山西芦芽山针叶林草本层群落谱系结构与多样性的海拔格局
赵鸣飞1,2, 薛峰1,2, 王宇航1,2, 王国义1,2, 邢开雄1,2, 康慕谊1,2,*, 王菁兰3
1北京师范大学地理科学学部地表过程与资源生态国家重点实验室, 北京 100875
2北京师范大学地理科学学部资源学院, 北京 100875
3清华大学生命科学学院,北京 100084

* 通信作者Author for correspondence (E-mail:kangmy@bnu.edu.cn)

摘要

物种多样性格局及其成因是生态学的经典问题之一。谱系方法将演化信息(即物种亲缘关系)整合到群落学研究中, 给群落学研究带来了全新视角。该研究以山西芦芽山针叶林冠层下草本群落为研究对象, 将群落物种组成数据、厘定有进化时间的谱系树与地形、土壤等生境数据相结合, 运用一般线性回归、基于距离矩阵的多元回归和谱系结构主坐标分析(PCPS)等统计方法, 探究了草本群落谱系结构与谱系多样性的海拔格局及其与生境因子的关系。结果表明: 所有样地的谱系结构指标均落在随机置信带之内, 说明中性因素对草本群落构建的影响较为强烈。同时,谱系结构与海拔梯度之间存在显著的正线性关系, 暗示确定性过程的作用也不可忽略, 种间竞争排斥可能促进了低海拔地区谱系结构呈发散趋势, 而高海拔的生境过滤引发了谱系聚集趋势。基于最近谱系距离的beta多样性与海拔差异之间呈极显著正相关关系, 同样反映了草本层物种集聚受到了生境过滤的显著影响。PCPS模型前两轴代表了55.9%的谱系-物种组成变异, 海拔是引起该变异的关键因子; PCPS二维排序图显示, 高海拔地区群落的谱系聚集可能由菊科、禾本科等大科所主导, 低海拔则显示出较高的演化分支多样性, 从而表现出谱系发散。综上所述, 该研究揭示了山西芦芽山山地针叶林冠层下草本群落谱系多样性格局, 并初步明晰了主要生态过程对物种集聚的影响规律。

关键词: 多样性海拔格局; 谱系alpha多样性; 谱系beta多样性; 谱系结构主坐标分析; 草本植物
doi: 10.17521/cjpe.2016.0247
Phylogenetic structure and diversity of herbaceous communities in the conifer forests along an elevational gradient in Luya Mountain, Shanxi, China
ZHAO Ming-Fei1,2, XUE Feng1,2, WANG Yu-Hang1,2, WANG Guo-Yi1,2, XING Kai-Xiong1,2, KANG Mu-Yi1,2,*, WANG Jing-Lan3
1State Key Laboratory of Earth Surface Processes and Resource Ecology, Faculty of Geographical Science, Beijing Normal University, Beijing 100875, China
2College of Resources Science & Technology, Faculty of Geographical Science, Beijing Normal University, Beijing 100875, China
and3School of Life Science, Tsinghua University, Beijing 100084, China
Abstract

Aims Incorporating phylogenetic data in the studies of species diversity patterns along elevational gradients can bridge the gap between ecological and evolutionary processes, and thus shed light on the issues related to community assemblage. We aim is to explore the elevational patterns of phylogenetic relatedness and phylodiversity in the herbaceous angiosperm assemblages of alpine conifer forest within mountain ecosystem, and to quantify the relationship between the patterns and habitat factors. Methods We sampled 17 plots (20 m × 30 m) in the coniferous forest communities at ca. 50 m altitudinal intervals along the elevation gradient. In each plot, we documented all species encountered and the environmental conditions. We used the Net Relatedness Index ( NRI) and Nearest Taxon Index ( NTI) to quantify the phylogenetic structures of each herbaceous assemblage, and used the Mean Pairwise Distance and Mean Nearest Taxon Distance to quantify phylogenetic beta diversity. Ordinary least square regression and multiple regression on distance matrices were employed respectively to explore the elevational trends of phylogenetic structure and phylobetadiversity. We analyzed the taxonomic composition of the herbaceous assemblages within forests, and demonstrated the relationship between the clustering of clades and the significant habitat descriptors using principal coordinates of phylogenetic structure (PCPS). Important findings The result showed that the herbaceous communities tended to be more phylogenetically overdispersed at lower elevations, suggesting that intraspecific competition potentially influences the local assemblages. In contrast, species occurring at high-elevation sites tended to be more closely related, implying that these communities are structured primarily by environmental filtering. However, we found that all of the NRI (or NTI) were confined within 95% confidence intervals, suggesting strong contributions of stochastic processes on species assembly. Phylogenetic beta diversity significantly increased with the elevational distance between community pairs, also suggesting that habitat filtering probably played an important role on structuring the herbaceous communities. The first two axes of PCPS contained 55.9% of total variation in phylogeny-weighted species composition, and were both significantly related with elevation and arboreal basal area. We found that species of large families, including Asteraceae and Poaceae, were phylogenetically clustered at high elevations, whereas the others were phylogenetically overdispersed at low elevation region. Our findings suggest that exploring the patterns of phylogenetic structures across elevational gradients is important, which can provide insights into the underlying mechanisms shaping community composition within montane ecosystems.

