Grazing enhances belowground carbon allocation,microbial biomass,and soil carbon in a subtropical grassland

Despite the large contribution of rangeland and pasture to global soil organic carbon (SOC) stocks, there is considerable uncertainty about the impact of large herbivore grazing on SOC, especially for understudied subtropical grazing lands. It is well known that root system inputs are the source of most grassland SOC, but the impact of grazing on partitioning of carbon allocation to root tissue production compared to fine root exudation is unclear. Given that different forms of root C have differing implications for SOC synthesis and decomposition, this represents a significant gap in knowledge. Root exudates should contribute to SOC primarily after microbial assimilation, and thus promote microbial contributions to SOC based on stabilization of microbial necromass, whereas root litter deposition contributes directly as plant‐derived SOC following microbial decomposition. Here, we used in situ isotope pulse‐chase methodology paired with plant and soil sampling to link plant carbon allocation patterns with SOC pools in replicated long‐term grazing exclosures in subtropical pasture in Florida, USA. We quantified allocation of carbon to root tissue and measured root exudation across grazed and ungrazed plots and quantified lignin phenols to assess the relative contribution of microbial vs. plant products to total SOC. We found that grazing exclusion was associated with dramatically less overall belowground allocation, with lower root biomass, fine root exudates, and microbial biomass. Concurrently, grazed pasture contained greater total SOC, and a larger fraction of SOC that originated from plant tissue deposition, suggesting that higher root litter deposition under grazing promotes greater SOC. We conclude that grazing effects on SOC depend on root system biomass, a pattern that may generalize to other C4‐dominated grasslands, especially in the subtropics. Improved understanding of ecological factors underlying root system biomass may be the key to forecasting SOC and optimizing grazing management to enhance SOC accumulation.     

亚热带山地草地退化系列上种群更新研究

从种子库,幼苗和种子雨等方面对亚热带山地草地退化系列上5种植物的种群更新进行了研究,结果表明：（1）引入种多年生黑麦草（Lolium perenne)和白三叶（Trifolium repens)种群在轻度和中度退化条件下能够正常更新,但不能忍受草地进一步的退化;（2）本地种对草地退化的胁迫表现出较强的承受能力。簇生卷耳（Cerastium caespitosum)、箭叶蓼（Polygonum sagittatum)和华蒲公英（Taraxacum sinicum)的种群更新分别在中度、重度和极度退化草地上处于最佳状态;（3）若要防治草地退化,就应当地引入种的种群更新受到抑制时采取有效措施。     

Vegetation and soil seed bank in a 23-year grazing exclusion chronosequence in a Mediterranean dry grassland

Long-term grazing shaped plant diversity in dry Mediterranean grasslands. Abandonment of grazing affects plant diversity especially in the northern Mediterranean. Considerable efforts are, therefore, under way for grassland conservation and restoration. Yet, we do not know at which temporal scales impacts of grazing abandonment appear and in particular how soil seed banks evolve after longer grazing abandonment. Here, we provide detailed data from one of the very few long-term experiments available. These experiments provide data for up to 23 years (1982–2005) of grazing exclusion built in 1982, 1989, 2000 and 2001. Grazing exclusion decreased species richness, modified vegetation structure and changed soil parameters. Decline in species richness appears in communities that experienced 16 and 23 years of grazing exclusion. Only four to nine plant species of this Mediterranean grassland built persistent soil seed banks appearing after grazing exclusion, compared to 40–50 species in the established vegetation of grazed plots. Hence, similarity between vegetation and soil seed bank decreased with time of grazing exclusion. Even 23 years after abandonment, no woody plants colonised the experiments. We conclude that vegetation will recover fast from grazing abandonment in the short-term. Nevertheless, longer abandonment will impact diversity due to reduced soil seed banks.     

