绿化植物的生活型对边坡植被物种多样性及护坡性能的影响

为了探讨绿化植物生活型构成对边坡植被物种多样性及护坡性能的影响, 揭示生活型构成-群落特征-物种多样性-生态系统功能间的内在联系, 借助三物种组装实验, 分别构建以草本、灌木或乔木为主体的草本型(HHX_i)、灌木型(SSX_i)、乔木型(AAX_i)或草-灌-乔混合型(HSA)配置模式的实验小区, 对实验区内边坡植被的群落特征、护坡性能进行持续4年的生态监测。结果表明: (1)边坡植被的物种丰富度与绿化植物生活型构成有关, AAX_i的物种丰富度总体高于其他模式, 呈AAX_i > SSX_i > HSA > HHX_i趋势。(2)不同配置模式边坡植被的群落盖度不一样, 年际间差异显著: 建坪初期(2010-2011年), HHX_i的群落盖度远高于其他模式, HSA次之, AAX_i最低; 2012-2013年间, HSA的群落盖度最高, HHX_i次之, AAX_i最低。(3)边坡植被的Shannon-Wiener指数、Pielou指数与绿化植物生活型构成有关, 其变化规律与群落盖度类似: 建坪初期, HHX_i的多样性水平远高于其他模式, 呈HHX_i > HSA > SSX_i > AAX_i趋势; 之后, 呈HSA > HHX_i > SSX_i > AAX_i趋势。(4)边坡植被的护坡性能与群落内的物种多样性密切相关: 多样性水平越高, 护坡性能越强。可见, 在生态护坡过程中, 绿化植物生活型构成对提高边坡植被物种多样性、改善护坡性能至关重要。     

公路建设对雾灵山自然保护区植被的影响

选取雾灵山自然保护区莲花池至燕山主峰段盘山公路周边的植被为对象,调查分析了公路建设对沿线植被组成、盖度、密度、生物量的影响。结果表明:1)公路干扰作用下,公路沿线乔木数量减少,但出现不同种类的幼龄乔木;草本植物群落生物量较高,物种丰富度指数、多样性指数较高。2)随样地与路肩距离的增加,草本与灌木层植物群落的生物多样性逐渐降低,乔木层植物群落的密度、高度、胸径均有所增加;距离路肩30m以上时,灌草植物群落组成趋向稳定;距离路肩150m以上时,乔木层植物群落结构趋于稳定。3)公路对植被的干扰作用包括正负2种,乔木层主要受负干扰影响,灌木和草本层主要受正干扰影响,且灌草植物群落所受的干扰作用大于乔木植物群落。4)公路干扰对于边坡上、下区域植被群落物种丰富度影响强度不同,对上边坡植物群落的影响大于下边坡植物群落。     

北京九龙山植物群落物种多样性特征对比分析

根据对北京九龙山植物群落的样地调查数据,采用Shannon-Wiener多样性指数、群落均匀度和生态优势度指标研究了九龙山封育植被乔灌草各层的物种多样性,并分别就阳坡和阴坡进行了乔木材、灌价群落的指标对比分析,结果表明,这3种指标能够有效地表征暖温带植物群落的组成结构特征,在九龙山封育植被中,阴阳坡的灌丛群落与乔木材相比,其灌木层的物种多样性Shannon-Wiener指数D高于乔木林中乔木层,而低于其灌木层;灌丛群落的草本层Shannon-Wiener指数小于乔木材中草本层;乔木材的草本层的物种多样性Shannon-Wiener指数大于灌木层,灌木层又大于乔木层,而生态优势度系数C在上述3种比较中与Shannon-Wiener指数表现相反,而群落均匀度指数J在比较中阴阳坡的表现不同,最后讨论了九龙坡地区植被演替序列及加速其进展演替的恢复措施。     