Key words: diversity elevational pattern; phylogenetic alpha diversity; phylogenetic beta diversity; principal coordinates of phylogenetic structure; herbaceous plants

探求群落物种组成以及群落间物种周转格局的形成原因一直以来是生态学的核心问题(Ricklefs, 2004), 学界对关键机制的认识目前尚存争议(Bryant et al., 2008; Telloet al., 2015)。目前, 生态学家已普遍达成共识, 认为物种多样性是生态位过程与中性因素等诸多生态过程共同作用的结果(Oommen & Shanker, 2005; McCain, 2009)。前者包括非生物因子(如气候条件或土壤营养状况等)和生物因子(如竞争、捕食、共生等种间相互作用)等确定性因素; 后者包括生态漂变和扩散限制等随机因素(Gilbert & Lechowicz, 2004; Soininenet al., 2007)。事实上, 基于宏观尺度, 与物种形成、灭绝以及长距离扩散有关的进化过程则是直接决定群落多样性水平的背景性动因(Wiens & Donoghue, 2004; Wienset al., 2007)。这一点正逐渐受到越来越多生态学工作者的重视(Bryantet al., 2008; Swensonet al., 2011)。

人们很早就发现, 物种的功能性状在演化上普遍具有内在关联(Elton, 1946)。具体来说, 由于物种对不同生境状况所做出的响应主要通过其功能性状来实现, 而演化的生态位保守性(niche conservatism)使近缘物种保持来自共同祖先的功能属性(Vamosiet al., 2009)。因此, 群落成员之间的谱系关系能够体现它们生态位的相似性。例如, 由生境过滤作用所主导的群落构建过程, 有利于筛选出具有耐受特定环境生理性状的物种(这些物种往往来自同一分支), 从而在谱系结构上表现出聚集(phylogenetically clustering)。相反, 如果群落成员间存在强烈的竞争排斥作用, 则有利于具有不同资源利用方式的物种稳定共存。这些物种之间的亲缘关系一般较为疏远, 从而使谱系结构呈现发散(phylogenetically over-dispersion) (Webbet al., 2002; Losos, 2008)。另外, 在生态位保守假设成立的情况下, 若谱系结构并未表现出统计意义上的显著聚集或发散, 则可能意味着物种集聚主要被中性过程所驱动(Kembel & Hubbell, 2006)。由此可见, 整合有谱系信息的多样性研究可为探究物种共存机制等核心生态学问题提供更为深刻的洞见(Webbet al., 2002; Cavender- Bareset al., 2009)。

山地生态系统通常在有限的空间内具备宽广的水热梯度和较高的生境异质性(Gaston, 2000; Kö rner, 2007; Sheldonet al., 2011)。这些特点一方面为新物种形成创造了有利条件, 使山地成为多样性的热点地域; 另一方面, 由于该系统对全球气候变化十分敏感, 从而增加了物种的灭绝风险。因此, 物种多样性沿海拔梯度的变化情况常被视作大尺度纬向格局的镜像, 长久以来为人们所关注(Stevens, 1992; Lomolino, 2001; Rahbek, 2005; Tanget al., 2012a)。然而, 有关群落谱系结构及其多样性沿海拔梯度变化规律的研究目前还处于起步阶段(Machac et al., 2011), 尚未形成统一的认识。现有经验案例显示, 研究结果会随着山地和生物类群的不同而存在显著差异。一般来说, 高海拔低温可对多数物种构成主要的生理胁迫, 因而群落谱系结构理论上应趋向聚集。这一预测已在蚂蚁(Machac et al., 2011)、蜂鸟(Graham & Mcguire, 2009)、蕨类植物(Kluge & Kessler, 2011)以及高山被子植物(Li et al., 2014)等多个类群得到验证。而Bryant等(2008)对美国落基山脉植物群落和土壤微生物群落谱系结构的海拔格局进行对比研究发现: 与理论预期相反, 该山地被子植物在低海拔地区谱系聚集, 而在高海拔地区谱系发散。卢孟孟等(2014)通过探索哀牢山森林群落谱系结构在海拔梯度上的变化, 发现采用不同指标得到的结论并不一致。

草本层是森林生态系统的关键组分, 对于温带森林植被的多样性尤为重要, 其物种数往往占整个群落物种丰富度的比例最大(Gilliam, 2007)。另外, 草本层在调控乔木物种更新、促进养分循环、增加生境异质性等诸多生态过程中发挥着重要作用(Gilliam, 2007; Cook, 2015; Má rialigeti et al., 2016)。地处山西西北部的芦芽山具有较大相对海拔高差, 并存在大面积完好、典型的寒温性针叶林。这些森林属于演替后期相对稳定的顶级群落, 建群种组成简单, 灌木层因林冠郁闭度较高而普遍不发达, 因此为其下的草本层物种创造了相对同质的生境条件(张金屯, 1989)。本文以该地区针叶林草本层为研究对象, 基于厘定有进化时间的谱系树, 结合物种组成与生境因子数据, 通过运用多种统计分析方法, 尝试回答以下科学问题: (1)冠层下草本层物种集聚的谱系结构沿海拔梯度呈怎样的格局? (2)谱系beta多样性与海拔差异的相互关系及解释。(3)群落物种组成、谱系关系与环境因子之间的定量关系如何?

