Species richness loss after nutrient addition as affected by N:C ratios and phytohormone GA3 contents in an alpine meadow community

Aims From the light-competition hypothesis, competition for light is asymmetric and the observed increases in plant-size variability with increasingly denser canopies are primarily due to competition for light. Greater plant height provides pre-emptive access to light and produces increased height differences among species. The question is what produces these differences in plant height or height growth response among species in response to fertilization.Methods In 2009, a field experiment of N, P and N + P enrichments at three levels each was initiated in an alpine meadow on the northeast Qinghai-Tibet Plateau. Effects of fertilization on species richness, aboveground net primary production (ANPP), relative light intensity and plant height of different plant functional groups were determined. Festuca ovina (grass), Kobresia humilis (sedge), Oxytropis ochrocephala (legume), Taraxacum lugubre (rosette forb) and Geranium pylzowianum (upright forb) were selected as exemplars of each of the indicated functional groups. The N:C ratios in aboveground biomass, gibberellic acid (GA 3) concentrations in leaves, plant heights and height relative growth rate (RGR) of these exemplar species were analyzed in detail.Important findings Species richness of grasses significantly increased with increasing N + P levels. Species richness of legumes and upright forbs decreased after N and N + P additions. P addition had no significant effect on species richness. The effects of N + P addition on species richness and ANPP were consistently stronger than those of the single N or P fertilization. Reductions in species richness caused by nutrient addition paralleled the increases in ANPP and decreases in light intensity under the canopies, indicating indirect effect of nutrient addition on species richness via ANPP-induced light competition. The exemplar species that responded most positively to fertilization in height and RGR also displayed stronger increases in their GA 3 content and N:C ratios. GA 3 concentrations and N:C ratios were positively correlated with height RGR when the data were pooled for all species. The tallest and the fastest-growing grass, F. ovina, had the largest increase in N:C ratios and the highest leaf GA 3 concentrations after nutrient addition. These results indicated that differential responses of GA 3 concentrations and N:C ratios to fertilization were related to the inequality in plant heights among species.     

沙坡头地区草本植物层片对施加氮素的响应

以沙坡头人工固沙区草本层片植物为研究对象,分析不同氮肥处理下,物种丰富度、多度、盖度、高生长以及地上部生物量等群落学特征对不同氮肥水平的响应。人工施加氮肥1年的试验结果表明:除50g·m-2的施肥水平外,其他各施肥水平下群落物种丰富度均高于对照;多度随施肥梯度的增加整体上呈下降趋势,施肥梯度越高,下降越明显,其中在50g·m-2的施肥水平下,多度相比于对照下降了86.7%;盖度随施肥水平梯度的增大呈先上升后下降的趋势,在20g·m-2的施肥水平下盖度达到最大值,其与对照间的差异达到了显著性水平(P<0.05);植物高生长对氮肥水平的响应存在种间差异;地上部生物量则在低肥水平下高于对照,而高肥水平下低于对照。总体而言,氮肥水平对荒漠生态系统群落物种多样性和生产力的影响呈低肥促进、高肥抑制的效应。     

Short-term responses of an alpine meadow community to removal of a dominant species along a fertilization gradient

Aims The relationship between biodiversity and ecosystem functioning has intrigued ecologists for several decades, but the effect of loss of a dominant species on community structure and functioning along a nutrient gradient remains poorly understood. The aim of this paper was to test the effect of a dominant species on community structure and function by conducting a species removal experiment along a fertilization gradient.Methods We removed the population of a dominant species (Elymus nutans) in a long-term fertilization field in an alpine meadow on the Tibetan Plateau, China. Univariate general linear models were used to evaluate the effects of fertilization and removal on above-ground vegetation characteristics, including photosynthetically active radiation in the understory, species richness, Shannon–Weiner diversity index, Simpson's dominance index, above-ground biomass (including different functional groups) and seedling richness and density.Important findings Results revealed that after two plant growing seasons, there was no significant effect of the removal of a dominant species on species richness and diversity of the remaining vegetation, but the biomass of forbs and seedling recruitment were significantly increased indicative of the potential for long-term effects. Moreover, removal had a large effect at high fertilization levels, but little effect when fertilization levels were low. Our studies indicated that community response to loss of a dominant species was mainly dependent on resource availability and the remaining functional group identities. We also found seedling recruitment was usually more sensitive to the influence of competition of dominant species than the established vegetation in the short term.     

