Effects of resource availability on seedling recruitment in a fire-maintained savanna

The herbaceous ground cover of the longleaf pine ecosystem harbors the highest plant species richness in North America, with up to 50 species per square meter, but the mechanisms that regulate this diversity are not well understood. In this system, variability in seedling recruitment events may best explain the extremely high small-scale species richness and its relationship to soil moisture and system net primary productivity. To understand the potential mechanistic controls on species richness, we used a long-term resource manipulation study across a natural soil moisture gradient to assess environmental controls on seedling recruitment. We considered the availability of resources to be an indicator of seedling safe-site supply, and also manipulated seed availability to examine the relative importance of recruitment limitations on seedling diversity. We found that water availability regulated the number of species in the seedling community regardless of the underlying natural moisture gradient, and that this effect may result from differential responses of seedling guilds to resource availability. Water supply was more important than seed supply in determining seedling establishment, suggesting that appropriate sites for regeneration are a factor limiting seedling success. This is the first study that shows that the episodic supply of microsites for recruitment could influence species richness in the highly threatened and biodiverse longleaf pine savanna.     

Pattern of local plant species richness along a gradient of landscape topographical heterogeneity: result of spatial mass effect or environmental shift?

Several processes are hypothesised to mediate the relationship between local (microsite) plant species richness and the topographical heterogeneity of the surrounding landscape. In a topographically heterogeneous landscape with various habitats occurring close to each other, local species richness may be enriched by species from surrounding habitats due to the spatial mass effect (sink‐source dynamics). In contrast, increased habitat fragmentation due to spatial heterogeneity may have a negative effect on local species richness. The spatial mass effect is thought to be more pronounced in communities with a higher ratio of generalists, as generalists are more likely to establish viable populations in sink habitats. To reveal the pattern of local species richness along a gradient of landscape topographical heterogeneity at middle altitudes of the Bohemian Massif, we used 2551 forest vegetation plots stored in the Czech National Phytosociological Database. We developed an analytical approach relating the pattern of local species richness of vegetation types to the gradient of landscape topographical heterogeneity. An increase or decrease in species richness with increasing landscape heterogeneity was related to changes in the generalist/specialist ratio, and also to changes in soil reaction and productivity estimated through Ellenberg indicator values. Local species richness along a gradient of increasing landscape heterogeneity increased in nutrient‐poor vegetation and decreased in nutrient‐rich vegetation. Nutrient‐poor vegetation types, such as thermophilous and acidophilous oak forests, also had a high proportion of habitat generalists, supporting the hypothesis that increased richness in heterogeneous landscapes may result from the spatial mass effect. However, the same pattern may be explained by a shift in environmental conditions along the landscape heterogeneity gradient, such as increasing productivity of nutrient‐rich vegetation types or increasing soil reaction of most vegetation types in more heterogeneous landscapes. We discuss available evidence and conclude that these two explanations need not be mutually exclusive.     

Composition and species diversity of pine-wiregrass savannas of the Green Swamp,North Carolina

Fire-maintained, species-rich pine-wiregrass savannas in the Green Swamp, North Carolina were sampled over their natural range of environmental conditions and fire frequencies. Species composition, species richness, diversity (Exp H′, 1/C), and aboveground production were documented and fertilization experiments conducted to assess possible mechanisms for the maintenance of high species diversity in these communities. Although savanna composition varies continuously, DECORANA ordination and TWINSPAN classification of 21 sites facilitated recognition of 3 community types: dry, mesic, and wet savannas. These savannas are remarkably species-rich with up to 42 species/0.25 m² and 84 species/625 m². Maximum richness occurred on mesic, annually burned sites. Aboveground production, reported as peak standing crop, was only 293 g · m^?2 on a frequently burned mesic savanna but was significantly higher (375 g · m^?2) on an infrequently burned mesic site. Production values from fertilized high and low fire frequency sites were equivalent. Monthly harvest samples showed that savanna biomass composition by species groups did not vary seasonally, but within groups the relative importance of species showed clear phenological progressions. The variation in species richness with fire frequency is consistent with non-equilibrium theories of species diversity, while phenological variation in production among similar species and the changing species composition across the moisture gradient suggest the importance of equilibrium processes for maintenance of savanna diversity.     