1 材料和方法
1.1 研究区概况

研究区位于山西省西北部吕梁山脉北端的芦芽山(38.60° -39.03° N, 111.77° -112.90° E), 其主峰荷叶坪海拔2 783 m。该地域属于暖温带半湿润区, 大陆性气候特征显著, 夏季凉爽多雨, 冬季寒冷干燥。根据研究区内五寨和宁武气象站的观测资料, 1971-2010年间平均年降水量为444 mm, 年平均气温为5.9 ℃ (李颖俊等, 2016)。芦芽山植被具有明显的垂直地带性, 低中山以暖温带落叶阔叶林和灌草丛为主; 海拔1 750-2 600 m覆盖有完整的寒温性针叶林, 其建群种由青扦(Picea wilsonii)、白扦(Picea meyeri)和华北落叶松(Larix principis-rupprechtii)组成; 海拔2 450 m以上开始出现亚高山灌丛草甸, 直至峰顶(张文涛等, 2015)。对应植被带变化, 土壤亦呈垂直地带性分布, 从低海拔起依次为: 山地褐土、山地淋溶褐土、棕色森林土和亚高山草甸土(张金屯, 1989)。

1.2 样地设置及取样方法

于海拔1 800-2 650 m的典型针叶林带布设样地, 其样方大小为20 m × 30 m, 海拔间隔约50 m, 共计17个样方。每个样方进一步划分为6个10 m × 10 m的样格。对样方内胸径大于3 cm的乔木进行每木调查, 记录其树名、胸径、高度和枝下高等特征。利用胸径计算每木胸高断面积, 按样方累加后获得胸高断面积和; 通过计算每木树高与枝下高的平均差值来估计样地的平均冠层高。选择两个对角位置的10 m × 10 m样格作为灌木样方, 调查其内灌木种类、高度和盖度。于每样格中间位置设置1 m × 1 m的草本样方(共6个), 统计种类、盖度与高度。同时利用GPS记录样地经纬度、海拔高度, 并利用罗盘记录其坡度、坡向等地形情况。分析时利用余弦公式(Roberts & Cooper, 1989)将坡向转换为[0, 1]区间的数据, 以反映生境的干湿程度。在进行植被调查的同时, 于每一样格随机选取3个点(共18个重复), 使用钢尺测量凋落物厚度。通过土壤剖面对样地土壤特征进行评估, 并利用环刀收集0-20 cm层土壤样品, 带回实验室经风干、研磨过筛后, 分别采用湿燃烧法、凯氏定氮法和钼锑抗比色法测定土壤有机碳、全氮与全磷含量。对种下分类阶元进行归并后, 共发现草本被子植物125种, 其中单子叶植物6科18属22种, 双子叶植物27科80属103种。

1.3 谱系树与谱系多样性指数

1.3.1 谱系树的建构

利用Phylomatic在线建树工具(Webb & Donoghue, 2005), 选择最新发表的Zanne维管植物进化树提取草本植物群落的谱系树(Zanne et al., 2014)。Zanne进化树以被子植物分类系统APG III (2009)为骨架, 其分支长度代表了演化时间(利用化石数据推定)。

1.3.2 群落谱系结构测度

选择净关系指数(NRI)和最近分类单元指数(NTI)来反映物种集聚的谱系结构。二者分别对应经典谱系alpha多样性指数— — 平均谱系距离(MPD)和最近谱系距离的(MNTD)的标准化测度(Webb et al., 2008)。MPD代表了物种间亲缘关系的平均情况, 而MNTD侧重指示分支末端的谱系关系(Webb, 2000)。NRINTI的计算公式分别如下:

$NRI=-1\times \frac{MP{{D}_{\text{observed}}}-MP{{D}_{\text{randomized}}}}{sdMP{{D}_{\text{randomized}}}}$ (1)

$NTI=-1\times \frac{MNT{{D}_{\text{observed}}}-MNT{{D}_{\text{randomized}}}}{sdMNT{{D}_{\text{randomized}}}}$ (2)

其中, MPDobserved(或MNTDobserved)为各样地观测值, MPDrandomized(或MNTDrandomized)和sdMPDrandomized(或sdMNTDrandomized)则分别对应由零模型得到的平均值和标准偏差(本文采用随机化谱系树末端分支方案, n= 999)。NRI(或NTI)显著大于0意味着群落成员的谱系关系比区域种库的随机化情形更紧密, 表现为谱系聚集, 反之则说明谱系关系发散(Webb et al., 2002)。

1.3.3 群落间谱系beta多样性测度

同样基于MPDMNTD, 分别计算了群落间谱系beta多样性指数betaMPDbetaMNTD。计算公式 (Swenson, 2011)如下:

