The relative importance of pre- and post-germination determinants for recruitment of an annual plant community on moving sandy land

• Background and Aims The relative importance of pre- and post-germination determinants for recruitment of natural plant communities is rarely explored. An annual plant community on moving sandy land was chosen for a case study. Answers to the following questions were sought: (a) Does recruitment of new individuals within the community of annual plants differ in time and space? (b) Is there spatial concordance between seed deposition, seedling emergence, survival and recruitment? (c) What are the direct and indirect effects of pre- and post-germination determinants on plant recruitment.• Methods An integrative approach combining investigation of natural recruitment processes with regression, correlation and path analyses was adopted. Data on seed deposition and seedling recruitment were collected by monitoring the number of seeds deposited in the top 5 cm of the soil and the numbers of seedlings emerged and recruited from all annual plants at sites to a range of distances from the existing shrub Artemisia halodendron (Asteraceae) in eight compass directions for two consecutive growing seasons.• Key Results Community-level recruitment was strongly affected by inter-annual rainfall variation and was highly site- and density-dependent. Low recruitment rate in this system was due to low emergence rate and low post-emergence survival rate. Of the pre- and post-germination determinants studied, it was the number of seedlings which emerged and the post-emergence survival rate that had the greatest direct effects on recruitment, with a combination of both variables explaining the majority of the variance (97 %) in recruitment.• Conclusions This study suggests that post-germination determinants (emergence and survival) rather than pre-germination determinants (seed deposition) substantially determined the final pattern of recruitment. Although the density of seeds deposited did not have a significant direct effect on recruitment, it contributed to observed variation in recruitment indirectly through density-dependent emergence of seedlings.     

科尔沁沙地盐蒿种群的空间分布格局及关联性

以科尔沁沙地半固定沙丘和流动沙丘两种不同沙丘上的盐蒿(Artemisia halodendron)种群为研究对象,采用空间点格局方法研究了 0～20 m尺度上盐蒿种群的空间分布格局及其关联性.结果表明,半固定沙丘上盐蒿种群的数量远大于流动沙丘,不同生长发育阶段的种群结构呈偏正态分布,属于稳定型种群;流动沙丘上不同生长发...     

生物土壤结皮对种子萌发和幼苗建植的影响研究进展

生物土壤结皮（BSC）在陆地生态系统中具有重要的生态地位,尤其是旱地生态系统中,BSC占据了种子植物之间的广阔地面。因此,BSC的发展必然影响种子植物更新过程与植被空间格局;但其作用方式、影响程度等因相关研究涉及多气候要素、土壤类型、BSC组成物种和种子植物物种的差异及其不同组合,导致目前的研究结论存在广泛争议。研究综合论述了BSC改变地表微形态对种子传播过程的影响;BSC改变土壤特性（物理、化学、生物学）对种子萌发和幼苗存活与建植等关键环节的影响;并结合种子形态特征及种子萌发、幼苗建植的性状等,综合分析了BSC对种子传播、种子萌发与幼苗建植等更新过程的潜在影响机理;探讨了目前研究矛盾性结论产生的原因。总体来说,深入研究并全面揭示BSC对种子植物更新过程的影响,应加强学科交叉,将分子生物学、植物生理学、生物化学等微观研究,与遥感、野外生态因子过程监测、控制实验等宏观、中观研究结合,从机理到过程方面动态研究BSC对种子植物更新过程的影响,并引入水文模型、气候模型、种群动态模型等模型预测方法,研究气候变化、各类干扰频发的情景下,BSC对种子传播、萌发及幼苗建植过程的潜在影响,以期促进对BSC与种子植物间相互关系的研究,加深对干旱脆弱生态系统植被发展规律的认识。     

Dominant plant species shape soil bacterial community in semiarid sandy land of northern China