Patterns of diversity in plant and soil microbial communities along a productivity gradient in a Michigan old-field

The relationship between plant diversity and productivity has received much attention in ecology, but the relationship of these factors to soil microbial communities has been little explored. The carbon resources that support soil microbial communities are primarily derived from plants, so it is likely that the soil microbial community should respond to changes in plant diversity or productivity, particularly if the plant community affects the quality or quantity of available carbon. We investigated the relationship of plant diversity and productivity to the composition of the soil microbial community along a topographic gradient in a mid-successional old-field in southwestern Michigan. Soil moisture, soil inorganic N, and plant biomass increased from the top to the base of the slope, while light at ground level decreased along this same gradient. We characterized the changes in resource levels along this gradient using an index of productivity that incorporated light levels, soil N, soil moisture, and plant biomass. Average plant species richness declined with this productivity index and there were associated compositional changes in the plant community along the gradient. The plant community shifted from predominantly low-growing perennial forbs at low productivities to perennial grasses at higher productivities. Although there was variation in the structure of the soil microbial community [as indicated by fatty acid methyl ester (FAME) profiles], changes in the composition of the soil microbial community were not correlated with plant productivity or diversity. However, microbial activity [as indicated by Biolog average well color development and substrate-induced respiration (SIR)] was positively correlated with plant productivity. The similarity between patterns of plant biomass and soil microbial activity suggests that either plant productivity is driving microbial productivity or that limiting resources for each of these two communities co-vary.     

Fire frequency and tree canopy structure influence plant species diversity in a forest-grassland ecotone

Disturbances and environmental heterogeneity are two factors thought to influence plant species diversity, but their effects are still poorly understood in many ecosystems. We surveyed understory vegetation and measured tree canopy cover on permanent plots spanning an experimental fire frequency gradient to test fire frequency and tree canopy effects on plant species richness and community heterogeneity within a mosaic of grassland, oak savanna, oak woodland, and forest communities. Species richness was assessed for all vascular plant species and for three plant functional groups: grasses, forbs, and woody plants. Understory species richness and community heterogeneity were maximized at biennial fire frequencies, consistent with predictions of the intermediate disturbance hypothesis. However, overstory tree species richness was highest in unburned units and declined with increasing fire frequency. Maximum species richness was observed in unburned units for woody species, with biennial fires for forbs, and with near-annual fires for grasses. Savannas and woodlands with intermediate and spatially variable tree canopy cover had greater species richness and community heterogeneity than old-field grasslands or closed-canopy forests. Functional group species richness was positively correlated with functional group cover. Our results suggest that annual to biennial fire frequencies prevent shrubs and trees from competitively excluding grasses and prairie forbs, while spatially variable shading from overstory trees reduces grass dominance and provides a wider range of habitat conditions. Hence, high species richness in savannas is due to both high sample point species richness and high community heterogeneity among sample points, which are maintained by intermediate fire frequencies and variable tree canopy cover.     