$betaMPD=\frac{1}{2}\left( \sum\limits_{i=1}^{{{n}_{a}}}{\overline{{{d}_{ib}}}}+\sum\limits_{j=1}^{{{n}_{b}}}{\overline{{{d}_{ja}}}} \right)$ (3)

$betaMNTD=\frac{1}{2}\left[ \sum\limits_{i=1}^{{{n}_{a}}}{\min ({{d}_{ib}})}+\sum\limits_{j=1}^{{{n}_{b}}}{\min ({{d}_{ja}})} \right]$ (4)

其中, min(d)和$\bar{d}$分别表示某样地出现的任意一个物种与另一样地所有物种间的最小及平均枝长; I(或j)表示遍历计算时的对象物种; na(或nb)表示样地 a(或b)中的物种总数。

1.4 统计分析

1.4.1 回归分析

使用散点图分别展示草本群落谱系结构、谱系beta多样性沿海拔梯度的变异情况。并利用普通最小二乘回归来量化谱系多样性指数与海拔之间的相互关系。由于谱系beta多样性的样对数据之间不独立, 采用基于距离矩阵的多元回归方法(MRM) (Lichstein, 2007)对回归系数进行置换检验(999次)。

1.4.2 谱系结构主坐标分析

为进一步从整体上分析群落谱系结构变异, 厘定主导因子, 本研究采用Duarte (2011)提出的谱系信息加权的排序方法, 即谱系结构主坐标分析(PCPS), 其主要分析步骤如下。首先, 通过谱系树生成种间谱系距离(以种对间的枝长度量)矩阵Dp, 由其中最大谱系距离归一化, 并转换为谱系相似性矩阵SM; 利用模糊集合算法(fuzzy set algorithm)由SM得到谱系隶属度(phylogenetic fuzzy belonging)矩阵Q; 以原始“ 样地-物种” 多度矩阵(或01矩阵) W作为权重右乘Q, 从而得到含有谱系信息的新“ 样地-物种” 矩阵P; P进一步转换为样地间beta多样性距离矩阵dP(本文在此选择Bray-Curtis指数); 接下来即可对dP进行主坐标分析(principal coordinates analysis, PCoA)。能够兼容更多距离、半距离测度是使用PCoA的优势所在。更多细节可参考(Pillar & Duarte, 2010; Duarte, 2011)。最后, 我们将解释变量被动加入PCPS排序结果。这些变量先通过计算膨胀因子排查严重共线性(本例中膨胀因子均小于10, 即该问题不存在), 后经基于置换检验的“ 向前选择式” 逐步回归以保留显著的生境变量。

因本文旨在探讨物种分布的限制因素(Lozupone et al., 2007), 所有指数计算时均未进行多度加权。以上统计分析由R软件完成, 谱系多样性指数计算、MRM和PCPS分析分别由Picante、ecodist和PCPS软件包实现。

2 结果

整体而言, NRI反映了芦芽山林下草本群落谱系结构随着海拔梯度的上升呈现由发散到聚集的变化趋势, 且回归系数达到统计显著水平(R2 = 0.447, p= 0.002, 图1A); 但NTI与海拔并无显著线性关系(R2 = 0.001, p= 0.702, 图2B)。具体来讲, 每个群落的谱系结构均处于随机置信区间内(NRINTI绝对值均小于1.96, 图1)。17个样方中有4个NRI大于0, 且均为高海拔地区群落; 而对于NTI, 虽然多数群落(76.5%)取值大于0, 但并无明显的海拔分布趋势。

图1 芦芽山针叶林草本群落谱系结构的海拔格局。Fig. 1 Elevational patterns of the phylogenetic structures of herbaceous assemblages in the coniferous forest plots in Luya Mountain.

回归分析发现, 代表群落间谱系近缘关系的betaMNTD与海拔差异存在极显著的正相关关系(R2 = 0.691, p< 0.001, 图2B)。而群落间平均谱系关系的betaMPD与样地海拔差异之间并不存在显著线性关系(R2 = 0.001, p= 0.668, 图2A)。

图2 芦芽山针叶林草本层谱系beta多样性与海拔差异之间的关系。Fig. 2 The relationships of phylogenetic beta diversity with elevational distance for the herbaceous plants assemblages in the coniferous forest plots in Luya Mountain.

PCPS模型全局F检验达到显著水平(F-pseudo = 1.173, p= 0.014, 999次置换)。前两轴(特征值分别为0.301和0.090)累积解释了55.9%的总变异。PCPS二维排序图显示, 环境因子中只有海拔、胸高断面积和对草本层物种集聚具有显著影响(图3; 表1), 而土壤、地形等其他局地因子的作用均不显著。由图3看出, 排序前两轴代表了全局种库谱系关系中演化早期分支的变异情况(例如单子叶分支和双子叶分支, 图1)。群落中物种丰富度大的科级分支(物种数大于5)多与海拔或胸高断面积和密切相关, 如菊科和禾本科存在高海拔聚集趋势, 而蔷薇科、石竹科、毛茛科等物种数少的科更多分布于中、低海拔地区。

图3 草本群落物种分布、谱系结构与环境因子之间相互关系的谱系结构主坐标分析(PCPS)排序图。●, 草本植物种, 包含5种以上的科以彩色显示; Ele, 海拔; BA, 胸高断面积和。Fig. 3 Scatter diagram between the first two axes of the principal coordinates of phylogenetic structure (PCPS) for herbaceous plants occurring in the coniferous forest plots in Luya Mountain. Color points represent large families (> 5 species) grouped in monocots and dicotyledon clades. Ele, elevation; BA, total basal area of breast height.

