDVI等生产力的代用指标)，包含了一些无效的数据，所以得出来的结果有很大偏差[27]。普遍认为在生产力相对较低时，多样性与生产力正相关；而在生产力相对较高时，多样性与生产力负相关[13]。对于负相关阶段的机理目前还没有明确的定论，引起的争议较多，包括干扰、竞争和资源等假说[13]。因此，目前有关对植物多样性-生产力关系的机理更多地停留在一些假设上(如资源比率变化以及生态位补偿假设等)，而缺乏一些直接的试验证据；特别是缺乏在自然状态下对不同草地生态系统和演替阶段过程中两者关系变化的机理研究，如根系的空间分布及其多样性如何影响生产力水平还知之甚少，严重制约了人们对通过植物多样性保护进而维持和提高植物生产力水平的理解和应用。

2 资源有效性对植物生产力的影响

一些经验证据表明，资源有效性控制着植物多样性和生产力水平[22]，因此，可以通过调控资源有效性达到提高多样性与生产力的目的[10,28-31]。由于植物在不同资源的利用能力方面存在“折衷”，因此提高一种限制性资源的供应水平可能会降低植物对该资源的利用效率，但会改善植物对另一资源的利用效率及植物生产力[32]。在干旱半干旱草地，水分是植物生长的主要限制因子，只有在湿润年份施肥(N)才能有效提高植物生产力[10,33-34]，而且N的利用效率随着灌溉增加而增加、但随着N施肥量增加而降低[34]。水分利用效率与初级生产力和生物量分配紧密相关[35]。研究表明，水分利用效率随降雨量增加而降低，但随土壤N有效性增加而增加[10,34,36]，在养分等胁迫消除以后植物水分利用效率将达到最大值[35]。因此，对于干旱半干旱草地而言，水分和土壤N的有效性对初级生产力形成存在交互作用[34,36-37]。资源供应状况改变对生产力和多样性的影响不同，养分添加提高了草地的生产力，但降低了植物多样性[10,31,34]。与添加某一种限制性的资源相比(如N)[10]，添加多种限制性资源(如N和P)使得植物多样性的丧失更多[31,38]，有人认为N的作用通常比P的作用更大[31,39]或一样大[40]。总之，最大限制性资源水平的提高会促进生产上力的发展，其它限制性资源水平的提高是否对生产力有促进作用取决于与最大限制性资源的交互作用。因此，对于天然草地而言，能否通过限制性资源的添加特别是平衡施肥达到同时维持较高的植物多样性和生产力水平，亟待深入研究。

3 放牧对生产力的影响

放牧通过选择性的采食、践踏和粪尿归还等过程而对草地产生了综合性的影响，包括改变了植物生产力及影响生产力形成过程中的诸多因子，如植物种类组成与多样性、土壤C/N库大小及土壤养分(如N)的有效性等，特别是过度放牧对上述各个方面几乎都产生了显著的负面效应[33,41-45]。不同放牧强度对土壤N有效性的影响随生长季节而变化。总体上，在生长早期中度以上的放牧提高了土壤N的矿化速率，但生长盛期和非生长季都降低了土壤N的矿化速率，因此不利于植物生产[46]。然而，适度放牧提高了土壤N可利用性[47]，则有利于维持草地植物多样性和生产力水平[41-42,48-51]，甚至产生补偿性生产[42,45]。有研究表明放牧提高了草地的异质性，从而增加植物多样性[52-53]，特别是不同放牧家畜的混牧有利于草地植物多样性的维持[49,54]。最近的许多研究表明，就生产力、牧草品质、凋落物量、抗旱性以及可恢复性等方面而言，当放牧强度超过中度以上都会造成内蒙古典型草原这些生产-生态指标的下降[45,55-58]。因此，与禁牧相比，适度放牧既可以获得畜产品生产，又可以维持甚至提高草地的生态功能，包括提高生产力。因此，如何确定不同草地生态系统适宜的放牧率水平仍将是放牧生态学的关键科学问题，这里，生产力应该是主要指标但不应该是唯一指标，必须考虑生态系统的其他功能，特别是生物多样性的维持和土壤质量的提高等方面。

