Vertical distribution pattern of mixed root systems of desert plants Reaumuria soongarica and Salsola passerina under different environmental gradients

植物种间相互作用及其对环境胁迫的响应一直是物种共存和生物多样性维持研究的一个热点, 从地下根系入手来探讨混生群落植物种间关系及其对环境胁迫响应的研究少见报道。该文以荒漠草原区(灵武)、典型荒漠区(张掖)和极端荒漠区(酒泉) 3个不同生境条件下单生与混生红砂(Reaumuria soongarica)和珍珠猪毛菜(Salsola passerina)为实验材料, 采用分层取样法对其垂直根系参数进行测定和分析, 探讨了两种植物根系分布对混生及荒漠环境梯度的响应。结果表明: 同一生境条件下, 混生红砂和珍珠猪毛菜比根长和比表面积均高于单生, 说明红砂、珍珠猪毛菜混生后其根系相互作用关系表现为互惠, 促进了植株对土壤养分和水分的吸收。不同生境条件下, 同一生长方式的红砂根系分布深度均大于珍珠猪毛菜, 且根系消弱系数也普遍高于珍珠猪毛菜, 说明二者在不同生境条件下占据不同生态位, 红砂表现为深根性, 根系位于土壤深层, 珍珠猪毛菜表现为浅根性, 根系分布于土壤浅层。随着荒漠环境胁迫增强, 单生和混生红砂与珍珠猪毛菜的比根长和比表面积均呈现出极端荒漠区>典型荒漠区>草原荒漠区的规律, 且生境越干旱, 混生群落根系分离越明显; 单生与混生红砂根系消弱系数也逐渐增大, 在极端干旱区达到最大值, 珍珠猪毛菜变化不大, 表明红砂-珍珠猪毛菜混生群落根系生态位分离随荒漠环境胁迫增强而加大, 验证了环境胁迫梯度假说。可见“地上聚生, 地下分离”的混生方式可能是红砂-珍珠猪毛菜混生群落适应干旱胁迫环境的生长策略。     

阿拉善高原2种荒漠植物根系构型及生态适应性特征

根系构型决定了植物对资源的吸收方式，根系构型的变化是植物对环境的生态适应和有效生存策略。在阿拉善高原西南缘红砂（Reaumuria soongarica）-珍珠猪毛菜（Salsola passerina）混生群落采用传统挖掘法收集两种植物根系，基于量化的根系形态指标，利用几何拓扑学及分形理论分析了根系构型特征，探讨了该地区2种植物对干旱生境的生态适应策略。结果表明：红砂和珍珠猪毛菜根系均以水平分布占优，根系浅层化分布明显，混生的两种植物占据不同的生态位；2种荒漠植物均具有较大的比根长（SRL）和比表面积（SRA），红砂SRL=21.3 cm/g，SRA=7.6 cm²/g，珍珠SRL=22.4 cm/g，SRA=6.5 cm²/g，有利于水分和养分的获取；红砂根系拓扑指数（TI）、修正拓扑参数（q_a和q_b）分别为0.86、0.52、0.49，珍珠猪毛菜对应参数分别为0.93、0.76、0.73，表明2种植物根系均趋向于鱼尾形分支结构；根系分形维数值（FD_红=1.488、FD_珍=1.422）较小，而分形丰度值（lgK_红=1.855、lgK_珍=1.774）较大，表明2种植物分支相对简单，但空间拓展能力强，有利于对营养空间的占有。上述特征可能是阿拉善西南缘红砂-珍珠猪毛菜群落2种荒漠植物植物对干旱贫瘠生境的重要生态适应策略。     