小兴安岭凉水国家级自然保护区植物beta多样性及其影响因素

自然保护区如何设置才能够最大程度保护生物多样性, 是保护生物学的研究热点; 阐明beta多样性特征、组分格局及其影响因素是保护生物学的重要基础。本研究选取小兴安岭凉水国家级自然保护区不同功能区(核心区、缓冲区、实验区)及毗邻地区(保护区外)共80块样方作为研究对象, 调查每块样方的保护位置(经纬度、海拔、坡位、坡度、坡向)和群落结构(郁闭度、林龄、乔木树高、胸径、灌木树高、地径), 并采集0-20 cm土壤样品, 测定土壤理化性质(有机碳、全氮、pH值、电导率、含水量、容重)。将样方间的beta多样性分解为物种周转和物种多度差异两种组分, 通过Mantel分析、冗余分析和方差分解分析解析非生物因子(地理地形、保护强度、土壤因子)和生物因子(群落结构)对beta多样性及其组分的影响。结果表明: (1)乔、灌、草3层中, 物种周转组分对于beta多样性的贡献均占主导地位(65%-73%), 物种多度差异贡献较小。(2) Mantel检验结果表明, 乔、灌、草3层beta多样性及其组分与地理地形指标显著相关的因子最多; 土壤因子只对乔木层和灌木层beta多样性及组分有影响, 对草本层影响不大。其中坡位、坡度、乔木树高和保护强度均与保护区乔、灌、草3层beta多样性显著正相关(P < 0.05)。(3)植物整体beta多样性受地理地形影响最大, 但存在乔、灌、草差异。乔木层beta多样性受生物因子影响最大; 灌木层的土壤因子解释力分别为地理地形和生物因子的2倍; 而草本层主要受地理地形的影响, 其解释力分别是土壤和生物因子的26倍和3倍。乔木胸径对植物beta多样性差异具有最大的解释作用。本研究结果表明, 未来保护区设置需要根据保护植物的类型, 选择适当的林分结构、土壤和地理地形等, 以增强保护区植物多样性保护的效果。     

河南典型草地碳氮库构成对植物多样性的响应

以河南4种典型草地类型(暖性草丛、暖性灌草丛、热性草丛与热性灌草丛)为研究对象,基于野外调查与室内测定,结合RDA冗余分析方法,分析了植物多样性与生态系统碳氮库构成之间的关系。结果表明:河南草地植物物种多样性整体上呈现出灌草丛类草地大于草丛类草地;河南草地具有符合地域特征的植物功能群类型,C3植物比例均远大于C4植物,杂类草占64%～73%,有性兼无性繁殖方式的植物类型多达49%～56%;而生活型组成较为复杂,暖性草地以多年生草本和灌木为主,热性草地以一或两年生草本以及多年生草本为主; RDA分析结果显示,暖性草丛、暖性灌草丛植被氮密度与Shannon和Pielou指数呈正相关,植被碳密度、土壤氮密度与Shannon和Pielou指数呈负相关;热性草丛植被碳氮密度与物种丰富度指数呈正相关,土壤氮密度与Shannon和Pielou指数呈正相关,植被氮密度与Shannon和Pielou指数呈负相关;热性灌草丛土壤碳氮密度与Shannon指数呈正相关,植被碳氮密度与物种丰富度和Shannon指数呈负相关,即物种多样性及不同草地类型植被的组成共同作用影响植被和土壤碳氮密度。总之,河南草地生态系统碳氮库构成是植物物种丰富程度和分布均匀程度综合作用的结果。     

泌桐高速公路生态护坡不同草灌混播10a后的植被群落特征

稳定的边坡群落是公路边坡生态防护的目标,为探究高速公路生态护坡工程10 a演替后群落特征,该研究以泌桐高速公路为依托,布设自然恢复、单一狗牙根播种和5个不同播种密度的草灌混播生态防护措施,分析不同恢复方式对边坡群落组成、生活型和物种多样性的影响。结果表明:(1)调查样方内共有52种植物,分属于17科49属。禾本科、菊科和豆科植物共26属29种,占种总数的55.77%,表明这三大科植物在边坡群落演替过程中起着重要作用。(2) 10 a演替后不同恢复方式下群落中多年生植物比例高于一年生草本植物。(3)在草灌混播样地中,群落物种多样性指数随着播种密度的增加呈先增加后降低的单峰变化趋势,在播种密度为每平方米500株时达到最大。(4)草灌混播的生态恢复效果优于纯草本种植和自然恢复方式。(5)播种密度对草灌混播群落类型,地上生物量和物种多样性指数没有显著影响。从植物的生长效果及成本方面考虑,初播密度每平方米为500~600株的草灌混播可构建较为稳定的边坡植物群落,实现最佳的边坡恢复效果,可应用于类似区域边坡生态恢复工程。     

