竹类植物对异质生境的适应——表型可塑性

竹类植物是一类以木本为主的克隆植物,凭借表型可塑性的优势,对异质生境具有很强适应能力。然而,目前对竹类植物表型可塑性的实现方式及其异质生境适应对策未见系统总结,从而在一定程度上限制了竹类生态学的发展。从形态可塑性、选择性放置、克隆整合和克隆分工等4个方面对竹类植物的表型可塑性研究进行分析和梳理,结果表明:竹类植物在异质生境中具有明显的表型可塑反应,主要采用形态可塑性、选择性放置和克隆整合来适应异质生境,而克隆分工的普遍性仍有待验证;目前侧重于研究构件形态和生物量分配格局,而很少深入探讨形态、生理和行为等可塑性机理。今后竹类植物表型可塑性研究重点在于:1)克隆整合的格局与机理;2)克隆整合对生态系统的影响;3)克隆分工的形成及其与环境关系;4)表型可塑性的等级性及环境影响;5)不同克隆构型的表型可塑性特征及其内在机制。     

异质生境中水生植物表型可塑性的研究进展

水生植物是一类以草本植物为主、与水紧密相关的生态类群, 大多数具有克隆性。面对水环境的变化, 水生植物在形态、行为和生理上表现出多样化的表型可塑性, 对异质生境具有很强的适应能力。表型可塑性研究已在陆生植物的多个类群展开, 然而目前对异质生境下水生植物的生态适应对策, 尤其是表型可塑性的研究尚重视不够。本文在阐明克隆植物表型可塑性主要实现方式及其关系、水生环境异质性及其特点的基础上, 重点从形态可塑性、觅食行为、克隆整合、克隆分工和风险分摊等5个方面讨论了水生植物如何通过表型可塑性适应异质性水生环境。在今后的水生植物表型可塑性研究中, 建议着重探讨以下问题: (1)表型可塑性的变化规律及机理; (2)克隆整合对群落和生态系统的影响; (3)克隆整合与克隆片段化的权衡; (4)不同克隆构型的表型可塑性及其内在机制; (5)表型可塑性的适应性进化; (6)水生植物与其他类群/营养级物种的关系; (7)水生生态系统对全球变化的响应。     

克隆植物中的劳动分工及其生态学效应

劳动分工是经济进步的发动机,克隆植物也具有与经济学相类似的劳动分工现象。环境异质性、分株专化与合作以及分株潜在的生长独立性是克隆植物劳动分工发生的基本条件。根据发生条件可以把克隆植物劳动分工分为环境诱导型和遗传型两种。克隆植物能够通过利用形态或生理可塑性和生理整合、劳动分工机制实现对生境中异质性资源的有效利用。克隆植物劳动分工的生态学效应在于：提高对局部资源的摄食效率、克服局域资源限制、实现生物量的增益与适合度的提升,上述效应的机理可以用经济学边际成本分析和规模报酬规律来解释。同时,劳动分工还能提高种间竞争力、增强觅食有效性、减弱种内自疏,但同时,克隆植物在不稳定环境下的劳动分工效应也会增加生存风险。随着现代生物学研究手段的不断应用,有关克隆植物劳动分工的研究将会得到更加深入的发展。     

异质性生境中的植物克隆生长：风险分摊

在异质生境中克隆生长使克隆分株处于不同的小生境中,从而将基株死亡风险以不同方式分摊。分摊有利于维持或提高基株适合度,因此,植物克隆生长被认为具有对小尺度生境异质性的生态对策性意义。拟—年生克隆草本Trientalis europaea对养分梯度的反应给出了基株风险分摊的实例。文中提出—个关于分株间连接维持时间对生境异质性反应的假说。     

克隆植物对种间竞争的适应策略

克隆植物种群因其寿命的持久性、空间上的可移动性和繁殖方式的多样化等特征与非克隆植物有很大区别, 在自然生态系统中占有重要地位, 甚至成为优势种或者建群种。该文通过归纳有关克隆植物的种间竞争适应策略研究案例, 阐述了克隆植物的竞争能力差异和影响竞争力的因素; 论述了克隆植物在构件形态、克隆构型、繁殖对策等方面对种间竞争的响应, 以及生理整合作用与种间竞争的关系; 分析了导致某些同类研究的结论不一致的原因, 认为实验对象差异、实验设计、生境条件与克隆植物形态及生理上的时空动态变化等都可能影响实验结果; 提出了全球变化背景下的克隆植物种间竞争及其分子生态学机制等可能是今后需要重点关注的问题。     