红砂根系分叉数和分支角度权衡关系的坡向差异

根系构型是决定根系分布及其对土体搜索效率的重要因素, 是植物与资源环境异质性相互适应的结果。利用ArcGIS建立研究区域的数字高程模型(digital elevation model), 采用全根系挖掘法, 研究了祁连山北坡荒漠草原不同坡向红砂(Reaumuria songarica)根系分叉数和分支角度的关系。结果表明: 不同坡向梯度草地群落的盖度、密度、高度和土壤含水量呈不同的变化趋势(p < 0.05), 红砂种群的密度、高度和根冠比等生物学特征的变化规律存在差异(p < 0.05); 不同坡向红砂根系分叉数和分支角度的相关性存在差异(p < 0.05), 在南坡和北坡红砂根系分叉数和分支角度之间存在极显著的负相关关系(p < 0.01), 在东坡和西坡根系分叉数和分支角度之间存在显著的负相关关系(p < 0.05), 红砂根系分配给根系分叉数和分支角度的资源存在着“此消彼长”的权衡关系; 随着坡向由南坡向西坡、东坡、北坡转变, 红砂根系分叉数和分支角度回归方程的标准化主轴斜率逐渐减小(p < 0.05), 红砂根系构型模式由扩散型转向聚集型。不同坡向红砂根系合理权衡连接长度和分支角度的资源配置模式, 反映了异质生境中植物种群应对资源多重竞争的环境生态适应机制。     

降水变化对红砂-珍珠碳、氮、磷化学计量特征的影响

以荒漠C3植物红砂(Reaumuria soongarica)和C4植物珍珠(Salsola passerina)为材料,在西北干旱荒漠区沿自然降水梯度,对不同降水条件下单生和混生红砂与珍珠根、茎、叶器官碳、氮、磷化学计量指标进行测定,分析其在不同生境下化学计量特征对种间关系及环境胁迫的响应规律。结果表明:(1)随干旱胁迫程度增加(降水量的减少),红砂各器官C含量平均升高7.73%,N、P含量分别平均降低6.20%、10.61%;珍珠各器官C含量平均升高7.36%,N、P含量分别平均降低5.93%、14.03%。两种植物叶片C含量升高表明其光合速率较低,生长缓慢,但对外界不利环境的防御能力增强,能更好地适应干旱环境。(2)干旱胁迫改变了红砂和珍珠的N、P含量在各器官的分配模式,两种植物N、P含量在叶部高于根部,在根、叶中N/P明显高于茎,表明两种植物不同器官受到的养分限制不同。(3)红砂各器官C、N、P含量高于珍珠,说明红砂防御能力较强,生长速率高,对资源的竞争和利用能力较珍珠强;珍珠C/N和C/P均高于红砂,表明珍珠比红砂有较强的碳同化能力和较高的营养利用效率。(4)在干旱胁迫条件下,红砂和珍珠均表现为碳素积累、氮磷素限制的格局,它们对于氮和磷的养分利用不活跃,受到氮和磷养分的限制较为均衡。     

盐胁迫下荒漠共生植物红砂与珍珠的根茎叶中离子吸收与分配特征

西北荒漠地区C3小灌木红砂(Reaumuria soongorica)和C4半灌木珍珠猪毛菜(Salsola passerina)在特定环境下混生在一起,分布面积广阔。以采自腾格里沙漠边缘荒漠地带的天然野生珍珠猪毛菜和红砂群落的幼苗为材料,经0、100、200、300、400mmol/L NaCl盐溶液共同胁迫10 d,检测它们的含水量、主要矿质离子在根茎叶的含量与分布,揭示二者耐盐的共生协同的离子平衡适应机制。试验结果发现,珍珠猪毛菜叶片具有"吸钾排钠的"的耐盐特征,红砂叶片具备"吸钠排钾"的特征,吸收利用无机矿质离子具备互补效应。二者耐盐Cl、Ca和Si离子吸收与累积能力存在很大差异:随着盐胁迫程度加剧,红砂的根茎叶中Cl离子含量持续增加,并且为珍珠猪毛菜的2—5倍;珍珠猪毛菜根中Ca离子含量为红砂的2—3倍,但含量变化不显著;红砂根中Si离子含量迅速降低后稳定,并且是珍珠猪毛菜根的3—5倍,其他器官变化差异较小。因此,红砂与珍珠猪毛菜的共培养盐胁迫下根中吸收的离子侧重不同,红砂以Na、Cl、Si为主,珍珠猪毛菜以K、Ca为主。随着盐胁迫的程度加强,离子选择吸收系数S k,Na的变化趋势降低,表明二者叶部对Na的选择性减小,K的选择性吸收积累增大,增强了它们的抗盐性,最终使叶片所受盐害减小。总之红砂与珍珠猪毛菜共生的耐盐离子稳态机制显著不同,离子吸收与分布具有互补互利的效应。     

