荒漠草原沙漠化植物群落及土壤物理变化

沙漠化是草地退化最严重的形式之一。以空间代替时间的方法,通过对宁夏中北部荒漠草原沙漠化过程中植物群落特征和土壤物理特性的研究,探讨草地植物群落与土壤物理特性对沙漠化的响应机制。结果表明:(1)潜在沙漠化阶段草地以牛枝子、猪毛蒿、中亚白草为优势种,轻度沙漠化阶段草地以中亚白草、苦豆子为优势种,中度沙漠化阶段草地以狗尾草、虫实为优势种,重度沙漠化阶段草地以沙米、赖草、狗尾草为优势种,极度沙漠化阶段草地以沙米为优势种。(2)随着沙漠化程度的加剧草地植物群落生物量、Shannon-Wiener指数、丰富度指数、盖度均呈降低趋势,但轻度沙漠化阶段草地植被生物量比潜在沙漠化增加了23%。(3)草地沙漠化导致土壤容重和土壤粗砂粒含量增加,而土壤水分,土壤细砂粒和粘粉粒含量降低。荒漠草原沙漠化导致了土壤环境和植被明显退化,草地生产力明显降低。     

玛曲高寒草甸沙化过程中群落结构与植物多样性

针对玛曲高寒草甸沙质荒漠化和水土流失日趋严重的问题,采用群落样方调查的方法,研究了潜在、轻度、中度和重度4种典型高寒沙化草甸的植被结构特征和α多样性与β多样性变化规律。结果表明:随着沙化程度的加剧,1)群落植被盖度持续下降(分别为99.216%,80.078%,49.895%,36.398%)、物种数逐渐减少(分别为53,32,14,13)、群落结构趋向简单(重要值大于1的物种分别有28,16,10,8种),典型高寒草甸优势种和伴生种逐渐退出群落,而适应沙质荒漠的物种逐渐占据群落中的优势地位,玛曲高寒草甸生态系统有向高寒沙质荒漠生态系统转化的趋势,群落以青藏苔草+高山早熟禾群落→青藏苔草+防风+高山嵩草群落→高山嵩草+防风+青藏苔草群落→毛穗赖草+藏虫实+青藏苔草+防风群落的方向演替;2)群落丰富度和植物多样性指数均呈下降趋势,差异显著;均匀度指数先增加后减小,在轻度沙化草甸达到最大,但差异不显著;而群落优势度逐渐增加,在中度沙化草甸增加极显著;3)潜在沙化草甸与沙化草甸之间的Whittaker指数差异极显著,中度沙化草甸与重度沙化草甸之间差异显著,而轻度沙化草甸与中度沙化草甸之间差异不显著。4)潜在沙化草甸与3种沙化草甸和轻度沙化草甸与重度沙化草甸之间群落相异性系数较高(0.705—0.937),群落共有度指数较低(0.034—0.173),而重度沙化草甸与中度沙化草甸、轻度沙化草甸与中度沙化草甸之间群落相异性系数相对较低(0.545—0.553),群落共有度指数相对较高(0.293—0.303)。以上分析表明,玛曲高寒草甸在沙化过程中存在轻度沙化和重度沙化两个关键过程,因此对潜在沙化草甸应采取封育、禁牧、轮牧、抚育等科学管护措施,而对沙化草甸应采取草皮移植、补播、施肥等植被快速恢复措施和流沙治理措施进行生态修复,防止草甸沙化的加剧和蔓延。     

宁夏东部风沙区沙化草地土壤水分和植被的空间特征

土壤水分作为土壤-植被-大气连续体的关键因子,对沙化草地的演化过程具有重要作用。为探讨宁夏东部风沙区沙化草地土壤水分、物种丰富度指数及植被盖度的空间变异及其相互关系,以哈巴湖自然保护区沙化草地为对象,采用样线法自潜在沙化草地至重度沙化草地进行植被调查和土壤取样,通过经典统计学和地统计学分析,得出以下结果：0—100 m各土层土壤水分含量、植被盖度和物种丰富度指数的分布范围分别为0.82%—28.22%、41.00%—93.00%和0.82—2.80,变异系数范围为0.20—0.48,均属于中等变异。各土层土壤水分和物种丰富度指数表现为中等的空间自相关性,植被盖度则表现为强烈的空间自相关性。Kriging插值结果表明,0—100 cm各土层土壤水分和植被盖度的空间插值图呈条带状和斑块状的梯度变化,物种丰富度呈斑块分布,自潜在沙化草地至重度沙化草地,表现为逐渐降低的趋势。相关分析表明,植被盖度与0—40 cm各土层土壤水分呈显著正相关,与40—100 cm各土层土壤水分呈极显著正相关。宁夏东部风沙区沙化草地土壤水分含量总体较低,由于结构因素和随机因素的共同作用,随草地沙化程度的加重,表现为...     