Plant species affect soil bacterial diversity and compositions. However, little is known about the role of dominant plant species in shaping the soil bacterial community during the restoration of sandy grasslands in Horqin Sandy Land, northern China. We established a mesocosm pots experiment to investigate short‐term responses of soil bacterial diversity and composition, and the related soil properties in degraded soils without vegetation (bare sand as the control, CK) to restoration with five plant species that dominate across restoration stages: Agriophyllum squarrosum (AS), Artemisia halodendron (AH), Setaria viridis (SV), Chenopodium acuminatum (CA), and Corispermum macrocarpum (CM). We used redundancy analysis (RDA) to analyze the association between soil bacterial composition and soil properties in different plant species. Our results indicated that soil bacterial diversity was significantly lower in vegetated soils independent of plant species than in the CK. Specifically, soil bacterial species richness and diversity were lower under the shrub AH and the herbaceous plants AS, SV, and CA, and soil bacterial abundance was lower under AH compared with the CK. A field investigation confirmed the same trends where soil bacteria diversity was lower under AS and AH than in bare sand. The high‐sequence annotation analysis showed that Proteobacteria, Actinobacteria, and Bacteroidetes were the most common phyla in sandy land irrespective of soil plant cover. The OTUs (operational taxonomic units) indicated that some bacterial species were specific to the host plants. Relative to bare sand (CK), soils with vegetative cover exhibited lower soil water content and temperature, and higher soil carbon and nitrogen contents. The RDA result indicated that, in addition to plant species, soil water and nitrogen contents were the most important factors shaping soil bacterial composition in semiarid sandy land. Our study from the pot and field investigations clearly demonstrated that planting dominant species in bare sand impacts bacterial diversity. In semiarid ecosystems, changes in the dominant plant species during vegetation restoration efforts can affect the soil bacterial diversity and composition through the direct effects of plants and the indirect effects of soil properties that are driven by plant species.     

毛乌素沙地4种沙生植物种子萌发及出苗对沙埋及单次供水的响应

飞播固沙作为一项快速而有效的现代化治沙措施,已成为毛乌素沙地植被恢复与重建的重要手段。但此项技术仍存在一些问题,如飞播后成苗率较低等。以毛乌素沙地优势植物柠条（Caragana korshinskii）、羊柴（Hedysarum laeve）、油蒿（Artemisia ordosica）与籽蒿（Artemisia sphaerocephala）为研究对象,人工控制7个沙埋深度和9个水分梯度,研究4种植物种子萌发和幼苗出土对沙埋和单次供水的响应,阐明多大的单次降雨才能保证种子萌发与幼苗出土及出土幼苗的维持时间。结果表明,对于柠条和羊柴质量相对较大的种子而言．种子主要在供水量10．20mm,埋藏深度为0．5．2cm的条件下出苗,但在埋藏深度为5cm时这两个物种仍有少量种子能够出苗;而对于油蒿和籽蒿质量相对较小的种子而言,种子主要在供水量10．20mm,埋藏深度为0．5cm时出苗,当埋藏深度为1．5cm和2cm时不能顺利出苗。沙埋深度0．5cm条件下,4个物种具有最高的出苗率和出苗速率。在试验期内（30d）,柠条、羊柴和油蒿出土幼苗分别在单次供水量为5、7．5mm和7．5mm时全部死亡,在单次供水量高于15mm时4个种的出土幼苗死亡率大多低于50％,且随供水量的增加出土幼苗死亡率降低。结合毛乌素沙地降雨特点与本次实验结果,0．5cm的沙埋及7．5mm以上的单次降雨是上述4种植物自然条件下出苗较好的重要条件。     

科尔沁沙地家榆林的种子散布及幼苗更新

家榆是典型的风播植物,在科尔沁沙地可以形成单一优势种林地.通过对林缘和孤立树不同方向从树基部向外的有序取样与林下和林窗的随机取样调查,分析了家榆林下和林窗不同生境的种子沉降和幼苗分布特征、林缘和孤立树种子散布方向的差异性与规律性.结果表明,家榆种子密度在林下生境阴坡和阳坡明显大于丘顶和丘底,在林窗生境丘顶明显大于丘底.家榆向周围不同方向种子散布的距离和累积数量均有较大差异,其中林缘和孤立树均以当地多逆风频率的西南方向最近,累积数量也最少;孤立树以多顺风频率的东北方向最远,累积数量也最多.林缘和孤立树共6个方向断面从树基部向外单位面积种子数量频度均适合于韦伯分布和对数-正态分布(X2(a)＜0.500)密度函数,具有相同的种子散布格局.家榆幼苗密度在林下阴坡明显大于阳坡,林下和林窗均为丘顶明显大于丘底.林缘19 m内家榆幼苗呈连续分布.在科尔沁沙地,如果不加限制或人为扰动,家榆林不仅可以很好地进行自然更新,而且可以在适宜的沙地生境逐渐扩大其分布空间.     

Negative density dependence of seed dispersal and seedling recruitment in a Neotropical palm

Negative density dependence (NDD) of recruitment is pervasive in tropical tree species. We tested the hypotheses that seed dispersal is NDD, due to intraspecific competition for dispersers, and that this contributes to NDD of recruitment. We compared dispersal in the palm Attalea butyracea across a wide range of population density on Barro Colorado Island in Panama and assessed its consequences for seed distributions. We found that frugivore visitation, seed removal and dispersal distance all declined with population density of A. butyracea, demonstrating NDD of seed dispersal due to competition for dispersers. Furthermore, as population density increased, the distances of seeds from the nearest adult decreased, conspecific seed crowding increased and seedling recruitment success decreased, all patterns expected under poorer dispersal. Unexpectedly, however, our analyses showed that NDD of dispersal did not contribute substantially to these changes in the quality of the seed distribution; patterns with population density were dominated by effects due solely to increasing adult and seed density.     