表1 环境因子与谱系结构主坐标分析(PCPS)前两排序轴的关系 Table 1 Correlations between the first two constrained ordination axes (PCPS 1 and PCPS 2) and environmental factors
3 讨论
3.1 谱系结构在海拔梯度上的变化

通过经验值与区域种库零模型的比较发现, 研究区林下草本群落的谱系结构均未超出随机阈值范围(p= 0.05, 图1)。这说明在局地尺度, 群落物种集聚受到较大的中性因素作用。换言之, 生境过滤、种间竞争等确定性过程不足以抵消种群随机波动等背景性因素的影响(Kembel & Hubbell, 2006)。这可能与草本植物的自身特点有关。与木本植物相比, 草本植物往往生活史周期短, 扩散能力强, 能够迅速占领适宜生境斑块(Latham & Ricklefs, 1993; Donoghue, 2008), 其种群具有较大的时空波动性, 势必掩盖了其他过程对群落谱系结构的影响。另外, 竞争排斥与生境过滤作用相互平衡的群落也能引起谱系结构不显著(Helmuset al., 2007)。

虽然具体样地并未发现统计显著的谱系结构, 但NRI随着海拔的升高存在由发散向聚集转变的趋势(图1A), 这与多数相似研究案例的结果相一致(Machacet al., 2011; Liet al., 2014; Qianet al., 2014)。此意味着, 除了中性过程以外, 种间竞争、生境过滤等生态位过程可能对草本群落的物种聚集也起到关键作用。生态位理论认为, 生境过滤与竞争排斥是促成物种共存且作用相反的两个主要过程。前者推动群落谱系结构趋向于聚集, 后者则利于谱系发散。对于山地生态系统来说, 一般低海拔地区具有相对良好的水热条件(也即生境过滤作用相对较弱), 有利于更多物种共存。亲缘关系越相近的物种, 其演化保守的关键性状也越相似(Wiens & Donoghue, 2004; Bryant et al., 2008)。由此带来较高的生态位重叠度, 促进了竞争排斥作用, 从而降低了种间稳定共存的概率, 使得谱系结构趋于发散(Machacet al., 2011)。高海拔地区则与此情况相反, 以冬季低温为代表的严峻气候条件可对多数物种形成生理限制。这种强烈的生境过滤将大多数尚未演化出冷适应性状的物种筛选排除, 仅剩少数耐寒分支(Wiens & Donoghue, 2004; Hawkins et al., 2014), 从而在谱系关系上呈现出聚集趋势。

3.2 谱系beta多样性与海拔差异之间的相关关系

谱系beta多样性能够度量群落之间物种演化关系在生境梯度或地理空间上的变化率, 可用于厘定区域整体受到生境过滤或扩散限制作用情况(Graham & Fine, 2008)。本研究发现, betaMNTD与海拔差异之间存在极显著正相关关系(图2B)。这说明生境过滤可能对群落构建过程起关键作用, 也即随着样地之间海拔差异的增大(通常意味着样地间热量条件的差异变大), 物种的亲缘关系也随之愈加疏远。同时, 这一结果暗示海拔梯度可能间接地解释了演化保守性状的大部分变异, 正是这些关键性状的变异使得生境差异对物种的筛选作用得以实现(Hardy & Senterre, 2007; Bryant et al., 2008; Fine & Kembel, 2011)。由于空间自相关现象普遍存在, 分离生境梯度与扩散限制这两类过程的独立影响通常较为困难(Tang et al., 2012b), 我们并不能直接排除与扩散限制有关的空间过程。然而, 由于本研究所涉及的空间尺度较小(最远样地之间距离小于10 km), 加之草本物种的扩散能力通常较强(见前文讨论), 我们认为扩散限制对草本群落物种集聚作用有限。这与Qian等(2014)对长白山森林案例类似结果的解释相一致。

3.3 谱系测度的特性及影响因素

需要强调的是, 本文基于两种谱系测度(MPDMNTD), 分别估计群落内部与群落之间成员亲缘关系的强度, 但它们对解释变量的灵敏度可能不尽相同。我们注意到, NRIbetaMNTD与海拔梯度之间存在显著线性关系, 而NTIbetaMPD并未发现显著趋势(图1, 图2), 这很可能跟指标本身的算法特性有关。MPD(或betaMPD)主要代表谱系关系的平均情况, 而MNTD(或betaMNTD)对谱系树末端分支的分布情况更为敏感(Webbet al., 2002)。被零模型标准化之后的NRINTI同样继承了上述特性。具体来说, 对区域种库整体上具有谱系聚集趋势的情况, 该两指标同时倾向于取得较高值; 而谱系聚集发生于谱系树的多个浅层分支上时, 仅NTI易于得到较高值(Vamosiet al., 2009)。此外, 取样误差、零模型的选择以及谱系树末端分支的解析程度都会给最终结果带来一定噪音。由此可见, 同时选取多个相关指标进行估计、比较, 有助于判别并量化关键过程。