喀斯特森林不同演替阶段植物群落物种多样性、功能性状、化学计量及其关联

探讨森林植物群落物种多样性、功能性状与化学计量特征及其内在关联,有助于构建结构-性状-功能的研究脉络,深入理解生态系统的过程与功能,为群落结构优化配置和调控提供科学依据。以贵州喀斯特高原峡谷区森林草灌、灌木、灌乔和乔木4个演替阶段的植物群落为对象,通过群落学调查和植物功能性状、生态化学计量测度,研究其物种多样性、功能性状和化学计量特征及其互作关系。结果表明:(1)乔木阶段的Shannon-Wiener指数与Pielou均匀度指数均为最高,依次达8.62、2.41,表明伴随着群落演替,物种分布的均匀程度增加。(2)植物群落物种多样性指数之间仅Margalef丰富度指数与Simpson优势度指数未呈显著相关(-0.644),其余均为显著或极显著相关关系,表明物种多样性指数之间存在较强的促进或抑制作用。(3)叶片厚度与其余指标,以及群落物种多样性指数与功能性状、化学计量之间均未呈显著相关,比叶面积(SLA)仅与土壤N∶P呈显著正相关(0.742),表明群落物种多样性、功能性状和生态化学计量特征之间的相关性较弱。(4)SLA为灌木阶段的272.13 cm2/g为最大,δ13C值在演替后期略高,草灌、灌木阶段倾向于受N、P共同限制,说明随演替进展,植物的适应策略和资源利用对策等均发生了一定的调整,表明植物和环境之间存在较强的协变关系。(5)该区森林培育时,应构建完整的乔木、灌木和草本层片,提高生态系统对养分矿化、转化和循环等的自调控能力。     

我国南方4种常见人工林林下植物多样性特征及影响因素

为了解长期植被恢复的成熟人工林林下植物组成与多样性特征及其影响因素,基于广东鹤山生态系统国家野外科学观测研究站的南亚热带人工林生态系统,对环境相似(坡度、坡向、海拔)、30 a生4种类型人工林(桉树混交林、马占相思纯林、乡土混交林、针叶混交林)进行调查研究,分析林下植物组成和物种多样性(Shannon-Wiener指数、Simpson指数、Pielou指数)特征。结果表明,人工林林下植物类型丰富,均可形成乔-灌-草垂直结构;4种林型林下植物组成既有相似性,也有差异性,桉树混交林与针叶混交林、马占相思纯林与乡土混交林的灌木层组成相似;桉树混交林与马占相思纯林、乡土混交林与针叶混交林的草本层组成相似,而桉树混交林与针叶混交林的草本层组成极不相似。林分类型影响林下植物多样性,马占相思纯林林下灌草多样性显著低于其他3种混交林(P<0.05),灌木物种数、个体数最少;针叶混交林林下物种丰富度最高。林分郁闭度与林下植物多样性呈正相关(P<0.001),林下植物分布与土壤养分含量相关,桉树混交林、马占相思纯林林下植物多样性与不同形态氮含量相关,有效磷、全磷影响乡土混交林林下物种的分布,针叶混交林受土壤酸碱度、全钾的影响较为明显。在4种人工林林下植物群落中,乡土混交林多样性,均匀度最高,优势度最低,具有更佳的保育和维持林下生物多样性功能。因此,乡土树种混交林更适用于生态公益林构建或对一些针叶林及外来树种纯林进行林分改造。     