4 退化草地恢复对植物生产力的影响

在生态系统恢复过程中，自然状态下净初级生产是提高土壤有机质累积的主要来源[59-60]，而土壤有机质增加又会改变其植物生产性能，即植物生产-土壤性质之间存在反馈作用[61-62]，在退化草地恢复过程中这种植物与土壤资源有效性的互作效应对植物生产力的变化起着关键作用[59,62-63]。一旦从退化状态开始恢复，土壤的C、N储量便从低水平的状态向高水平的状态演替并最终达到当地生态系统顶级状态的稳定水平[59,64-65]。在此过程中，随着根系生物量、根系中的C/N比、微生物C量以及土壤碳库的增加，土壤净N矿化速率和N的生物有效性快速下降[66]，植物-土壤的互作表现为负反馈，从而反过来限制了植物生产力的进一步提高[59]。Baer等[63]的研究证实，在退化草地的8a恢复过程中，施N肥显著提高了生产力，同时该研究也看到生态系统的其它功能都没有受到施N肥的影响。在内蒙古退化草场恢复过程中，由于物种组成的变化，施N肥仅在湿润年份提高了地上初级生产力，但对地下生产力没有显著影响，因而即使施用N肥，土壤C、N库也没有能够快速恢复到先前的水平[33]。草地在恢复初期生产力逐渐提高，在恢复的中后期达到最大。如果没有干扰和放牧，恢复后期生产力会下降。因此，如何利用恢复过程中植物-土壤的反馈关系，通过针对性的干扰措施来设计和加速恢复进程，是未来恢复生态学的重点研究内容之一。

5 气候变化对植物生产力的影响

尽管许多研究表明，增温直接促进了冻原植物生长和物种组成的变化、延长了植物生长季[58,67-69]、以及间接增加了土壤N有效性[29,70]，但到目前为止，增温对土壤N的有效性和植物生产力的影响仍然没有一致的结论，因地点和生态系统的不同而异[67-68,71-75]。总体而言，增温对地下生产力的影响大于对地上生产力的影响，显著提高了地上和地下总生产力[75]。有研究表明，只有在土壤N有效性和水分不是制约因子时，增温才提高了植物生产力[74-75]。放牧作为天然草地的主要利用方式之一，在放牧条件下，放牧与增温的互作效应对草地植物生产力、物种组成和土壤养分有效性产生了显著影响，放牧甚至改变了群落组成对增温的反应模式[76-78]。由于土壤N的有效性对生产力的影响随物种组成变化而不同[31,33,63]，因此，在放牧条件下，未来增温对不同草地植物多样性和生产力等生态过程的影响仍然存在很大的不确定性，目前这方面的研究很少。Klein等[76,79]利用OTC试验研究表明，增温而不是刈割降低了植物多样性和生产力，特别是降低了禾草的比例、增加了阔叶草的比例，因此，刈割可以缓解增温对高寒草甸的负面影响。然而，通过6a红外增温和放牧试验研究表明，增温和放牧都提高了凋落物和粪便的分解速率[47,66]，增加了土壤水溶液中DOC的含量[66]。与Klein[76,79]等的研究结果不同，发现增温对植物多样性影响不大，但提高了高寒草甸植物地上生产力达40%左右，但放牧降低了地上生产力对增温的反应程度[78]。特别是发现了是过度放牧而不是增温导致了高寒草甸的退化，因为增温提高了禾草和豆科牧草的比例[78]。因此，放牧与增温对不同草地生态系统的各自影响及其可能的互作效应的程度及其过程，将是未来气候变化生态学的关键研究内容之一，也是认识气候变化对草地生态系统的影响、发展适应性的管理措施的核心内容之一。

6 存在的科学和实践问题

综上所述，尽管国内外对影响草地植物生产力的主要影响因素进行了深入研究，但与国外其它草地分布区相比，国内这方面的研究不仅在研究数量上明显不足，更重要是欠缺机理上的深入研究。因此，在放牧和未来气候变化背景下如何维持和提高草地生产力，如何加速退化草地生态系统的恢复，进而实现生态安全建设和经济社会协调发展，是当前急需解决的理论和实践问题。为此，提出草地植物生产力研究存在以下一些科学问题及其基本假设(图1)。

图1 草地生态系统植物多样性与生产力之间的关系对适度放牧和资源有效性变化的响应示意图

1)资源有效性-植物多样性-生产力三者间的关系

假设在低资源有效性下植物多样性对生产力的维持有正效应；然而，由于不同植物对资源有效性改变的反应不同，根据现有多数研究发现，自然状态下(不放牧)资源有效性的提高将降低植物多样性，但提高了生产力，由此可推论在资源有效性高的状况下，植物多样性与生产力呈负相关；由于植物多样性降低，高资源有效性不能长期维持高生产力，从而对其它生态功能产生负面影响。