沙化草地土壤碳氮磷化学计量特征及其对植被生产力和多样性的影响

沙化草地土壤碳(C)、氮(N)、磷(P)化学计量特征及其对植被生产力与多样性的影响对于认识草地沙漠化过程中土壤与植被的互馈关系,以及沙漠化发展的生态学机理具有重要的意义。通过对科尔沁沙地75个沙化样地的野外调查,研究了科尔沁沙地不同程度沙化草地的表层土壤C、N、P化学计量特征及其与生产力和多样性的相关关系。结果表明:1)科尔沁沙地沙化草地表层土壤具有较低的有机C、全N、全P含量及C∶N、N∶P和C∶P,平均值分别为1.39 mg/g、0.117 mg/g、0.079 mg/g和7.50、2.22、16.91;草地沙漠化过程中,土壤有机C、全N、全P含量显著降低的同时,C∶N、N∶P和C∶P亦显著降低,表明土壤有机C、全N、全P在沙漠化过程中的损失是不同步的;2)科尔沁沙地沙化草地表层土壤有机C、全N、全P元素间均呈显著正相关,具有一定的耦合关系,且土壤有机C和全P间的耦合关系不随沙漠化的发展而发生改变;3)草地沙化过程中,土壤养分的损失限制着草地生产力,而土壤N∶P较全N、全P含量更能反映土壤养分对生产力的限制作用;4)沙化草地土壤全N含量与物种丰富度间具有显著正相关关系,而土壤全P含量与其无显著相关性;多样性指数与全N、全P含量间均具有显著正相关关系;相对于土壤全N、全P含量,N∶P能更好地反映养分平衡对物种多样性的影响作用。     

鄱阳湖沙地沙漠化过程中单叶蔓荆群落结构和功能特征

在鄱阳湖沙地不同沙化程度区开展单叶蔓荆群落组成及其物种多样性变化调查.结果表明:随着沙漠化程度的增加,多年生草本和灌木逐渐减少,物种多样性减少,至重度沙化时,沙地植被明显退化,群落结构趋于简单和不均匀;在不同程度沙化区和植被恢复演替过程中,物种组成结构具有递进性和渐变性;在沙山治理过程中,可进行单叶蔓荆和禾本科植物配植...     

川西北高寒草地沙化过程中土壤氮素变化特征

草地沙化是我国最严重的环境问题之一,但关于草地沙化过程中氮素变化特征的研究报道多集中于干旱半干旱地区,而半湿润地区的相关报道还比较缺乏。通过野外调查,研究了川西北半湿润地区高寒沙质草地沙化过程中土壤氮素变化特征。结果表明,草地沙化对0—100cm土层土壤氮素具有显著影响,全氮、碱解氮、铵态氮(NH_4~+-N)、硝态氮(NO_3~--N)和微生物量氮(MBN)均呈现极显著下降的变化特征,极度沙化阶段较未沙化阶段分别减少了73.95%、77.72%、76.75%、79.77%和84.12%。其中,0—20cm土层变化最显著,全氮、碱解氮、NH_4~+-N、NO_3~--N和MBN含量分别减少了86.43%、83.52%、82.11%、88.82%和91.77%。随着土层深度增加,不同程度沙化草地土壤氮素含量及其变化量逐渐减少;草地沙化过程中,不同沙化阶段土壤氮素损失数量不尽相同,其中,以轻度沙化阶段氮素损失最严重,全氮、碱解氮、NH_4~+-N、NO-3-N和MBN含量分别降低了41.18%、35.17%、46.74%、43.46%和46.88%。草地沙化过程中,土壤全氮、碱解氮、NH_4~+-N、NO_3~--N和MBN含量与土壤粉粒、粘粒含量和植被群落盖度均呈极显著正相关特征,与土壤沙粒含量呈极显著负相关特征。研究区土壤氮素损失与风蚀选择性吹蚀土壤粉粒、粘粒及地表植物覆盖状况逐渐变差密切相关,因此该区域治沙的关键是采取措施降低风蚀对地表土壤吹蚀作用,提高沙化草地地表植被覆盖。同时,还应及时对沙化前期阶段及潜在沙化的草地进行生态治理,从而避免草地沙化继续恶化。     

