High levels of diversity in Fusarium oxysporum from non-cultivated ecosystems in Australia

The Fusarium oxysporum species complex (FOSC) is a ubiquitous ascomycetous group that includes both pathogenic and non-pathogenic strains, the former being responsible for disease in over 100 cultivated plant species. Previous phylogenetic studies have uncovered at least four major clades within the FOSC, with Clade 1 hypothesised as being ancestral. However, the origin of these clades and pathogenic strains is poorly understood. Due to an emphasis on agricultural isolates in previous studies, the underlying diversity of this species complex in non-cultivated soils is largely unknown. To address this imbalance an extensive survey of isolates associated with native vegetation geographically isolated from cultivation throughout the Australian continent was conducted. A multi-gene phylogenetic analysis of the translation elongation factor (EF-1α) and the mitochondrial small subunit (mtSSU) rDNA loci did not recover any novel clades. However, the Australian isolates had high levels of intra-Clade diversity based on EF-1α sequence type (ST) comparison with a global dataset. The ST diversity was not equally distributed across the four clades, with the majority of novel STs recovered from Clade 1. Implications on the origin of the FOSC are discussed.     

基于个体的中国森林生态系统碳收支模型FORCCHN及模型验证

以植物生理学、森林生态学和土壤环境学的基本原理为基础,建立了基于个体的中国森林生态系统碳收支模型FORCCHN。模型通过两种步长运行:在步长为天时的基本过程包括林分(个体)的光合、呼吸、分配和凋落,以及凋落物和土壤有机物的呼吸和转移;在步长为年时的基本过程包括林分的同化物分配、树高和胸径增长、大凋落物生成。通过对模型样地水平上与全国总量上NPP、NEP的验证,说明该模型在考虑幼龄林基础上,能较好的模拟出中国森林生态系统的碳收支,因此可以用来模拟中国森林生态系统碳收支的过去动态和未来发展趋势。     

No support for the emergence of lichens prior to the evolution of vascular plants

The early‐successional status of lichens in modern terrestrial ecosystems, together with the role lichen‐mediated weathering plays in the carbon cycle, have contributed to the long and widely held assumption that lichens occupied early terrestrial ecosystems prior to the evolution of vascular plants and drove global change during this time. Their poor preservation potential and the classification of ambiguous fossils as lichens or other fungal–algal associations have further reinforced this view. As unambiguous fossil data are lacking to demonstrate the presence of lichens prior to vascular plants, we utilize an alternate approach to assess their historic presence in early terrestrial ecosystems. Here, we analyze new time‐calibrated phylogenies of ascomycete fungi and chlorophytan algae, that intensively sample lineages with lichen symbionts. Age estimates for several interacting clades show broad congruence and demonstrate that fungal origins of lichenization postdate the earliest tracheophytes. Coupled with the absence of unambiguous fossil data, our work finds no support for lichens having mediated global change during the Neoproterozoic‐early Paleozoic prior to vascular plants. We conclude by discussing our findings in the context of Neoproterozoic‐Paleozoic terrestrial ecosystem evolution and the paleoecological context in which vascular plants evolved.     

An analytical framework for estimating aquatic species density from environmental DNA

Environmental DNA (eDNA) analysis of water samples is on the brink of becoming a standard monitoring method for aquatic species. This method has improved detection rates over conventional survey methods and thus has demonstrated effectiveness for estimation of site occupancy and species distribution. The frontier of eDNA applications, however, is to infer species density. Building upon previous studies, we present and assess a modeling approach that aims at inferring animal density from eDNA. The modeling combines eDNA and animal count data from a subset of sites to estimate species density (and associated uncertainties) at other sites where only eDNA data are available. As a proof of concept, we first perform a cross‐validation study using experimental data on carp in mesocosms. In these data, fish densities are known without error, which allows us to test the performance of the method with known data. We then evaluate the model using field data from a study on a stream salamander species to assess the potential of this method to work in natural settings, where density can never be known with absolute certainty. Two alternative distributions (Normal and Negative Binomial) to model variability in eDNA concentration data are assessed. Assessment based on the proof of concept data (carp) revealed that the Negative Binomial model provided much more accurate estimates than the model based on a Normal distribution, likely because eDNA data tend to be overdispersed. Greater imprecision was found when we applied the method to the field data, but the Negative Binomial model still provided useful density estimates. We call for further model development in this direction, as well as further research targeted at sampling design optimization. It will be important to assess these approaches on a broad range of study systems.     

