黄土高原子午岭植物群落物种多样性的时空格局与过程

【作者】 王世雄；

【导师】 王孝安；

【作者基本信息】 陕西师范大学 ， 生态学， 2013， 博士

【摘要】 目的与意义：物种多样性的时空格局一直是群落生态学的研究热点,尤其是针对目前人工林物种多样性恢复功能的普遍争议,本研究以群落构建过程为主线,以三个功能群(草本层、灌木层和乔木层)为研究单位,探讨自然群落和人工群落中物种多样性在时间(演替梯度)和空间尺度上的多尺度格局,确定环境筛选(生态位理论)和扩散限制(中性理论)对物种多样性格局的相对贡献,比较人工油松群落和天然辽东栎群落在物种多样性格局以及群落构建过程中的差异,旨在阐明黄土高原群落的构建过程及其机制,评价人工油松林对物种多样性的恢复效果,探索有效指导人工林和天然次生林恢复演替的理论依据。对于物种丰富度较低的黄土高原森林生态系统的群落构建机制研究,不仅可以促进该地区的植被建设和人工林多样性恢复,同时也是揭示物种共存及群落生物多样性维持机制的必需研究内容之一。更重要的是,黄土高原存有我国独特的植被类型,生物多样性较低,群落结构相对简单,加速该地区退化生态系统的恢复与重建,无论对于改善区域生态环境还是对于整个西北地区生态系统生产力的提高均具有极其重大的意义。研究地点：子午岭林区,位于黄土高原中部研究方法：采用空间代替时间的方法,确定演替时间尺度上的物种多样性的变化；将p多样性进一步分解为物种更替和内嵌两种成分,阐明群落演替过程；采用物种多样性的加性分配方法,建立物种多样性与空间尺度间的关系；通过构建基于个体随机的零假说模型,揭示物种多样性的非随机格局；采用典范变异分解和Mantel检验两种互补的方法,通过分离环境因子/距离(度量环境筛选)和空间因子／距离(间接度量扩散限制)对群落物种组成/距离的相对贡献,确定环境筛选和扩散限制在该区植物群落构建中的相对重要性；通过非度量多维标度(NMDS)以及多响应置换过程(MRPP),揭示人工林与天然林间的物种组成差异；采用典范变异分解的方法,连结物种组成变异与空间和环境间的关系,揭示扩散限制和生境筛选在群落构建中的相对重要性；采用典范冗余分析和Mantel检验方法,连结人工林和天然林两个林型间的物种组成差异与两个林型间的环境差异和空间距离,寻求导致两个林型间物种差异的直接原因。研究结果：(1)黄土高原植物群落的不同层次在演替梯度上表现了不同的α多样性格局：随演替进展,草本层物种丰富度、均匀度及Shannon多样性均逐渐降低(P<0.05)；灌木层的物种丰富度与Shannon多样性呈现相似的“J”形增长趋势,而均匀度在各演替阶段间的差异均不显著(P>0.05)；乔木层物种丰富度、均匀度以及Shannon多样性差异均不显著(P>0.05)；群落总体物种丰富度在演替梯度上呈现了明显的单峰变化趋势,与中期物种丰富度假说一致,生境异质性可能是决定群落演替过程中α多样性变化的主要因素。(2)与α多样性一样,不同的层次的β多样性在演替梯度上也呈现了不同的变化规律：草本层物种呈现明显的单峰变化趋势,即随演替进展,β多样性逐渐增加,在灌丛演替到白桦群落阶段时达最大值,之后p多样性逐渐减小。灌木层β多样性随演替进展逐渐减小,而乔木层p多样性随演替进展逐渐增大。黄土高原群落演替过程中的B多样性变化由物种内嵌和物种更替两种过程共同完成,二者的相对贡献依赖于演替阶段和具有不同物种性状的群落层次,但以物种更替为主导过程。(3)不同群落类型间的物种组成差异显著,空间和环境因子共同解释了草本、灌木、乔木三层29%-65%的物种组成变异(P<0.01)。但是,二者的贡献变异于群落的不同层次,乔木层和灌木层主要以空间因子为主导；而在草本层中,空间和环境因子具有相似的贡献率。草本、灌木、乔木三层的物种非相似性距离与环境距离和地理距离都显著相关(P<0.05),草本层具有较大的环境距离相关系数,而灌木层和乔木层具有较大的地理距离相关系数。(4)辽东栎(Quercus wutaishanica)林物种多样性在多有尺度上均呈现了显著的非随机格局(P<0.05)。物种丰富度随尺度的增加而增加,其中样地尺度上的p多样性贡献最大。而Shannon多样性呈现了物种丰富度截然相反的格局,其最大贡献来自于小样方尺度上的α多样性。尽管各层具有相似物种多样性分配格局,但是不同的尺度由不同的生态过程所主导。草本层由β多样性主导,灌木层和乔木层呈现了相似的α和B多样性大小。相反地,Shannon多样性格局一直由α多样性主导。结果表明,大尺度决定物种丰富度格局,稀有种的分布对该格局有较强的影响作用,而小尺度决定Shannon多样性格局,这主要由常见种的分布决定。(5)在人工油松(Pinus tabulaeformis)群落中,大尺度(样地尺度)决定物种丰富度格局,物种丰富度随尺度的增加而增加,在样地尺度上的p多样性贡献最大；Shannon多样性指数呈现了与丰富度截然相反的格局,小尺度(小样方尺度)决定Shannon多样性格局,其最大贡献来自于小样方尺度上的a多样性。扩散限制和环境筛选都是人工油松林群落构建的重要过程,二者相互作用,共同决定了其林下的多样性格局。(6)经过50年的自然恢复演替,人工油松林已经显著地改善了林下群落的物种组成,因此人工油松林不能被认为是“绿色荒漠”,特别是当考虑了空间尺度效应后。人工油松林和天然辽东栋林的物种多样性具有相似的多尺度格局,都表现为随尺度的增加,物种多样性逐渐增加。大尺度上的p多样性都是两种林型物种多样性重要支撑成分,而样地尺度是进行物种多样性保护的较适宜尺度。但是要有效的恢复人工油松林,我们应该加强小尺度上的物种共存,即小样方和样方尺度上的α多样性。只有同时具有较高的a多样性和p多样性,人工林的物种多样性才能得到真正的有效恢复。研究表明,综合环境特化和扩散限制在群落构建研究中的重要性,即使在生境严酷的黄土高原也不例外。对于每层物种而言,环境特化和扩散限制具有相似的效应大小,但是二者的效应大小依赖于具有不同性状的不同层次。结论与启示：(1)p多样性及其成分在群落构建中的重要性。群落的构建过程实际上就是物种多样性的动态变化过程,而p多样性连结了群落格局与过程能很好地反映群落动态,特别是将β多样性进一步分解,可以洞察群落构建的内在机制。同时,p多样性也是黄土高原人工群落和天然群落总物种多样性的重要支持成分,是应该优先考虑的物种多样性的保护成分。(2)物种的更替是该区群落演替、群落构建的重要过程。因此,要有效恢复群落中的物种多样性,降低物种间的竞争强度(特别是局域尺度上),是物种多样性恢复的有效途径。如前所述,保持生境间的斑块状分布是一种有效途径,即延迟种间竞争的排除过程。另外,该区较严酷的生境条件也是该区物种更替强度较大的重要原因。因此,改善生境的资源数量及质量以及生境的异质性都将是指导该区无论是人工林还是天然林恢复实践的重要依据。(3)扩散限制和环境筛选相互作用,共同影响着黄土高原森林群落的构建过程。本研究的结果并没有否定环境筛选在黄土高原群落构建中的重要性,而是更加强调了扩散限制在该区的主导性。因此,在黄土高原森林群落的恢复重建中,应该同时考虑环境异质性和地理空间的差异性。(4)空间尺度在人工林物种多样性恢复评价中的重要性。仅在一个尺度上进行评价可能会低估或者高估人工林的物种多样性功能。当引入空间尺度后,人工林并不是“绿色荒漠”,其具有较强的物种多样性恢复潜力,如何加强和引导人工林的恢复是今后很长一段时间需要重点研究的科学论题。同时恢复较高的α和p多样性是我们恢复实践的终极目标。人工油松林的恢复实践应该加强a多样性的恢复,例如改善林下环境、增加林下异质性等都可能是有效的办法。(5)生境的破碎化以及植被的斑块状分布对黄土高原物种多样性的重要性。一般认为,生境的破碎化限制了群落间物种的扩散以及改变了有效生境的数量和大小,对物种的共存和物种多样性的维持都是十分不利的。但是本研究的结果与我们的预期相反,该区的生境破碎化以及斑块状分布反而有益于该区物种的共存,至少对于该区的植物种来说是有益的。由于该区较严酷的生境条件,生境间的不连续性有效的阻止了群落间物种的扩散,从而有效地延缓了物种间的竞争排斥,至少在较大的尺度上使更多的物种得以共存。更多还原