Cumulative nitrogen input drives species loss in terrestrial ecosystems

Aim Elevated inputs of biologically reactive nitrogen (N) are considered to be one of the most substantial threats to biodiversity in terrestrial ecosystems. Several attempts have been made to scrutinize the factors driving species loss following excess N input, but generalizations across sites or vegetation types cannot yet be made. Here we focus on the relative importance of the vegetation type, the local environment (climate, soil pH, wet deposition load) and the experimentally applied (cumulative) N dose on the response of the vegetation to N addition. Location Mainly North America and Europe. Methods We conducted a large‐scale meta‐analysis of in situ N addition experiments in different vegetation types, focusing on the response of biomass and species richness. Results Whereas the biomass of grasslands and salt marshes significantly increased with N fertilization, forest understorey vegetation, heathlands, freshwater wetlands and bogs did not show any significant response. Graminoids significantly increased in biomass following N addition, whereas bryophytes significantly lost biomass; shrubs, forbs and lichens did not significantly respond. The yearly N fertilization dose significantly influenced the biomass response of grassland and salt marshes, while for the other vegetation types none of the collected predictor variables were of significant influence. Species richness significantly decreased with N addition in grasslands and heathlands [Correction added on 23 March 2011, after first online publication: ‘across all vegetation types’ changed to ‘in grasslands and heathlands’]. The relative change in species richness following N addition was significantly driven by the cumulative N dose. Main conclusions The decline in species richness with cumulative N input follows a negative exponential pathway. Species loss occurs faster at low levels of cumulative N input or at the beginning of the addition, followed by an increasingly slower species loss at higher cumulative N inputs. These findings lead us to stress the importance of including the cumulative effect of N additions in calculations of critical load values.     

Vegetation structure,species life span,and exotic status elucidate plant succession in a limestone quarry reclamation

An understanding of the processes involved in plant succession is pivotal in achieving an effective site restoration. In a former limestone quarry (northeastern Italy), we explored the effects of a technical reclamation on the plant community using changes in cover of vegetation layers and two sensitive plant traits (i.e. exotic status and life span), with a chronosequence approach. Four reclaimed areas of different ages (from 8 to 35 years old) and natural vegetation in the surroundings were investigated with seven permanent plots each, for a total of 35. Changes in vegetation layers and species richness of both exotic status and life span were analyzed by generalized linear (mixed) models. Relations with plant community assembly were also considered, using a multivariate approach. Both vegetation layers and plant traits were affected by the age of reclaimed areas, evidencing the main changes in plant succession. Annual and exotic species decreased toward the mature stages of reclamation and target vegetation, whereas overall plant diversity (species richness) was stable. Our findings show that both vegetation layer changes and plant traits can be used to assess the degree to which reclamation efforts produce results that approach the restoration of a natural vegetation reference. Implementation of management practices aimed at favoring native perennial species (e.g. appropriate seed mixtures, mowing, tree, and shrub planting) could limit weed‐control efforts, representing a reasonable trade‐off between biodiversity promotion and invasive plant control.     

Recovery of subalpine dwarf shrub heath after neighbour removal and fertilization

We tested if subalpine heath vegetation in northern Italy recovered after experimental perturbation of soil nutrient availability (fertilization) and species composition (removal of co-dominant dwarf shrubs). Species cover was assessed non-destructively before the start of the treatments (1995), at the end of the treatments (1999) and 4 years after the treatments ended (2003). Shrub removal had rather modest effects on heath vegetation, except for mosses which decreased significantly in removal plots. Fertilization decreased the cover of shrubs, mosses, and some graminoids but increased the cover of Festuca rubra. Fertilization converted heath to grassland, but the response of graminoid species was individualistic. Fertilization decreased vascular species richness and evenness, probably through negative effects of shading and litter accumulation on plant growth or recruitment. The vegetation had not recovered completely 4 years after the perturbations had stopped. This suggests that, in contrast to rapid responses to species removal and fertilization, recovery from these perturbations was rather slow, presumably because recovery was affected by long-term biotic interactions and species controls on ecosystem properties.     