放牧对内蒙古典型草原生态系统植物及土壤δ15N的影响

植物和土壤中的¹⁵N自然丰度值(δ¹⁵N)是评价生态系统N循环的一个重要指标, 而放牧是草原生态系统的主要土地利用方式, 对草原生态系统的N循环过程的改变起着重要作用。该研究测定了内蒙古锡林河流域放牧和围封条件下草原群落主要优势植物和土壤的δ¹⁵N值, 探讨放牧对草原N循环的影响。研究中所测定的8种植物叶片δ¹⁵N变化很大(-4.04‰-4.34‰), 但与植物功能型有一定的相关性。放牧显著降低了大针茅(Stipa grandis)、杂类草和小半灌木木地肤(Kochia prostrata)的δ¹⁵N值。具有潜在共生固氮能力的豆科植物δ¹⁵N偏低负值(-4.04‰ - -1.90‰), 但在放牧和围封条件下无显著差异; 而被认为具有联合固氮能力的羊草(Leymus chinensis), 放牧后δ¹⁵N显著增加, 一定程度上表明了豆科植物和羊草生物固氮能力的存在。所有植物中, 除无菌根侵染的木地肤外, 其他有丛枝菌根真菌侵染记录的物种δ¹⁵N值较低, 通常接近0或为负值, 说明在N限制的内蒙古草原, 菌根转运N可能也是一种重要的N源途径。放牧显著降低了0-20 cm土壤δ¹⁵N值, 这也与过去的研究结果不同。δ¹⁵N的测定为生态系统提供了一个整合时空N循环过程的综合指标, 反映出放牧改变了草原生态系统的N循环。     

城市林地与非林地大气SO２季节动态变化

SO2作为主要的大气污染物之一,对人体与环境具有严重危害,导致酸雨后危害更大,尤其是长江以南省区污染严重。对长沙和株洲市区内2种类型区域———城市林地与城市非林地空气SO2浓度通过近1a(2004-01~2005-01)的同时进行对比定位观测,用甲醛吸收副玫瑰苯胺分光光度法分析,结果表明:从两市2种类型采样地空气SO2浓度水平总体与分别来看,SO2浓度年内变化均具有明显季节性波动特征(p=0·001),冬季最高,秋季最低;除冬季二者SO2浓度较为接近外,其他各季株洲非林地观测区SO2浓度均高于同季节长沙非林地观测区SO2浓度。空气SO2浓度季节性变化与当地的地理环境、气候条件、采暖期与工业生产布局等因素有较大的关系。其中,燃煤、降水、风速风向和气温是影响空气SO2浓度变化主要的污染源与气候条件因素。空气SO2浓度水平还与所在地有无林木覆盖关系密切。无林地空气SO2浓度年均值(0·18±0·08)mg/m3,有林地空气SO2浓度年均值(0·09±0·07)mg/m3,二者间存在极其显著差异(p=0·001)。林木生理活性季节性变化对植物调节空气SO2浓度季节变化的能力有一定的影响。按林地SO2浓度减缓效应大小排序,依次是夏季(55·4%)>冬季(54·1%)>秋季(49·3%)>春季(29·6%)。城市森林作为一种有效的生物措施在控制和治理城市大气SO2污染实践中具有重要作用,不失为一种经济可行、高效的环境保护措施,应着力提高城市森林覆盖率。     

退化生态系统恢复与恢复生态学

恢复生态学起源于100a前的山地、草原、森林和野生生物等自然资源的管理研究,形成于20世纪80年代。它是研究生态整合性的恢复和管理过程的科学。恢复生态学的研究对象是在自然或人为干扰下形成的偏离自然状态的退化生态系统。生态恢复的目标包括恢复退化生态系统的结构、功能、动态和服务功能,其长期目标是通过恢复与保护相结合,实现生态系统的可持续发展。恢复生态学的理论与方法较多,它们均源于生态学等相关学科,但自我设计和人为设计是唯一源于恢复生态学研究和实践的理论。由于生态系统的复杂性,退化生态系统恢复的方向和时间具有不确定性,其恢复的机理可用临界阈值理论和状态跃迁模型进行解释。中国森林恢复中存在的问题包括：大量营造种类和结构单一的人工林忽视了生物多样性在生态恢复中的作用;大量使用外来种;忽视了生态系统健康所要求的异质性;忽略了物种间的生态交互作用;造林时对珍稀濒危种需要缺乏考虑;城镇绿化忽略了植被的生态功能等问题。此外,还介绍了生态恢复的方法、成功恢复的标准,并提出了恢复生态学的发展趋势：恢复生态学尚未形成理论和方法体系,要成熟还有很长的路要走;恢复生态学正在强调自然恢复与社会、人文的耦合;对森林恢复研究要集中在恢复中的障碍和如何克服这些障碍两个方面;鉴于生态系统复杂性和动态性,应停止期待发现能预测恢复产出的简单定律,相反,应该根据恢复地点及目标多样性而强调适应性恢复。     

High level of genetic differentiation in the marine isopod Sphaeroma terebrans (Crustacea Isopoda Sphaeromatidae) as inferred by mitochondrial DNA analysis