【Abstract】 Aims Plantations are established for a variety of reasons including wood production, soil and water conservation, and carbon sequestration. However, their implications for species diversity are considerably debated. To assess restoration effect of species diversity in plantations, we characterized and compared species diversity between plantations and natural forests at multiple scales, including sucessional time scales; quantifying the relative contributions of space and the environments on community composition to demonstrate different structuring processes. At last, the program intends to answer the following research questions:what governs community assembly and the maintenance of biodiversity? Plantations should be considered as ’green deserts’or valuable habitats for indigenous species? How to strengthen the restorational functions of plantations? Importantly, we address the long-term recovery of the plantations while taking into account the multiple spatial scales, compared with old growth forests as a baseline, providing a comprehensive understanding to recover in plantation forests. Meanwhile, it will be endued with practical significance for vegetation restoration on the Loess Plateau.Location Ziwu Mountains, in the middle of the Loess Plateau, northwestern ChinaMethods Using the method of’substituting temporal changes with spatial difference’, alpha and beta diversity patterns along a successional gradient were investigated in the Ziwu Mountain of Loess Plateau, northwestern China. The patterns of two beta diversity components (i.e. nestedness component and species replacement component) were also studied using additive partitioning. Species diversity patterns of Liaodong oak (Quercus wutaishanica) forests and Chinese pine (Pinus tabulaeformis) plantations were analyzed by additive partitioning across three spatial scales (subplot, plot and site scales). We examined how community assembly was affected by environmental filtering and dispersal limitation using canonical variation partitioning and Mantel tests, i.e. partition the variation of community composition/dissimilarity distance between environmental and spatial factors/distance. We repeated the analysis for three species groups (herb, shrub and tree layers) that differ in traits of likely importance for environmental filtering and dispersal limitation. Results (1) Herb layer species richness, evenness and species diversity decreased gradually during community succession (P<0.05); Shrub layer species diversity and richness showed a J-shaped trend, while evenness did not change significantly (P>0.05); Tree layer species richness, evenness and species diversity did not change significantly (P>0.05); There was a significant hump-shaped pattern in species richness along the temporal gradient, which agrees with the intermediate species-richness hypothesis.(2) There was a significant hump-shaped pattern in beta diversity of herb layer (P<0.05) during the process of community succession. Beta diversity of shrub layer decreased gradually over the successional stages, while that in tree layer increased gradually. Both species replacement and nestedness component had significant effects on beta diversity, and varied throughout the successional stages and among different layers. Overall, replacement component had significant dominant effects on beta diversity at all stages and layers. For conservation purposes, we suggest devoting efforts to a large number of different sites, but not necessarily the ones with richest diversity.(3) All forest types were simultaneously governed by environmental control and spatial processes; together, these processes explain29%to65%of the species composition variation (P<0.01). However, the effects of these two processes were varied among species groups; shrub layer and tree layer species were dominated by spatial process while there was similar importance between two processes for herb layer. Mantel and partial Mantel tests showed significant correlations between community dissimilarity and environmental dissimilarity/geographical distance (P<0.05). Herb layer showed high correlation with environmental dissimilarity while shrub and tree layer displayed high correlations with geographical distance.