根茎克隆植物生态学研究进展

根茎在植物的无性繁殖、克隆分株间信息交流和物质交换、预测资源斑块的质量等方面具有重要意义,并且根茎克隆植物的研究涉及生物入侵、全球变化等诸多生态学前沿领域。作为一种重要的克隆植物类型,根茎克隆植物在资源异质性生境中表现出特有的适应方式,这种方式可以通过形态可塑性、觅食行为、生理整合以及适合度来具体表征。着眼于根茎克隆植物,总结和分析了国内外近年来的研究案例,并对形态可塑性起源与多样性的限制假说和适应假说、觅食行为中的强度觅食和广度觅食策略、克隆分株间间隔子保持和断裂的利益权衡等热点内容进行了讨论。最后联系生态学学科前沿,提出了本领域在未来需要重视的研究方向。     

基于根系形态可塑性的空心莲子草克隆分工特征

资源在空间和时间上不均匀分布现象往往形成资源异质性斑块,克隆植物凭借强大的侧向生长能力占据广阔空间,分株间的生理连接促进了其对异质性生境的适应。克隆分株首先通过资源获取结构的功能特化来提高从各种资源富养斑块中的养分获取,然后通过克隆整合作用实现分株间的养分传输,这种功能特化和资源共享模式被称为‘分工’。该文以入侵克隆植物空心莲子草(Alternanthera philoxeroides)为研究对象,研究其根系对资源异质性分布的形态可塑性响应;通过调节光照强度和土壤养分来实现资源的异质性分布,共设置4个处理:1近端分株高光低养—远端分株高光低养(HL-HL),2近端分株低光高养—远端分株低光高养(LH-LH),3近端分株高光低养—远端分株低光高养(HL-LH),4近端分株低光高养—远端分株高光低养(LH-HL);使用WinRHIZO Pro软件分析相关根系指标,SPSS 18.0单因素方差(one-way ANOVA)分析方法分析异质性条件对近、远端分株以及整个克隆片段的影响。结果表明:异质性斑块中经历高光低养的分株分配更多的生物量到地上部分,经历低光高养的分株分配更多的生物量到地下部分,空心莲子草通过调整对地上和地下部分的生物量分配比例实现了克隆分工;异质性斑块中,生长在富养斑块中的空心莲子草分株根系有更高的根生物量、根长、根表面积、根体积以及分枝系数等,表明空心莲子草分株根系通过对异质性斑块的形态可塑性变化提高了土壤养分的吸收能力。由此可见,空心莲子草通过对资源获取结构的功能特化提高了其资源吸收能力,这可能是其具强入侵能力的重要原因。     

葡匐茎草本蛇莓对基质分条件的克隆可塑性

为了研究葡匐茎草本植物对基质养分供应水平的克隆可塑性,在一栽培实验中葡茎草本植物蛇莓（Ｄｕｃｈｅｓｎｅａ ｉｎｄｉｃａ Ｆｏｃｋｅ）经历了高、中、低养分（Ｎ,Ｐ,Ｋ）水平的环境处理。实验表明,蛇莓植株生物量在较高养分条件下较高养冠比在低养分条件下最大,高欠之,中养分量小;对铺匐茎的生物量投资在中养分条件下最大;对叶柄的生物量投资没有对养分处理发生反应。蛇莓植株在人工养分下比在高养分和中分条件下形成     

异质光照环境下克隆整合对南美蟛蜞菊和蟛蜞菊种间关系的影响

很多入侵植物具有克隆性,克隆整合对入侵克隆植物生长和繁殖具有重要的贡献。自然界中,植物生长和繁殖所需的各种资源如光照、水分和矿质养分等在空间上分布通常是异质的,但关于异质环境下克隆整合对入侵植物和本土同属植物种间关系影响的研究相对缺乏。通过温室控制实验,将入侵植物南美蟛蜞菊(Wedelia trilobata)和同属本土植物蟛蜞菊(W.chinensis)的分株对单独种植或者混合种植在异质性光照条件下,同时通过保持或者切断分株之间的连接来控制克隆整合效应的有无,研究异质光照环境下克隆整合对南美蟛蜞菊和蟛蜞菊种间关系的影响。克隆整合对南美蟛蜞菊和蟛蜞菊的生长和繁殖都有促进作用,且南美蟛蜞菊比蟛蜞菊从克隆整合中获益更多。与单独种植相比,两者混种对南美蟛蜞菊的叶生物量有显著影响,而对本地种蟛蜞菊的根生物量有显著影响。克隆整合和种间关系对南美蟛蜞菊的总生物量和叶生物量产生了显著的交互作用,而对蟛蜞菊各指标无显著影响。克隆整合状态显著影响了南美蟛蜞菊和蟛蜞菊的种间关系。这些结果表明,异质环境下克隆整合可以改变入侵植物南美蟛蜞菊和本土植物蟛蜞菊的生长性状及种间关系。     