松嫩草原中度退化草地不同植被恢复方式下土壤螨类群落特征的差异

应用类群数、个体密度、多样性指数和MI指数等多个群落参数,研究不同植被恢复方式下松嫩草原中度退化草地土壤螨类群落特征的差异。研究结果表明,与过度放牧样地相比,种植苜蓿和围栏封育样地的土壤环境相对优越,它们拥有较高的土壤螨类类群数、个体密度和群落多样性以及中气门螨类MI指数。在土壤螨类群落结构所有参数中,多样性指数(H′)和中气门螨类MI指数,种植苜蓿和围栏封育样地均明显高于过度放牧样地,这些差异反映了种植苜蓿和围栏封育对中度退化草地土壤螨类的群落结构具有明显改善作用。然而研究结果也显示,围栏封育样地土壤螨类群落多样性指数(H′)和中气门螨类MI指数尽管明显高于过度放牧样地,但是依然显著低于种植苜蓿样地,表明选择种植苜蓿较围栏封育可能更利于松嫩草原中度退化草地土壤螨类群落的恢复与重建。     

Small‐scale spatial dynamics of vegetation in a grazed Uruguayan grassland

We explored the small‐scale plant species mobility in a subhumid native grassland subjected to grazing by cattle in south‐western Uruguay. We established four permanent plots of 40 × 40 cm, divided in 16 × 16 cells. In each cell, the presence of species was seasonally recorded for 2 years and annually recorded for 4 years. By nesting the cells, we studied the mobility at different scales, from 6.25 cm² to 400 cm². At each scale we measured species richness, cumulative richness and the turnover rates of the dominant species. We found that the cumulative species richness was an increasing power function, with higher accumulation rates with smaller spatial scale. Although species richness showed seasonal fluctuations, the mean species richness was constant during the study period. We detected significant spatio‐temporal variability in mobility patterns among species. Certain species showed a high capacity to colonize new sites, whereas other species rotate among sites that they previously occupied. Grazed communities in Uruguayan Campos are structured as a dense matrix of perennials grasses and forbs, where vegetative propagation is the main form of growth of the species. The small‐scale dynamics and the high variability in the mobility characteristics could be linked with the diversity of growth forms and spatial strategies of the species in this community. We believe that a high degree of small‐scale spatial dynamics contribute to explain the species coexistence and the apparent stability of communities at local scales.     

Dynamics of cryptogamic soil crusts in a derived grassland in south‐eastern Australia

We examined the dynamics of cryptogamic soil crusts in a derived (disclimax) grassland near Orange in south‐eastern Australia. Changes in the cover of cryptogamic crusts and floristics and abundance of the constituent species were measured on four treatments with two levels each of grazing and cultivation. Twenty‐two lichens, mosses and liverworts were found at the study site and, of these, 13 were collected in the quadrats. Three moss species (Barbula calycina, Eccremidium arcuatum and Bryum pachytheca) and one lichen species (Cladonia tessalata) accounted for 67% of total cover‐abundance scores. Generally, cover‐abundance was significantly higher in the unvegetated microsites than in the vegetated microsites. Species richness was not significantly different between the four grazing‐cultivation treatments but, on average, there were significantly more species in the unvegetated microsites (mean = 3.2 species) than in the vegetated microsites (0.54 species). Grazing and cultivation resulted in significantly greater cover of bare ground and consequently significantly greater crust cover. Averaged across all treatments, approximately half of the area of unvegetated soil was occupied by cryptogams. Overall, the results indicate that lichens and bryophytes are important components of derived temperate grasslands, surviving in even densely vegetated swards. This study suggests that strategies which disturb the soil surface (e.g. grazing and cultivation) will stimulate the abundance and cover of soil crust organisms by increasing the availability of unvegetated microsites.     

