中国科学院新疆生态与地理研究所机构知识库

竞争、抑制等种间负相互作用被认为是群落结构与功能的决定性因素， 但近期的实验生态学表明，胁迫环境下种间正相互作用对群落同样具有重要意义。 在干旱、 半干旱地区， 灌木对草本植物的“保护效应”正是种间正相互作用的典型,即灌木可通过改善生境有利于冠下植物的存活与定居。本项研究以古尔班通古特沙漠南缘为研究区域，以梭梭(Haloxylon ammodendron)对冠下草本植物的“保护效应”为切入点， 通过野外对不同生活型植物分布的调查、 不同微生境草本植物在不同生活史阶段的观测、梭梭“保护效应”发生过程的分解， 研究梭梭与不同生活型植物的空间关联性， 不同生活史阶段冠下与灌丛间地草本植物的特征，及梭梭“保护效应”作用机理， 研究结果对促进理解胁迫环境下种间关系对群落结构与动态的意义。具体结果如下：梭梭群落内不同生活型物种空间分布类型在总体上呈现出： 小尺度范围内的聚集分布，随着尺度的增大， 空间分布趋向于随机分布, 在更大尺度上表现为均匀分布。梭梭与不同生活型植物在小尺度上呈现一定的相关性：梭梭与一年生草本呈现出正相关；小梭梭与灌木状草本呈现负相关，但随着梭梭形态的增大，与灌木状草本呈现出正相关；梭梭与短命植物在任何尺度上无相关性或相关性较弱。梭梭与不同生活型植物的空间关联性表明梭梭对冠下植物的“保护效应”主要作用在小尺度范围内， “保护效应”强度与植物生活型有关，受不同大小梭梭的影响。梭梭“保护效应”可因环境变化而迅速改变， 在植物不同生活史阶段作用效果不同。 土壤种子库和地上幼苗库中均以一年生草本植物和短命植物占绝对优势。土壤种子库密度从梭梭冠下经冠缘至灌丛间地逐渐减小，梭梭灌丛对植物种子具有明显的聚集效应（P < 0.05），但这种聚集效应在沙丘顶部不明显。梭梭冠缘处自然条件下相对萌发率最高，冠下和灌丛间地则差异不明显， 表明灌丛覆盖对种子萌发的影响在灌丛中心至灌丛间地这一小尺度上具有分异， 冠缘处植物从梭梭“保护效应”中获益最大。 土壤种子库和幼苗库的空间格局表明梭梭通过对种子的聚集效应及对种子库萌发成苗的促进作用，对其冠下植物在萌发期这一生活史早期阶段具有明显的“保护效应”。 在草本植物生长期， 灌丛间地沙漠绢蒿（Seriphidiam santolinum） 和刺毛碱蓬（Suaeda acuminate）对光强的可利用范围显著大于冠下（P < 0.05），表明灌丛间地沙漠绢蒿和刺毛碱蓬可通过降低光补偿点、增大光饱和点来提高对弱光和强光的利用能力。 沙漠绢蒿和刺毛碱蓬冠下光和速率显著小于灌丛间地（P < 0.05），表明梭梭对冠下草本植物的“保护效应”表现为中性或者负相互作用。在草本植物生长末期， 冠下生物量随形态增加增大，大梭梭冠下生物量最大，与中梭梭无显著差异，但显著高于小梭梭（P < 0.05），且所有梭梭冠下草本生物量均显著高于灌丛间地（P < 0.05）。较高的生物量可能与冠下较多植物成活定居有关。梭梭”保护效应”可通过多种机制同时作用于冠下草本植物， 不同作用机理会导致不同的作用结果： “肥岛效应”可对冠下一年生草本生长产生正效应。而遮阴效应对冠下一年生草本密度、生物量以及个体生物量的影响是中性的。 “灌木”下一年生草本密度显著高于其他样地（P < 0.05），表明所有机制协同作用对一年生草本成活定居影响显著。较高的密度使地上草本层生长受到空间限制，相互竞争资源，因此“灌木”下个体生物量较小。

Negative interaction was thought as the key factor of community structure andfunction. But recent experimental ecology shows that facilitation among plant speciesalso has an important significance to the community at stress environments. In aridand semi-arid regions, there is a typical concept of facilitation is nurse effect. Theseare shrubs can improve habitat environment for plant survival and settled under crown.To learn the nurse effect of Haloxylon ammodendron on understory plants, our studylocated in south margin of the Gurbantunggut Desert, with nurse effect between H.ammodendron and understory plants as the breakthrough point, through investigatingthe distribution of different life form understory plants, observing the understoryplants in the life stage at different microhabitats, untangling the nurse effect of H.ammodendron , to study the spatial association between H. ammodendron anddifferent life forms understory plants, understory plants in different life stagecharacteristics at different microhabitats and the nurse effect mechanisms of H.ammodendron. Our findings provide insights into the significance of communitystructure and dynamic underlying the plant-plant facilitation commonly observed instress environments, raise awareness about the facilitation. The main results were asfollows:The distribution pattern of different life form plants general present aggregateddistribution at the small scale distribution, with the increase of scale, population tendsto random distribution, a larger scale for the regular distribution. The spatialassociation of H. ammodendron and different life form plants presents certainassociation at a small scale: H. ammodendron and annual herb presents positiveassociation; small H. ammodendron and shurbs present negative association, but withthe increase size of H. ammodendron, big H. ammodendron and shurbs presentedpositive association; H. ammodendron