Sphaeroma terebrans Bate 1866 is a marine isopod belonging to the large family Sphaeromatidae, which normally colonises the aerial roots of the mangrove genus Rhizophora in tropical and subtropical areas. S. terebrans is part of a group of species whose complete life cycle occurs within the same mangrove wood. In this paper, we provide clear evidence of significant genetic differentiation among geographic populations of the taxon S. terebrans. The consistently low internal variation and the large interpopulation distances indicate that almost all the mitochondrial variation (cytochrome oxidase I) in S. terebrans is apportioned among populations rather than within them. The mean haplotype diversity (h) is 0.71%, and the mean nucleotide diversity (π) is 0.34%. The Minimum Spanning Tree (MST) reveals a complex pattern: three principal haplotype groups corresponding to the geographic locations investigated are distributed in a network. This suggests an ancient evolutionary history and very restricted gene flow between populations. The large genetic distances between the populations of S. terebrans could suggest that this taxon is not a single species but a species complex whose taxonomic status must be revaluated.     

Allometry of fine roots in forest ecosystems

Theoretical predictions regarding fine root production are needed in many ecosystem models but are lacking. Here, we expand the classic pipe model to fine roots and predict isometric scaling relationships between leaf and fine root biomass and among all major biomass production components of individual trees. We also predict that fine root production scales more slowly against increases in leaf production across global forest ecosystems at the stand level. Using meta‐analysis, we show fine root biomass scales isometrically against leaf biomass both at the individual tree and stand level. However, despite isometric scaling between stem and coarse root production, fine root production scales against leaf production with a slope of about 0.8 at the stand level, which probably results from more rapid increase of turnover rate in leaves than in fine roots. These analyses help to improve our understandings of allometric theory and controls of belowground C processes.     

Vascular plant 15N natural abundance in heath and forest tundra ecosystems is closely correlated with presence and type of mycorrhizal fungi in roots

In this study we show that the natural abundance of the nitrogen isotope 15, δ¹⁵N, of plants in heath tundra and at the tundra-forest ecocline is closely correlated with the presence and type of mycorrhizal association in the plant roots. A total of 56 vascular plant species, 7 moss species, 2 lichens and 6 species of fungi from four heath and forest tundra sites in Greenland, Siberia and Sweden were analysed for δ¹⁵N and N concentration. Roots of vascular plants were examined for mycorrhizal colonization, and the soil organic matter was analysed for δ¹⁵N, N concentration and soil inorganic, dissolved organic and microbial N. No arbuscular mycorrhizal (AM) colonizations were found although potential host plants were present in all sites. The dominant species were either ectomycorrhizal (ECM) or ericoid mycorrhizal (ERI). The δ¹⁵N of ECM or ERI plants was 3.5–7.7‰ lower than that of non-mycorrhizal (NON) species in three of the four sites. This corresponds to the results in our earlier study of mycorrhiza and plant δ¹⁵N which was limited to one heath and one fellfield in N Sweden. Hence, our data suggest that the δ¹⁵N pattern: NON/AM plants > ECM plants ≥ ERI plants is a general phenomenon in ecosystems with nutrient-deficient organogenic soils. In the fourth site, a␣birch forest with a lush herb/shrub understorey, the differences between functional groups were considerably smaller, and only the ERI species differed (by 1.1‰) from the NON species. Plants of all functional groups from this site had nearly twice the leaf N concentration as that found in the same species at the other three sites. It is likely that low inorganic N availability is a prerequisite for strong δ¹⁵N separation among functional groups. Both ECM roots and fruitbodies were ¹⁵N enriched compared to leaves which suggests that the difference in δ¹⁵N between plants with different kinds of mycorrhiza could be due to isotopic fractionation at the␣fungal-plant interface. However, differences in δ¹⁵N between soil N forms absorbed by the plants could also contribute to the wide differences in plant δ¹⁵N found in most heath and forest tundra ecosystems. We hypothesize that during microbial immobilization of soil ammonium the microbial N pool could become ¹⁵N-depleted and the remaining, plant-available soil ammonium ¹⁵N-enriched. The latter could be a main source of N for NON/AM plants which usually have high δ¹⁵N. In contrast, amino acids and other soil organic N compounds presumably are ¹⁵N-depleted, similar to plant litter, and ECM and ERI plants with high uptake of these N forms hence have low leaf δ¹⁵N. Further indications come from the δ¹⁵N of mosses and lichens which was similar to that of ECM plants. Tundra cryptogams (and ECM and ERI plants) have previously been shown to have higher uptake of amino acid than ammonium N; their low δ¹⁵N might therefore reflect the δ¹⁵N of free amino acids in the soil. The concentration of dissolved organic N was 3–16 times higher than that of inorganic N in the sites. Organic nitrogen could be an important N source for ECM and, in particular, ERI plants in heath and forest tundra ecosystems with low release rate of inorganic N from the soil organic matter. Received: 8 June 1997 / Accepted: 28 February 1998