宁夏云雾山典型草原灌丛化过程中植被和土壤演变特征

干旱半干旱区灌木、半灌木等木本植物的分布面积持续增加(灌丛化)显著改变了原有植被景观及生态过程,但缺乏对长期封育过程中灌木物种取代草本物种进而影响草地群落组成、结构和功能的研究。为揭示长期封育措施下典型草原灌丛化特征及地上/地下植物群落的响应规律,研究通过对宁夏云雾山典型草原同一封育草地不同时间(2010年、2016年、2021年)植物群落(物种重要值、丰富度、多样性和生物量)和土壤特性(土壤容重、土壤水分及储水量、养分、pH)的定点观测,探讨了长期封育过程中灌木物种矮脚锦鸡儿(Caragana brachypoda)驱动的植被和土壤特性变化。结果表明,(1)灌木矮脚锦鸡儿已经取代草本植物成为封育草地的优势种,群落中灌木生物量、总生物量以及盖度、高度显著增加,但对植物密度的影响较小;(2)灌丛化过程中草地群落Patrick丰富度指数、Simpson优势度指数和Shannon-Wiener多样性指数显著降低,而Pielou均匀度指数显著增加;(3)灌丛化引起土壤深层(60-100 cm)容重显著增大,土层储水量随土层深度的增加呈先减少后增加的趋势;(4)灌丛化使得土壤有机碳含量在浅层(0...     

黄河源区沙化草地区域景观特征

以黄河源区玛曲县沙化草地为对象,运用景观生态学原理,借助地理信息系统技术,选取景观异质性指数(景观多样性、景观优势度)、景观破碎化指数(斑块密度、分离度)、形状指数(伸长指数、分维数)3类指标,分析了黄河源区沙化草地不同景观类型的空间分布及景观格局现状特征.结果表明,区域整体景观结构简单,局部变化复杂,自然景观仍是景观变化的主体,景观结构受草地、湿地、灌木林地等少数自然景观类型控制.区域景观斑块呈条带状分布,各景观条带差异较大,从河流阶地带到低山草地带到高山草甸带,景观斑块类型减少,景观多样性减小,景观破碎化程度减弱,景观结构呈简单化趋势.沙化景观尤其是流动沙地对景观结构的影响较大,在区域景观异质性增强及破碎化加剧过程中起主要作用.固定沙地、半固定沙地和潜在沙化土地也不同程度地影响着景观的构成及变化;人工景观比沙化景观对景观结构的影响小,但其潜在变化空间较大.人工景观和沙化景观将是未来区域景观结构变化的共同主导因素.     

石羊河中游沙漠化逆转过程土壤种子库的动态变化

在石羊河中游,应用空间代替时间的方法,选择流动沙丘以及封育恢复5a、15a和25a的沙漠化逆转过程序列,研究了沙漠化逆转过程土壤种子库的变化特征。结果表明:石羊河中游沙漠化土地土壤种子库由4科12种植物组成,种子库主要分布于表层0-5cm。在沙漠化逆转过程中,土壤种子库物种数趋于增加,物种组成以1年生草本植物占优势逐渐向多年生草本植物和半灌木植物转变;种子库密度、表层土壤种子比例、物种多样性指数、与地上植被的相似性呈现先增大后降低趋势;沙漠化土地生态恢复间隔的时间越长,土壤种子库间的相似性程度越低;沙漠化土地与地带性植被区种子库的相似性逐渐增大,但是恢复25a沙漠化土地也仅达到0.36。研究认为,沙漠化逆转过程也是沙漠化土地土壤种子库向地带性植被土壤种子库演变的过程,而且是一个十分缓慢的过程,该研究有助于丰富干旱区土壤种子库的理论和指导干旱内陆河流域沙漠化土地的生态恢复实践。     