and ephemeral plants showed independence orweak association. The nurse plant, H. ammodendron was mainly effect at the small scale from on the understory plants. The intensity of nurse effect was associated withlife forms, and the strong of association was influenced by the form of H.ammodendron.Nurse effect of H. ammodendron can be rapidly changing due to environmentalchanges on plant different life stage. Soil seed bank and underground seedling bankwere dominated by annuals and ephemerals. Density of the soil seed bank decreasedgradually for shrub cores to shrub periphery and to inter-shrub open areas, showing anenrichment effect of the shrub on seeds. However, this effect was not significant at thedune crest. Germination rate of soil seed bank was highest at the shrub peripheries,but the difference was not significant between shrub cores and inter-shrub open areas,indicating a fine-scale zonation of the shrub effect on germination from shrub cores tointer-shrub open areas. Correspondingly, seedling occurring at shrub peripheriesbenefited greatest from the nurse plant, H. ammodendron. Through aggregating seedsand promoting seedling settling, H. ammodendron have significantly nurse effect onthe understory plants’ spatial patterns of soil seed bank and seedling in theGurbantunggut Desert at early stage of the life history. In plant growing period,Seriphidiam santolinum and Suaeda acuminate under the open on the light intensityrange of use was significantly greater than Crown (P < 0.05), suggesting that S.santolinum and S. acuminate can reduce the light compensation point and increaselight saturation point to improve the ability to use low light and strong light. The S.santolinum and S. acuminate under the canopy has a significantly lowerphotosynthetic rate than open (P < 0.05), indicate that H. ammodendron is neutral ornegative interactions to Crown herbaceous plants. In plant late period, understoryplants biomass increased with the increase of size classes, the biomass of understoryplants under big H. ammodendron were maximum, and no significant differenceunder middle H. ammodendron but was significantly higher than that of small H.ammodendron. And understory biomass all age classes were significantly higher than open (P < 0.05). Higher biomass may be associated with the survival of more plantsunder the crown.Nurse plants promote their understorey beneficiaries through severalmechanisms that operate simultaneously, the net effects can be positive or neutral,even or negative. The soil nutrient enrichment under canopies of H. ammodendron isable to alone exert positive effects on the production of annual plants by promotingindividual growth rather than establishment, either with or without canopy shading.Unexpectedly, the separate effects of the shading on the annual plant density,production and individual growth are neutral. When all mechanisms involved in thenurse plant paradigm, i.e., exerted by living shrubs, are combined, the synergisticeffects were found only on the desert annual establishment rather than the productivity,as evidenced by the significantly higher density under living shrubs. As a result, thedensity limitation of growth likely reduced the individual size of annuals under shrubsbecause the spatially lustered understorey beneficiary individuals may competeamong each other for resources.