基于NDVI绿洲-荒漠过渡带宽度识别——以河西走廊中部荒漠绿洲为例

绿洲-荒漠过渡带是荒漠与绿洲之间的生态缓冲区,在维持绿洲能量流动、物质循环和景观稳定方面具有重要作用。过渡带宽度和属性直接影响到了其在整个绿洲系统中的功能发挥。以河西走廊中部张掖绿洲的Landsat(OLI)NDVI(归一化植被指数,Normalized Difference Vegetation Index)数据为基础,结合野外调查将过渡带类型划分为绿洲与石质裸山、砾质荒漠、沙质荒漠和人工固沙区4类。采用缓冲分析、分段线性趋势分析和尺度分别为30 m、90 m、210 m、330 m焦点分析等方法研究了不同类型过渡带宽度和尺度依赖特征。结果表明,在不同尺度上绿洲外缘NDVI变化存在二种线性回归趋势,趋势线交点至绿洲边界距离可确定为过渡带宽度。不同尺度分析表明,绿洲-石质裸山过渡带宽度为165—220 m,在其内NDVI线性变化趋势显著(P0.05)。绿洲-砾质荒漠过渡带宽度保持在330 m,在其内NDVI变化趋势极显著(P0.001)。绿洲-沙质荒漠过渡带宽度变化在230—290 m,NDVI变化趋势也为极显著(P0.001)。绿洲-人工固沙区过渡带宽度变化在570—580 m,与其它类型不同地是在过渡带内存在二种变化趋势,在210—240 m范围内变化趋势极显著(P0.001),超出此范围线性回归趋势不显著(P0.05)。     

黑河中游荒漠绿洲过渡带土壤水分与植被空间变异

荒漠绿洲过渡带是绿洲生态系统的重要组成部分,对维持绿洲稳定具有重要作用。过渡带土壤湿度和植被之间的相互关系是了解荒漠绿洲过渡带的重要科学问题,从而开始受到重视。研究是在黑河中游荒漠绿洲过渡带,选择一条1700m×200m的样带（包括3条平行样线）,在对土壤湿度、植被高度、盖度等调查的基础上,应用统计分析和地统计的方法,研究了荒漠绿洲过渡带土壤水分和植被的空间变异性特征。结果表明：在荒漠绿洲过渡带上,0～200cm土层中土壤平均湿度介于1．45％～3．85％,变异系数在27．7％～83．2％;植被盖度介于9％～80％,变异系数为80％。植被盖度、冠幅与0～20、20～40cm两层土壤湿度显著负相关（p〈0．05）,与120～140、140～160cm两层土壤湿度显著正相关（p〈0．01）。土壤水分和植被空间分布在样带上存在明显的空间异质性,表现在小于100m的尺度上随机分布,而在100～3110m的尺度上呈聚集分布格局。     

绿洲-荒漠过渡带土壤蓄水量的空间分布及其时间稳定性

掌握土壤水分动态变化规律是绿洲地区水资源管理与生态建设的关键.本文在黑河中游绿洲与荒漠交错地带设置面积约50 km²的研究区域,持续监测2012—2014年间植被生长季节土壤水分含量,利用地统计分析与时间稳定性分析探索土壤水分的时空分布特征.结果表明: 黑河北岸绿洲-荒漠过渡带土壤蓄水量8月最大、10月最小,0～2 m土层土壤平均蓄水厚度约367 mm.绿洲-荒漠过渡带土壤水分梯度除受河流影响外,还与土地利用方式密切相关.不同土地利用类型土壤蓄水量依次为农地>乔木林地>灌木林地>荒漠,农地土壤蓄水量平均高出外围荒漠土壤蓄水量300%以上.研究区内各采样点土壤水分时间稳定性具有明显的空间差异,不同土地利用类型土壤水分稳定程度依次为灌木林地>荒漠>农地>乔木林地.河流对沿岸绿洲土壤水分的直接影响集中在3 km以内,在河岸绿洲与新垦沙地农田之间存在一条土壤水分较为稳定的样带,可以用于指示绿洲生态系统的发展与演变.     

