Variation characteristics of an invasive <i>Alternanthera philoxeroides</i> community along latitudinal gradients in spring

WU Hao

doi:10.11833/j.issn.2095-0756.2017.05.007

Volume 34 Issue 5

Sep. 2017

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WU Hao. Variation characteristics of an invasive Alternanthera philoxeroides community along latitudinal gradients in spring[J]. Journal of Zhejiang A&F University, 2017, 34(5): 816-824. doi: 10.11833/j.issn.2095-0756.2017.05.007

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WU Hao. Variation characteristics of an invasive Alternanthera philoxeroides community along latitudinal gradients in spring[J]. Journal of Zhejiang A&F University, 2017, 34(5): 816-824. doi: 10.11833/j.issn.2095-0756.2017.05.007

Variation characteristics of an invasive Alternanthera philoxeroides community along latitudinal gradients in spring

doi: 10.11833/j.issn.2095-0756.2017.05.007

WU Hao^{1,2,3
,}

1.
College of Life Science, Xinyang Normal University, Xinyang 464000, Henan, China
2.
Wuhan Botanical Garden, Chinese Academy of Sciences, Wuhan 430074, Hubei, China
3.
University of Chinese Academy of Sciences, Beijing 100049, China

Received Date: 2016-10-17
Rev Recd Date: 2016-11-02
Publish Date: 2017-10-20

Abstract

Latitude and season are the crucial environmental factors that affect biological invasions, such as Alternanthera philoxeroides, a malignant invasive weed native to South America and now widely found in several provinces of China. To explore variation characteristics of an A. philoxeroides community along latitudinal gradients and also to test its invasive effect on native plant diversity (α diversity indices) and community stability (Godron coordinate values) in spring, a latitudinal field survey was conducted in April with 15 invasive and 15 control plots selected in the 22°-30° N (spanning Guangdong, Guangxi, Hunan, and Hubei Provinces) terrestrial habitats of South China. Analysis included the ordination method of canonical correspondence analysis (CCA) based on species and environmental matrixes, as well as the curve regression between A. philoxeroides cover and α diversity indices (the Simpson, Shannon, Pielou, and Patrick indices). Results showed that: 1) species cover of Alternanthera philoxeroides increased with increasing latitude from 22°-30° N of China with the Simpson, Shannon, and Pielou indices all significantly and negatively correlated (P=0.012, r=-0.629; P=0.020, r=-0.591; P=0.024, r=-0.579, respectively) to latitude in the invasive community but significantly and positively correlated (P=0.041, r=0.533; P=0.020, r=0.591; P=0.010, r=0.641, respectively) in the control community. The Patrick index had no significant correlation with latitude. 2) Invasion decreased species diversity at the whole community level (t=7.811, P < 0.001). However, a slight invasion (A. philoxeroides cover < 25%) improved diversity, and with invasion cover increasing, species diversity indices were unimodal. In spring the Godron values between invasive and control communities were similar. 3) Invasive plots recorded 45 plant species; whereas, control plots recorded 125 plant species. Plant species of these two community types formed three separate distribution areas in the CCA two-dimensional diagrams with the invasive community having a higher distribution density. A. philoxeroides was near to the central position indicating it had a wider latitudinal adaptability. These findings would be helpful for predicting dynamics of an invasive plant community and also very important for biological invasion control and native biodiversity protection.
- plant ecology,
- biological invasion,
- Alternanthera philoxeroides,
- spring,
- species diversity,
- community stability,
- latitude,
- canonical correspondence analysis(CCA)