3.4 整合谱系信息的群落物种组成变异与环境因子的关系

自Webb等(2000)首次提出以NRINTI作为群落谱系关系的量化指标以来, 越来越多的生态学者将其纳入相关研究。如上文所述, 这些指标可从不同层次表征演化历史信息, 然而单一测度难以兼顾谱系树连续变化情况, 即等级结构, 因而无法获得群落谱系结构与多个环境梯度的协同渐变关系(Duarte, 2011)。

作为一种全新的多元分析方法, PCPS能够将谱系信息整合到群落物种组成变异中, 并进一步与环境因子等多维变量相结合, 利用排序降维技术, 定量地将主要信息投影到二维空间(Duarte, 2011), 从而有助于人们理解关键生态过程如何连续地影响群落物种集聚与谱系结构。例如, 在本研究中我们发现, 随着海拔的升高, 建群种郁闭度亦有增加的趋势(由胸高断面积和指征), 二者的交互效应相互叠加后可能会增强对草本层物种的生境过滤作用, 导致适应了高海拔暗针叶林冠层下低温、低照度生境的演化分支呈减少趋势(如图3显示, 菊科物种可能是少数能够适应如此生境的分支之一); 相反, 低海拔地区则汇集有更多远缘类群(谱系发散)。由此可见, PCPS不但与本研究谱系多样性分析所得结论相吻合, 还能使我们更加明晰地知晓具体哪些演化分支引发了谱系结构的集聚或发散, 以及这些分支与关键生境因子具有怎样的相互关系。

4 结论

综上所述, 本研究揭示了芦芽山针叶林冠层下草本群落谱系结构随海拔升高而呈现由发散到聚集的趋势; 海拔差异是驱动群落间谱系beta多样性递减格局的主因; 通过PCPS方法综合展示了群落谱系分支构成及其与主要生境因子之间的相互关系。上述结果表明研究区草本群落构建过程可能是生境过滤、竞争排斥以及随机因素等生态因素共同作用的结果。将谱系理念引入群落生态学研究, 可为全球气候变化背景下的生物多样性保育与维持工作带来全新的理论视角和重要的应用价值。

The authors have declared that no competing interests exist.

作者声明没有竞争性利益冲突.

