内蒙古典型草原小叶锦鸡儿灌丛化对水分再分配和利用的影响

灌丛化是全球草原地区存在的主要环境问题。通过对内蒙古典型草原区小叶锦鸡儿灌丛和草地斑块冠层降雨再分配、地表径流、土壤含水量的对比观测,研究了小叶锦鸡儿灌丛化对该区水分再分配和利用的影响。结果表明,灌丛和草地斑块的冠层截留量分别占降雨量的20.86%和7.88%,灌丛和草地斑块的平均地表径流系数分别为5.95%和17.19%。土壤含水量观测结果显示,0—60 cm土层中,降雨事件过程中,灌丛斑块较草地斑块能捕获更多水分,灌丛斑块植被冠层下方土壤含水量高于草地斑块;而在雨后无有效降水补充土壤水分的前提下,0—60 cm土层中,灌丛斑块土壤水分蒸散发量高于草地斑块,其中0—10cm土层中灌丛斑块土壤水分蒸散发速率低于草地斑块,10—60 cm土层中灌丛斑块土壤水分蒸散发速率高于草地斑块。研究认为,在水分为关键性限制因子的干旱半干旱区,小叶锦鸡儿灌丛化过程增加草原生态系统中水分分布的空间异质性,灌丛斑块能捕获、利用更多水分以维持更多的生物量。     

小叶锦鸡儿(Caragana microphylla)灌丛对草本群落特征的影响

基于干旱、半干旱草原灌丛化现象的发展，探究灌木与草本之间的关系对深刻理解灌丛化对草原植物群落特征的影响机制具有重要意义。然而，很少有研究考虑草地类型和灌丛斑块大小对灌木和草本植物之间关系的影响。在内蒙古典型草原和荒漠草原的自由放牧草地上选择了18个小叶锦鸡儿灌丛化样地进行野外采样，并调查了不同大小灌丛和周围草地的植物群落特征。运用单因素方差分析(one-way ANOVA)和最小显著差异法(LSD)来检验不同大小灌丛斑块内外的植被群落特征(高度、密度、地上生物量、凋落物量)和植物多样性指数(Shannon-Wiener指数、Pielou指数、丰富度指数、群落Sprensen相似性系数)的差异性。结果表明：(1)与草地相比，两种草原类型的灌丛产生、捕获了更多的凋落物，灌丛的保育作用显著提高了斑块内草本植物的高度，但灌丛占用了生存空间和资源，从而降低了斑块内草本植物的密度和地上生物量。(2)灌丛对多年生草本植物的地上生物量有明显的抑制作用，但对两种草原类型的一、二年生草本植物没有显著影响(典型草原中的小灌丛除外)。(3)两种草原类型的灌丛对草本植物多样性的影响存在差异。典型草原灌丛内多样性...     

小叶锦鸡儿灌丛化对典型草原群落结构与生态系统功能的影响

草原灌丛化是全球干旱半干旱地区面临的重要生态问题。灌丛化对草原生态系统结构与功能的影响较为复杂, 有待于在更广泛的区域开展研究。该研究在内蒙古锡林郭勒典型草原选择轻度、中度和重度灌丛化草地, 通过群落调查, 结合植物功能性状和土壤理化性质观测, 研究了小叶锦鸡儿(Caragana microphylla)灌丛化对草原群落结构(物种多样性、功能多样性和功能群组成)和生态系统功能(初级生产力、植被和土壤养分库)的影响。结果表明: 1)不同程度灌丛化草地的物种丰富度、功能性状多样性和群落加权性状平均值差异显著, 其中, 中度灌丛化草地的物种多样性和功能多样性较高, 表明一定程度的灌丛化有利于生物多样性维持。2)重度灌丛化草地的地上净初级生产力(ANPP)显著高于轻度和中度灌丛化草地, 其原因主要是随着灌丛化程度加剧, 群落内一/二年生草本植物显著增加, 而多年生禾草和多年生杂类草显著减少。三个灌丛化草地的植被叶片和土壤碳、氮库差异均不显著。3)灌丛化对草原生态系统功能包括ANPP、植被和土壤养分库均没有直接的影响, 而是通过影响功能群组成、土壤理化性质和功能多样性, 间接地影响生态系统功能; 灌丛化导致功能群发生替代和土壤旱碱化是最重要的生物和非生物因素。     