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生物入侵是当代社会面临的最严峻的生态问题之一，给环境保护、生态系统功能维持及全球生物多样性造成巨大威胁^[1]。外来入侵植物通过资源竞争、干扰等途径影响本土生物群落物种分布、生物多样性及群落稳定性，而诸多环境因子在生物入侵过程中起到重要作用^[2-3]。自然界中，纬度包含了复杂的环境因素，如温度、光周期、生长季长度等^[4]，纬度梯度引起的水、热条件非均衡性导致入侵植物区系分布、入侵生物群落结构、入侵效应等特征发生显著变化^[5-6]。吴晓雯等^[7]发现，中国大陆低纬度地区入侵植物的组成以热带区系植物为主，而高纬度地区的入侵植物则以温带区系植物为主；RICHARDSON等^[8]发现，在美国草原的低纬度区域，入侵杂草能够快速生长并抑制本土植物建群，而在高纬度地区，随着环境压力增大，入侵杂草反而可以作为先锋物种改善生境提高本土植物多样性。探讨纬度梯度对入侵物种群落特征的影响有助于深入理解外来生物入侵机理及预测其入侵态势。空心莲子草Alternanthera philoxeroides原产于南美洲，强大的克隆能力及水陆两生属性使它在全球大纬度梯度范围内(42°S~37°N)广为分布^[9]。空心莲子草于20世纪30年代被引入中国，现已入侵南北方20余省，给当地生态环境和经济建设带来巨大危害^[10]。林金成等^[11]、郭连金等^[12-13]、蔡一村等^[14]、WU等^[9]对不同区域空心莲子草群落特征、入侵-物种多样性关系进行了研究，但这些调查多集中在入侵态势旺盛的秋季；林金成等^[10]虽然探讨了春季空心莲子草入侵对本土群落的影响，但其调查的纬度梯度较小(仅局限于南京市)。由于不同季节入侵植物的种群动态变化较大，导致入侵群落的物种组成、多样性等特征差异显著^[15-16]，因此，在春季(入侵初期)沿大尺度纬度变化的入侵植物空心莲子草的群落特征亟待研究。本研究基于纬度梯度(22°~30°N)的野外群落学调查，对春季空心莲子草群落物种多样性沿纬度变化、入侵对群落多样性及稳定性的影响以及群落物种沿纬度的分布特征进行了探讨，以期为空心莲子草入侵防治及本土生物多样性保护提供科学依据。

1. 材料与方法

1.1. 野外调查

于2013年4月下旬在中国南方22°~30°N纬度梯度范围内(广东、广西、湖南、湖北)对陆生空心莲子草群落进行野外调查。选择空心莲子草入侵面积超过100 m²的陆生区域布设调查样地(极少数延伸至水体中)，间隔1°设置1~3个样地(10 m × 10 m，间隔10 km以上)。同时，采用同样方法在每个入侵样地附近设置对照样地(无空心莲子草入侵)进行群落调查，并保证对照样地与入侵样地周围的生境条件基本一致。共设置15个入侵样地和15个对照样地。研究样地处于中国亚热带季风气候区，水热条件优越，年平均气温为21.0 ℃，年降水量为1 500 mm。土壤质地以赤红壤和黄壤为主，土层较厚，质地黏重，多呈酸性。空心莲子草在此区域内大量入侵的生境主要为远离城市的撂荒地、田埂、河岸带以及部分林下空地，人为干扰程度较小。

在每个样地中央均匀布设2条10 m长的样带，每条样带均匀布设5个面积为0.5 m × 0.5 m的样方。调查时，记录各样方中植物物种名称。由于春季研究区内空心莲子草等植物处于生长初期，紧贴地面生长，其高度、个体数均难以准确测量，故采用盖度作为衡量群落中植物优势度的唯一指标。利用具有100分格的0.5 m × 0.5 m样方框精确计算每种植物的盖度。利用全球定位系统(GPS)定位仪(Garmin eTrex 20) 记录各样地的地形因子(经度、纬度、海拔)。对于未知的植物，拍照或采标本带回实验室，使用在线中国植物志(Flora of China, http://frps.eflora.cn/)以及中国数字植物标本馆(Chinese Virtual Herbarium, http://www.cvh.org.cn/)进行物种鉴定。

1.2. 数据计算

1.2.1. α物种多样性^[17-18]

利用植物盖度计算草本群落物种多样性指数。Patrick丰富度指数：R=S；Simpson多样性指数：$\lambda = 1 - \sum\limits_{i = 1}^n {P_i^2}$ ；Shannon-Wiener多样性指数：$H = - \sum\limits_{i = 1}^n {{P_i} \times \ln {P_i}}$ ；Pielou均匀度指数：E=H/lnS。其中：S为样地内所有植物的物种数，P_i为第i种植物的相对盖度。