贺兰山西坡植被群落特征及其与环境因子的关系

通过SPSS聚类分析和CANOCO排序研究了贺兰山西坡植被分布与环境因子的关系,结果表明:在海拔梯度上,贺兰山西坡植被大致可以划分珍珠猪毛菜(Salsola passerina)-红砂(Reaumuria soongorica)群落、短花针茅(Stipa breviflora)-大针茅(Stipa grandis)群落和蒙古扁桃(Prunus mongolica )-灰榆(Ulmus glaucescens)群落3种类型.从典范对应分析(CCA)排序结果来看,不同植被群落与环境因子的关系存在明显的分异.在珍珠猪毛菜-红砂群落,第一排序轴反映了土壤盐碱化梯度,沿着藏锦鸡儿群落-珍珠猪毛菜、猫头刺群落-珍珠猪毛菜、红砂群落序列,土壤盐碱化程度不断增强;第二排序轴则反映了土壤结构梯度,沿着藏锦鸡儿群落-珍珠猪毛菜、红砂群落-珍珠猪毛菜、猫头刺群落序列,土壤质地逐渐粗化;在短花针茅-大针茅群落,第一排序轴反映了土壤水分梯度,第二排序轴反映了海拔梯度上的水热组合梯度;在蒙古扁桃-灰榆群落,第一排序轴反映了土壤pH梯度,沿着灰榆、蒙古扁桃群落-蒙古扁桃、金露梅群落-蒙古扁桃群落序列,土壤pH值逐渐下降;第二排序轴主要反映了土壤结构梯度,沿着蒙古扁桃群落-灰榆、蒙古扁桃群落-蒙古扁桃、金露梅群落序列,土壤中粉粒、粘粒含量逐渐增加,土壤质地呈细化趋势.     

红砂幼苗根系形态特征和水分利用效率对土壤水分变化的响应

为探讨干旱与半干旱区受损红砂种群幼苗适宜生长的土壤水分条件,采用盆栽方法,研究了红砂幼苗在充分灌溉(FI)、适度灌溉(MI)、干旱处理(DT)3个水分处理下根系形态和水分利用效率的变化特征。结果表明:(1)红砂幼苗根系形态因水分条件和根序的不同而各异;随灌溉量的减少红砂幼苗根系直径和根体积均表现为FIMIDT,但干旱处理促进了根系的伸长生长和比表面积和比根长增加,根系形态的可塑性是红砂幼苗获取水分适应干旱环境的重要策略之一。(2)随根序的升高,各处理水平下红砂幼苗根长、比根长均显著减少,而其根直径和体积却显著增加,表明红砂幼苗根系内部具有高度的形态异质性。(3)与FI处理相比,MI和DT处理下红砂幼苗根系总生物量分别增加了50.00%、19.23%,但MI和DT处理却显著降低了红砂幼苗地上生物量,特别是叶片生物量下降幅度最大,分别降低了62.15%、83.28%,导致根冠比随灌溉量的减少而逐渐增加。(4)干旱处理显著提高了红砂幼苗的水分利用效率。研究认为,在灌溉量减少的情况下,红砂幼苗可通过根长、根系表面积和体积、直径等形态变化来优化其空间分布构型,以调节植株对水分的利用,提高水分利用效率。     