(4) Species diversity in Liaodong oak (Quercus wutaishanica) forests displayed significant non-random patterns at all scales. Species richness increased with increasing spatial scales, and β richness at site scale contributed the most. While Shannon diversity varied across scales and the greatest contribution to y diversity was from a at the finest scale (i.e., subplot scale). Species richness pattern in forest floor was beta-dominated across scales while for canopy layer and understory layer showed a similar importance between α and β components. By contrast, Shannon diversity patterns in all three layers were alpha-dominated. In conclusion, larger scales (plot and site scales) drive overall species richness patterns. While finer scale (subplot scale) determine Shannon diversity patterns. Habitat specialization explains a larger proportion of the variation in species composition between plots than does dispersal limitation; together, these processes explain28%to60%of the variation. The significant environmental variables differed among different layers and mainly functioned as spatially structured habitat. The effects of between habitat specialization and dispersal limitation were also varied among species groups, and both processes were stronger for the herb layer than for the shrub layer and canopy layer species.(5) Species diversity of both herb and shrub layers in the Chinese pine plantations (Pinus tabulaeformis) displayed significant non-random patterns at all scales. Species richness increased with the spatial scales, and beta richness at the site scale contributed the most. While Shannon diversity displayed a distinct opposite trend as compared with species richness patterns, and the greatest contribution to y diversity was from a at the finest scale (i.e., subplot scale). The spatial distribution of diversity in Chinese pine plantations was simultaneously driven by environmental filter and dispersal limitation. However, the relative contributions of these two processes varied with different layers. The herb layer was dominated by spatial process (spatial structured environmental factors and purely spatial variables), while the shrub layer dominated by environmental filter, including spatial structured environmental factors and purely environmental factors. In the herb layer, altitude, slope position, soil organic matter, and soil available potassium were significant environmental factors; in the shrub layer, altitude, slope aspect, soil available nitrogen, available potassium and available phosphorus were the significantly contributing environmental variables. In conclusion, larger scales drive overall species richness patterns while finer scales determine Shannon diversity patterns, which may be explained by distribution patterns of the common and rare species. These results suggest that the pine plantation alters plant species composition considerably after long-term succession and should not be considered as’green deserts’. High beta diversity at the site scale demonstrates the importance of this scale in preserving biodiversity. However, different protection measures should be applied to species in different layers with different species traits.