匍匐茎草本植物形态可塑性、整合作用与觅食行为研究进展

综述了匍匐茎型克隆植物在形态可塑性、整合作用及觅食行为方面的研究进展。资源斑块性分布是生境异质性的特征之一,适应于异质性生境,匍匐茎植物对环境资源表现了一系列可塑性反应。本文着重从匍匐茎植物对光、水、肥的可塑性反应及其整合作用以及觅食行为等方面的研究进行总结分析,以期对匍匐茎型克隆植物进行更广泛深入的研究。     

Two types of division of labour in clonal plants: benefits, costs and constraints

Clonal plants spread vegetatively within their habitats by forming rooted ramets on stolons or rhizomes. Each of these ramets is capable of an independent existence after establishment. Nevertheless, ramets remain physically connected by stolon or rhizome internodes for variable periods of time, thereby allowing for resource movement and signal transduction within clones.Interconnected ramets of clonal plants, though potentially independent and totipotent, can specialize functionally in the performance of limited numbers of tasks such as the uptake of resources from above- vs below-ground sources, carbohydrate storage, vegetative spread and sexual reproduction. Such specialization and cooperation is comparable to a division of labour in economic systems or in colonies of social animals. The ecological significance of division of labour in clonal plants may be found in the increased efficiency of entire clones in exploiting their environments.Two different types of division of labour in clonal plants will be discussed in this review. The first type is an environmentally-induced specialization of ramets in the uptake of locally abundant resources (plastic division of labour), which can be found in several stoloniferous species. Evidence exists that this response increases resource uptake in spatially heterogeneous environments. The second type of division of labour, which occurs mainly in rhizomatous species, relates to a developmentally-programmed specialization and cooperation between interconnected ramets. This response pattern is thought to enhance plant performance by restricting the number of tasks for individual ramets and thereby significantly increasing the efficiency of task performance. In some plants, such an inherent division of labour is likely to contribute to nutrient extraction from poor and unpredictably variable sources.In this article not only benefits but also potential costs and constraints on division of labour in clonal plants are shown. The aim is to provide a review of existing knowledge and to develop concepts and hypotheses for future research.     

Clonal plants as cooperative systems: Benefits in heterogeneous environments

All natural environments are spatially and temporally heterogeneous. Consequently, their ability to provide essential resources for the growth of plants is variable. Modular plant species produce repeated basic structures which, in the case of clonal species, are called ramets. Ramets belonging to the same clone are distributed throughout the environment in space and time, and therefore they may be located in sites which differ in resource-providing quality. The connections between ramets may allow resources to be shared, enabling the clone to behave as a cooperative system. As a result of such physiological integration, ramets can survive in conditions where there is lethal shortage of a resource because they are connected to, and supported by, ramets located in conditions where there is ample supply of the same resource. Physiological integration between connected ramets presents opportunities for heterogeneous environments to be exploited to an extent that is only just becoming apparent. As heterogeneity is ubiquitous in natural environments, it may be expected that plants, as relatively immobile organisms, will have evolved the capacity to cope with it by making appropriate localized morphological and/or physiological plastic responses. Recent studies suggest that such responses not only enable clonal species to cope with environmental heterogeneity, but that under some circumstances they can benefit more from environments which are heterogeneous rather than homogeneous, even when both types of environment contain the same amount of resources. Studies on Glechoma hederacea (Lamiaceae) that illustrate this phenomenon are described.     

Importance of clonal plants and plant species diversity in the Northeast China Transect

In plant communities, the internal (genet-level) control mechanisms on a spatio-temporal scale of clonal plants impose strong constraints on spatial pattern as well as on competitive relations and, thus, species coexistence. Therefore, the presence of clonal species within a plant community affects spatio-temporal dynamics and plant species diversity. We examined the distribution of plants with different clonal growth forms in the Northeast China Transect (NECT) and correlated plant species diversity with the importance of clonal plants, and the importance of phalanx and guerilla clonal plants. Phalanx clonal plants were more abundant in western communities where the altitude was higher and both the soil nitrogen contents and precipitation were relatively low. Whereas guerilla clonal plants were more abundant in the middle of the NECT where the precipitation, mean annual temperature and photosynthetically active radiation were relatively high. In the relatively productive temperate typical steppe, plant species diversity was negatively correlated with the importance of phalanx clonal plants and positively correlated with the importance of guerilla clonal plants. In relatively unproductive temperate desert steppe, plant species diversity was positively correlated with the importance of both phalanx and guerilla clonal plants.     