Intrinsic biotic factors and microsite conditions drive seedling survival in a species with masting reproduction

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Effects of watering regime and depth of burial on seedling emergence of four dominant psammophytes in the Mu Us sandy land, Inner Mongolia, China, and relevance to revegetation of a desertified region

Four dominant psammophyte species are air-seeded in attempts to revegetate the Mu Us sandy land in Inner Mongolia, but seedling emergence is low. This study sought to clarify the behaviour of seedling emergence under different water supply and sand burial regimes to improve the technology of air seeding. Seeds were buried in sand at depths of 0, 0.5, 1, 1.5, 2, 3 and 5 cm and supplied one time with 2.5, 5, 7.5, 10, 15, 20, 30 or 40 mm of water or with 2.5 mm once every 3 days for 30 days. Our study showed that a 0.5-cm burial depth and under 10- to 20-mm single-watering regimes resulted in highest seedling emergence percentage and rate of these four species. All emerged seedlings of Caragana korshinskii , Hedysarum laeve and Artemisia ordosica died under 2.5, 5 and 7.5 mm of watering, respectively, but seedling mortality was <50% when the single-watering regime was >10 mm. In the field, germination of the four species increased from the top of the dune to the middle of the leeward side and then to the bottom. Based on precipitation pattern in the Mu Us sandy land, our study indicates that 0.5-cm burial depth and a single-watering regime of >7.5 mm is the threshold condition for seedling emergence.     

Dynamics of buried seed population and seedling cohorts of two dominant weeds in a hill agroecosystem of the humid subtropics of India

Dynamics of the buried seeds and plant population of two dominant weeds, viz.,Emilia sonchifolia (Linn.) DC. andRichardsonia pilosa HBK were studied in the crop fields of Meghalaya, north-east India during radish and maize cropping and intervening fallow periods. The total buried seed population ofR. pilosa was always larger than that ofE. sonchifolia, but the germinable fraction was invariably greater in the latter. A major portion (39–41%) of the viable (germinable+dormant) seed population in both weeds was confined to the surface soil layer (0–5 cm). The viable seed population ofE. sonchifolia peaked during April, while that ofR. pilosa showed two peaks (during August and December). The survival pattern and half-lives of seedling cohorts showed, some differences in the two weed species, but both being summer annuals, their populations behaved in a similar manner by showing higher seedling recruitment (K) and survivorship (p) rates in the summer crop (maize) than in the winter crop (radish). However, the density of plants that could attain adulthood was significantly higher inE. sonchifolia thanR. pilosa which might have resulted in greater seed input of the former to the soil leading to its greater abundance in the crop fields.     

Intraspecific variation in seed dispersal of a Neotropical tree and its relationship to fruit and tree traits

The distribution of wind‐dispersed seeds around a parent tree depends on diaspore and tree traits, as well as wind conditions and surrounding vegetation. This study of a neotropical canopy tree, Platypodium elegans, explored the extent to which parental variation in diaspore and tree traits explained (1) rate of diaspore descent in still air, (2) distributions of diaspores dispersed from a 40‐m tower in the forest, and (3) natural diaspore distributions around the parent tree. The geometric mean rate of descent in still air among 20 parents was highly correlated with geometric mean wing loading^1/2 (r = 0.84). However, diaspore traits and rate of descent predicted less variation in dispersal distance from the tower, although descent rate⁻¹ consistently correlated with dispersal distance. Measured seed shadows, particularly their distribution edges, differed significantly among six parents (DBH range 62–181 cm) and were best fit by six separate anisotropic dispersal kernels and surveyed fecundities. Measured rate of descent and tree traits, combined in a mechanistic seed dispersal model, did not significantly explain variation among parents in natural seed dispersal distances, perhaps due to the limited power to detect effects with only six trees. Seedling and sapling distributions were at a greater mean distance from the parents than seed distributions; saplings were heavily concentrated at far distances. Variation among parents in the distribution tails so critical for recruitment could not be explained by measured diaspore or tree traits with this sample size, and may be determined more by wind patterns and the timing of abscission in relation to wind conditions. Studies of wind dispersal need to devote greater field efforts at recording the “rare” dispersal events that contribute to far dispersal distances, following their consequences, and in understanding the mechanisms that generate them.