不同沙化阶段高寒草甸植物群落与表土环境因子的关系

沙漠化是玛曲高寒草甸最为严重的生态问题之一,明晰区域内不同沙化阶段植物群落与表土环境因子的关系,有助于为玛曲高寒草甸沙漠化的监测与防治提供科学参考。鉴于此,研究应用方差分析与典范对应分析,厘清了潜在、轻度、中度、重度和极重度沙化阶段植物的组成格局与变化规律,并对其与表土环境因子的关系进行了分析。结果表明：（1）伴随沙漠化的发展,表土水分与电导率呈逐步降低的趋势,下降幅度显著（P<0.05）;温度呈先降低后增加的变化规律,其中轻度沙化阶段最低,为24.32℃,极重度阶段最高,达30.79℃;（2）伴随沙漠化的发展,植物的盖度、多度依次降低,下降幅度显著（P<0.05）;物种丰富度、Shannon-Wiener指数、Simpson指数与Pielou指数的变化趋势相近,呈潜在至轻度阶段上升,之后逐步下降的规律;（3）典范对应分析（CCA）表明,表土水分和电导率对群落组成的影响程度达到极显著水平（P<0.01）。潜在与轻度沙化阶段,CCA前两轴的方差解释率分别是71.91%和55.25%,表土环境因子能够较好的描述植物群落特征与多样性水平的变化规律;中度、重度和极重度沙化阶段,CCA前两轴的方差解释率不足40%,表土环境因子难以有效表征植物群落特征,预示放牧等人类活动在这些演替阶段的影响更为突出。     

Bird Communities and Biomass Yields in Potential Bioenergy Grasslands

Demand for bioenergy is increasing, but the ecological consequences of bioenergy crop production on working lands remain unresolved. Corn is currently a dominant bioenergy crop, but perennial grasslands could produce renewable bioenergy resources and enhance biodiversity. Grassland bird populations have declined in recent decades and may particularly benefit from perennial grasslands grown for bioenergy. We asked how breeding bird community assemblages, vegetation characteristics, and biomass yields varied among three types of potential bioenergy grassland fields (grass monocultures, grass-dominated fields, and forb-dominated fields), and assessed tradeoffs between grassland biomass production and bird habitat. We also compared the bird communities in grassland fields to nearby cornfields. Cornfields had few birds compared to perennial grassland fields. Ten bird Species of Greatest Conservation Need (SGCN) were observed in perennial grassland fields. Bird species richness and total bird density increased with forb cover and were greater in forb-dominated fields than grass monocultures. SGCN density declined with increasing vertical vegetation density, indicating that tall, dense grassland fields managed for maximum biomass yield would be of lesser value to imperiled grassland bird species. The proportion of grassland habitat within 1 km of study sites was positively associated with bird species richness and the density of total birds and SGCNs, suggesting that grassland bioenergy fields may be more beneficial for grassland birds if they are established near other grassland parcels. Predicted total bird density peaked below maximum biomass yields and predicted SGCN density was negatively related to biomass yields. Our results indicate that perennial grassland fields could produce bioenergy feedstocks while providing bird habitat. Bioenergy grasslands promote agricultural multifunctionality and conservation of biodiversity in working landscapes.     

科尔沁沙地沙漠化过程中的物种多样性

对内蒙古科尔沁地区沙漠化过程中物种多样性的变化特征进行了分析 ,依据植物种在各类型沙漠化土地的出现频率和不同沙漠化阶段各生活型植物种的组成 ,探讨了沙漠化过程中植物种绝灭和定居特点 .物种多样性指数分析表明 ,沙漠化过程是一个物种多样性衰减的过程 ,沙漠化首先导致特有种的绝灭 ,其次为稀有种和普通种 .从植物的生活型来看 ,多年生草本和灌木类植物受沙漠化影响最大 ,而 1年生草本和半灌木类植物可存活于沙漠化过程各个阶段 .在沙漠化过程中植物种的绝灭速率大于定居速率 .     