2)适度放牧-植物多样性-生产力三者间的关系

根据已有多数研究发现，在自然状态下(即资源有效性低的情况下)，适度放牧提高了植物多样性和生产力；然而，当资源有效性提高时，由于适度放牧部分抑制了对资源获取能力强的植物生长(容易被采食)，从而能继续维持或提高植物多样性和生产力。因此，在提高资源有效性的同时进行适度放牧，仍然可以同时维持较高的植物多样性和生产力水平。

因此，低水平生产力的维持(如靠现有的植物多样性水平就可以维持)和提高(如提高资源有效性等)可能相对比较容易，而怎样长期维持高水平的生产力、同时又不损害其它生态功能，则比较困难。

:

[1]White R P, Murray S, Rohweder M, Prince S D. Pilot Analysis of Global Ecosystems (PAGE): Grassland Ecosystems. Washington DC: World Resources Institute, 2000: 1-12.

[2]Wrage N, Strodthoff J, Cuchillo H M, Isselstein J, Kayser M. Phytodiversity of temperate permanent grasslands: ecosystem services for agriculture and livestock management for diversity conservation. Biodiversity and Conservation, 2011, 20(14): 3317-3339.

[3]O′Mara F P. The role of grasslands in food security and climate change. Annals of Botany, 2012, 110(6): 1263-1270.

[4]Huyghe C. Multi-function grasslands in France: I. Production functions. Cahiers Agricultures, 2008, 17(5): 427-435.

[5]Loreau M, Naeem S, Inchausti P, Bengtsson J, Grime J P, Hector A, Hooper D U, Huston M A, Raffaelli D, Schmid B, Tilman D, Wardle D A. Biodiversity and ecosystem functioning: Current knowledge and future challenges. Science, 2001, 294(5543): 804-808.

[6]Hooper D U, Chapin F S III, Ewel J J, Hector A, Inchausti P, Lavorel S, Lawton J H, Lodge D M, Loreau M, Naeem S, Schmid B, Setälä H, Symstad A J, Vandermeer J, Wardle D A. Effects of biodiversity on ecosystem functioning: A consensus of current knowledge. Ecological Monographs, 2005, 75(1): 3-35.

[7]Knapp A K, Smith M D. Variation among biomes in temporal dynamics of aboveground primary production. Science, 2001, 291(5503): 481-484.

[8]Fang J Y, Yu S Y, Wu P C, Huang Y B, Tsai Y H. In vitro skin permeation of estradiol from various proniosome formulations. International Journal of Pharmaceutics, 2001, 215(1/2): 91-99.

[9]Swemmer A M, Knapp A K. Defoliation synchronizes aboveground growth of co-occurring C4grass species. Ecology, 2008, 89(10): 2860-2867.

[10]Bai Y F, Wu J G, Qi X, Pan Q M, Huang J H, Yang D L, Han X G. Primary production and rain use efficiency across a precipitation gradient on the Mongolia plateau. Ecology, 2008, 89(8): 2140-2153.

[11]Yang Y H, Fang J Y, Ma W H, Wang W. Relationship between variability in aboveground net primary production and precipitation in global grasslands. Geophysical Research Letters, 2008, 35(23): 23710-23720.

[12]Yahdjian L, Sala O E. Vegetation structure constrains primary production response to water availability in the Patagonian steppe. Ecology, 2006, 87(4): 952-962.

[13]Adler P B, Seabloom E W, Borer E T, Hillebrand H, Hautier Y, Hector A, Harpole W S, O′Halloran L R, Grace J B, Anderson T M, Bakker J D, Biederman L A, Brown C S, Buckley Y M, Calabrese L B, Chu C J, Cleland E E, Collins S L, Cottingham K L, Crawley M J, Damschen E I, Davies K F, DeCrappeo N M, Fay P A, Firn J, Frater P, Gasarch E I, Gruner D S, Hagenah N, Hille Ris Lambers J, Humphries H, Jin V L, Kay A D, Kirkman K P, Klein J A, Knops J M H, La Pierre K J, Lambrinos J G, Li W, MacDougall A S, McCulley R L, Melbourne B A, Mitchell C E, Moore J L, Morgan J W, Mortensen B, Orrock J L, Prober S M, Pyke D A, Risch A C, Schuetz M, Smith M D, Stevens C J, Sullivan L L, Wang G, Wragg P D, Wright J P, Yang L H. Productivity is a poor predictor of plant species richness. Science, 2011, 333(6050): 1750-1753.