不同荒漠植物根际土壤微生物群落结构特征

于2015年7月在甘肃安西极旱荒漠国家级自然保护区采集膜果麻黄(Ephedra przewalskii)、红砂(Reaumuria songarica)、合头草(Sympegma regelii)、泡泡刺(Nitraria sphaerocarpa)和珍珠猪毛菜(Salsola passerina)5种典型荒漠植物根际土壤样品,利用磷脂脂肪酸(PLFA)法结合Sherlock微生物鉴定系统分析了土壤微生物群落结构。结果表明,5种荒漠植物根际土壤微生物磷脂脂肪酸种类和组成差异显著,其中表征革兰氏阳性菌的18:0 iso、16:0 iso和17:1 isoω9c分别为红砂、珍珠猪毛菜特有表征放线菌的18:1ω7c 10-methyl仅在珍珠猪毛菜根际存在。总PLFAs、真菌、放线菌和真菌/细菌在珍珠猪毛菜中显著最高,革兰氏阳性菌、革兰氏阴性菌在膜果麻黄和珍珠猪毛菜根际显著高于其他植物,AM真菌在合头草根际有最高值。结构方程模型分析表明,与植物相比,土壤因子对微生物群落结构影响更为显著,其中易提取球囊霉素对放线菌有显著影响,土壤碱解氮是革兰氏阳性菌和革兰氏阴性菌的主要影响因子。同时,土壤微生物群落结构可用于检测不同荒漠植物根际微环境土壤退化状况。     

我国西北地区芨芨草-苦豆子群落根系分布与种间关系

以宁夏沙湖的一个典型的干旱区草本型植物群落——芨芨草(Achnatherum splendens)-苦豆子(Sophora alopecuroides) (禾草-非禾草型)群落为研究对象, 自芨芨草-苦豆子聚生丛的基部向丛间裸地依次划分为植冠区、过渡区和空旷区3个分区, 通过比较分析两个优势种根系生物量密度在这3个分区的空间分布格局, 研究了干旱胁迫生境中芨芨草和苦豆子的根际关系。结果表明: 在水平方向上, 芨芨草根系总生物量主要集中分布于植冠区, 而苦豆子根系总生物量密度在3个分区间无显著差异; 在垂直方向上, 芨芨草的根系总生物量密度和细根生物量密度最大的土壤层次从植冠区到空旷区埋深渐深, 而苦豆子的根系总生物量在3个分区主要分布于10~30 cm土层, 但其细根生物量密度最大的土层从植冠区向外呈上升趋势; 同时发现, 在植冠区, 根系总生物量密度较高, 两种植物细根生物量密度最大的土壤层次不同, 但在空旷区和过渡区, 两种植物细根生物量密度的垂直分布格局趋于一致。群落中两个主要植物种的根系在植冠区具有垂直层次的分异性, 在不同分区间具有水平分布格局的差异性, 这可能是群落中两种主要植物种间为了避免根系间直接竞争共同适应干旱胁迫生境的重要机制。     

降雨格局变化对白刺幼苗根系形态特征的影响

为探讨荒漠植物白刺幼苗根系形态对降雨格局变化的响应特征,设置3个降雨量梯度(W-、W、W+)和2个降雨间隔时间梯度(T、T+)进行人工模拟试验,结果表明,1)降雨量和降雨间隔时间对白刺幼苗根系形态有不同程度的影响,且降雨量的作用效应更大。2)降雨量相同时,延长降雨间隔时间均使白刺幼苗主根长、根系平均直径、根体积和根表面积减小,但总根长和根系生物量和总生物量却增加,在高降雨量条件下(W+)延长降雨间隔时间白刺幼苗比根长和比表面积分别增加了45.09%和20.20%,但差异均不显著。3)降雨间隔时间相同时,降雨量减少30%仅使主根长平均增加12.06%,总根长、根平均直径、根体积和根表面积等根系形态指标均显著减少,比根长和比表面积变化不大;降雨量增加30%仅使比表面积显著增加,其余各形态指标差异均不显著,低降雨量条件下(W-)主根长与根冠比达到最大,其他指标均在高降雨量条件下(W+)达到最大。4)对8个根系形态参数进行主成分分析,根系生物量、总根长、总根表面积、比根长、比表面积和根体积6个根系生态参数受降雨格局影响显著。     