Size-related deterioration of semi-natural grassland fragments in Sweden

Abstract. One of the most dramatic landscape changes during the 20th century in Sweden, like in most of Europe, has been the reduction and fragmentation of semi-natural grasslands. Using a set of remnant semi-natural grasslands, chosen to be as similar as possible, but differing in size, we have examined whether size of remnant fragments of traditionally managed semi-natural grasslands in Sweden is related to patterns of species richness and composition. We focused on edge-to-interior relationships, since we expected that a possible impact from invasive habitat generalists would be manifested in a gradient from the edge of fragments to their interior. We found no relationship between size of grassland fragments and (a) overall species richness, (b) species richness at different spatial scales, and (c) abundance of some typical invader species or species characteristic of semi-natural grasslands. However, the results indicated that larger grasslands have a comparatively larger number of species in the edges, whereas the opposite pattern was found in smaller grasslands. The similarity in species composition between the edge and the interior of the pastures also increased with grassland size. Thus, even though the overall species richness is still unaffected by reduction in grassland fragment size, the edges of smaller grasslands show signs of degradation, i.e. reduction in species richness and a decreased similarity to the grassland interior. We suggest that these kinds of effects may be early signs of fragmentation effects that in the future will result in species loss even if the present distribution of semi-natural grasslands is maintained.     

Trophic-level differences in functional composition of the Nardus grassland vegetation

In this study, we examined floristic and functional composition of Nardus grassland of the highlands of NE Slovenia. The data-set included 55 relevés, 59 plant species, and 17 plant functional traits (PFT). The TWINSPAN classification resulted in two plant communities; calcifuge species (G1_oligotr) and another group of species characteristic of mesotrophic meadow (G2_mesotr). On the basis of selected PFT 11 out of 17 differ significantly between the groups. Group G1_oligotr had higher community-weighted mean trait values for chamaephytes (G1 = 0.09; G2 = 0.02), geophytes (G1 = 0.03; G2= 0.01), competitors (G1 = 0.43; G2 = 0.41) and plants that start flowering later (G1 = 142.84; G2 = 136.69). On the other side was G2_mesotr represented with more plants that are therophytes (G1 = 0.04; G2 = 0.09), creeping (G1 = 0.01; G2 = 0.12) and short-lived (G1 = 0.04; G2 = 0.11) and have longer flowering period (G1 = 3.24; G2 = 3.60). Differences may reflect the stronger effect of disturbance and eutrophication in G2_mesotr, probably due to intensification of grassland management (grazing) in the region. Our findings are significant in understanding the relative influence of environmental stress and disturbance within Nardus grasslands, and this may have important implications for their conservation management.     

Acceleration of vegetation succession on eroded land by reforestation in a subtropical zone

Based on field investigations in the East River basin in Guangdong Province, south China, the processes of vegetation development and vegetation succession on bare slopes with high erosion rates are studied. Different reforestation measures have been applied to the slopes, which have resulted in very different processes of vegetation development and succession. On an experimental plot with burned forest but surviving roots, the vegetation restored naturally and quickly because there was little soil erosion. However, in the plots suffering from long-term severe soil erosion, natural vegetation recovery on these barren slopes is very slow. After 26 years, a barren slope has only been partly colonized by poorly developed vegetation composed of heliophilous herbages and scattered shrubs. On the other hand, plantation of some selected wood species on the slope land has dramatically accelerated the vegetation recovery and succession. The plots were reforested with plantation of Acacia auriculiformis in the early 1980s. Twelve years later, the vegetation cover of the artificial forests reached 90% and an understory vegetation community consisting of local species had naturally developed. Reforestation with suitable strategies may control erosion and greatly accelerate vegetation succession in the eroded slope land in the subtropical zones.     