[14]Maestre F T, Quero J L, Gotelli N J, Escudero A, Ochoa V, Delgado-Baquerizo M, Garcia-Gomez M, Bowker M A, Soliveres S, Escolar C, Garcia-Palacios P, Berdugo M, Valencia E, Gozalo B, Gallardo A, Aguilera L, Arredondo T, Blones J, Boeken B, Bran D, Conceicao A A, Cabrera O, Chaieb M, Derak M, Eldridge D J, Espinosa C I, Florentino A, Gaitan J, Gatica M G, Ghiloufi W, Gomez-Gonzalez S, Gutierrez J R, Hernandez R M, Huang X, Huber-Sannwald E, Jankju M, Miriti M, Monerris J, Mau R L, Morici E, Naseri K, Ospina A, Polo V, Prina A, Pucheta E, Ramirez-Collantes D A, Romao R, Tighe M, Torres-Diaz C, Val J, Veiga J P, Wang D, Zaady E. Plant species richness and ecosystem multifunctionality in global drylands. Science, 2012, 335(6065): 214-218.

[15]Foster B L, Dickson T L. Grassland diversity and productivity: The interplay of resource availability and propagule pools. Ecology, 2004, 85(6): 1541-1547.

[16]Tilman D, Reich P B, Knops J M H. Biodiversity and ecosystem stability in a decade-long grassland experiment. Nature, 2006, 441(7093): 629-632.

[17]Gillman L N, Wright S D. The influence of productivity on the species richness of plants: A critical assessment. Ecology, 2006, 87(5): 1234-1243.

[18]Weigelt A, Schumacher J, Roscher C, Schmid B. Does biodiversity increase spatial stability in plant community biomass?. Ecology Letters, 2008, 11(4): 338-347.

[19]Hector A, Hautier Y, Saner P, Wacker L, Bagchi R, Joshi J, Scherer-Lorenzen M, Spehn E M, Bazeley-White E, Weilenmann M, Caldeira M C, Dimitrakopoulos P G, Finn J A, Huss-Danell K, Jumpponen A, Mulder C P H, Palmborg C, Pereira J S, Siamantziouras A S D, Terry A C, Troumbis A Y, Schmid B, Loreau M. General stabilizing effects of plant diversity on grassland productivity through population asynchrony and overyielding. Ecology, 2010, 91(8): 2213-2220.

[20]Mittelbach G G, Steiner C F, Scheiner S M, Gross K L, Reynolds H L, Waide R B, Willig M R, Dodson S I, Gough L. What is the observed relationship between species richness and productivity?. Ecology, 2001, 82(9): 2381-2396.

[21]Hector A, Schmid B, Beierkuhnlein C, Caldeira M C, Diemer M, Dimitrakopoulos P G, Finn J A, Freitas H, Giller P S, Good J, Harris R, Högberg P, Huss-Danell K, Joshi J, Jumpponen A, Körner C, Leadley P W, Loreau M, Minns A, Mulder C P H, O′Donovan G, Otway S J, Pereira J S, Prinz A, Read D J, Scherer-Lorenzen M, Schulze E D, Siamantziouras A S D, Spehn E M, Terry A C, Troumbis A Y, Woodward F I, Yachi S, Lawton J H. Plant diversity and productivity experiments in European grasslands. Science, 1999, 286(5442): 1123-1127.

[22]Fridley J D. Resource availability dominates and alters the relationship between species diversity and ecosystem productivity in experimental plant communities. Oecologia, 2002, 132(2): 271-277.

[23]Guo Q F, Shaffer T, Buhl T. Community maturity, species saturation and the variant diversity-productivity relationships in grasslands. Ecology Letters, 2006, 9(12): 1284-1292.

[24]Davidson A, Csillag F, Wilmshurst J. Diversity-productivity relations at a northern prairie site: An investigation using spectral data. Community Ecology, 2007, 8(1): 87-102.

[25]Venail P A, Maclean R C, Meynard C N, Mouquet N. Dispersal scales up the biodiversity-productivity relationship in an experimental source-sink metacommunity. Proceedings of the Royal Society B-Biological Sciences, 2010, 277(1692): 2339-2345.