先锋种丰富度对边坡植被群落特征及其护坡效益的影响

乡土植物灌木化建植是高速公路边坡防护的重要途径。以成渝高速(G85)永川段为例,以西南地区两种典型的边坡绿化初始配置"慈竹(Neosino calamus affinis)+野牛草(Buchloe dactyloides)"、"棉槐(Amorpha fruticosa)+紫羊茅(Festuca rubra)"为基础,分别与马棘(Indigofera pseudotinctoria Mats.)、黄荆(Vitex negundo)、狗牙根(Cynodon dactylon)、芒(Miscanthus sinensis)等路域优势种以不同方式组合,研究一定立地条件下初始绿化植物(先驱植物)的丰富度对边坡植被群落生长发育、早期演替行为及护坡性能的影响。结果表明:1)先驱植物丰富度可直接影响边坡植被的成坪时间、生物量积累:相同建植条件(边坡条件、播种密度等)下,先锋种种类越多,成坪时间就越短,被积累的生物量也越多;2)先锋种数量与群落的生物多样性水平有关(R20.954):相同演替阶段内,先锋种越多,群落的多样性水平(如物种丰富度、Shannon-Wiener指数、Pielou指数)则越高;3)群落中物种多样性水平与植被的护坡性能密切相关(R20.998):多样性水平越高,植被的蓄水、保土能力越强,边坡内径流系数、土壤侵蚀模数越小。可见,在边坡条件、播种密度等建植条件与建植措施一致的情形下,提高初始绿化植物的物种数可有效改善边坡植被的护坡性能。     

植被状况对乔木幼苗物种多样性的影响

运用灰色关联度分析亚热带常绿阔叶林植被状况对乔木幼苗物种多样性的影响。结果表明,所选取的植被状况参数对乔木幼苗物种多样性有不同的影响,对乔木幼苗物种多样性影响较大的是草本层的Simpson指数、灌木层Simpson指数和灌木层盖度,影响较小的为草本层盖度和灌木层Shannon-Wiener指数。乔木幼苗4个物种多样性指数受植被状况影响的顺序为：Pielou均匀性指数＞Shannon-Wiener指数＞物种丰富度指数＞Simpson指数。草本层、灌木层和乔木层对乔木幼苗的物种多样性有不同的影响,其影响方式也不一样。     

黄土高原蓝藻和藓类生物结皮对草本植物多样性及生物量的影响

生物结皮在干旱和半干旱气候区占据了广阔的生态位,深刻影响草本植物的空间格局,但其影响程度和作用途径目前仍存有争议。分别以黄土高原风沙土和黄绵土上的蓝藻结皮和藓结皮为对象,以无结皮为对照,通过样方法调查并测定了生物结皮与无结皮样地中草本植物的多样性和生物量,继而结合灰色关联分析和结构方程模型,对比分析了两类生物结皮对草本植物多样性和生物量的影响程度及作用途径。结果表明：（1）风沙土上,无结皮样地的优势科为禾本科和菊科,蓝藻结皮样地的优势科为藜科,藓结皮样地的优势科为禾本科,而黄绵土上无结皮样地的优势科为菊科、禾本科和豆科,蓝藻结皮样地的优势科与无结皮相似,藓结皮样地的优势科为禾本科和菊科;（2）与无结皮相比,风沙土上蓝藻结皮和藓结皮样地中多年生草本的株数分别增加了30%和16%,而黄绵土上两类生物结皮样地中多年生草本株数则分别增加了44%和减少了46%;（3）与无结皮相比,风沙土上蓝藻结皮和藓结皮样地中草本植物的Patrick和Shannon指数分别降低了1.0、0.2和3.4、0.5;而黄绵土上蓝藻结皮和藓结皮样地中两种多样性指数分别降低了4.8、0.2和4.2、0.4;（4）风沙土上蓝藻结皮和藓结皮样地的草本植物生物量比无结皮减少了111.5 g/m²和145.0 g/m²,而在黄绵土上则分别减少了127.8 g/m²和172.2 g/m²;（5）灰色关联分析显示,土壤穿透阻力对草本植物的多样性影响最大（关联系数分别为0.960和0.996）,而NH₄⁺-N和NO₃^--N含量对草本植物的生物量影响最大（两种土壤上关联系数分别为0.727和0.752）。综上,黄土高原生物结皮显著降低了草本植物的多样性和生物量,其途径为通过增加土壤穿透阻力以降低草本植物的种子萌发几率并限制幼苗根系伸展,同时通过土壤氮竞争限制了草本植物生长。     