Perennial biomass feedstocks enhance avian diversity

Federal mandates to increase biofuel production in North America will require large new tracts of land with potential to negatively impact biodiversity, yet empirical information to guide implementation is limited. Because the temperate grassland biome will be a production hotspot for many candidate feedstocks, production is likely to impact grassland birds, a group of major conservation concern. We employed a multiscaled approach to investigate the relative importance of arthropod food availability, microhabitat structure, patch size and landscape‐scale habitat structure and composition as factors shaping avian richness and abundance in fields of one contemporary (corn) and two candidate cellulosic biomass feedstocks (switchgrass and mixed‐grass prairie) not currently managed as crops. Bird species richness and species density increased with patch size in prairie and switchgrass, but not in corn, and was lower in landscapes with higher forest cover. Perennial plantings supported greater diversity and biomass of arthropods, an important food for land birds, but neither metric was important in explaining variation in the avian community. Avian richness was higher in perennial plantings with greater forb content and a more diverse vegetation structure. Maximum bird species richness was commonly found in fields of intermediate vegetation density and grassland specialists were more likely to occur in prairies. Our results suggest that, in contrast to corn, perennial biomass feedstocks have potential to provide benefits to grassland bird populations if they are cultivated in large patches within relatively unforested landscapes. Ultimately, genetic improvement of feedstock genets and crop management techniques that attempt to maximize biomass production and simplify crop vegetation structure will be likely to reduce the value of perennial biomass plantings to grassland bird populations.     

Restoring Western (Basalt) Plains grassland. 2. Field emergence, establishment and recruitment following direct seeding

Summary   Both reservation of small remnants and ecological restoration of degraded areas will be crucial if the Victorian Western (Basalt) Plains grassland community is to be conserved in the long term. This study examined the potential of direct seeding as a technique for grassland restoration by recording the initial establishment and subsequent recruitment success of 64 (predominantly perennial) grassland species direct sown onto a constructed site. Forty-three (67%) of the sown species emerged and established during the 2-year study and a further three species were recorded in subsequent years. In the second year, 32 species increased their number either through seedling or vegetative recruitment and 30 species dispersed beyond their original sown plot. Seed size was not correlated with field emergence but life form did influence initial field success for some groups. The finding that many species are able to establish and recruit under the study conditions supports the need for further investigation of direct seeding in the restoration of grassland communities.     

Life form and life history explain variation in population processes in a grassland community invaded by exotic plants and mammals

The existence of general characteristics of plant invasiveness is still debated. One reason we may not have found these characteristics is because we do not yet understand how processes underlying population dynamics contribute to community composition in invaded communities. Here I modify Ricker stock-recruitment models to parameterize processes important to community dynamics in an invaded grassland community: immigration, maximum intrinsic growth rate, self-regulation, and limitation by other species. I then used the parameterized models in a multi-species stochastic simulation to determine how processes affected long-term community dynamics. By parameterizing the models using the frequency of the 18 most common species in the grassland, I determined that life history and life form are stronger predictors of underlying processes than is native status. Immigration maintains exotic annual grasses and the dominant native perennial grass in the community. Growth rate maintains other perennial species. While the model mirrors the frequency of native species well, exotic species have lower observed than parameterized frequencies, suggesting that they are not reaching their potential frequency. These results, combined with results from past research, suggest that disturbance may be key to maintaining exotic species in the community. Here I showed that a continuous modified Ricker model fit discrete grassland frequency data well. This allowed me to model the dominant species in the community simultaneously and gain insight into the processes that determine community composition.     

土壤非保护性有机碳对荒漠草原沙漠化的响应

沙化草地的恢复与重建是干旱半干旱区生态建设的重要内容,分析荒漠草原沙漠化过程中土壤非保护性有机碳分配比例的变异规律对于探讨沙化草地恢复机制具有重要的理论价值。以干旱、半干旱地区荒漠草原不同沙化阶段的土壤为研究对象,分析土壤粗颗粒有机碳、细颗粒有机碳、轻组有机碳含量和分配比例的分布特征、土壤非保护性有机碳转化为保护性有机碳的速率。结果表明:荒漠草原发生逆向演替后,土壤细颗粒有碳含量和分配比例表现为固定沙地荒漠草地半固定沙地流动沙地;粗颗粒有机碳含量表现为荒漠草地半固定沙地流动沙地固定沙地,粗颗粒有机碳分配比例表现为流动沙地半固定沙地荒漠草地固定沙地;轻组有机碳含量和分配比例递减。颗粒有机碳、轻组有机碳、土壤有机碳对草地沙漠化的敏感性不同,颗粒有机碳较轻组有机碳和土壤有机碳敏感性强,其中细颗粒有机碳较粗颗粒有机碳敏感性强。随着草地沙漠化程度的增强土壤非保护性有机碳分配比例呈下降趋势,表明草地沙漠化降低土壤质量。荒漠草地退化至流动沙地土壤非保护性有机碳转化为保护性有机碳的速率整体呈上升趋势。     