(6) Overall, these two forest types showed significant different species compositions at the subplot and plot scales, but displayed similar accumulation of species diversity across spatial scales. The contribution of species diversity components increased with the spatial scales, and both types displayed lower alpha diversity at the subplot scale but higher beta diversity at the plot and site scales. Alpha diversity was not significantly different between these two forests at all three scales. By contrast, the pine plantations displayed significant higher beta diversity than old-growth forests at finer scales (i.e. subplot and plot scales). The diversity of both two forest types were simultaneously governed by environmental control and spatial processes, but dominated by spatial processes, with the exception of a greater contribution of each component in the pine plantations. The litter depth and altitude were two significant environmental factors that differed species composition between the two forests.Main conclusions (1) The importance of beta diversity. As mentioned above, beta diversity is the most important component to support total species diversity, no matter old-growth forests or plantations, especially at larger scales. In addition, beta diversity links ecological patterns with ecological processes. Assigning the different beta diversity patterns to their respective biological phenomena is essential for analyzing the causality of the processes underlying biodiversity. Thus, the differentiation of the spatial turnover and nestedness components of beta diversity is crucial for our understanding of central biogeographic, ecological and conservation issues.(2) Overall, replacement component had significant dominant effects on beta diversity at all stages and layers, which may be caused by the harsh environments in this area. Thus, reducing the intensity of interspecific competition is the most important task at present to effectively restore species diversity, especially at local scales (such as the subplot and plot scales in present study). Therefore, improving the quality and quantity of availability resource, and even habitats heterogeneity may be an important approach. In addition, as mentioned above, keep the spatial discontinuity of habitats may be also one of effective ways.(3) The assembly of plant communities in the Loess Plateau appears to be simultaneously driven by environmental filtering and dispersal limitation. This case study shows the importance of the dispersal limitation in understanding the forests assembly in the Loess Plateau. Consequently, it is important that management planning for restorations of natural vegetations take into account both habitat heterogeneity and geographical differences. (4) Our study did reveal a significant role of spatial scales in assessing biodiversity restoration function of plantations. Although beta diversity is a key component supporting total biodiversity in the both forest types, higher contribution beta diversity in pine plantations at finer scales (at the subplot and plot scales) suggested that beta diversity was more important than that in the old-growth forests to support total species diversity. These results support the argument that pine plantations alter plant species composition considerably and should not be considered as’green deserts’. However, to effectively restore species diversity, pine plantations should also be managed for enhancing high levels of species co-existence at the local scale (i.e. alpha diversity at the subplot and plot scales in the present case); a pattern that characterizes by not only the higher beta diversity but also higher alpha diversity may be our final restoration goals.(5) Our results also indicate that the spatial discontinuity of habitats acts in concert with stark environmental gradients to dictate species coexistence and diversity for both forest types. Although habitat fragmentation is often associated with a loss in biodiversity, studies indicate that fragmentation per se, as opposed to habitat loss, may actually have positive effects on biodiversity. Similarly, in our study system, natural fragmentation of the landscape promoted beta diversity at the regional scale (between sites), which probably enhances regional coexistence and diversity beyond what would be expected in contiguous habitats.更多还原