西藏高寒草原群落植物根系属性在降水梯度下的变异格局

根系功能属性及其变异性能够介导物种共存及环境适应策略, 但强烈的环境约束作用能够引起不同物种间根系属性的趋同性。为了研究西藏高寒草原群落中植物根系属性变异规律, 并阐明不同物种资源获取和适应策略的多样性, 该文对西藏高寒草原不同的环境梯度进行了研究。作者自东向西沿着降水梯度在那曲、班戈和尼玛3个自然草原群落进行群落调查, 并采集了共计22种植物。测定了每种植物的一级根直径、一级侧根长度和根系分支强度3个关键根系属性。结果表明: 在西藏高寒草原群落中, 不同物种根系直径普遍较小, 且种间变异非常小(22.76%), 其中86%的物种一级根直径集中在0.073 mm到0.094 mm之间; 相较于直径较粗的物种, 直径越细的物种分支强度越高, 侧根越短。在群落尺度上, 植物主要通过增加根系直径、侧根长度, 降低分支强度的方式来适应水分的减少; 而在物种尺度上, 植物适应水分变化的策略则呈现多样性。     

Variations of root traits in three Xizang grassland communities along a precipitation gradient

根系功能属性及其变异性能够介导物种共存及环境适应策略, 但强烈的环境约束作用能够引起不同物种间根系属性的趋同性。为了研究西藏高寒草原群落中植物根系属性变异规律, 并阐明不同物种资源获取和适应策略的多样性, 该文对西藏高寒草原不同的环境梯度进行了研究。作者自东向西沿着降水梯度在那曲、班戈和尼玛3个自然草原群落进行群落调查, 并采集了共计22种植物。测定了每种植物的一级根直径、一级侧根长度和根系分支强度3个关键根系属性。结果表明: 在西藏高寒草原群落中, 不同物种根系直径普遍较小, 且种间变异非常小(22.76%), 其中86%的物种一级根直径集中在0.073 mm到0.094 mm之间; 相较于直径较粗的物种, 直径越细的物种分支强度越高, 侧根越短。在群落尺度上, 植物主要通过增加根系直径、侧根长度, 降低分支强度的方式来适应水分的减少; 而在物种尺度上, 植物适应水分变化的策略则呈现多样性。     

Effects of environmental factors on the distribution of plant communities in a semi-arid region of the Qinghai-Tibet Plateau

Against the background of global climate warming, the relationship between plant communities in high-cold ecosystems and environmental gradients has attracted much attention. We investigated the relationship between the distribution of plant communities and environmental factors in a semi-arid region of the Qinghai-Tibet Plateau. We analyzed the effects of environmental factors on the distribution of plant communities using two-way indicator species analysis and canonical correspondence analysis. The most important factor explaining the distribution of plant communities was the depth of the active layer of permafrost (denoted as PF), followed by soil water content at 40-cm soil depth. There was a strong correlation between these two factors. With changes in the PF, the dominant species in plant communities showed an obvious transition. The indices of species richness and species diversity decreased markedly with increasing PF, whereas biomass and vegetation coverage showed weaker responses to changes in the PF. The distribution structure for plant communities in this area mainly results from changes in the PF. Furthermore, the PF has remarkable and important effects on the characteristics of the plant community.     

Phylogenetic and functional structures of succession in plant communities on mounds of Marmota himalayana in alpine regions on the northeast edge of the Qinghai–Tibet Plateau

Few studies have examined the succession of plant communities in the alpine zone. Studying the succession of plant communities is helpful to understand how species diversity is formed and maintained. In this study, we used species inventories, a molecular phylogeny, and trait data to detect patterns of phylogenetic and functional community structure in successional plant communities growing on the mounds of Himalayan marmots (Marmota himalayana) on the southeast edge of the Qinghai-Tibet Plateau. We found that phylogenetic and functional diversities of plant communities on marmot mounds tended to cluster during the early to medium stages of succession, then trended toward overdispersion from medium to late stages. Alpine species in early and late stages of succession were phylogenetically and functionally overdispersed, suggesting that such communities were assembled mainly through species interactions, especially competition. At the medium and late stages of succession, alpine communities growing on marmot mounds were phylogenetically and functionally clustered, implying that the communities were primarily structured by environmental filtering. During the medium and late stages of succession the phylogenetic and functional structures of plant communities on marmot mounds differed significantly from those on neighboring sites. Our results indicate that environmental filtering and species interactions can change plant community composition at different successional stages. Assembly of plant communities on marmot mounds was promoted by a combination of traits that may provide advantages for survival and adaptation during periods of environmental change.     