Positive effects of plant diversity on soil microbial biomass and activity are associated with more root biomass production

This study aims to explore relationships between plant diversity and soil microbial function and the factors that mediate the relationships. Artificial plant communities (1, 2, 4 and 8 species) were established filled with natural and mine tailing soils, respectively. After 12 months, the plant species richness positively affected the soil microbial functional diversity in both soil environments but negatively affected microbial biomass and soil basal respiration in the natural soil. The root biomass positively correlated with the microbial biomass, cultural bacterial activity and soil basal respiration in both soil environments. Moreover, the D_i (deviations between observed performances and expected performances from the monoculture performance of each species of mixture) of microbial biomass, cultural bacterial activity and soil basal respiration positively correlated with the D_i of root biomass in both soil environments. Consistent with stress-gradient hypothesis, the D_mix (over-function index) of aboveground biomass positively correlated plant species richness in the mine tailing soil. Results suggest that the root biomass production is an important mechanism that affects the effects of plant diversity on soil microbial functions. Different responses of soil microbial function to increasing plant diversity may be due to root biomass production mediated by other factors.     

莽山浪畔湖国家重要湿地植物多样性及环境解释

研究莽山高山湖泊湿地植物多样性对环境因子的响应分析,不仅有助于深入了解湿地植物对环境变化下的适应策略,同时对保护当地湿地珍稀濒危物种具有重要价值。采用系统聚类分析、典范对应分析(CCA)和Pearson相关性分析等方法,对研究区30个植物群落样方进行分析,研究植物多样性与环境因子(土壤、水)之间的相互关系。研究结果表明:浪畔湖国家重要湿地植物群落可分为蕨状苔草+宽叶泽苔草群丛、雪白睡莲+莼菜群丛、水毛花+宽叶泽苔草群丛、莼菜+宽叶泽苔草群丛等4类。雪白睡莲+莼菜群丛Patrick丰富度指数和Pielou均匀度指数最低,Simpson多样性指数最高,水毛花+宽叶泽苔草群丛与之相反。浪畔湖国家重要湿地植物多样性与土壤速效钾和水深呈极显著相关性,与水质氨氮呈显著相关性,水深和土壤速效钾是影响湿地植物群落分布格局的主要因子。群落物种多样性指数与环境因子之间存在显著相关性,群落物种丰富度指数随水深增加呈先增后降的趋势,群落物种均匀度指数与水质氨氮浓度之间呈负相关关系。本研究旨在揭示浪畔湖国家重要湿地植物多样性与环境因子之间的生态关系,为保护湿地珍稀物种及维持湿地生态系统稳定提供理论依据。     

Contrasting Cloud Forest Restoration Potential Between Plantations of Different Exotic Tree Species

The role of exotic tree plantations for biodiversity conservation is contested. Such plantations nevertheless offer various ecosystem service benefits, which include carbon storage and facilitation of indigenous tree species regeneration. To assess forest restoration potential in tropical exotic tree plantations, we assessed native cloud forest tree regeneration in 166 plots in ca. 50‐year‐old plantations of five timber species that are widely used in tropical plantations (Pinus patula, Eucalyptus saligna, Cupressus lusitanica, Grevillea robusta and Acacia mearnsii). Differences in species abundance, diversity and composition were compared among plantations, and between plantations and disturbed and undisturbed indigenous Afromontane cloud forest (southeast Kenya) relicts after controlling for environmental variation between plots (i.e. altitude, distance to indigenous forest, soil depth, slope, aspect) and for environmental and stand structural variation (i.e. dominant tree height and basal area). Regenerating trees were mostly early‐successional species. Indigenous tree species regeneration was significantly higher in Grevillea plantations, where the seedling community also included late‐successional tree species. Regeneration under Eucalyptus was particularly poor. Acacia had a strong invasive nature, reducing its potential role and usefulness in indigenous forest restoration. Our study underlined that exotic tree plantations have differential effects on native tree species regeneration, with high potential for Grevillea plantations and low potential for invasive exotic species.     