1.2.2. 群落稳定性

群落稳定性代表了生物群落耐性、持久性、恒定性以及弹性等抵抗外界干扰的特征。本研究利用Godron坐标值测度群落稳定性状况，具体计算方法为^[19]：将群落中所有植物频度(某一植物在样方中出现频次的总和)按从大到小顺序排列，并按其相对频度由大到小逐步累积，然后将植物种类百分数与累积相对频度对应画出散点图，并对散点进行曲线方程模拟[方程(1)：y= ax² + bx + c]；在横、纵坐标轴100处连线，其直线方程为[方程(2)：y=100-x]。将方程(2) 带入方程(1) 可得方程(3)：ax² +(b+1)x + c-100 = 0，方程(3) 的2个解中一个远高于100，另一个为0~100，第2个解为稳定点横坐标值，将它代入方程(1) 或(2) 中即可得到群落稳定性纵坐标值。Godron坐标值越接近(20，80)，则群落稳定性越高。

1.3. 统计分析

用SPSS 16.0(SPSS Inc, Chicago，美国)对纬度梯度与空心莲子草入侵盖度、入侵区和对照区群落的4个多样性指数分别进行回归分析。分析时，使用软件中“curve estimation”提供的2种曲线模拟方程(线性、二项式)，选择回归显著且拟合系数最高者进行讨论。同时，也对空心莲子草入侵盖度与入侵区植物多样性进行回归分析。

为了检验春季空心莲子草入侵对本土群落多样性的影响，对入侵区、对照区的4个多样性指数分别进行“独立样本t检验”(independent-samples t test)。

分别计算入侵区、对照区群落的稳定性坐标点，并比较两者与理想稳定点的欧氏距离。

分别建立植物物种相对盖度矩阵、调查样地纬度梯度矩阵，利用数量生态学排序软件CANOCO 4.5(Microcomputer Power，美国)提供的“典范对应分析”(canonical correspondence analysis，CCA)，探讨春季空心莲子草入侵区及对照区植物物种沿纬度梯度的分布状况。

3. 讨论

3.1. 纬度对入侵群落的影响

空心莲子草原产于南美洲，共有6个种群，其中位于29°S附近的Santa Fé种群极有可能是入侵中国的空心莲子草的源种群^[20]。在研究区22°~30°N范围内，随着纬度上升，入侵盖度逐步增大，这可能是由于研究区的亚热带生态环境和同处于该气候带29°S附近源种群的生境条件相似，有利于空心莲子草的大量入侵爆发。

由赤道到两极，随着纬度上升，物种多样性锐减，这种“纬度多样性梯度”变化趋势(latitudinal diversity gradient, LDG)是全球生物多样性的基本特征^{[8, 21]}。本研究发现春季空心莲子草入侵群落物种多样性也符合LDG模式，一方面是由于在研究区内随着纬度上升空心莲子草入侵态势加剧，只有禾本科等耐受性较强的物种能够在受空心莲子草入侵的群落中继续维持较好的发育，如结缕草、李氏禾等，而某些耐受能力较弱的本土植物优势度则被削弱甚至丧失，如车轴草等，最终导致群落多样性严重下降；另一方面，纬度上升所带来热量和水分变化对物种分布均匀程度产生很大影响。随着纬度上升，太阳辐射量降低，水分条件有所改善^[22]，这可能降低了本土植物与空心莲子草的可用性水资源竞争，本土种只需要消耗较少的能量来应对种间竞争，因此可以将更多的能量用于优化其生命进程和拓展生态位，在群落内形成多种类型的植物生态种组^[23]，从而加剧物种空间分布的斑块化。相对于本土群落，入侵群落中不同习性物种间分布界限更为明显，群落均匀度随纬度上升而降低，导致入侵群落整体多样性下降。

本研究中，对照群落在22°~30°N范围内的植物多样性随纬度上升而增加，这与经典的全球物种多样性LDG模式相反，主要原因可能是研究区样地所处的气候带较为集中，均处于亚热带气候范围内，热季节性较为稳定，物种沿纬度梯度的种间替代速率降低^[24]，使其无法呈现出完整的“热带—亚热带—温带”植物区系多样性随纬度上升而下降的特征。此外，本研究发现纬度并未显著影响春季入侵群落和对照群落的物种丰富度(Patrick指数)，可能是由于较多的植物在春季尚未萌发生长，导致群落物种数目对纬度变化不敏感。