小叶锦鸡儿灌丛化对典型草原群落结构与生态系统功能的影响

草原灌丛化是全球干旱半干旱地区面临的重要生态问题。灌丛化对草原生态系统结构与功能的影响较为复杂,有待于在更广泛的区域开展研究。该研究在内蒙古锡林郭勒典型草原选择轻度、中度和重度灌丛化草地,通过群落调查,结合植物功能性状和土壤理化性质观测,研究了小叶锦鸡儿(Caragana microphylla)灌丛化对草原群落结构(物种多样性、功能多样性和功能群组成)和生态系统功能(初级生产力、植被和土壤养分库)的影响。结果表明:1)不同程度灌丛化草地的物种丰富度、功能性状多样性和群落加权性状平均值差异显著,其中,中度灌丛化草地的物种多样性和功能多样性较高,表明一定程度的灌丛化有利于生物多样性维持。2)重度灌丛化草地的地上净初级生产力(ANPP)显著高于轻度和中度灌丛化草地,其原因主要是随着灌丛化程度加剧,群落内一/二年生草本植物显著增加,而多年生禾草和多年生杂类草显著减少。三个灌丛化草地的植被叶片和土壤碳、氮库差异均不显著。3)灌丛化对草原生态系统功能包括ANPP、植被和土壤养分库均没有直接的影响,而是通过影响功能群组成、土壤理化性质和功能多样性,间接地影响生态系统功能;灌丛化导致功能群发生替代和土壤旱碱化是最重要的生物和非生物因素。     

内蒙古退化草原中与小叶锦鸡儿相关的小尺度土壤碳、氮资源异质性动态

小叶锦鸡儿在内蒙古典型草原中有着广泛分布,从而对小空间尺度土壤资源异质性产生强烈影响。退化草原中小叶锦鸡儿能够通过克隆生长形成灌丛斑块,不同大小斑块对应于其发育的不同阶段。因此,通过对与不同大小灌丛斑块相关的土壤有机碳（SOC）、全氮（STN）空间分布状况的分析,能够推断内蒙古退化草原中、与小叶锦鸡儿相关的小尺度SOC与STN异质性动态。结果表明：在内蒙古退化草原中,就0-5cm,5-10cm,10-20cm 3个土壤层次,小叶锦鸡儿均导致了与灌丛斑块相关小尺度上SOC与STN分布的空间异质性,且随着灌丛斑块的发育,这种异质性均不断增强,其中土壤表层0-5cm碳氮资源异质性的增强最为强烈,表现在灌丛斑块内部相对于外部（或边缘）对SOC与STN富集程度均显著增加。这表明内蒙古退化草原中,随着灌丛斑块自身的扩展,与小叶锦鸡儿相关小尺度上土壤碳氮资源空间异质性趋于增强。     

内蒙古温带典型草原围封十年草灌景观格局动态

通过样方调查和差分GPS法,研究内蒙古典型草原围封10年后灌木和草本盖度、生物量动态以及植被时空分布格局的变化。结果显示:草本盖度和生物量以2010年为拐点先减少后增加,灌木盖度和生物量呈现增加的趋势,样地整体植被生产力显著恢复。2010年以后,样地景观格局发生变化,小叶锦鸡儿灌丛斑块表现出破碎化程度和蔓延度先增加后减少的趋势。研究认为:(1)2012年之前为干旱期,草本生产力下降,且灌、草之间的竞争关系加剧了这一过程。(2)2012年之后降水增加,草本生产力先于灌木迅速恢复;景观尺度上小叶锦鸡儿灌丛斑块破碎化程度达到最高,是小叶锦鸡儿克隆生长的扩张过程所致。(3)2012年之后为湿润时期,小叶锦鸡儿对草本的生长存在促进作用,使生态系统逐渐恢复和重建。     

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...     