Use of indigenous ecological knowledge of the Maasai pastoralists for assessing rangeland biodiversity in Tanzania

This paper incorporates the indigenous ecological knowledge (IEK) of the Maasai pastoralists and ecological methods to assess effects of grazing and cropping on rangeland biodiversity at macro‐ and micro‐landscape scales in northern Tanzania. The joint surveys with pastoralists identified indicator plant species and their associations with micro‐landscapes and livestock grazing suitability (i.e. for cattle and small ruminant grazing), while traditional calf‐pasture reserves (alalili pl. alalilia) were evaluated for preservation of rangeland biodiversity. The macro‐landscapes comprising the cool high plateau (osupuko pl. isipuki) and montane forest highland (endim) were included in the survey. At micro‐landscape scales, the osupuko was classified into uplands (orkung'u), slopes (andamata) and dry valley bottomlands (ayarata). The micro‐landscapes were assessed in terms of herbaceous plant species and woody species richness and risks of soil erosion. Biodiversity varied at both the macro‐ and micro‐landscape scales and in accordance with the land‐use types. Greater plant species diversity and less erosion risks were found in the pastoral landscapes than in the agro‐pastoral landscapes. The calf‐grazing pastures had greater herbaceous species richness than the non‐calf pastures, which in turn had more woody species. The study concludes that the indigenous systems of landscape classification provides a valuable basis for assessing rangeland biodiversity, which ecologists should incorporate into ecological surveys of the rangelands in East Africa in the future.     

Effects of soil fertility on early development of wetland vegetation from soil seed bank: Focusing on biomass production and plant species diversity

To examine the effects of soil fertility on biomass production, plant species diversity, and early vegetation development, we performed a mesocosm experiment using soil seed bank under three soil fertility levels (ombrotrophic, mesotrophic, and eutrophic). Biomass production linearly increased (P < 0.01), whereas plant species diversity significantly decreased (P < 0.001) as soil fertility increased. Soil fertility seemed to play a role as an environmental sieve in early vegetation development, in turn, lead the patterns of biomass production and plant species diversity. Several Poaceae species which are forming tall and dense canopy, such as Phalaris arundinacea, Miscanthus sacchariflorus, and Zizania latifolia, showed higher importance values under fertilized condition, whereas relatively small macrophytes of Eleocharis acicularis, Carex dickinsii, and Isachne globosa mainly survived under ombrotrophic condition. Although plant species richness decreased as soil fertility increased, the percentages of perennials and exotics increased rather than annuals and natives. Predominance of perennials and exotics under eutrophic condition in early vegetation development seemed to lead a continuous decrease in plant species diversity as a result of competitive effect. A significant negative relationship between plant species diversity and biomass production (P < 0.0001) was obtained when all experimental plots are included in correlation analysis.     

城市化对本土植物多样性的影响——以廊坊市为例

虽然城市化对生物多样性影响的研究在发达国家是一个重要的研究领域,但是在发展中国家这方面的研究不多。通过案例研究,分析本土植物多样性沿着城市化梯度的变化,及其与生境土壤因子的关系。在廊坊市,中国北部一个快速城市化的地区,沿着中心城区、城区、郊区、远郊区城市化梯度,每个梯度选取6个样地进行本土植物多样性调查,记录种类数、多样性和种类组成,并分析了多样性指数。与远郊区相比,中心城区失去了88%的物种,物种多样性下降了78%,城区物种多属于禾本科、藜科等耐践踏、耐土壤紧实度的物种。远郊区的本土植物属于45个科,科数大于城区物种。相似性指数表明,城区和郊区大部分物种相同,但是远郊区差异较大。DCCA分析表明,土壤总氮、有机质含量是影响物种城市化分布的主要因素。城市化促进了物种分布的匀质化。     

Local and regional assessments of the impacts of plant invaders on vegetation structure and soil properties of Mediterranean islands