科尔沁沙地封育恢复过程中植物群落特征变化及影响因素

封育是退化沙地植被恢复与生态重建的重要措施, 理解长期处于封育状态下不同类型沙地植物群落特征变化及其影响因素有利于沙地植被恢复和生态重建。该文基于对科尔沁沙地长期封育的流动沙丘(2005年封育)、固定沙丘(1985年封育)和沙质草地(1997年封育)连续多年(2005-2017年)的植物群落调查, 结合土壤种子库、土壤养分以及气象数据, 分析了植物群落特征变化及其对环境变化的响应。研究结果表明流动沙丘植被盖度显著增加, 群落生物量和物种多样性年际间波动变化, 但无明显趋势; 固定沙丘植物群落存在逆行演替趋势, 具体表现为群落生物量、灌木和半灌木以及豆科优势度显著下降, 而一年生和多年生杂类草优势度显著增加; 沙质草地群落物种丰富度和多年生禾草优势度存在降低趋势, 并且一年生杂类草优势度明显高于其他功能群, 群落存在退化现象。3类沙地土壤种子密度变化不显著, 而种子丰富度在流动沙丘显著增加, 在固定沙丘和沙质草地有下降趋势, 土壤养分仅有有效氮和有效磷含量增加。回归分析结果表明气温和降水是影响年内生物量积累的主要因素, 但对年际间群落生物量和物种丰富度变化影响不大。除趋势对应分析结果显示土壤种子库与植物群落之间存在很高的相似性, 典型相关分析结果表明沙质草地植物群落与土壤养分紧密相关, 而固定沙丘群落主要与土壤水分紧密相关。综合以上结果可知, 封育33年的固定沙丘群落和封育21年的沙质草地群落都存在退化现象, 而封育11年的流动沙丘群落正在缓慢恢复, 因此封育年限的设定对退化沙地植被恢复至关重要, 封育时间过长不仅不利于植物群落恢复, 反而会使群落发生逆行演替, 建议封育年限的设定应综合考虑植被退化程度、土壤养分状况、土壤种子库基础以及气候条件等因素的影响。     

Spatial heterogeneity in soil particle size: does it affect the yield of plant communities with different species richness?

Aims Soil heterogeneity is ubiquitous in many ecosystems. We hypothesized that plant communities with higher species richness might be better adapted to soil heterogeneity and produce more biomass than those with lower richness. This is because there is niche differentiation among species and different species can complement each other and occupy a broader range of niches when plant species richness is high. However, no study has tested how soil particle heterogeneity affects the yield of plant communities, and whether such effects depend on the spatial scale of the heterogeneity and the species richness within the communities.Methods In a greenhouse experiment, we sowed seeds of four-species or eight-species mixtures in three heterogeneous treatments consisting of 32, 8 or 2 patches of both small (1.5mm) and large quartz (3.0mm) particles arranged in a chessboard manner and one homogeneous treatment with an even mixture of small and large quartz particles.Important findings Biomass production was significantly greater in the communities with high species richness than those with low species richness. However, soil particle heterogeneity or its interactions with patch scale or species richness did not significantly affect biomass production of the experimental communities. This work indicates that plant species richness may have a bigger impact on plant productivity than soil particle heterogeneity. Further studies should consider multiple sets of plant species during longer time periods to unravel the potential mechanisms of soil heterogeneity and its interactions with the impacts of species richness on community yield and species coexistence.     

黄土高原典型草原群落结构和土壤水分对划破的响应

划破是草原改良的基础措施之一，划破强度是划破措施的关键环节，划破对草原健康持续管理有重要意义。目前的研究主要集中在划破对植物群落结构和生产力的影响上，然而草原植物群落与土壤水分对划破强度的响应尚不清楚。在黄土高原典型草原开展不同程度的草地划破试验，探究不同划破强度（27.4%、46.3%和61.9%）对草地植物群落物种多样性、生物量和土壤水分的影响。结果表明：3个划破强度下划破带物种丰富度显著低于未划破带1-3种/m²，划破带和未划破带群落相似性分别低于整区23.85%-119.23%和44.43%-84.55%。地上生物量随物种丰富度的增加而增大，且地上生物量与Simpson指数和Shannon Weiner指数显著负相关。3个划破强度下未划破带地下生物量和总生物量分别高于划破带88.2%-134.6%、52.4%-67.8%、2.5%-16.6%和103.9%-152.9%、59.3%-75.8%、9.1%-22.6%。植物群落物种丰富度和地上生物量随划破强度的增加呈"驼峰"型曲线变化，当划破强度分别为43.7%-55.3%和43.8%-45.7%时，植物群落物种丰富度和生物量均最高。本试验阐明了划破对典型草原植物群落特征和土壤水分的作用机制，研究结果为采用划破措施实现草地培育和草原修复提供了科学依据，对保护草地生物多样性和提高生产力具有重要意义。     

Experimental analysis of patch dynamics and community heterogeneity in tallgrass prairie