松嫩草原中度退化草地不同植被恢复方式下土壤线虫的群落特征

2005年5月～10月对松嫩草原中度退化草地不同植被恢复方式样地进行土壤线虫调查,淘洗-过筛-蔗糖离心法提取土壤线虫,应用类群属数、个体密度、多样性指数和功能类群指数等多个群落参数,研究中度退化草地不同植被恢复方式间土壤线虫群落特征的差异.研究共捕获线虫13 713条,分别隶属于线虫动物门2纲7目24科40属,个体密度平均113条/100g干土.研究结果表明,不同植被恢复方式间土壤线虫群落特征存在一定差异,围栏封育和种植苜蓿均能明显改善中度退化草地土壤线虫群落环境,但围栏封育较种植苜蓿更能显著提高土壤线虫的个体密度和群落多样性.土壤线虫个体密度垂直分布,围栏封育样地线虫表聚性最明显,种植苜蓿样地表聚性次之,过度放牧样地表聚性最差,其夏季和秋季土壤线虫向土壤下层移动明显.此外,研究结果还表明,功能类群指数能够指示中度退化草地植被恢复方式间的差异,其中∑MI指数和PPI指数对于不同植被恢复方式下土壤线虫群落变化反映最敏感,根据功能类群指数统计,与围栏封育相比,种植苜蓿和过度放牧显著改变了土壤线虫群落中r-和k-选择植物寄生线虫的比例.对于松嫩草原中度退化草地,选择围栏封育方式可能更利于草地土壤线虫群落的恢复与重建.     

Changes in plant species diversity along a chronosequence of vegetation restoration in the humid evergreen broad-leaved forest in the Rainy Zone of West China

Plant species diversity has been recognized as one of the vital attributes for assessing vegetation restoration. Changes in the diversity may be related to different stages of succession. In this study, 54 sites of humid, evergreen, broad-leaved forest were selected in the Rainy Zone of West China. A chronosequence of the sites was used to study the successive patterns of the diversity in the forest that had undergone natural regeneration for 5 to 350 years and to test the hypothesis that the diversity is maximized in mid-succession. Data were collected simultaneously at different stages of succession, and four α-diversity indices (species richness, Margalef index, Shannon-Wiener index, Pielou Evenness index) and two β-diversity indices (Whittaker index, Sørensen’s index) were calculated for each stratum in each plot. A total of 394 vascular plant species were recorded. From the β-diversity indices, the forest succession may be divided into the early-successional stage (before 50 years), mid-successional stage (from 50 to 300 years), and late-successional stage (after 300 years). In this community, the species diversity and richness were found to be the greatest at the mid-successional stage, followed by the late- and early-successional stages. The results of regression analysis indicated that the richness and Margalef index peaked around the 175th and 165th year, respectively. Shannon-Wiener index values also appeared to follow an approximately humped pattern of succession and were maximal around the 100th year. However, the species evenness did not show any significant relationship with successional age. Our results demonstrate (1) forest restoration is a long-term process and the formation of climax forest requires at least 300 years and (2) the forest has a strong capacity for restoration. Our results also suggest Lindera limprichitii and Machilus pingii as ideal tree species for afforestation because of their wide niche.     

植被对亚热带城市生态系统CO2通量的影响

城市是陆地生态系统的主要碳源,而城市植被是城市区域缓解人类活动所释放的二氧化碳的主要碳汇,但对城市植被对城市大气二氧化碳的影响方面的研究比较缺乏,尤其是发展中国家。发展中国家多数处于亚热带气候区,且发展中国家城市化进程较快,为推进不同生态系统类型碳循环的研究,该研究以位于中国东南部的上海市奉贤大学城为案例,研究该区域植被对亚热带城市生态系统CO_2通量的影响。使用上海市奉贤大学城的涡动相关通量观测站点所观测和记录的2016年10月1日至2017年9月30日共计12个月的通量,气象数据结合遥感数据分析了该研究区的CO_2通量动态特征及其影响因子,主要结论是:(1)整个生态系统全年CO_2通量总交换量为9664.06μmol m~(-2)a~(-1)即表现为碳源。CO_2通量增长率在2017年5月6日达到最低为-4.48μmol m~(-2)d~(-1)在2017年7月30日的CO_2通量增长率为0,在2017年8月30日达到最高为2.24μmol m~(-2)d~(-1),生长季CO_2通量交换量为2169.58μmol m~(-2)月~(-1)低于非生长季的CO_2通量交换量(7494.48μmol m~(-2)月~(-1));(2)不同风区的CO_2通量特征不同,主要表现为随着植被面积的上升CO_2通量有下降的趋势,生长季CO_2通量均值的最低值出现在西北风区为0.09μmol m~(-2)s~(-1);(3) CO_2通量与叶面积指数呈现负相关关系,即随着叶面积指数的上升CO_2通量有下降的趋势。植物的生长状况和其生理活动影响亚热带城市生态系统的碳循环过程,该研究可以为量化城市植被对大气二氧化碳的影响提供参考,同时为亚热带地区建设绿色低碳城市提供服务。     