内蒙古半干旱草原灌丛化过程中小叶锦鸡儿引起的土壤碳、氮资源空间异质性分布

过度放牧下内蒙古半干旱草原,由于小叶锦鸡儿多度增加导致植被灌丛化,这已经成为该地草原退化时的普遍现象。草原灌丛化过程中,灌丛内凋落物的累积使得养分循环区域化,草原土壤有机碳(SOC)、土壤全氮(STN)空间异质性增强,导致灌丛沃岛形成。半干旱草原灌丛化过程机制的假说认为:灌丛斑块扩展与其引起的土壤空间异质性的增强之间存在着正反馈作用,正是这种反馈作用促使草地向灌丛地的转变。小叶锦鸡儿通过克隆生长形成不同大小的斑块,它们对应于其发育的不同阶段。因此,将不同大小的灌丛沃岛划分为小灌丛组与大灌丛组,它们代表着灌丛沃岛发育的早期与晚期两个阶段。结果表明:内蒙古半干旱草原灌丛化过程中,小叶锦鸡儿灌丛斑块引起了土壤有机碳(SOC)与土壤全氮(STN)的空间异质性,表现在水平方向上,对于0～5cm,5～10cm,10～20cm3个土壤层次,由灌丛斑块内部向外部SOC与STN均趋于降低;而在垂直方向上,对于灌丛斑块内部、边缘和外部3个位置,由0～10cm到10～20cm再到20～30cm,随着土壤深度增加SOC与STN均趋于降低。随着小叶锦鸡儿灌丛斑块扩展,SOC与STN空间异质性的分布不断增强,表现在灌丛斑块内部相对于外部(或边缘)对SOC与STN富集程度均显著增加。草原灌丛化过程SOC与STN空间异质性变化在0～5cm,5～10cm,10～20cm3个土壤层次中,以表层0～5cm的表现最为强烈。表层0～5cm土壤空间异质性的形成与灌丛自身凋落物的生产以及对灌丛外凋落物的截留有关。由此推断:退化草原灌丛化的过程中,小叶锦鸡儿灌丛斑块引起SOC与STN空间异质性的变化是一个自我增强过程,灌丛斑块扩展与其导致的SOC与STN空间异质性增强之间存在着正反馈作用。因此,以上关于半干旱草原灌丛化机制的假说对我国内蒙古半干旱草原灌丛化也是适用的。     

灌丛化对黄土高原草地植物群落结构和地上生物量的影响

中国北方草地普遍出现灌丛化现象,灌丛化改变植物群落结构、植物多样性和生产力,直接影响着草地生态保护与可持续利用。该研究以黄土高原灌丛化草地为研究对象,通过植被调查,分析比较不同坡向的灌丛斑块与禾草斑块植物群落结构(物种组成、优势种及物种多样性)和地上生物量的差异。结果发现:(1)灌丛化草地不同坡向对物种多样性及地上生物量均无显著影响(P 0.1),但不同斑块植物群落结构(P=0.001)及地上生物量(P0.001)存在显著差异。(2)灌丛化草地共出现植物29种,其中禾草斑块有27种,灌丛斑块有18种;灌丛化显著改变了植物群落的物种组成,优势种由长芒草(Stipa bungeana)更替为矮脚锦鸡儿(Caragana brachypoda),且灌丛化降低了草地物种丰富度,增加了群落均匀度。(3)灌丛化显著改变了草地地上生物量,其中灌丛斑块地上生物量较禾草斑块地上生物量增加251.2 g·m~(-2),灌丛斑块中灌木/半灌木地上生物量提高了452.1 g·m~(-2),多年生丛生禾草减少了176.5 g·m~(-2),其余功能群植物的地上生物量减少了24.4 g·m~(-2)。(4)灌丛化过程(从禾草斑块—灌丛斑块)中,植物种丢失对地上生物量减少的影响较小,新增物种和群落优势种更替促进了灌木斑块地上生物量增加;虽然灌丛化导致草地地上生物量增加,但植物物种丰富度降低和优势种更替很有可能改变草地多样性和稳定性维持机制。     

内蒙古锡林河流域植被退化的格局及驱动力分析

草地退化是中国北方草原面临的主要生态问题。该文以1984和2004年草原植被群落调查数据为主要依据, 以优势种、建群种和群落类型及其比例的变化作为主要指标, 并与20世纪80年代的植被类型图比较, 分析了内蒙古锡林河流域草地的退化趋势及其空间分布。根据流域内草地退化的实际情况, 将其分为未退化、轻度退化、中度退化、重度退化和极度退化5种退化类型。另外, 根据近20年前后草地植被变化的实际情况, 又划分了恢复和盐化两个类型。结果表明: 草地退化呈现明显的空间分布, 以锡林河为标志, 总体上从上游到下游退化程度逐渐加剧, 表现为浑善达克沙地进入锡林河的部分及沿河地区为重度退化, 锡林浩特市以北的区域, 尤其是流域的西北部, 已经达到了极度退化; 流域中部的白音锡勒牧场主要是中度退化; 轻度退化则均匀地分布于整个流域; 未退化类型多分布于锡林河的西部, 锡林河中下游地区由于农田退耕、草地围封出现了一定程度的恢复; 锡林浩特市以北沿锡林河植被的盐化程度较重。不同的植被类型退化程度也不同, 沙地灌丛植被轻度退化比例较高, 占植被的43%; 羊茅(Festuca ovina)草原和榆树(Ulmus pumila)疏林沙地中度退化草地的比重较高, 超过50%; 贝加尔针茅(Stipa baicalensis)草原和无芒雀麦(Bromus inermis)杂类草草甸的重度退化面积达50%以上; 极度退化比例较大的有克氏针茅(S. krylovii)草原和小叶锦鸡儿(Caragana microphylla)灌丛化草原。对于羊草(Leymus chinensis)草原, 以轻度和中度退化为主。导致不同区域和不同植被类型草地退化的原因也不尽相同, 从近20年的时间尺度来看, 过度放牧、不合理的居民点布局, 以及道路等是草地退化的主要人为驱动因子。研究结果还显示, 仅以生物量的变化来划分草地退化存在一定的局限性, 因此, 该文以群落优势种和建群种来表征草地的退化类型更具有客观性和实际的应用价值。     