塔克拉玛干沙漠南缘荒漠绿洲过渡带不同土地利用影响下土壤化学计量特征

荒漠绿洲过渡带在维护绿洲生态安全和绿洲稳定上具有重要作用。垦荒等土地利用的增强使得荒漠绿洲过渡带的健康稳定受到了巨大的挑战。以塔克拉玛干沙漠南缘典型荒漠绿洲过渡带为研究对象,系统分析了不同土地利用方式(桑田、半自然柽柳林、瓜地、棉花-玉米地)对土壤养分化学计量特征的影响。土壤取样沿农田到荒漠方向进行,分5层进行。研究发现,土壤各养分指标均受土地利用方式(4种)、土层(5层)和与农田边缘垂直距离(4梯度)的显著影响,且存在一定的交互作用。土地利用方式显著影响土壤各养分元素含量。随土层由浅到深,有机碳(C)、有效氮(N)和有效磷(P)基本呈下降趋势,全N具有波动变化,而全P变化不显著。随与农田边缘垂直距离的增加,各养分含量基本呈递减趋势。对同一土层(共选择三层)不同土地利用方式下土壤养分具体分析表明,棉花-玉米地这一利用方式在农田内部具有最高的土壤有机C和全N含量,其次为桑田。随着与农田边缘垂直距离的增加,土壤C、N含量优势减弱。除农田内部样地(0 m)外,三层土壤全P含量基本呈桑田柽柳林棉花-玉米地趋势。表层有效N含量在农田内部样地(0 m)瓜地最高,其他距离处(大于等于20 m)棉花-玉米地高,下层土壤有效氮含量在农田内部各土地利用方式间无差异。在各距离样点处不同土地利用类型间土壤有效P含量的变化无明显规律,在农田内部以瓜地有效P含量最高。棉花-玉米地土壤全量N/P在农田内部和与农田边缘垂直距离20 m处含量最高。三土层土壤有效N/P在农田内部以柽柳林最高,随着与农田边缘垂直距离增加,土壤有效N/P显著改变。综合来看,土地利用对荒漠绿洲过渡带土壤营养含量的增加具有正向作用,由土壤养分变化带来的生态效应值得关注。     

干旱荒漠白刺灌丛植被演替过程土壤种子库变化特征

土壤种子库是反映气候与土地利用变化的指示器,也是荒漠植被更新与恢复的基础。典型荒漠白刺灌丛植被广泛分布于我国西北绿洲边缘地带,对保护绿洲免受风沙危害发挥着重要作用。但是,对干旱荒漠白刺灌丛植被土壤种子库仍未见研究报道。应用空间代替时间的方法,选择石羊河下游白刺灌丛植被演替的初始阶段、稳定阶段、衰退阶段与严重衰退阶段样地,研究了白刺灌丛植被演替过程的土壤种子库物种组成、时空分布和数量变化特征。结果表明:石羊河下游白刺灌丛植被演替过程中,土壤种子库共出现9科18种植物,其中草本植物占到55%—80%,是白刺灌丛植被土壤种子库的主体。不同演替阶段土壤种子库密度以稳定阶段最高,达到660.7粒/m2,是初始阶段、衰退阶段和严重衰退阶段的5.6、14.5倍和6.2倍。不同沙堆部位,土壤种子库密度以迎风坡中部与背风坡中部最高,分别达到329粒/m2和309粒/m2;沙堆土壤种子库密度是堆间地的1.9倍,土壤种子库主要分布于灌丛沙堆上。不同演替阶段土壤种子库间的相似性系数均在0.6以下,除稳定阶段外,其他阶段间均属于中等相似水平;土壤种子库与地上植被的相似性系数均在0.5以上,达到中等相似或极相似水平。该研究进一步证明了灌丛沙堆和土壤种子库在白刺灌丛植被中的重要性,也说明人为干扰损害了白刺灌丛地下潜在植被及其恢复潜力。因此,需要开展人工种子库或人工植被建设以控制干旱荒漠白刺灌丛沙堆活化,并实现退化白刺灌丛植被恢复。     