[26]Bradford J B, Lauenroth W K, Burke I C, Paruelo J M. The influence of climate, soils, weather, and land use on primary production and biomass seasonality in the US Great Plains. Ecosystems, 2006, 9(6): 934-950.

[27]Garbulsky M F, Peuelas J, Gamon J, Inoue Y, Filella I. The photochemical reflectance index (PRI) and the remote sensing of leaf, canopy and ecosystem radiation use efficiencies: a review and meta-analysis. Remote Sensing of Environment, 2011, 115(2): 281-297.

[28]Harpole W S, Tilman D. Grassland species loss resulting from reduced niche dimension. Nature, 2007, 446(7137): 791-793.

[29]LeBauer D S, Treseder K K. Nitrogen limitation of net primary productivity in terrestrial ecosystems is globally distributed. Ecology, 2008, 89(2): 371-379.

[30]Gao Y Z, Chen Q, Lin S, Giese M, Brueck H. Resource manipulation effects on net primary production, biomass allocation and rain-use efficiency of two semiarid grassland sites in Inner Mongolia, China. Oecologia, 2011, 165(4): 855-864.

[31]Ren Z W, Qi L, Chu C J, Zhao L Q, Zhang J Q, Dexiecuo A, Yang Y B, Wang G. Effects of resource additions on species richness and ANPP in an alpine meadow community. Journal of Plant Ecology, 2011, 3(1): 25-31.

[32]LeBauer D S, Treseder K K. Nitrogen limitation of net primary productivity in terrestrial ecosystems is globally distributed. Ecology, 2008, 89(2): 371-379.

[33]Chen Q, Hooper D U, Shan L. Shifts in species composition constrain restoration of overgrazed grassland using nitrogen fertilization in Inner Mongolian Steppe, China. PLoS One, 2011, 6(3): e16909, doi: 10.1371/journal.pone.0016909.

[34]Li J Z, Shan L, Friedhelm T, Pan Q M, Klaus D. Above and belowground net primary productivity of grassland influenced by supplemental water and nitrogen in Inner Mongolia. Plant and Soil, 2011, 340(1/2): 253-264.

[35]Huxman T E, Smith M D, Fay P A, Knapp A K, Shaw M R, Loik M E, Smith S D, Tissue D T, Zak J C, Weltzin J F, Pockman W T, Sala O E, Haddad B M, Harte J, Koch G W, Schwinning S, Small E E, Williams D G. Convergence across biomes to a common rain-use efficiency. Nature, 2004, 429(6992): 651-654.

[36]Brueck H, Erdle K, Gao Y Z, Giese M, Zhao Y, Peth S, Lin S. Effects of N and water supply on water use-efficiency of a semiarid grassland in Inner Mongolia. Plant and Soil, 2010, 328(1/2): 495-505.

[37]Bell C, McIntyre N, Cox S, Tissue D, Zak J. Soil microbial responses to temporal variations of moisture and temperature in a Chihuahuan desert grassland. Microbial Ecology, 2008, 56(1): 153-167.

[38]Niinemets Ü, Kull K. Co-limitation of plant primary productivity by nitrogen and phosphorus in a species-rich wooded meadow on calcareous soils. Acta Oecologica, 2005, 28(3): 345-356.

[39]Fynn R W S, O′Connor T G. Determinants of community organization of a South African mesic grassland. Journal of Vegetation Science, 2005, 16(1): 93-102.

[40]Elser J J, Bracken M E S, Cleland E E, Gruner D S, Harpole W S, Hillebrand H, Ngai J T, Seabloom E W, Shurin J B, Smith J E. Global analysis of nitrogen and phosphorus limitation of primary producers in freshwater, marine and terrestrial ecosystems. Ecology Letters, 2007, 10(12): 1135-1142.

[41]Milchunas D G, Lauenroth W K. Quantitative effects of grazing on vegetation and soils over a global range of environments. Ecological Monographs, 1993, 63(4): 327-366.

[42]Wang S P, Wang Y F, Hu Z Y, Chen Z Z, Fleckenstein J, Schnug E. Status of iron, manganese, copper, and zinc of soils and plants and their requirement for ruminants in inner Mongolia steppes of China. Communications in Soil Science and Plant Analysis, 2003, 34(5): 655-670.