3.2. 不同季节入侵效应的比较

林金成等^[9]、郭连金等^[11]、WU等^[13]均发现秋季小规模的空心莲子草入侵能够提高群落物种多样性。本研究亦得出相似结论，即随着入侵盖度的增加，空心莲子草对本土群落多样性先促进后抑制的生态效应存在于其整个入侵进程。这可能是由于在入侵初期，空心莲子草优越的表型可塑性和生理适应性使它能作为先锋物种，改善土壤性能并优化小生境条件，有利于本土植物的定居与建群^[25]，而当空心莲子草入侵爆发时，则可通过资源竞争、释放化感物质等途径严重削弱威胁本土群落多样性^{[9, 13, 26]}。但与前人研究结果不同的是，春季空心莲子草入侵并未对群落物种丰富度产生影响，这是由于受不同物候期影响，春、秋两季空心莲子草入侵群落植物组成存在较大差异^[9-10]。例如，在大纬度尺度上，秋季陆生空心莲子草的优势伴生种为植株高大的马唐Digitaria sanguinalis^[9]，但马唐出苗、抽穗时间较晚(一般为5-6月)^[27]，物候期的差异使得在春季空心莲子草入侵初期，缺乏竞争力较强的马唐植物与之相抗衡，野外样地中多为空心莲子草的单优势种群丛，导致春季空心莲子草入侵对群落物种丰富度影响不显著。

郭连金等^[12]发现，秋季空心莲子草入侵会导致本土植物群落稳定性严重下降，是因为在入侵旺盛的秋季，空心莲子草聚集成大量的斑块状群落镶嵌体，并通过克隆生长迅速增加其盖度，从而破坏群落中各植物之间稳定的平衡关系。本研究结果与之不同，在春季，空心莲子草入侵并未对本土群落稳定性造成威胁。一方面，春季研究区本土物种多样性较高，共记录125种植物，且物种沿纬度梯度的分布较为均匀，较高的群落多样性水平对于抵抗外来物种入侵及维持自身群落稳定性起到一定的缓冲作用；另一方面，生物入侵一般需要经历较长的滞后期，而后才进入爆发性的扩展阶段。在春季，空心莲子草还处于生长初期，其无性分株数及分枝数比较少，难以形成密集的斑块镶嵌体，这削弱了它对本土植物的干扰、制约作用，导致入侵、对照群落的稳定性无显著差异。

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编号	对照区主要植物		编号	入侵区主要植物
编号	植物	总盖度	编号	植物	总盖度
1	结缕草Zoysia japonica	1.376	1	空心莲子草Alternanthera philoxeroides	10.265
2	鹅观草Roegneria kamoji	0.883	2	结缕草Zoysia japonica	0.649
3	專草Humulus scandens	0.652	3	李氏禾Leersiahexandra swartz	0.593
4	酢浆草Oxalis cornicwlata	0.634	4	双穗雀稗Paspalumpas paloides	0.568
5	苦宦菜Sonchus oleraceus	0.597	5	四叶萍Marsilea quadrifolia	0.522
6	双穗雀稗Paspalum paspaloides	0.572	6	赛草Humulus scandens	0.317
7	艾蒿Artemisia argyi	0.571	7	铁觅菜Acalypha australis	0.199
8	水蓼Polygonum hydropiper	0.529	8	鹅观草Roegneria kamoji	0.194
9	车轴草Trifolium repens	0.42	9	荩草Arthraxon hispidus	0.183
10	碎米莽Cardamine hirsuta	0.406	10	狗牙根Cynodon dactylon	0.155
11	苏门白酒草Conyza sumatrensis	0.373	11	莠竹Microstegium nodosum	0.121
12	慈菇Sagittaria trifolia	0.371	12	鬼针草Bidens pilosa	0.114
13	节节麦Aegilops tauschii	0.342	13	酸模叶蓼Polygonum lapathifolium	0.094
14	香附子Cyperus rotundus	0.328	14	龙爪茅Dactyloctenium aegyptium	0.091
15	牵牛PharHitis nil	0.263	15	水蓼Polygonum hydropiper	0.09
16	苣契菜Sonchus brachyotus	0.259	16	艾嵩Artemisia argyi	0.08
17	鬼针草Bidens pilosa	0.253	17	老鹤草Geranium carolinianum	0.075
18	豚草Ambrosia artemisiifolia	0.242	18	地稔Melastoma dodecandrum	0.07
19	老鹤草Geranium carolinianum	0.241	19	酔楽草Oxalis corniculafa	0.068
20	莎草Cyperus microiria	0.223	20	虫几子草Leptochloa panicea	0.06

Variation characteristics of an invasive Alternanthera philoxeroides community along latitudinal gradients in spring

doi: 10.11833/j.issn.2095-0756.2017.05.007