沙冬青灌丛地的土壤颗粒大小分形维数空间变异性分析

通过土壤颗粒大小分形维数对土壤质地定量分析表明,土壤颗粒大小分形维数与沙粒含量（＞0.1mm）呈显著的线性负相关,与粘粉粒含量（＜0.05mm）呈显著的线性正相关关系（p＜0.0001）,沙粒和粘粉粒含量每增加1﹪,分形维数则降低或升高0.022个单位,沙粒增加导致分形维数的降低和粘粉粒增加导致分形维数升高幅度一致,所以土壤颗粒分形维数可以作为评价土地沙质荒漠化程度的定量指标之一.荒漠地带以沙冬青为优势种的沙生植被地段,地表土壤颗粒粗粒化和50～70cm土层土壤颗粒的细粒化,为该区有限降水的深层渗漏提供了基质,为沙冬青的存活提供了保障,一定程度上支持了一些专家提出的概念模型,验证了粗质土壤质地支持以灌木为主的群落的假说.但从空间变异性分析来看,荒漠区沙冬青群落因地表物质的吹蚀和堆积过程频繁以及灌丛沙堆发育程度引起的分枝形态差异,导致了土壤颗粒含量空间变异尺度并未集中在冠幅范围,并不支持灌丛在沙漠生态系统对降尘等细粒物质拦截所形成的＂沃岛＂作用.正是因为以沙冬青为优势种的沙生植被地段土壤颗粒组成的高度空间异质性,并且空间变异的范围并不局限于灌木冠幅范围和冠幅间的裸地,没有为其它植物种的入侵创造土壤基质条件,才使得沙冬青群落在该区稳定存在,为荒漠残遗植物种--沙冬青的迁地保护和干旱沙区植被恢复过程中合理地利用土壤资源,以避免营林失败等问题提供了理论依据.     

孔雀河下游断流河道的环境特征及物种间关系

基于孔雀河下游断流河道的环境因子和植被样地数据,采用聚类与CCA排序法,分析了生境的退化特征以及物种间的相互关系,结果表明:1)断流河道退化生境分为绿洲-荒漠过渡类型、轻度荒漠化类型和盐土荒漠化类型。绿洲-荒漠过渡类型地下水位低、盐分含量相对较低,植被盖度相对较高,土壤维持着原砂质壤土,为潜在退化型;轻度荒漠化类型地下水位、土壤质地与含盐量与前者基本相同,土壤未明显退化,但植被盖度低于10%,植物种类与个体数目都较低,属于轻度退化型;盐土荒漠化类型地下水位高、盐分含量高,土壤机械组成中砂粒比重较大、无建群种幸存,属于重度退化型。2)绿洲-荒漠过渡类型总体联结性为显著正联结,正负联结比小于1,生态系统表现为建群种维系物种关系的不稳定状态;轻度荒漠化类型总体联结性为不显著负联结,正负联结比小于1,表现出生态系统进入退化演替的阶段;盐土荒漠化类型总体联结性为显著正联结,正负联结比大于1,表现出重度退化群落的种间平衡状态,物种间以达到稳定共存,其中,真盐生植物对这种平衡的维持起着重要的作用。3)CCA排序表明,绿洲-荒漠过渡类型形成以胡杨为中心的种间正联结,幸存于盐分适中、水分养分相对较高的生境;轻度荒漠化类型,形成以多枝柽柳与刚毛柽柳相互依存的不显著负联结,幸存于土壤养分、水分相对较低的生境;盐土荒漠化类型形成以盐爪爪、盐节木、盐穗木等真盐生植物维系的显著正联结,幸存于土壤贫瘠、地下水位浅、盐分含量高、沙化严重的生境。     