Restoring Structure in Late-Successional Sagebrush Communities by Thinning with Tebuthiuron

Late successional, dense Artemisia tridentata (big sagebrush) stands restrict associated plant species production, resulting in a monotypic, shrub‐dominated community that threatens biodiversity preservation and ecosystem function. Traditional practices to control A. tridentata can severely reduce or temporarily eliminate A. tridentata and other plant species. Thinning A. tridentata with low rates of the herbicide tebuthiuron enhances herbaceous plant production, community structure, ecosystem functioning, and biodiversity. Tebuthiuron was applied at rates of 0.11–1.0 kg ai/ha to A. tridentata‐dominated plots at Ten Sleep (1979), Lander (1993), and Waltman (1993), Wyoming. Changes in A. tridentata canopy cover, associated plant species biomass, and community composition were evaluated 13 and 14 years post‐treatment at Ten Sleep, and 2 and 4 years post‐treatment at Lander and Waltman. At all sites A. tridentata canopy cover decreased proportionally with increased tebuthiuron rate. Biomass of grasses increased as shrub biomass and cover decreased with increasing tebuthiuron rate. Forb biomass varied between treatments across sample year and site. Shifts from shrub‐dominated control to grass‐dominated treatment plots were attributable to biomass differences of A. tridentata and 2–3 grass species among treatments. The number of plant species was not significantly different between treatments at any site. Similarity indexes revealed progressively greater dissimilar plant composition between the control and sequential treatment plots of increased tebuthiuron rate. Incremental rates of tebuthiuron produce gradual changes in plant species composition without reducing species richness, which may have utility in certain restoration projects.     

Effect of grazing on plant patterns in arid ecosystems of Patagonian Monte

Our objective was to assess the relationship between the spatial patterning of perennial grasses (total, grazed, and non‐grazed) and shrub patches in rangelands under different grazing pressures of the Patagonian Monte. We selected three grazed paddocks with the usual stocking rate for the area, where previous studies showed that a piosphere formation is common. At each paddock, we analysed the grain of heterogeneity at sites located at two distances from the single watering point (near, far), using high‐resolution aerial photographs. At these sites, we also assessed in the field the density, size, cover, and spatial patterning of grazed and non‐grazed perennial grasses and shrub patches. The grain of heterogeneity of shrub patches was coarser in sites near the watering point than in those distant from it, as a consequence of the increase in size of both, bare soil and shrub patches. Field sampling showed that a coarse grain of heterogeneity relative to fine‐grained sites resulted from changes in species composition, increased bare soil areas and reduced perennial grass cover. In coarse‐grained sites, lower perennial grass cover resulted from lower density and/or smaller size of grass bunches than in fine‐grained sites. We did not find significant differences among sites in the proportion of perennial grazed grasses. Since the density and cover of perennial grasses was higher in fine‐ than in coarse‐grained sites, we suggested that fine‐grained sites are more important as feeding stations than coarse‐grained sites. The consequences of this differential use could lead to degradation of fine‐grained sites and to higher homogeneity in spatial plant structure and floristic composition within paddocks with respect to the condition observed at present, increasing the size of the highly degraded zone within the piosphere. At the patch level, we found that at about one third of the sampled transects, both total and non‐grazed perennial grasses were spatially aggregated with shrub patches. However, in most transects grazed perennial grasses were indifferently distributed in relation with shrub patches, showing that grazers display high selectivity of foraging sites at macro level (i.e. high and low grazing pressure sites at the paddock level), but random occupancy of vegetation units (randomness in the distribution of grazed perennial grasses at the patch level). The intensity of the positive association between non‐grazed grasses and shrub patches was higher in fine‐grained than in coarse‐grained sites and may be attributed to higher protection against herbivores associated to denser shrub patches in fine‐ relative to coarse‐grained sites. We concluded that a feedback exists between the spatial distribution of species preferred by grazers and the spatial patterning of use of these species.     