[43]Hwang B C, Lauenroth W K. Effect of nitrogen, water and neighbor density on the growth ofHesperismatronalisand two native perennials. Biological Invasions, 2008, 10(5): 771-779.

[44]Sasaki T, Okayasu T, Jamsran U, Takeuchi K. Threshold changes in vegetation along a grazing gradient in Mongolian rangelands. Journal of Ecology, 2008, 96(1): 145-154.

[45]Schönbach P, Wan H W, Martin G, Bai Y F, Müller K, Lin L J, Susenbeth A, Taube F. Grassland responses to grazing: effects of grazing intensity and management system in an Inner Mongolian steppe ecosystem. Plant and Soil, 2011, 340(1/2): 103-115.

[46]Shan Y M, Chen D M, Guan X X, Zheng S X, Chen H J, Wang M J, Bai Y F. Seasonally dependent impacts of grazing on soil nitrogen mineralization and linkages to ecosystem functioning in Inner Mongolia grassland. Soil Biology and Biochemistry, 2011, 43(9): 1943-1954.

[47]Xu Y Q, Li L H, Wang Q B, Chen Q S, Cheng W X. The pattern between nitrogen mineralization and grazing intensities in an Inner Mongolian typical steppe. Plant and Soil, 2007, 300(1/2): 289-300.

[48]Altesor A, Oesterheld M, Leoni E, Lezama F, Rodriguez C. Effect of grazing on community structure and productivity of a Uruguayan grassland. Plant Ecology, 2005, 179(1): 83-91.

[49]Bakker C, Van Bodegom P M, Nelissen H J M, Ernst W H O, Aerts R. Plant responses to rising water tables and nutrient management in calcareous dune slacks. Plant Ecology, 2006, 185(1): 19-28.

[50]Klimek S, Richter gen Kemmermann A, Hofmann M, Isselstein J. Plant species richness and composition in managed grasslands: The relative importance of field management and environmental factors. Biological Conservation, 2007, 134(4): 559-570.

[52]Questad E J, Foster B L. Coexistence through spatio-temporal heterogeneity and species sorting in grassland plant communities. Ecology Letters, 2008, 11(7): 717-726.

[53]Marion B, Bonis A, Bouzillé J B. How much does grazing-induced heterogeneity impact plant diversity in wet grasslands?. Ecoscience, 2010, 17(3): 229-239.

[54]Sebastià M T, de Bello F, Puig L, Taull M. Grazing as a factor structuring grasslands in the Pyrenees. Applied Vegetation Science, 2008, 11(2): 215-223.

[55]Liu Y S, Pan Q M, Liu H D, Bai Y F, Simmons M, Dittert K, Han X G. Plant responses following grazing removal at different stocking rates in an Inner Mongolia grassland ecosystem. Plant and Soil, 2011, 340(1/2): 199-213.

[56]Schönbach P, Wan H W, Schiborra A, Gierus M, Bai Y F, Müller K, Glindemann T, Wang C J, Susenbeth A, Taube F. Short-term management and stocking rate effects of grazing sheep on herbage quality and productivity of Inner Mongolia steppe. Crop and Pasture Science, 2009, 60(10): 963-974.

[57]Wan H W, Bai Y F, Schönbach P, Gierus M, Taube F. Effects of grazing management system on plant community structure and functioning in a semiarid steppe: scaling from species to community. Plant and Soil, 2011, 340(1/2): 215-226.

[58]Wu Z T, Dijkstra P, Koch G W, Peuelas J, Hungate B A. Responses of terrestrial ecosystems to temperature and precipitation change: a meta-analysis of experimental manipulation. Global Change Biology, 2011, 17(2): 927-942.

[59]Baer S G, Kitchen D J, Blair J M, Rice C W. Changes in ecosystem structure and function along a Chronosequence of restored grasslands. Ecological Applications, 2002, 12(6): 1688-1701.

[60]Baer S G, Blair J M, Collins S L, Knapp A K. Soil resources regulate productivity and diversity in newly established Tallgrass prairie. Ecology, 2003, 84(3): 724-735.

[61]Kardol P, Bezemer T M, van der Putten W H. Temporal variation in plant-soil feedback controls succession. Ecology Letters, 2006, 9(9): 1080-1088.

[62]Kulmatiski A, Beard K H, Stevens J R, Cobbold S M. Plant-soil feedbacks: a meta-analytical review. Ecology Letters, 2008, 11(9): 980-992.