藏锦鸡儿群落土壤资源空间异质性

在内蒙古高原, 藏锦鸡儿(Caragana tibetica)群落分布在草原向荒漠的过渡带上, 在植被区划中常作为由草原进入荒漠的指示物种。藏锦鸡儿是一种旱生垫状矮灌木, 由于其垫状生物学特性, 在地表覆沙的生境中常形成较均匀的、非常醒目的灌丛沙堆。不同大小的灌丛沙堆可对应其不同的发育阶段, 选取不同大小的藏锦鸡儿灌丛沙堆, 划分成小、中、大3组, 小、中两组灌丛沙堆对应其发育阶段, 大沙堆组对应于稳定阶段, 通过测定沙堆间、沙堆内(顶部、中部)及沙堆下的土壤有机质(Soil organic matters, SOM)、全磷(Total phosphorus, TP)和土壤质量含水量(Mass water content of soil, W_m), 分析了藏锦鸡儿群落土壤资源的特点。结果显示, 随着沙堆的发育, 沙堆内和堆下0~20 cm处的土壤有机质、全磷和土壤质量含水量的平均含量呈递增趋势, 大沙堆、中沙堆及堆下0~20 cm处的土壤有机质、全磷和土壤质量含水量的平均含量均大于沙堆间, 小沙堆的土壤有机质、堆下0~20 cm的土壤全磷和质量含水量的平均含量大于沙堆间。不同大小的灌丛沙堆, 在垂直方向上, 土壤有机质、全磷含量基本均呈先增加后减少的趋势, 土壤质量含水量呈递减趋势; 在水平方向, 随着灌丛沙堆的发育, 土壤有机质、全磷和土壤质量含水量在同一部位的同一土层基本呈递增趋势; 不同大小的沙堆表层0~20 cm 处的土壤有机质由顶部、中部、沙堆间依次呈先增加后减小的趋势; 大沙堆和中沙堆的全磷由沙堆顶部、中部、沙堆间依次呈递减趋势, 而小沙堆土壤全磷呈先增加后减少的趋势。随着灌丛沙堆的发育, 藏锦鸡儿灌丛截获的植物残体量呈增加趋势。研究显示, 藏锦鸡儿在发育过程中形成“沃岛效应”, 这种效应不仅表现在沙堆内, 也表现在沙堆下, 同时也引起土壤有机质、全磷和土壤水分的空间异质性分布。     

绿洲―荒漠交错带土壤水分变化特征初步研究

 以甘肃省的民勤绿洲为例,选择土壤水分最为亏缺的7月,对白刺(Nitraria tangutorum)沙包不同迎风面,以及绿洲和荒漠交错地带的土壤水分进行了取样分析。结果表明,对于白刺沙包而言,其上层和深层土壤水分是迎风坡大于背风坡,中间层次（40～80 cm）则刚好相反。在绿洲与荒漠的交错带上,灌丛沙包与沙丘间低地水分变化有很大的不同,总体而言,丘间地水分含量大于灌丛沙包,但在距离绿洲406 m处,二者均出现一个土壤水分的最低值。在丘间地表层,水分变化是距离绿洲愈近,土壤含水量愈高;而深层土壤水分的变化趋     

荒漠-绿洲土壤微生物群落组成与其活性对比

结合野外观测与实验室研究方法,对比研究了准葛尔盆地南缘盐生荒漠与绿洲农田土壤微生物活性与其群落组成的变化特征,并分析了土壤温度与湿度对荒漠-绿洲土壤微生物活性的影响。结果表明:荒漠开垦为绿洲后,土壤细菌明显增加,真菌无明显变化,放线菌显著减少。细菌在绿洲农田土壤矿化作用中占主导,真菌则在荒漠中占优势,绿洲农田土壤微生物活性(包括真菌与细菌活性)明显高于荒漠。温度对荒漠-绿洲土壤微生物活性的影响只在一定土壤湿度范围内作用显著,绿洲农田受其影响较大;荒漠有机质含量明显高于绿洲农田,但水分与盐分因素抑制了微生物对其的分解和矿化。不同土地利用方式导致了荒漠绿洲间土壤湿度及盐份的较大差异,加之与土壤温度极显著的交互作用,使得开垦后土壤有机碳的易得性增强,微生物群落结构发生显著改变,进而有机碳的矿化速率加快,土壤碳库随之消减。     

半干旱地区天然草地灌丛化与土壤异质性关系研究进展

 回顾了半干旱地区天然草地灌丛化的成因和机理、灌丛化导致草地土壤水分和养分空间异质性及其对生态系统生物地球化学过程的影响,以及土壤异质性与土地退化关系等方面的研究进展,周期性气候干旱和过度放牧是天然草地灌丛化的主要原因,伴随灌木入侵而出现的草地土壤水分和养分的空间异质性,是造成生态系统水分和养分流失,以及土壤加速侵蚀的原因之一。因此,半干旱地区天然草地的灌丛化应得到一定的控制,使群落中灌丛保持适宜的密度,以避免生态系统水分和养分的损失。     