The mycorrhizal community in a forest chronosequence of Sitka spruce [Picea sitchensis (Bong.) Carr.] in Northern England

Demography and fungal diversity of the belowground ectomycorrhizal community in a chronosequence of Sitka spruce [Picea sitchensis (Bong.) Carr.] in Northumberland, Northern England, were analysed; mycorrhizal root samples were taken from 6-, 12-, 30- and 40-year-old stands, and fungal fruiting bodies were collected in autumn to complement the survey. Naturally germinated seedlings less than 1 year of age (taken from the 30-year-old stand) were also examined. A total of 118,000 mycorrhizal root tips were extracted from 40 soil cores (ten per age class) and from the complete root systems of 25 seedlings and separated into active and senescent root tips according to their morphology and anatomy. Active tips were distinguished according to their mycobionts which were characterised and identified microscopically. Although almost 100% of all fine roots were mycorrhizal, EM fungal diversity throughout the chronosequence was low, consisting of a total of 16 species of which three were only found as fruiting bodies. Of the six mycobionts found most regularly below ground, Tylospora fibrillosa was the most common, colonising about 70% of all root tips and more than 90% of those of seedlings and young trees. Root density and mycorrhizal diversity increased, but percentage of vital root tips decreased with increasing tree age, levelling off in the 30- and 40-year-old stand. Among the five subdominant fungal species, Dermocybe crocea was found to have its peak of distribution in the 12-year-old stand and Russula emetica, Lactarius rufus, Hymenoscyphus ericae agg. and the unidentified Piceirhiza sulfo-incrustata in the 30- and 40-year-old stands. The possible correlations between the mycorrhizal community structure and biotic and abiotic factors are discussed.     

南亚热带混交人工林树种丰富度与土壤微生物多样性和群落组成的关系

森林植被与土壤微生物作为森林生态系统的重要组成部分,它们之间的相互作用对维持森林生态系统功能和稳定性起着重要作用。以往多在天然草地和森林生态系统开展植物多样性与土壤微生物多样性关系的研究,但人工构建的多树种混交林生态系统中树种多样性对土壤微生物群落组成的影响及其机制尚不完全清楚。因此,以南亚热带人工块状造林后自然恢复形成的多树种混交森林生态系统为研究对象,利用高通量测序技术研究了随树种丰富度(1—10种)变化土壤细菌和真菌多样性的变化规律及主要影响因子。结果表明,随树种丰富度增加,土壤真菌α多样性显著提高,但土壤细菌α多样性差异不显著;不同树种丰富度梯度间土壤细菌和真菌的群落结构组成均差异显著;Pearson相关分析表明土壤细菌α多样性主要受土壤pH和土壤铵态氮影响,而土壤pH和有效磷是土壤真菌α多样性的主要影响因子。距离冗余分析(db-RDA)表明,对土壤细菌群落组成产生显著影响的环境因子分别为土壤pH、硝态氮和芳香碳组分,而土壤有机碳、硝态氮、细根生物量和氧烷基碳组分是影响土壤真菌群落组成的主要因子。本研究的结果说明了南亚热带人工林不同树种混交后形成多树种混交林生态系统的过程中,树...