Germinable soil seed banks of anthropogenic native grasslands and grassy forest remnants in temperate south-eastern Australia

Soil seed banks of anthropogenic native grassland and grassy forest remnants on the Gippsland Plain in south-eastern Australia were studied using the seedling emergence method. Intact examples of both ecosystems are rare, owing to extensive agricultural development. Both ecosystems are assumed to have been derived since European settlement from the same original, grassy forest ecosystem. It was hypothesised that species now restricted to grassland remnants might persist in the soil seed bank of forest remnants, and vice versa. This hypothesis was not supported. In total, 25554 seedlings of at least 155 species emerged from forest and grassland seed banks collectively. Small-seeded, annual and perennial herbs were most abundant. Forest seedbanks differed substantially in composition from grassland seedbanks, and both essentially contained a subset of the species in the vegetation. Forest seedbanks contained significantly more species and individuals of annual and perennial native dicotyledons than grassland seedbanks. Seedbanks made a major contribution to local diversity, since many species in the seed bank at each quadrat were not recorded from the vegetation, but only a minor contribution to regional diversity. Species in the seedbank that were absent from all vegetation samples were recorded at few quadrats, except Juncus species, which were widespread and abundant in the seedbank. The apparent absence from forest seed banks of species that are best represented in grassland remnants (and vice versa), suggests that there is little opportunity of recruiting grassland-restricted species in forest remnants by instigating grassland management practices.     

Early-successional plants regulate grassland productivity and species composition: a removal experiment

Grime's (1998) "mass-ratio" hypothesis holds that ecosystem processes depend in the short term on functional properties of dominant plants and in the longer term on how resident species influence the recruitment of dominants. The latter of these effects may be especially important among early-successional species in disturbed ecosystems, but experimental tests are few. We removed two groups of early-successional species, an annual forb Gutierrezia dracunculoides (DC.) S. F. Blake and annual species (mostly grasses) that complete growth early in the growing season [early-season (ES) species], from a heavily-grazed grassland in central Texas, USA dominated by a C₄ perennial grass. We sought to determine effects of annuals on grassland functioning [productivity, water balance, soil and plant nitrogen (N)] and composition. Removals did not impact N retention in the soil/plant system during the two years of this study, but removing ES annuals increased the amount of water between 30 and 120 cm in the soil profile early in each growing season. Production and N accumulation by vegetation declined following the removal of ES annuals in approximate proportion to the contribution of annuals to aboveground biomass and N, consistent with the mass-ratio hypothesis. By the second year, production and N uptake by initially sub-dominant species increased to fully compensate for the loss of annuals. These results are consistent with the view that ecosystem functions are more strongly linked to species attributes than to diversity per se. Longer-term effects of annuals on grassland composition were evident in a dramatic increase in biomass of perennial forbs after annuals were removed. Because perennial forbs differ from the dominant grass in this grassland in traits that influence ecosystem functioning, ES annuals may affect grassland functioning more by regulating the composition of vegetation than by directly affecting process rates.     

Structure and seasonal dynamics of humid tropical grasslands in Meghalaya,India

Abstract. Four humid grassland communities at three different locations in Meghalaya, India were analysed during 1988 and 1989 for species and life-form composition, diversity and dominance in relation to altitude, soil and prevailing disturbances. Due to the adverse interactive influences of exceptionally high annual rainfall (> 10 000 mm), topography and human interference on soil fertility, the grassland at Cherrapunji, at 1300 m altitude, had a low species diversity (H'= 1.74) and was dominated by three perennial grass species. Similar grasslands, at both higher and lower altitudes on fertile soil and with lower rainfall (ca. 2000 mm), showed higher diversity values (H'= 2.28 at Burnihat and 2.31 at Upper Shillong). The proportion of perennial species and chamaephytes increased with elevation. At the high altitude site a grassland under short-term protection from fires and grazing had a higher species richness, density and basal cover than an unprotected grassland. All grasslands show a clear seasonality, albeit with different patterns, with a maximum in density and basal cover in August. The differences in structure and seasonality are discussed in terms of different levels of stress.