Species interactions at the level of fine roots in the field: influence of soil nutrient heterogeneity and plant size

Interference at the level of fine roots in the field was studied by detailed examination of fine root distribution in small soil patches. To capture roots as they occur in natural three-dimensional soil space, we used a freezing and slicing technique for microscale root mapping. The location of individual roots intersecting a sliced soil core surface was digitized and the identity of shrub and grass roots was established by a chemical technique. Soil patches were created midway between the shrub, Artemisia tridentata, and one of two tussock grasses, Pseudoroegneria spicata or Agropyron desertorum. Some soil patches were enriched with nutrients and others given only deionized water (control); in addition, patches were located between plants of different size combination (large shrubs with small tussock grasses and small shrubs with large tussock grasses). The abundance of shrub and grass roots sharing soil patches and the inter-root distances of individual fine roots were measured. Total average rooting density in patches varied among these different treatment combinations by only a factor of 2, but the proportion of shrub and grass roots in the patches varied sixfold. For the shrub, the species of grass roots sharing the patches had a pronounced influence on shrub root density; shrub roots were more abundant if the patch was shared with Pseudoroegneria roots than if shared with Agropyron roots. The relative size of plants whose roots shared the soil patches also influenced the proportion of shrub and grass roots; larger plants were able to place more roots in the patches than were the smaller plants. In the nutrient-enriched patches, these influences of grass species and size combination were amplified. At the millimeter- to centimeter-scale within patches, shrub and grass roots tended to segregate, i.e., avoid each other, based on nearest-neighbor distances. At this scale, there was no indication that the species-specific interactions were the result of resource competition, since there were no obvious patterns between the proportion of shrub and grass roots of the two species combinations with microsite nutrient concentrations. Other potential mechanisms are discussed. Interference at the fine-root level, and its species-specific character, is likely an influential component of competitive success, but one that is not easily assessed.     

平茬处理对小叶锦鸡儿灌丛邻居植物群落空间格局的影响

近几十年来,内蒙古典型草原小叶锦鸡儿(Caragana microphylla)灌丛化现象普遍发生,严重影响了草地生产力。平茬作为灌丛化草地主要管理方式之一,已受到广泛关注;然而平茬如何影响灌丛邻居植物群落的格局动态及其下土壤理化性质变化,仍缺少相应研究。以内蒙古草原小叶锦鸡儿灌丛化草地为研究对象,进行平茬处理,并依据距离小叶锦鸡儿灌丛的远近位置(远—2.5 m,近—0.5 m)设置样方进行植物群落调查和土壤样品的采集,经分析发现:(1)平茬处理显著提高了灌丛邻居植物群落均匀度及邻居植物群落下土壤全碳、全氮含量。(2)借助零模型(Null-model)分析,得出未平茬处理条件下,距离灌丛远近两种位置群落均表现为竞争性格局;而平茬处理条件下,群落竞争性格局作用弱化,且距离灌丛2.5 m位置处群落表现为促进性格局。(3)调查群落中物种间的关系多为中性作用;仅有少数物种对表现为显著正(或负)相互作用关系,且多由群落中优势物种组成;平茬处理条件下群落中显著负相互作用关系物种对比例下降,支持群落整体竞争性格局弱化这一结论。所得结果不仅为小叶锦鸡儿灌丛平茬处理条件下邻居植物群落及土壤环境的动态变化的探讨提供了实验依据,而且对灌丛化草地的恢复具有一定的实践指导意义。     

Determinants of Caragana microphylla density distribution in the Mongolian steppe

Regional differences in Caragana microphylla density in the Mongolian steppes were explained by considering multiple abiotic and biotic factors collectively, including aridity gradients, grazing regimes, fire disturbance, and interspecific interactions. In the central and eastern Mongolian steppes, we collected vegetation data from 127 sites. Along 250-m line transects, the hit frequencies of C. microphylla and tall-grass species were recorded. Ancillary data included weather information, livestock populations, fire occurrence maps, and herder camp locations. Based on the steppe types and disturbance regimes, the sites were classified into 12 sub-groups. The data were statistically analyzed at the site, county, and sub-group levels. The natural C. microphylla density decreased with climatic aridity from forb-steppes to semi-desert steppes, but this pattern was not observed at grazed and burned sites. Livestock grazing decreased C. microphylla density, but this effect was considerably confounded by aridity effects, especially in the central steppes, making the relationship between C. microphylla and livestock densities complex. Although fire appeared to be an important factor in the eastern steppes, the mechanism of its effect on C. microphylla density was unclear, because the fire–shrub interaction is influenced by the tall-grass recovery process after fire. Based on our results, we propose that two different confounding effects, namely aridity versus grazing and fire versus interspecific interactions, play important roles in determining the spatial distribution of C. microphylla density in the central and eastern Mongolian steppes, respectively.     