Effects of fire and small-scale soil disturbances on species richness, community heterogeneity, and microsuccession were investigated in a central Oklahoma tallgrass prairie. In the fall of 1985, 0.2 m² soil disturbances were created on burned and unburned tallgrass prairie. Vegetation on and off disturbances was sampled at monthly intervals over two growing seasons. During the first growing season, the cover of forbs and annuals, and species richness were significantly greater on versus off disturbances, but these differences did not persist through the second year. The variation in species composition among disturbed plots (heterogeneity) was significantly greater compared to undisturbed areas throughout the study. Fire had no consistent effect on richness and heterogeneity of vegetation on soil disturbances but fire reduced heterogeneity on undisturbed vegetation. Rate of succession, based on an increase in cumulative cover of perennial grasses over time, did not differ among treatments during the first growing season. During the second year, rate of succession was significantly greater on burned soil disturbances compared to unburned soil disturbances. These results suggest that while small-scale soil disturbances have primarily short-lived effects on grassland community structure, disturbances do help to maintain spatial and temporal variation in tallgrass prairie communities. Unlike in undisturbed vegetation, however, species richness and heterogeneity on soil disturbances were little effected by fire, but the rate of colonization onto disturbances appeared to be enhanced by fire.     

The richness–heterogeneity relationship differs between heterogeneity measures within and among habitats

The positive monotonic relationship between habitat heterogeneity and species richness is a cornerstone of ecology. Recently, it was suggested that this relationship should be unimodal rather than monotonic due to a tradeoff between environmental heterogeneity and population sizes, which increases local species extinctions at high heterogeneity levels. Here, we studied the richness–heterogeneity relationship for an avian community using two different environmental variables, foliage‐height diversity and cover type diversity. We analyzed the richness–heterogeneity within different habitat types (grasslands, savannas, or woodlands) and at the landscape scale. We found strong evidence that both positive and unimodal relationships exist at the landscape scale. Within habitats we found positive relationships between richness and heterogeneity in grasslands and woodlands, and unimodal relationships in savannas. We suggest that the length of the environmental heterogeneity gradient (which is affected by both spatial scale and the environmental variable being analyzed) affects the type of the richness–heterogeneity relationship. We conclude that the type of the relationship between species richness and environmental heterogeneity is non‐ubiquitous, and varies both within and among habitats and environmental variables.     

The influence of Prosopis canopies on understorey vegetation: Effects of landscape position

Abstract. The influence of canopy trees and shrubs on under‐storey plants is complex and context‐dependent. Canopy plants can exert positive, negative or neutral effects on production, composition and diversity of understorey plant communities, depending on local environmental conditions and position in the landscape. We studied the influence of Prosopis velutina (mesquite) on soil moisture and nitrogen availability, and understorey vegetation along a topographic gradient in the Sonoran Desert. We found significant increases in both soil moisture and N along the gradient from desert to riparian zone. In addition, P. velutina canopies had positive effects, relative to open areas, on soil moisture in the desert, and soil N in both desert and intermediate terrace. Biomass of understorey vegetation was highest and species richness was lowest in the riparian zone. Canopies had a positive effect on biomass in both desert and terrace, and a negative effect on species richness in the terrace. The effect of the canopy depended on landscape position, with desert canopies more strongly influencing soil moisture and biomass and terrace canopies more strongly influencing soil N and species richness. Individual species distributions suggested interspecific variation in response to water‐ vs. N‐availability; they strongly influence species composition at both patch and landscape position levels.     