Effects of soil heterogeneity and clonal integration on Alternanthera philoxeroides populations with a radial ramet aggregation

Some clonal plants can spread their ramet populations radially, and soil heterogeneity and clonal integration may greatly affect the establishment of these types of populations. We constructed Alternanthera philoxeroides populations with a radial ramet aggregation, allowing old ramets of clonal fragments to concentrate in central pots and younger ramets to root in peripheral pots. The peripheral pots were supplemented either with three different levels (high, medium and low) of soil nutrients to simulate a heterogeneous soil environment, or only one medium level of soil nutrients to simulate a homogeneous environment. Stolon connections between the central older ramets and the peripheral younger ramets were left intact or severed to test the effect of clonal integration. The maintenance of stolon connection could induce the division of labor between different‐aged ramets, by increasing the root investment in central ramets and the above‐ground growth in peripheral ramets. The maintenance of stolon connection could improve the growth of the central and peripheral ramets, clonal fragments and even the whole population. However, the positive consequence in peripheral ramets and whole fragments was only detected in the high‐nutrient patch of heterogeneous treatment. In sum, in the population with the radial ramet aggregation, clonal integration can play a key role in the rapid recruitment of young ramets of A. philoxeroides fragments, as well as the expansion of the whole population. The magnitude of clonal integration also became more obvious in the peripheral young ramets and whole fragments that experienced high‐nutrient patches.     

Asexual and sexual reproductive strategies in clonal plants

Most plants can reproduce both sexually and asexually (or vegetatively),and the balance between the two reproductive modes may vary widely between and within species.Extensive clonal growth may affect the evolution of life history traits in many ways.First,in some clonal species,sexual reproduction and sex ratio vary largely among populations.Variation in sexual reproduction may strongly affect plant's adaptation to local environments and the evolution of the geographic range.Second,clonal growth can increase floral display,and thus pollinator attraction,while it may impose serious constraints and evolutionary challenges on plants through geitonogamy that may strongly influence pollen dispersal.Geitonogamous pollination can bring a cost to plant fitness through both female and male functions.Some co-evolutionary interactions,therefore,may exist between the spatial structure and the mating behavior of clonal plants.Finally,a trade-off may exist between sexual reproduction and clonal growth.Resource allocation to the two reproductive modes may depend on environmental conditions,competitive dominance,life span,and genetic factors.If different reproductive modes represent adaptive strategies for plants in different environments,we expect that most of the resources should be allocated to sexual reproduction in habitats with fluctuating environmental conditions and strong competition,while clonal growth should be dominant in stable habitats.Yet we know little about the consequence of natural selection on the two reproductive modes and factors which control the balance of the two reproductive modes.Future studies should investigate the reproductive strategies of clonal plants simultaneously from both sexual and asexual perspectives.     

Asexual and sexual reproductive strategies in clonal plants

Most plants can reproduce both sexually and asexually (or vegetatively), and the balance between the two reproductive modes may vary widely between and within species. Extensive clonal growth may affect the evolution of life history traits in many ways. First, in some clonal species, sexual reproduction and sex ratio vary largely among populations. Variation in sexual reproduction may strongly affect plant’s adaptation to local environments and the evolution of the geographic range. Second, clonal growth can increase floral display, and thus pollinator attraction, while it may impose serious constraints and evolutionary challenges on plants through geitonogamy that may strongly influence pollen dispersal. Geitonogamous pollination can bring a cost to plant fitness through both female and male functions. Some co-evolutionary interactions, therefore, may exist between the spatial structure and the mating behavior of clonal plants. Finally, a trade-off may exist between sexual reproduction and clonal growth. Resource allocation to the two reproductive modes may depend on environmental conditions, competitive dominance, life span, and genetic factors. If different reproductive modes represent adaptive strategies for plants in different environments, we expect that most of the resources should be allocated to sexual reproduction in habitats with fluctuating environmental conditions and strong competition, while clonal growth should be dominant in stable habitats. Yet we know little about the consequence of natural selection on the two reproductive modes and factors which control the balance of the two reproductive modes. Future studies should investigate the reproductive strategies of clonal plants simultaneously from both sexual and asexual perspectives. Translated from Acta Phytoecologica Sinica, 2006, 20(1): 174–183 [译自: 植物生态学报]