荒漠草原“土岛”生境群落物种共存机制

经过长期破碎化, 荒漠草原原生硬质灰钙土斑块散布在广大沙化土地中, 形成类似“土岛”的土被结构。为揭示土岛生境的群落物种共存机制, 2016年在宁夏盐池县皖记沟村选取大(200-300 m²)、中(约100 m²)、小(约50 m²)土岛各3个开展调查, 采用Jaccard相异系数、物种生态位宽度和生态位重叠度、零模型、Meta分析, 综合计算和分析土岛内部与外部植物群落相似性、物种生态位宽度和生态位重叠、物种共存格局及其影响因子。研究发现, (1)随着破碎化加剧, 土岛内部植物多样性整体呈现下降趋势, 群落优势种从短花针茅(Stipa breviflora)转变为猪毛蒿(Artemisia scoparia)和短花针茅共优种, 土岛内外群落相似性增加。(2)土岛内外绝大多数物种生态位重叠较小, 生态位重叠在土岛内呈集中分布, 而土岛外则呈均匀发散分布。(3)环境过滤为主的生态过程决定了土岛生境群落物种的共存格局, 随着土岛面积减小, 环境因子对群落物种共存的调控强度降低, 关键性环境因子由土壤细砂粒和黏粒转变为粗砂粒, 显著性竞争物种共存格局在小岛出现。综上所述, 土岛生境对于维持草原物种具有重要作用, 环境过滤主导了荒漠草原物种共存格局。随着生境破碎化加剧或土岛面积减小, 物种共存格局及其调控因子发生转变。保护面积在200 m²以上的大土岛对于恢复荒漠草原区草原成分种和其物种多样性机制都十分必要。     

Plasticity in above‐ and belowground resource acquisition traits in response to single and multiple environmental factors in three tree species

Functional trait plasticity is a major component of plant adjustment to environmental stresses. Here, we explore how multiple local environmental gradients in resources required by plants (light, water, and nutrients) and soil disturbance together influence the direction and amplitude of intraspecific changes in leaf and fine root traits that facilitate capture of these resources. We measured population‐level analogous above‐ and belowground traits related to resource acquisition, i.e. “specific leaf area”–“specific root length” (SLA–SRL), and leaf and root N, P, and dry matter content (DMC), on three dominant understory tree species with contrasting carbon and nutrient economics across 15 plots in a temperate forest influenced by burrowing seabirds. We observed similar responses of the three species to the same single environmental influences, but partially species‐specific responses to combinations of influences. The strength of intraspecific above‐ and belowground trait responses appeared unrelated to species resource acquisition strategy. Finally, most analogous leaf and root traits (SLA vs. SRL, and leaf versus root P and DMC) were controlled by contrasting environmental influences. The decoupled responses of above‐ and belowground traits to these multiple environmental factors together with partially species‐specific adjustments suggest complex responses of plant communities to environmental changes, and potentially contrasting feedbacks of plant traits with ecosystem properties. We demonstrate that despite the growing evidence for broadly consistent resource‐acquisition strategies at the whole plant level among species, plants also show partially decoupled, finely tuned strategies between above‐ and belowground parts at the intraspecific level in response to their environment. This decoupling within species suggests a need for many species‐centred ecological theories on how plants respond to their environments (e.g. competitive/stress‐tolerant/ruderal and response‐effect trait frameworks) to be adapted to account for distinct plant‐environment interactions among distinct individuals of the same species and parts of the same individual.     