内蒙古高原典型草原区河漫滩湿地植物群落退化表征

草原区河流河漫滩草甸是生物多样性表现最充分和生物生产力最高的地段, 但由于过度放牧利用, 绝大部分草甸处于退化状态。该文以锡林河流域中游的河漫滩草甸为研究对象, 比较分析了围封保育湿地与放牧退化湿地的群落组成、地上生物量, 以及共有植物种的植株高度、节间长、叶长、叶宽, 土壤含水量、容重, 群落地下根量及根的分布, 土壤微生物生物量碳、氮的变化。结果表明: 1)放牧使得湿地植物群落优势种发生变化, 原有湿生植物逐渐向旱生化转变, 同时地上及地下生物量明显降低。2)退化湿地的植物呈现显著小型化现象。3)放牧退化湿地的土壤含水量较围封保育湿地低, 其垂直分布及地下根的垂直分布也发生变化。在低河漫滩, 土壤水分随土层的增加而增加, 根量也趋于深层化。但在高河漫滩湿地, 土壤含水量接近典型草原, 根未出现深层化分布趋势。4)放牧践踏引起土壤容重和土壤紧实度增加。5)放牧使得低河漫滩湿地土壤微生物生物量增加, 而在过渡区及高河漫滩湿地, 放牧使得土壤微生物生物量碳、氮含量显著降低。     

Are the eucalypt and non-eucalypt components of Australian tropical savannas independent?

Eucalypts (Eucalyptus and Corymbia spp.) dominate (>60%) the tree biomass of Australia's tropical savannas but account for only a fraction (28%) of the tree diversity. Because of their considerable biomass and adaptation to environmental stressors, such as fire, the eucalypts may drive tree dynamics in these savannas, possibly to the exclusion of non-eucalypts. We evaluated whether the eucalypt and non-eucalypt components in tropical savannas are dependent so that changes in one component are matched by opposite trends in the other. Using tree inventory data from 127 savanna sites across the rainfall and fire frequency gradients, we found that eucalypt and non-eucalypt basal area and species richness had a negative relationship. This relationship was maintained across the rainfall gradient, with rainfall having a positive effect on the basal area and species richness of both components, but with a greater effect in non-eucalypts. Fire frequency negatively affected basal area, but not species richness, although basal area and species richness of eucalypts and non-eucalypts did not differ in their response to fire. Rainfall appears to set the upper bounds to woody biomass in these mesic savannas, while fire maintains woody biomass below carrying capacity and facilitates coexistence of the components. The magnitude of the component responses, particularly for non-eucalypts, is determined by rainfall, but their dependence is likely due to their differential response to both rainfall and fire, but not to competition for resources. Thus, while eucalypts dominate biomass overall, at high rainfall sites non-eucalypt basal area and diversity are highest, especially where fire frequency is low.     

Density may alter diversity–productivity relationships in experimental plant communities

Contemporary biodiversity experiments, in which plant species richness is manipulated and aboveground productivity of the system measured, generally demonstrate that lowering plant species richness reduces productivity. However, we propose that community density may in part compensate for this reduction of productivity at low diversity. We conducted a factorial experiment in which plant functional group richness was held constant at three, while plant species richness increased from three to six to 12 species and community density from 440 to 1050 to 2525 seedlings m⁻². Response variables included density, evenness and above- and belowground biomass at harvest. The density gradient converged slightly during the course of the experiment due to about 10% mortality at the highest sowing density. Evenness measured in terms of aboveground biomass at harvest significantly declined with density, but the effect was weak. Overall, aboveground, belowground and total biomass increased significantly with species richness and community density. However, a significant interaction between species richness and community density occurred for both total and aboveground biomass, indicating that the diversity–productivity relationship was flatter at higher than at lower density. Thus, high species richness enabled low-density communities to reach productivity levels otherwise seen only at high density. The relative contributions of the three functional groups C₃, C₄ and nitrogen-fixers to aboveground biomass were less influenced by community density at high than at low species richness. We interpret the interaction effects between community density and species richness on community biomass by expanding findings about constant yield and size variation from monocultures to plant mixtures.     