高寒草甸植物群落物种多样性和生产力关系的光竞争研究

通过施肥形成的生产力由低到高的过程中,物种多样性往往降低。总体竞争假说认为对所有资源的竞争作用对多样性的影响随着生产力提高而加剧,导致物种多样性的下降;光竞争假说则认为随着生产力提高,种间竞争从低生产力时的地下竞争转向高生产力时的光竞争,是光竞争导致了物种多样性的下降。为了验证这两种假说,本文通过在甘南玛曲高寒草甸的均匀施肥实验,研究了光竞争对高寒草甸植物群落物种多样性和生产力关系的影响。结果表明：(1)随着施肥梯度的增加,大部分植物的生长速率加快,高度和叶面积增加;(2)随着施肥梯度的增加,植物群落地上总的生物量提高,叶面积指数增加,透光率降低,物种多样性减少;(3)个体大小不对称的光竞争导致了高寒草甸植物群落物种多样性随施肥梯度的增加而减少。     

Linking Above- and Belowground Responses to 16 Years of Fertilization,Mowing, and Removal of the Dominant Species in a Temperate Grassland

Species-rich oligotrophic meadows are affected by a wide range of management interventions that influence their functioning and capacity to deliver ecosystem services, but long-term studies on the above- and belowground adaptations to different management tools are still scarce. We focused on the interactive effects of NPK fertilization, mowing, and removal of the initially dominant species (Molinia caerulea) on plant, soil, and microbial responses in wet oligotrophic grassland in a 16-year full-factorial manipulative experiment. Changes in vegetation composition, soil pH, and nutrient availability were accompanied by altered microbial phospholipid fatty acid (PLFA) composition, whereas treatment effects on soil microbial biomass and carbon (C) mineralization were mainly related to changes in soil organic matter (SOM) content and nutrient availability. Fertilization decreased plant species richness aboveground and lowered SOM storage and microbial activity belowground. Mowing preserved high plant diversity and led to more efficient recycling of N within the grassland, whereas Molinia removal significantly affected only plant community composition. Mowing combined with fertilization maintained high species richness only in the short term. Belowground, mowing reduced N leaching from the fertilized system but did not prevent SOM depletion, soil acidification, and concomitant adverse effects on soil microbes. We conclude that annual mowing is the appropriate type of extensive management for oligotrophic species-rich meadows, but the concomitant nutrient depletion should not be compensated for by regular NPK fertilization due to its adverse effects on soil quality.     

草原化荒漠草本植物对人工施加磷素的响应

荒漠地区因土壤水分和养分含量较低而限制了植被的生长,且磷素对于植物的生长具有重要的作用。利用人工施加磷素的控制实验研究了不同磷肥处理对荒漠区草本植物物种丰富度、多度、盖度、生物量、植物株高等群落学特征的影响。实验表明：物种丰富度和多度在施肥量分别为12.5,25和50 g?m-2的梯度下,施肥当年和第二年相较于对照均有所降低,且施肥梯度越高,降低越明显;植被盖度和地上部生物量则在两年的实验中表现出相似的规律,在不同的施肥梯度下均有所提高,高肥处理对其促进作用更大,且在降水充足的07年高于降水较少的08年,说明水肥耦合更有助于群落生产力的提高;优势种株高则对磷素的响应存在种间差异,年际间的差异也较大,这或许与荒漠植物本身特有的生物学特性有关。     

Accelerated vegetation succession but no hydrological change in a boreal fen during 20 years of recent climate change

Northern mires (fens and bogs) have significant climate feedbacks and contribute to biodiversity, providing habitats to specialized biota. Many studies have found drying and degradation of bogs in response to climate change, while northern fens have received less attention. Rich fens are particularly important to biodiversity, but subject to global climate change, fen ecosystems may change via direct response of vegetation or indirectly by hydrological changes. With repeated sampling over the past 20 years, we aim to reveal trends in hydrology and vegetation in a pristine boreal fen with gradient from rich to poor fen and bog vegetation. We resampled 203 semi‐permanent plots and compared water‐table depth (WTD), pH, concentrations of mineral elements, and dissolved organic carbon (DOC), plant species occurrences, community structure, and vegetation types between 1998 and 2018. In the study area, the annual mean temperature rose by 1.0°C and precipitation by 46 mm, in 20‐year periods prior to sampling occasions. We found that wet fen vegetation decreased, while bog and poor fen vegetation increased significantly. This reflected a trend of increasing abundance of common, generalist hummock species at the expense of fen specialist species. Changes were the most pronounced in high pH plots, where Sphagnum mosses had significantly increased in plot frequency, cover, and species richness. Changes of water chemistry were mainly insignificant in concentration levels and spatial patterns. Although indications toward drier conditions were found in vegetation, WTD had not consistently increased, instead, our results revealed complex dynamics of WTD as depending on vegetation changes. Overall, we found significant trend in vegetation, conforming to common succession pattern from rich to poor fen and bog vegetation. Our results suggest that responses intrinsic to vegetation, such as increased productivity or altered species interactions, may be more significant than indirect effects via local hydrology to the ecosystem response to climate warming.     