Plant functional group identity differentially affects leaf and root decomposition

Losses of species and changes in the composition of plant communities are likely to influence numerous ecosystem functions. Changes in the plant‐soil interactions that control decomposition, in particular, could alter carbon and nutrient cycling in soils and further alter other ecosystem functions. The effects of plant communities on decomposition may depend both on the type of tissue being decomposed and also on the different stages of the decomposition process. We used an experimental design where single plant functional groups were removed from a northern grassland to examine the role of plant identity in determining both short‐term and long‐term above‐ and belowground decomposition rates. Plant removals were conducted across fertilization and fungicide treatments to examine environmental context‐dependency of functional group identity effects on decomposition. There were significant effects of plant functional group identity on aboveground decomposition, with the loss of grasses and forbs slowing decomposition, whereas the effects on belowground decomposition were rare and transient. Effects of plant identity on decomposition were consistent in both short‐ and long‐term decomposition studies indicating that the influences of identity on the decomposition environment remained consistent throughout the different stages of the decomposition process. Both fertilizer and fungicide treatments affected overall decomposition rate, but there were few interactions between these treatments and plant removals. Although current species loss is likely to be happening in concert with environmental changes, the role a species plays in determining ecosystem functions such as decomposition may not be context‐dependent in these northern environments, and this may provide greater predictive power in determining the effects of species loss with changing environments. Further, as plant identity shows significant effects on litter decomposition rates, the effects of current and predicted future biodiversity losses may depend specifically on which species are lost.     

The plant communities and environmental gradients of Pitcairn Island: the significance of invasive species and the need for conservation management

Quantitative surveys of the vegetation of south-east Polynesian Islands are rarely undertaken owing to time and logistical restrictions; however they are fundamental in determining the conservation status of fragile island ecosystems. The aim of the research was to document quantitatively the vegetation of Pitcairn Island by investigating whether clearly definable plant communities existed on the island, and the underlying environmental gradients influencing these communities. Initially, 10 x 10 m quadrats were taken from all areas of the island, with environmental parameters recorded for each quadrat. The vegetation was then mapped from high altitude vantage points. Two-way indicator species analysis was used to identify distinct plant communities, and canonical correspondence analysis was used to determine the underlying environmental gradients. The vegetation consists of 14 plant communities: four coastal, six forest, two fernland and two scrub communities. Large areas are covered by non-native scrub vegetation, and by monospecific Syzygium jambos (rose-apple) plantations. Less than 30 % of the island is covered by native forest, and these areas are limited to remote valleys. Fernlands also cover large areas, including both eroding areas and ridge tops. Coastal vegetation comprises rock and cliff communities with limited strand vegetation. The major environmental gradient affecting the composition of the plant communities is altitude, but anthropogenic influences also have a large effect, owing to forest clearance and introduced species. The light environment is affected by the canopy species, and determines what ground flora can develop. Identification of distinct plant communities has allowed for a system of nature reserves to be suggested, which conserve all of these plant communities and a significant proportion of the threatened plant species.     

不同放牧强度下克氏针茅(Stipa krylovii)草原的根系特征

以呼伦贝尔克氏针茅草原不同放牧强度下的演替群落为对象,开展群落及其群落建群种的地下生物量和根系形态特征研究.结果表明:从轻度放牧到重度放牧,群落种类组成和根系功能群类型趋于简单化;群落地下生物量的空间分布形态呈“T”型;不同放牧强度下草原群落的建群种出现了明显替代现象,轻度放牧样地群落建群种为密丛型根系的克氏针茅,中度放牧为疏丛型根系的糙隐子草(Cleistogenes squarrosa,重度放牧为鳞茎型根系的碱韭(Allium polyrhizum);随着放牧强度的增大,群落建群种根冠比逐渐增加,分别为0.47、1.0、4.1,并且群落建群种根系数量、根系体积、根系生物量、比根长及根长密度等各指标均发生了明显变化.另外,3种放牧强度样地群落建群种根冠比、根长密度均与土壤速效氮含量呈现显著正相关(P＜0.05).