Sawdust Addition Reduces the Productivity of Nitrogen‐Enriched Mountain Grasslands

Anthropogenic nutrient enrichment of mountain grasslands has boosted grasses and fast‐growing unpalatable plants at the expense of slow‐growing species, resulting in a significant loss in biodiversity. A potential tool to reduce nutrient availability and aboveground productivity without destroying the perennial vegetation is carbon (C) addition. However, little is known about its suitability under severe climatic conditions. Here, we report the results of a 3‐year field study assessing the effects of sawdust addition on soil nutrients, aboveground productivity, and vegetational composition of 10 grazed and ungrazed mountain grasslands. Of particular interest was the effect of C addition on grasses and on the tall unpalatable weed Veratrum album. After 3 years, soil pH, ammonium, and plant‐available phosphorus were not altered by sawdust application, and nitrate concentrations were marginally higher in treatment plots. However, the biomass of grasses and forbs (without V. album) was 20–25% lower in sawdust‐amended plots, whereas the biomass of V. album was marginally higher. Sawdust addition reduced the cover of grasses but did not affect evenness, vegetation diversity, or plant species richness, although species richness generally increased with decreasing biomass at our sites. Our results suggest that sawdust addition is a potent tool to reduce within a relatively short time the aboveground productivity and grass cover in both grazed and ungrazed mountain grasslands as long as they are not dominated by tall unpalatable weeds. The technique has the advantage that it preserves the topsoil and the perennial soil seed bank.     

三峡水库消落带植被高程梯度分异及其对生境胁迫的响应

三峡水库蓄水运行形成的极端生境胁迫深刻改变了消落带植被结构和功能。在消落带横向断面高程梯度上,植被生境具有典型的空间异质性。选取三峡水库典型自然恢复消落带,通过野外调查和室内分析,揭示了消落带自然演替植被的群落构成、物种多样性和生物量随高程梯度的分异特征,系统分析了极端淹水、侵蚀-沉积、土壤环境等生境胁迫类型对消落带植被高程梯度分异的影响。结果表明：①研究消落带适生植被以草本为主,共有15科25种23属,其中禾本科种类较多,但单属单种、单优群落现象明显;一年生和多年生草本分别占52%和48%,且一年生草本多在消落带上部定居,多年生草本主要在消落带下部聚集。②物种多样性指数与高程呈正相关趋势,在145-150m范围内较低,在160-170m范围内较高。③植被生物量为199.68-1211.2g/m²,总体呈现随高程增加而显著增加的趋势;受多种生境因子的复合胁迫影响,生物量随高程存在局部波动。④水库水位变动形成淹水时长、出露时令、淹水强度等是影响消落带植被生物量高程梯度分异的主导因子;侵蚀/沉积过程改变土层厚度、根层土壤持水能力和肥力条件,对植被生长产生重要影响;土壤水分和氮是植被生长的限制性因子。因此,优势生境适宜性物种选育、土壤基质保育和植被格局功能优化是三峡水库消落带植被恢复和生态功能重建的重要任务。     

Grazing, Environmental Heterogeneity, and Alien Plant Invasions in Temperate Pampa Grasslands

Temperate humid grasslands are known to be particularly vulnerable to invasion by alien plant species when grazed by domestic livestock. The Flooding Pampa grasslands in eastern Argentina represent a well-documented case of a regional flora that has been extensively modified by anthropogenic disturbances and massive invasions over recent centuries. Here, we synthesise evidence from region-wide vegetation surveys and long-term exclosure experiments in the Flooding Pampa to examine the response of exotic and native plant richness to environmental heterogeneity, and to evaluate grazing effects on species composition and diversity at landscape and local community scales. Total plant richness showed a unimodal distribution along a composite stress/fertility gradient ranging several plant community types. On average, more exotic species occurred in intermediate fertility habitats that also contained the highest richness of resident native plants. Exotic plant richness was thus positively correlated with native species richness across a broad range of flood-prone grasslands. The notion that native plant diversity decreases invasibility was supported only for a limited range of species-rich communities in habitats where soil salinity stress and flooding were unimportant. We found that grazing promoted exotic plant invasions and generally enhanced community richness, whereas it reduced the compositional and functional heterogeneity of vegetation at the landscape scale. Hence, grazing effects on plant heterogeneity were scale-dependent. In addition, our results show that environmental fluctuations and physical disturbances such as large floods in the pampas may constrain, rather than encourage, exotic species in grazed grasslands.     

