Encroachment of woody plants into grasslands has generated considerable interest among ecologists. Syntheses of encroachment effects on ecosystem processes have been limited in extent and confined largely to pastoral land uses or particular geographical regions. We used univariate analyses, meta-analysis and structural equation modelling to test the propositions that (1) shrub encroachment does not necessarily lead to declines in ecosystem functions and (2) shrub traits influence the functional outcome of encroachment. Analyses of 43 ecosystem attributes from 244 case studies worldwide showed that some attributes consistently increased with encroachment (e.g. soil C, N), and others declined (e.g. grass cover, pH), but most exhibited variable responses. Traits of shrubs were associated with significant, though weak, structural and functional outcomes of encroachment. Our review revealed that encroachment had mixed effects on ecosystem structure and functioning at global scales, and that shrub traits influence the functional outcome of encroachment. Thus, a simple designation of encroachment as a process leading to functionally, structurally or contextually degraded ecosystems is not supported by a critical analysis of existing literature. Our results highlight that the commonly established link between shrub encroachment and degradation is not universal.© 2011 Blackwell Publishing Ltd/CNRS and State Government of NSW.

灌木/半灌木扩张能够显著改变原有植被景观及其生态学过程，但目前仍缺乏对半灌木扩张驱动下草地植物多样性、地上生产力变化特征及其二者关系的系统研究。本研究以黄土高原宁夏云雾山典型草原为研究对象，通过选择3个半灌木扩张强度草地（轻度、中度和重度），并以未扩张的天然草地作为对照， 分析物种多样性、群落水平植物功能性状、功能多样性和地上生产力的变化特征及其内在关系。结果表明：1）半灌木扩张驱动下草地群落组成、结构发生了明显改变，半灌木物种白莲蒿已取代了原有的草本植物成为新的草地优势种，且导致物种多样性显著降低；2）与对照相比，半灌木扩张显著降低了群落水平叶干物质含量，但增加了群落水平比叶面积、叶碳、叶氮和叶磷含量；功能丰富度无显著性差异，而功能离散度和功能均匀度在不同扩张强度下差异显著；3）半灌木扩张显著增加了群落地上生产力和凋落物生物量，具体表现为杂草生物量显著增加，豆科生物量显著降低，而禾类草生物量在重度扩张下显著增加；4）半灌木扩张驱动下草地群落地上生产力与物种多样性、群落水平叶干物质含量及功能离散度呈负相关关系，而与群落水平叶碳、叶氮和叶磷含量呈正相关关系。随机森林模型和方差分析结果进一步显示，群落水平功能性状对地上生产力变异解释度为68.28%，其中群落水平叶氮含量和叶干物质含量为最主要的解释因子，物种多样性指数解释度为64.38%，功能多样性指数解释度为11.16%。研究表明半灌木扩张驱动下，物种向快速生长、资源获取型策略转变，且物种生态位分化程度更高；植物功能性状和物种多样性比功能多样性更能解释群落地上生产力的变异。本研究结果可为黄土高原天然草地的稳态转变理论研究和新时期灌丛化草地的适应性和可持续管理提供数据支撑。

Forest diversity-productivity relationships have been intensively investigated in recent decades. However, few studies have considered the interplay between species and structural diversity in driving productivity. We analyzed these factors using data from 52 permanent plots in southwestern Germany with more than 53,000 repeated tree measurements. We used basal area increment as a proxy for productivity and hypothesized that: (1) structural diversity would increase tree and stand productivity, (2) diversity-productivity relationships would be weaker for species diversity than for structural diversity, and (3) species diversity would also indirectly impact stand productivity via changes in size structure. We measured diversity using distance-independent indices. We fitted separate linear mixed-effects models for fir, spruce and beech at the tree level, whereas at the stand level we pooled all available data. We tested our third hypothesis using structural equation modeling. Structural and species diversity acted as direct and independent drivers of stand productivity, with structural diversity being a slightly better predictor. Structural diversity, but not species diversity, had a significant, albeit asymmetric, effect on tree productivity. The functioning of structurally diverse, mixed forests is influenced by both structural and species diversity. These sources of trait diversity contribute to increased vertical stratification and crown plasticity, which in turn diminish competitive interferences and lead to more densely packed canopies per unit area. Our research highlights the positive effects of species diversity and structural diversity on forest productivity and ecosystem dynamics.

Hill prairies are remnant grasslands perched on the bluffs of major river valleys, and because their steep slopes make them unsuitable for traditional row crop agriculture, they have some of the lowest levels of anthropogenic disturbance of any prairie ecosystems in the Midwestern USA. However, many decades of fire suppression have allowed for shrub encroachment from the surrounding forests. While shrub encroachment of grasslands can modify soil respiration rates and nutrient storage, it is not known whether shrubs also alter the community composition of soil microorganisms. We conducted transect sampling of nine different hill prairie remnants showing varying degrees of shrub encroachment, and we used DNA-based community profiling (automated ribosomal intergenic spacer analysis) to characterize the composition of bacterial and fungal communities in the open prairie habitat, the shrub-encroached border, and the surrounding forest. While both bacterial and fungal communities showed statistically significant variation across these habitats, their predominant patterns were different. Bacterial communities of forest soils were distinct from those of the open prairie and the shrub-encroached areas, while fungal communities of the open prairie were distinct from those of the forest and the shrub-encroached border. Shrub encroachment significantly altered the community composition of soil fungal communities. Furthermore, fungal communities of heavily encroached prairie remnants more closely resembled those of the surrounding forest than those of lightly encroached prairies. Thus, shrub encroachment can cause soil fungi to shift from a "grassland" community to a "woody" community, with potential consequences for soil processes and plant-microbe interactions.

草原灌丛化已成为全球性问题,也是生态学、畜牧学研究的热点问题。紧扣草原灌丛化过程、发生机理及其生态学后效等关键问题,介绍了国内外的最新研究进展。灌木入侵途径包括有意识引进入侵和自然入侵2类,入侵过程分为传播到达、定居建群和扩散入侵3个阶段。放牧、火烧、温室气体增加,干旱以及降水格局改变等均可导致草本向木本的转化,且机制复杂,主要有影响草本,木本植物盖度、水分利用效率、化感作用等。木本植物入侵打破了草原生态系统的稳定,表现出沃岛效应,影响动植物的分布以及生态系统的功能和服务。阐明草原“草本-木本”群落演替机制,可为我国研究草原灌丛化和管理提供理论借鉴。