树干径流对梭梭"肥岛"和"盐岛"效应的作用机制

为探明荒漠灌木个体周围土壤养分和盐分的空间分布及其成因,以荒漠灌木梭梭(Haloxylon ammodendron Bge.)为研究对象,对其周围土壤pH值,电导率,有机碳,全氮和有效磷的空间异质性以及树干径流的化学性质进行了研究。结果表明:树干周围出现显著的"肥岛"效应,且土壤有机碳,全氮和有效磷的"肥岛"作用范围发生在距主根大约20—40 cm以内。土壤pH值和电导率在主根周围25 cm以内的值却显著低于外围,而在离主根25 cm处出现了"盐岛",即在梭梭主根中心形成了一种高养分、低盐和低pH值的环境。树干径流和自然降雨化学性质的对比研究表明:除pH值和CO23-外,树干径流中其他化学离子的含量均显著高于自然降雨中的含量,说明这种高养分、低盐分的环境是由树干径流引起的,也就是树干径流带给土壤养分的同时将盐分带走。     

干旱和半干旱地区灌木下土壤“肥岛”研究进展

世界许多干旱、半干旱地区草地生态系统逐渐被灌木生态系统取代,这种取代过程的发展是由灌木冠幅下土壤中“肥岛”的形成和灌木的扩散相互作用直接造成的,这种“肥岛”的形成和灌木的扩散之间的反馈作用能够改变该系统中的植被组成、结构和土壤养分分布格局,从而改变该生态系统的结构和功能,文中从“肥岛”的概念和形成机制出发,对“肥岛”现象形成的原因、“肥岛”现象的研究意义、研究方法、灌木扩散与“肥岛”之间的相互关系及生物对“肥岛”的响应进行了总结,并分析了“肥岛”研究中应注意的问题,希望为中国干旱、半干旱区域土地退化的成因和过程以及植被恢复的研究提供一些参考。     

Spatial relations between shrubs and Prosopis glandulosa canopies

Abstract. Prosopis glandulosa, an arborescent legume, may act as a nurse plant that facilitates the establishment of other woody species. We hypothesized that attenuation of radiant energy and increased soil nutrients beneath P. glandulosa canopies facilitate establishment of subordinate shrubs and shrub cluster development. We determined the spatial distribution pattern of shrubs under P. glandulosa at three locations in southern Texas. Density of Celtis pallida, Zanthoxylum fagara, and total woody plants were comparable among the four cardinal directions at each location, which countered the prediction that shrub density would be greater on the north side of P. glandulosa canopies if attenuation of solar energy was a factor in cluster development. Total woody plant density increased with increasing P. glandulosa basal diameter, canopy radius, and height only at one location. Total woody plant density decreased with increasing total N in the upper 15 cm of soil at two of the three locations. Late in shrub cluster development, extraction of N from the soil and incorporation of N into plant tissue in dense shrub clusters may operate to inhibit further increases in subordinate shrub density.     

Multiple effects of shrubs on annual plant communities in arid lands of South Australia

Abstract The presence of shrubs in arid lands creates spatial heterogeneity that affects the distribution and performance of annual plants; several possible mechanisms have been implicated. A preliminary survey in a chenopod shrubland in South Australia showed differences in the distribution of annual plants under canopies of Atriplex vesicaria and Maireana sedifolia (the two dominant shrub species) and open spaces. A series of experiments were conducted to test the potential contribution to these patterns of nutrient enrichment under shrubs, differential seed accumulation, stress reduction by the canopy, competition by shrub roots, and protection against grazing. The germinable soil seed‐bank under A. vesicaria and M. sedifolia was different from that in open spaces, but these differences can only explain a fraction of the differences observed in the growing annual plant community in different microsites. The soil under A. vesicaria had higher total nitrogen content than soil in open spaces, whereas soil under M. sedifolia had lower available phosphorus than open spaces. Although annual plant densities under A. vesicaria were higher than in open spaces, experimental removal of shrubs increased their density, suggesting that shrub canopies inhibit annual plants in this system. Surprisingly, trenching of open areas close to shrubs (severing lateral shrub roots) decreased annual plant density. We suggest that water moves laterally through shrub roots, in a process akin to a hydraulic lift, increasing water availability for the annual plants. Exclusion of vertebrate grazers had a stronger effect on annual plant biomass in open spaces than under M. sedifolia, suggesting that this shrub provides shelter against herbivory. Overall our results show that shrubs can have simultaneously facilitative and inhibitory effects on the annual plant community through different mechanisms, but more importantly that different shrub species have different effects. This is a potential mechanism allowing for species coexistence of annual plants.     