[63]Baer S G, Blair J M. Grassland establishment under varying resource availability: A test of positive and negative feedback. Ecology, 2008, 89(7): 1859-1871.

[64]McLauchlan K K. The nature and longevity of agricultural impacts on soil carbon and nutrients: a review. Ecosystems, 2006, 9(8): 1364-1382.

[65]McLauchlan K K, Hobbie S E, Post W M. Conversion from agriculture to grassland builds soil organic matter on decadal timescales. Ecological Applications, 2006, 16(1): 143-153.

[66]Luo C Y, Xu G P, Wang Y F, Wang S P, Lin X W, Hu Y G, Zhang Z H, Chang X F, Duan J C, Su A L, Zhao X Q. Effects of grazing and experimental warming on DOC concentrations in the soil solution on the Qinghai-Tibet Plateau. Soil Biology and Biochemistry, 2009, 41(12): 2493-2500.

[67]Arft A M, Walker M D, Gurevitch J, Alatalo J M, Bret-Harte M S, Dale M, Diemer M, Gugerli F, Henry G H R, Jones M H, Hollister R D, Jónsdóttir I S, Laine K, Lévesque E, Marion G M, Molau U, Mølgaard P, Nordenhäll U, Raszhivin V, Robinson C H, Starr G, Stenström A, Stenström M, Totland Ø, Turner P L, Walker L J, Webber P J, Welker J M, Wookey P A. Responses of tundra plants to experimental warming: Meta-analysis of the international tundra experiment. Ecological Monographs, 1999, 69(4): 491-511.

[68]Walker R D, Pastor J, Dewey B W. Effects of wild rice (Zizaniapalustris) straw on biomass and seed production in northern Minnesota. Canadian Journal of Botany, 2006, 84(6): 1019-1024.

[69]Aerts R, Cornelissen J H C, Dorrepaal E. Plant performance in a warmer world: General responses of plants from cold, northern biomes and the importance of winter and spring events. Plants and Climate Change, 2006, 41: 65-78.

[70]Dormann C F, Woodin S J. Climate change in the Arctic: Using plant functional types in a meta-analysis of field experiments. Functional Ecology, 2002, 16(1): 4-17.

[71]Rustad L. Global change-matter of time on the Prairie. Nature, 2001, 413(6856): 578-579.

[72]Hovenden M J, Newton P C D, Wills K E, Janes J K, Williams A L, Vander Schoor J K, Nolan M J. Influence of warming on soil water potential controls seedling mortality in perennial but not annual species in a temperate grassland. New Phytologist, 2008, 180(1): 143-152.

[73]Hudson J M G, Henry G H R. Increased plant biomass in a High Arctic heath community from 1981 to 2008. Ecology, 2009, 90(10): 2657-2663.

[74]Natali S M, Schuur E A G, Rubin R L. Increased plant productivity in Alaskan tundra as a result of experimental warming of soil and permafrost. Journal of Ecology, 2012, 100(2): 488-498.

[75]Wu H H, Dannenmann M, Fanselow N, Wolf B, Yao Z S, Wu X, Brüggemann N, Zheng X H, Han X G, Dittert K, Butterbach-Bahl K. Feedback of grazing on gross rates of N mineralization and inorganic N partitioning in steppe soils of Inner Mongolia. Plant and Soil, 2011, 340(1/2): 127-139.

[76]Klein J A, Harte J, Zhao X Q. Experimental warming, not grazing, decreases rangeland quality on the Tibetan plateau. Ecology Application, 2007, 17(2): 541-557.

[77]Post E, Pedersen C. Opposing plant community responses to warming with and without herbivores. Proceedings of the National Academy of Sciences of the United States of America, 2008, 105(34): 12353-12358.

[78]Wang S P, Duan J C, Xu G P, Wang Y F, Zhang Z H, Rui Y C, Luo C Y, Xu B, Zhu X X, Chang X F, Cui X Y, Niu H S, Zhao X Q, Wang W Y. Effects of warming and grazing on soil N availability, species composition, and ANPP in an alpine meadow. Ecology, 2012, 93(11): 2365-2376.

[79]Klein J A, Harte J, Zhao X Q. Decline in medicinal and forage species with warming is mediated by plant traits on the Tibetan plateau. Ecosystems, 2008, 11(5): 775-789.