Aims Although biological invasions occur throughout the world, and some invaders are widespread in many habitats, few studies on the ecological impact of invaders have examined multiple sites. We tested how the impact of three widespread plant invaders changed depending on the identity of the species and the invaded island. We also tested whether relative species loss was lower in species‐rich communities than in species‐poor ones. Location We conducted floristic surveys and soil analyses in eight Mediterranean Basin islands: Crete and Lesbos (Greece), Sardinia (Italy), Corsica, Bagaud and Porquerolles (France), and Mallorca and Menorca (Spain). Methods We compared native species richness and diversity, proportion of life forms, soil percentage nitrogen, percentage organic carbon, C/N, and soil pH in nearby paired plots of 2 × 2 m: one control and one invaded by either the deciduous tree Ailanthus altissima, the succulent subshrubs Carpobrotus spp. or the annual geophyte Oxalis pes‐caprae, across eight Mediterranean Basin islands. Results On average, the presence of invaders reduced species diversity, Carpobrotus spp. exhibiting the largest impact and Oxalis the least. However, the relative impact was island‐dependent, and was positively but weakly associated with the species richness of the recipient community. Therophytes were the life form that experienced the largest decrease across islands. The effects of invasion on soil properties were very variable. Total N changed (increased) only in plots invaded by Ailanthus, significantly decreasing the C/N ratio. The presence of this tree increased soil pH, whereas the opposite was found in plots invaded by the other two species. Organic C increased in plots invaded by Ailanthus and Carpobrotus species. Main conclusions By conducting an analysis at multiple sites, we found that the three plant invaders had an impact on plant community structure not entirely concordant with changes in soil properties. The impacts depended on the identity of the species and of the invaded island, suggesting that impact of invaders is context‐specific. The impact in terms of species loss was not lower in species‐rich than in species‐poor communities.     

Scale-dependent relationships between the spatial distribution of a limiting resource and plant species diversity in an African grassland ecosystem

One cornerstone of ecological theory is that nutrient availability limits the number of species that can inhabit a community. However, the relationship between the spatial distribution of limiting nutrients and species diversity is not well established because there is no single scale appropriate for measuring variation in resource distribution. Instead, the correct scale for analyzing resource variation depends on the range of species sizes within the community. To quantify the relationship between nutrient distribution and plant species diversity, we measured NO₃^- distribution and plant species diversity in 16 paired, modified Whittaker grassland plots in Serengeti National Park, Tanzania. Semivariograms were used to quantify the spatial structure of NO₃^- from scales of 0.4–26 m. Plant species diversity (Shannon-Weiner diversity index; H ) was quantified in 1-m² plots, while plant species richness was measured at multiple spatial scales between 1 and 1,000 m². Small-scale variation in NO₃^- (<0.4 m) was positively correlated with 1-m² H , while 1,000-m² species richness was a log-normal function of average NO₃^- patch size. Nine of the 16 grassland plots had a fractal (self-similar across scales) NO₃^- spatial distribution; of the nine fractal plots, five were adjacent to plots that had a non-fractal distribution of NO₃^-. This finding offered the unique opportunity to test predictions of Ritchie and Olff (1999): when the spatial distribution of limiting resources is fractal, communities should display a left-skewed log-size distribution and a log-normal relationship between net primary production and species richness. These predictions were supported by comparisons of plant size distributions and biomass-richness relationships in paired plots, one with a fractal and one with a non-fractal distribution of NO₃^-. In addition, fractal plots had greater large-scale richness than paired non-fractal plots (1,0–1000 m²), but neither species diversity (H ) nor richness was significantly different at small scales (1 m²). This result is most likely explained by differences in the scale of resource variation among plots: fractal and non-fractal plots had equivalent NO₃^- variation at small scales but differed in NO₃^- variation at large scales (as measured by the fractal dimension). We propose that small-scale variation in NO₃^- is largely due to the direct effects of plants on soil, while patterns of species richness at large scales is controlled by the patch size and fractal dimension of NO₃^- in the landscape. This study provides an important empirical step in understanding the relationship between the spatial distribution of resources and patterns of species diversity across multiple spatial scales.