Are fine roots of both shrubs and perennial grasses able to occupy the upper soil layer? A case study in the arid Patagonian Monte with non-seasonal precipitation

We tested whether both shrubs and grasses are able to develop similar active fine-root systems in the upper soil layer of the arid Patagonian Monte ecosystem with non-seasonal precipitation. We selected in the field shrub patches consisting of one isolated modal plant of the dominant shrub Larrea divaricata Cav., grass patches formed by one or more bunches of the dominant grass Stipa tenuis Phil. (15 cm diameter), and mixed patches consisting of one individual of L. divaricata with bunches of S. tenuis under its canopy. We assessed the biomass, regrowth, and activity of fine roots (diameter <1.4 mm) of each species in the upper soil (50 cm depth) of each patch type at 3-month intervals. We also measured the N concentration in fine roots to estimate the relative contribution of each species to fine-root biomass of mixed patches. We injected Li⁺ in the soil as a chemical tracer to detect fine-root activity of each species in the upper soil. Fine-root biomass was higher in mixed patches than in grass patches while fine-root biomass in shrub patches did not differ from the two former. We did not find differences in fine-root regrowth among patch types. Li⁺ injection provided evidence of active fine roots of both species in the upper soil when it was wet. N concentration in fine roots suggested the prevalence of fine roots of L. divaricata in the upper soil of mixed patches. Our results support evidence of the ability of fine roots of both the shrub and the grass species to occupy the upper soil. These findings did not support the two-layer model (H Walter, Ecology of tropical and subtropical vegetation, Oliver and Boyd, Edinburgh, 1971) and provide evidence of this model would be less applicable to arid ecosystems with non-seasonal precipitation. Further, our results highlighted some issues deserving more research such as the outcome of belowground competition between neighboring plants of both contrasting life forms, the eventual limited fine-root carrying capacity of the upper soil, and differences in fine-root lifespan between species of both contrasting life form.     

Desertification alters patterns of aboveground net primary production in Chihuahuan ecosystems

Abstract The Chihuahuan desert of New Mexico, USA, has changed in historical times from semiarid grassland to desert shrublands dominated by Larrea tridentata and Prosopis glandulosa. Similar displacement of perennial grasslands by shrubs typifies desertification in many regions. Such structural vegetation change could alter average values of net primary productivity, as well as spatial and temporal patterns of production. We investigated patterns of aboveground plant biomass and net primary production in five ecosystem types of the Jornada Basin Long‐Term Ecological Research (LTER) site. Comparisons of shrub‐dominated desertified systems and remnant grass‐dominated systems allowed us to test the prediction that shrublands are more heterogeneous spatially, but less variable over time, than grasslands. We measured aboveground plant biomass and aboveground net primary productivity (ANPP) by species, three times per year for 10 years, in 15 sites of five ecosystem types (three each in Larrea shrubland, Bouteloua eriopoda grassland, Prosopis dune systems, Flourensia cernua alluvial flats, and grass‐dominated dry lakes or playas). Spatial heterogeneity of biomass at the scale of our measurements was significantly greater in shrub‐dominated systems than in grass‐dominated vegetation. ANPP was homogeneous across space in grass‐dominated systems, and in most growing seasons was significantly more patchy in shrub vegetation. Substantial interannual variability in ANPP complicates comparison of mean values across ecosystem types, but grasslands tended to support higher ANPP values than did shrub‐dominated systems. There were significant interactions between ecosystem type and season. Grasslands demonstrated higher interannual variation than did shrub systems. Desertification has apparently altered the seasonality of productivity in these systems; grasslands were dominated by summer growth, while sites dominated by Larrea or Prosopis tended to have higher spring ANPP. Production was frequently uncorrelated across sites of an ecosystem type, suggesting that factors other than season, regional climate, or dominant vegetation may be significant determinants of actual NPP.