参考文献
[1] APG III (2009). An update of the Angiosperm Phylogeny Group classification for the orders and families of flowering plants: APG III. Botanical Journal of the Linnean Society, 161, 105-121. [本文引用:1]
[2] Bryant JA, Lamanna C, Morlon H, Kerkhoff AJ, Enquist BJ, Green JL (2008). Microbes on mountainsides: Contrasting elevational patterns of bacterial and plant diversity. Proceedings of the National Academy of Sciences of the United States of American, 105, 11505-11511. [本文引用:3]
[3] Cavender-Bares J, Kozak KH, Fine PV, Kembel SW (2009). The merging of community ecology and phylogenetic biology. Ecology Letters, 12, 693-715. [本文引用:1]
[4] Cook JE (2015). Structural effects on understory attributes in second-growth forests of northern Wisconsin, USA. Forest Ecology and Management, 347, 188-199. [本文引用:1]
[5] Donoghue MJ (2008). Phylogenetic perspective on the distribution of plant diversity. Proceedings of the National Academy of Sciences of the United States of American, 105, 11549-11555. [本文引用:1]
[6] Duarte LDS (2011). Phylogenetic habitat filtering influences forest nucleation in grassland s. Oikos, 120, 208-215. [本文引用:3]
[7] Elton C (1946). Competition and the structure of ecological communities. Journal of Animal Ecology, 15, 54-68. [本文引用:1]
[8] Fine PVA, Kembel SW (2011). Phylogenetic community structure and phylogenetic turnover across space and edaphic gradients in western Amazonian tree communities. Ecography, 34, 552-565. [本文引用:1]
[9] Gaston KJ (2000). Global patterns in biodiversity. Nature, 405, 220-227. [本文引用:1]
[10] Gilbert B, Lechowicz MJ (2004). Neutrality, niches, and dispersal in a temperate forest understory. Proceedings of the National Academy of Sciences of the United States of America, 101, 7651-7656. [本文引用:1]
[11] Gilliam FS (2007). The ecological significance of the herbaceous layer in temperate forest ecosystems. BioScience, 57, 845-858. [本文引用:2]
[12] Graham CH, Fine PV (2008). Phylogenetic beta diversity: Linking ecological and evolutionary processes across space in time. Ecology Letters, 11, 1265-1277. [本文引用:1]
[13] Graham CH, Mcguire JA (2009). Phylogenetic structure in tropical hummingbird communities. Proceedings of the National Academy of Sciences of the United States of American, 106, 19673-19678. [本文引用:1]
[14] Hardy OJ, Senterre B (2007). Characterizing the phylogenetic structure of communities by an additive partitioning of phylogenetic diversity. Journal of Ecology, 95, 493-506. [本文引用:1]
[15] Hawkins BA, Rueda M, Rangel TF, Field R, Diniz-Filho JAF (2014). Community phylogenetics at the biogeographical scale: Cold tolerance, niche conservatism and the structure of North American forests. Journal of Biogeography, 41, 23-38. [本文引用:1]
[16] Helmus M, Savage K, Diebel MJ, Ives A (2007). Separating the determinants of phylogenetic community structure. Ecology Letters, 10, 917-925. [本文引用:1]
[17] Kembel SW, Hubbell SP (2006). The phylogenetic structure of a neotropical forest tree community. Ecology, 87, 86-99. [本文引用:2]
[18] Kluge J, Kessler M (2011). Phylogenetic diversity, trait diversity and niches: Species assembly of ferns along a tropical elevational gradient. Journal of Biogeography, 38, 394-405. [本文引用:1]
[19] Körner C (2007). The use of “altitude” in ecological research. Trends in Ecology & Evolution, 22, 569-574. [本文引用:1]
[20] Latham RE, Ricklefs RE (1993). Global patterns of tree species richness in moist forests: Energy-diversity theory does not account for variation in species richness. Oikos, 67, 325-333. [本文引用:1]
[21] Li XH, Zhu XX, Niu Y, Sun H (2014). Phylogenetic clustering and overdispersion for alpine plants along elevational gradient in the Hengduan Mountains Region, southwest China. Journal of Systematics and Evolution, 52, 280-288. [本文引用:2]
[22] Li YJ, Wang SY, Niu JJ, Fang KY, Li XL, Li Y, Bu WL, Li YH (2016). Climate-adial growth relationship of Larix principis-rupprechtii at different altitudes on Luya Mountain. Acta Ecologica Sinica, 36, 1608-1618. (in Chinese with English abstract)
[李颖俊, 王尚义, 牛俊杰, 方克艳, 李晓岚, 栗燕, 布文丽, 李玉晗 (2016). 芦芽山华北落叶松(Larix principis-rupprechtii)树轮宽度年表对气候因子的响应. 生态学报, 36, 1608-1618. ] [本文引用:1]
[23] Lichstein JW (2007). Multiple regression on distance matrices: A multivariate spatial analysis tool. Plant Ecology, 188, 117-131. [本文引用:1]
[24] Lomolino MV (2001). Elevation gradients of species-density: Historical and prospective views. Global Ecology Biogeography, 10, 3-13. [本文引用:1]
[25] Losos JB (2008). Phylogenetic niche conservatism, phylogenetic signal and the relationship between phylogenetic relatedness and ecological similarity among species. Ecology Letters, 11, 995-1003. [本文引用:1]
[26] Lozupone CA, Hamady M, Kelley ST, Rob K (2007). Quantitative and qualitative beta diversity measures lead to different insights into factors that structure microbial communities. Applied & Environmental Microbiology, 73, 1576-1585. [本文引用:1]
[27] Lu MM, Huang XC, Ci XQ, Yang GP, Li J (2014). Phylogenetic community structure of subtropical forests along elevational gradients in Ailao Mountains of southwest China. Biodiversity Science, 22, 438-448. (in Chinese with English abstract)
[卢孟孟, 黄小翠, 慈秀芹, 杨国平, 李捷 (2014). 沿海拔梯度变化的哀牢山亚热带森林群落谱系结构. 生物多样性, 22, 438-448. ] [本文引用:1]
[28] Machac A, Jand a M, Dunn RR, Sand ers NJ (2011). Elevational gradients in phylogenetic structure of ant communities reveal the interplay of biotic and abiotic constraints on diversity. Ecography, 34, 364-371. [本文引用:4]