Consequences of anthropogenic disturbances on soil seed bank diversity and nurse shrub effect in a semiarid rangeland

What is the effect of common land use histories on the diversity, richness, spatial distribution and abundance of the soil seed bank (SSB)? Does the effect change between different microsites under shrub canopy? To address these questions we selected seven sites with different grazing and fire histories in the Patagonian Monte desert. We took soil samples in seven microsites at different distances of the trunk along a windward/leeward transect through shrub patches to estimate the SSB of perennial grasses, shrubs and annuals. Shrubs SSB was scarce. The nurse shrub effect on perennial grass SSB was evident at sites with a low disturbance intensity (as sites without continuous grazing), and higher on the leeward side under the shrub canopy. High disturbance intensity (such as permanent grazing) promoted a decrease in perennial grass SSB and an increase in annuals SBB (especially non-native). Land use histories related to fire showed a moderated disturbance response (medium values for both groups). Differences between land use histories varied depending on the windward/leeward microsite from which the SSB was analysed. Our results suggest a nurse effect of the shrub patches on the spatial distribution and abundance of the SSB, but this effect decrease under continuous grazing. Fire seems to have a positive effect on perennial grass SSB. Nevertheless, fire characteristics must be controlled. Moreover, perennial grass SSB was almost depleted under continuous grazing, driving the grazed system towards a vulnerable state; and annual non-native species take advantage of this disturbance and dominate the SSB.     

Extreme drought stress shifts net facilitation to neutral interactions between shrubs and sub‐canopy plants in an arid desert

The stress gradient hypothesis (SGH) predicts that the importance or intensity of competition and facilitation will change inversely along abiotic stress gradients. It was originally postulated that increasing environmental stress can induce a monotonic increase in facilitation. However, more recent models predicted that the relationship between severity and interaction exhibits a hump‐shaped pattern, in which positive interactions prevail under moderate stress but decline at the extreme ends of stress gradients. In the present study, we conducted a field experiment along a temporal rainfall gradient for five consecutive years, in order to investigate interactions in a shrub‐herbaceous plant community at the southern edge of the Badain Jaran Desert, and, more specifically, investigated the effects of Calligonum mongolicum, a dominant shrub species, on both abiotic environmental variables and the performance of sub‐canopy plant species. We found that shrubs can improve sub‐canopy water regimes, soil properties, plant biomass, density, cover, and richness and, more importantly, that the positive effect of shrubs on sub‐canopy soil moisture during the summer diminishes as rainfall decreases, a pattern that partly explains the collapse of the positive interaction between shrubs and their understory plants. These results provide empirical evidence that the positive effect of shrubs on understory plant communities in extreme arid environments may decline and become neutral with increasing drought stress.     

Impacts of the removal of vascular plants on physiological and biochemical characteristics of Syntrichia caninervis during winter season in a temperate desert

Aims Patchily distributed biological soil crusts and shrubs is one of the main vegetation cover types in Gurbantünggüt desert. The existence of shrubs in desert areas serves not only as a shelter for small animals, but also a good living condition for cryptogams and some herbs. Syntrichia caninervis, a dominant moss species in Gurbantünggüt desert, is patchily distributed under shrub canopy and open spaces between vascular plants. To our knowledge, the impacts of the removal of shrub canopy on physiological and biochemical characteristics of S. caninervis during the winter is still unknown.
Methods We simulated grazing of animals on Ephedra distachya at various rates (shrubs left intact, 50% shrubs removed, and shrubs removed completely) by cutting different percent of above-ground shoots of E. distachya. The shoot water content, chlorophyll fluorescence, proline content, soluble sugar content, soluble protein content, malonyldialdehyde (MDA) content, peroxidase (POD) and superoxide dismutase (SOD) activity were compared.
Important findings The results show that, as for proline and soluble sugar contents of tested S. caninervis, there were no significant differences among three treatments of shrub canopy. However, the MDA content, and the activities of POD, SOD, CAT were significantly higher than nature shrubs in snow cover periods. During snow-melting periods, soil moisture and temperature did not served as limiting factors on the growth of S. caninervis. The soluble sugar content, and the activities of POD, CAT with shrub removal were significantly higher than shrubs left intact. This may indicate that the decline of the function of maintaining constant soil temperature due to shrub canopy increased the damage of temperature on moss plants. In addition, as for physiological and biochemical characteristics, there were no significant differences between two treatments of 50% shrub canopy and shrub canopy left intact. In snow covered periods and snow melting periods, the MDA content, the activities of POD, SOD and CAT of S. caninervis under shrub canopy removed completely were significantly higher than that of 50% shrub canopy and shrub canopy left intact, except for soluble protein content. The duration of chlorophyll fluorescence activity of S. caninervis growing in habitats with shrubs removed completely was significantly shorter than that of S. caninervis growing in habitats of 50% shrub canopy and shrub canopy left intact. This result indicated that the removal of shrubs may increase the UV-B radiation and weaken the “moisture island effects” developed by the existence of shrub canopy.     