放牧扰动下高寒草甸植物多样性、生产力对土壤养分条件变化的响应

为了揭示高寒小嵩草草甸群落在放牧扰动下,探讨土壤养分供给水平的变化对生态系统初级生产力和多样性影响,为高寒草地的退化演替机理研究提供依据,以野外样地调查和室内分析法研究了放牧扰动下高寒草甸植物多样性、生产力对土壤养分条件变化的响应.结果表明,放牧干扰不仅改变了高寒小蒿草草甸群落土壤根系和蕴育土壤根系的"载体"量及根土比例,改变了植物群落的结构和功能,而且使土壤的物理和化学特性发生了明显的改变.随着放牧强度的增加,蕴育土壤根系的基质量逐渐减少,根土比特别是0～10 cm土层的根土比例增加;"载体"量减少导致大部分地下根系由于营养供给水平的降低而死亡,归还土壤中有机质的数量逐渐减少,加之地上部分持续利用,土壤养分也在不断消耗,土壤基质量的减少和土壤资源持续供给能力的下降,草地发生逆向演替(退化),表现在:物种数减少、多样性下降、能量的分配转向地下等;土壤性状上的某些改变(土壤容重、土壤湿度等),也会引起植被组成、物种多样性变化;放牧主要通过影响土壤环境及其养分含量来改变草地群落生物量(地上、地下);土壤表面的适度干扰和原有植物的适度破坏为新成员提供了小生境,从而允许新的植物侵入群落,并提高了植物的丰富度.但是,在受到强度干扰时,草地植物群落的主要物种的优势地位发生明显的替代变化.     

Arbuscular mycorrhizal fungi associated with Populus-Salix stands in a semiarid riparian ecosystem

This study examined the activity, species richness, and species composition of the arbuscular mycorrhizal fungal (AMF) community of Populus-Salix stands on the Verde River (Arizona, USA), quantified patterns of AMF richness and colonization along complex floodplain gradients, and identified environmental variables responsible for structuring the AMF community. Samples from 61 Populus-Salix stands were analyzed for AMF and herbaceous composition, AMF colonization, gravimetric soil moisture, soil texture, per cent organic matter, pH, and concentrations of nitrate, bicarbonate phosphorus and exchangeable potassium. AMF species richness declined with stand age and distance from and elevation above the channel and was positively related to perennial species cover and richness and gravimetric soil moisture. Distance from and elevation above the active channel, forest age, annual species cover, perennial species richness, and exchangeable potassium concentration all played a role in structuring the AMF community in this riparian area. Most AMF species were found across a wide range of soil conditions, but a subset of species tended to occur more often in hydric areas. This group of riparian affiliate AMF species includes several not previously encountered in the surrounding Sonoran desert.     

呼伦贝尔草原不同草地利用方式下生物多样性与生物量的关系

生物多样性与生产力的关系是当前生态学中研究的重点之一,以呼伦贝尔草原为研究对象,通过连续两个生长季的野外监测,从草地植物功能型的角度探讨了在不同利用方式下草地物种丰富度与地上生物量的关系,结果表明：（1）不同草地利用方式显著影响草地生物多样性和生产力,在3种不同利用方式中,生物多样性总体的趋势是割草〉围封〉放牧,其中Shannon-Wiener指数、Simpson指数和物种丰富度均差异显著;割草草地地上生物量最高,围封草地次之,放牧草地最少。（2）将草地植物按照植物功能型分类,放牧草地1、2年生植物占优势,随着物种丰富度的增加,1、2年生植物生物量没有明显的变化趋势;割草草地以禾本科植物和非禾本科植物为主,随着物种丰富度的增加,禾本科植物生物量呈下降趋势,而非禾本科植物变化不明显;围封草地中禾本科植物占优势,其他功能型植物分布较均匀,多度、频度和生物量等差异不显著。（3）3种草地利用方式中只有围封草地物种丰富度和地上生物量存在显著的正相关,即随着物种丰富度的增加,生物量也随之升高。其他两种利用方式下,物种丰富度对地上生物量没有显著影响。