[29] Márialigeti S, Tinya F, Bidló A, Ódor P (2016). Environmental drivers of the composition and diversity of the herb layer in mixed temperate forests in Hungary. Plant Ecology, 217, 549-563. [本文引用:1]
[30] McCain CM (2009). Global analysis of bird elevational diversity. Global Ecology and Biogeography, 18, 346-360. [本文引用:1]
[31] Oommen MA, Shanker K (2005). Elevational species richness patterns emerge from multiple local mechanisms in Himalayan woody plants. Ecology, 86, 3039-3047. [本文引用:1]
[32] Pillar VD, Duarte LDS (2010). A framework for metacommunity analysis of phylogenetic structure. Ecology Letters, 13, 587-596. [本文引用:1]
[33] Qian H, Hao Z, Zhang J (2014). Phylogenetic structure and phylogenetic diversity of angiosperm assemblages in forests along an elevational gradient in Changbaishan, China. Journal of Plant Ecology, 7, 154-165. [本文引用:1]
[34] Rahbek C (2005). The role of spatial scale and the perception of large-scale species-richness patterns. Ecology Letters, 8, 224-239. [本文引用:1]
[35] Ricklefs RE (2004). A comprehensive framework for global patterns in biodiversity. Ecology Letters, 7, 1-15. [本文引用:1]
[36] Roberts DW, Cooper SV (1989). Concepts and techniques of vegetation mapping. In: Ferguson D, Morgan P, Johnson FD eds. Land Classifications Based on Vegetation: Applications for Resource Management, General Technical Report INF-257. Department of Agriculture, Forest Service, Intermountain Research Station, Odgen, USA. 90-96. [本文引用:1]
[37] Sheldon KS, Yang S, Tewksbury JJ (2011). Climate change and community disassembly: Impacts of warming on tropical and temperate montane community structure. Ecology Letters, 14, 1191-1200. [本文引用:1]
[38] Soininen J, McDonald R, Hillebrand H (2007). The distance decay of similarity in ecological communities. Ecography, 30, 3-12. [本文引用:1]
[39] Stevens GC (1992). The elevational gradient in altitudinal range: An extension of Rapoport’s latitudinal rule to altitude. The American Naturalist, 140, 893-911. [本文引用:1]
[40] Swenson NG (2011). Phylogenetic beta diversity metrics, trait evolution and inferring the functional beta diversity of communities. PLOS ONE, 6, e21264. doi: DOI:10.1371/journal.pone.0021264. [本文引用:2]
[41] Swenson NG, Anglada-Cordero P, Barone JA (2011). Deterministic tropical tree community turnover: Evidence from patterns of functional beta diversity along an elevational gradient. Proceedings of the Royal Society of London B: Biological Sciences, 278, 877-884. [本文引用:1]
[42] Tang ZY, Fang JY, Chi XL, Feng JM, Liu YN, Shen ZH, Wang XP, Wang ZH, Wu XP, Zheng CY (2012 a). Patterns of plant beta-diversity along elevational and latitudinal gradients in mountain forests of China. Ecography, 35, 1083-1091. [本文引用:1]
[43] Tang ZY, Fang JY, Chi XL, Yang YY, Ma WH, Mohhamot A, Guo ZD, Liu YN, Gaston KJ (2012 b). Geography, environment, and spatial turnover of species in China’s grassland s. Ecography, 35, 1103-1109. [本文引用:1]
[44] Tello JS, Myers JA, Macía MJ, Fuentes AF, Cayola L, Arellano G, Loza MI, Torrez V, Cornejo M, Mirand a TB (2015). Elevational gradients in β-diversity reflect variation in the strength of local community assembly mechanisms across spatial scales. PLOS ONE, 10, e0121458. doi: DOI:10.1371/journal.pone.0121458. [本文引用:1]
[45] Vamosi SM, Heard SB, Vamosi JC, Webb CO (2009). Emerging patterns in the comparative analysis of phylogenetic community structure. Molecular Ecology, 18, 572-592. [本文引用:2]
[46] Webb CO (2000). Exploring the phylogenetic structure of ecological communities: An example for rain forest trees. The American Naturalist, 156, 145-155. [本文引用:1]
[47] Webb CO, Ackerly DD, Kembel SW (2008). Phylocom: Software for the analysis of phylogenetic community structure and trait evolution. Bioinformatics, 24, 2098-2100. [本文引用:1]
[48] Webb CO, Ackerly DD, McPeek MA, Donoghue MJ (2002). Phylogenies and community ecology. Annual Review of Ecology and Systematics, 33, 475-505. [本文引用:4]
[49] Webb CO, Donoghue MJ (2005). Phylomatic: Tree assembly for applied phylogenetics. Molecular Ecology Notes, 5, 181-183. [本文引用:1]
[50] Wiens JJ, Donoghue MJ (2004). Historical biogeography, ecology and species richness. Trends in Ecology & Evolution, 19, 639-644. [本文引用:3]
[51] Wiens JJ, Parra-Olea G, García-París M, Wake DB (2007). Phylogenetic history underlies elevational biodiversity patterns in tropical salamand ers. Proceedings of the Royal Society of London B: Biological Sciences, 274, 919-928. [本文引用:1]
[52] Zanne AE, Tank DC, Cornwell WK, Eastman JM, Smith SA, FitzJohn RG, McGlinn DJ, O’Meara BC, Moles AT, Reich PB (2014). Three keys to the radiation of angiosperms into freezing environments. Nature, 506, 89-92. [本文引用:1]
[53] Zhang JT (1989). Vertical zone of vegetation in Luya Mountain in Shanxi Province. Scientia Geographica Sinica, 9, 346-353. (in Chinese with English abstract)
[张金屯 (1989). 山西芦芽山植被垂直带的划分. 地理科学, 9, 346-353. ] [本文引用:2]
[54] Zhang WT, Jiang Y, Wang MC, Zhang LN, Dong MY (2015). Responses of radial growth in Larix principis-rupprechtii to climate change along an elevation gradient on the southern slope of Luya Mountain. Acta Ecologica Sinica, 35, 1-10. (in Chinese with English abstract)
[张文涛, 江源, 王明昌, 张凌楠, 董满宇 (2015). 芦芽山阳坡不同海拔华北落叶松径向生长对气候变化的响应. 生态学报, 35, 1-10. ] [本文引用:1]