Shrub canopies influence soil temperatures but not nutrient dynamics: An experimental test of tundra snow–shrub interactions

Shrubs are the largest plant life form in tundra ecosystems; therefore, any changes in the abundance of shrubs will feedback to influence biodiversity, ecosystem function, and climate. The snow–shrub hypothesis asserts that shrub canopies trap snow and insulate soils in winter, increasing the rates of nutrient cycling to create a positive feedback to shrub expansion. However, previous work has not been able to separate the abiotic from the biotic influences of shrub canopies. We conducted a 3‐year factorial experiment to determine the influences of canopies on soil temperatures and nutrient cycling parameters by removing ~0.5 m high willow (Salix spp.) and birch (Betula glandulosa) shrubs, creating artificial shrub canopies and comparing these manipulations to nearby open tundra and shrub patches. Soil temperatures were 4–5°C warmer in January, and 2°C cooler in July under shrub cover. Natural shrub plots had 14–33 cm more snow in January than adjacent open tundra plots. Snow cover and soil temperatures were similar in the manipulated plots when compared with the respective unmanipulated treatments, indicating that shrub canopy cover was a dominant factor influencing the soil thermal regime. Conversely, we found no strong evidence of increased soil decomposition, CO₂ fluxes, or nitrate or ammonia adsorbtion under artificial shrub canopy treatments when compared with unmanipulated open tundra. Our results suggest that the abiotic influences of shrub canopy cover alone on nutrient dynamics are weaker than previously asserted.     

灌丛移除对荒漠齿肋赤藓越冬过程中生理生化特性的影响

灌丛和生物土壤结皮镶嵌分布是古尔班通古特沙漠一种重要的地表覆被类型, 灌丛的存在不仅为小型动物提供了庇护场所, 也为隐花植物及部分草本植物创造了良好的生存条件。然而在初冬时期移除灌丛是否会影响这些隐花植物的越冬及生长还不得而知。该研究通过模拟放牧及鼠害, 移除50%双穗麻黄(Ephedra distachya)灌丛、移除全部双穗麻黄灌丛及自然对照, 测定齿肋赤藓(Syntrichia caninervis)植株脯氨酸、可溶性糖和可溶性蛋白含量, 以及丙二醛(MDA)含量和超氧化物歧化酶(SOD)、过氧化物酶(POD)和过氧化氢酶(CAT) 3种抗氧化酶活性, 并测定原位光化学效率, 探讨初冬灌丛丧失对荒漠藓类植物越冬的影响。结果表明, 在积雪期3种处理下齿肋赤藓的游离脯氨酸和可溶性糖含量之间差异并不显著, 但去灌丛后其MDA含量、POD、SOD和CAT 3种抗氧化酶活性均显著高于自然灌丛下齿肋赤藓的相关生理指标。即使是在融雪期, 去灌丛下齿肋赤藓的可溶性糖含量及POD和CAT活性仍显著高于自然灌丛, 而可溶性蛋白明显较低, 这可能是由于灌丛的丧失造成的温度波动加剧了冬季低温对藓类植物的伤害。灌丛的部分移除(50%)对齿肋赤藓的生理生化特性影响不显著, 就积雪融化期叶绿素荧光活性持续时间而言, 与自然灌丛和移除50%灌丛相比, 完全移除灌丛的齿肋赤藓植株叶绿素荧光活性持续时间显著缩短。这可能是由于灌丛移除导致其UV-B辐射增加及“湿岛效应”消失所致, UV-B辐射的增加加剧了对植物的伤害, 而春季融雪期保水能力的下降也是其叶绿素荧光活性时间缩短的重要原因。