Ungulates increase forest plant species richness to the benefit of non‐forest specialists

Large wild ungulates are a major biotic factor shaping plant communities. They influence species abundance and occurrence directly by herbivory and plant dispersal, or indirectly by modifying plant‐plant interactions and through soil disturbance. In forest ecosystems, researchers’ attention has been mainly focused on deer overabundance. Far less is known about the effects on understory plant dynamics and diversity of wild ungulates where their abundance is maintained at lower levels to mitigate impacts on tree regeneration. We used vegetation data collected over 10 years on 82 pairs of exclosure (excluding ungulates) and control plots located in a nation‐wide forest monitoring network (Renecofor). We report the effects of ungulate exclusion on (i) plant species richness and ecological characteristics, (ii) and cover percentage of herbaceous and shrub layers. We also analyzed the response of these variables along gradients of ungulate abundance, based on hunting statistics, for wild boar (Sus scrofa), red deer (Cervus elaphus) and roe deer (Capreolus capreolus). Outside the exclosures, forest ungulates maintained higher species richness in the herbaceous layer (+15%), while the shrub layer was 17% less rich, and the plant communities became more light‐demanding. Inside the exclosures, shrub cover increased, often to the benefit of bramble (Rubus fruticosus agg.). Ungulates tend to favour ruderal, hemerobic, epizoochorous and non‐forest species. Among plots, the magnitude of vegetation changes was proportional to deer abundance. We conclude that ungulates, through the control of the shrub layer, indirectly increase herbaceous plant species richness by increasing light reaching the ground. However, this increase is detrimental to the peculiarity of forest plant communities and contributes to a landscape‐level biotic homogenization. Even at population density levels considered to be harmless for overall plant species richness, ungulates remain a conservation issue for plant community composition.     

Short-term responses of rehabilitating coastal dune forest ground vegetation to livestock grazing

We investigated the responses of the ground vegetation in a 17‐year‐old coastal dune forest plant community to four levels of experimentally applied livestock grazing (three grazing levels and one ungrazed control) from May 1994 to March 1996. The effects of grazing were apparently subordinate to site‐specific intrinsic vegetation change and there were some indications that rainfall interacted with grazing level. Grazing had some apparent but no significant effects on plant species composition, significantly affected plant species richness over time, and significantly increased the range of species richness and vegetation cover values as well as the relative abundance and numbers of plant species with erect growth forms. Vegetation cover changed significantly over time, independently of grazing. Our results point to two important, easily measured mechanisms for the conservation management of coastal dune forests – the interaction of disturbance type with plant growth form and the increase of variation in community structural variables under disturbance. These mechanisms, although they potentially have wide application and predictive power, have not been studied adequately.     

Plant species richness in ephemeral and perennial reaches of a dryland river

Ephemeral reaches are common along desert rivers but are less well studied than those with perennial stream flow. This study contrasted riparian plant species richness and composition (extant vegetation and soil seed bank) between stream reaches with different low-flow conditions (perennial vs. ephemeral flow) but similar flood patterns and similar watershed-derived species pools. Data were collected at Cienega Creek (Arizona, USA) over a 2 year period spanning drought conditions and wetter conditions. Consistent with expectations relating to water limitation effects on diversity, species richness in the riparian zone was lower at ephemeral-flow sites during a season with minimal precipitation and no overbank flooding; under these conditions, the more permanent water sources of the perennial-flow sites sustain the larger number of species. During seasons with greater precipitation and elevated stream flows, in contrast, species richness at ephemeral-flow sites increased to levels at or slightly above those of perennial-flow sites. For values pooled across two wet seasons of a calendar year, year-round richness was greater at the two ephemeral-flow sites (total of 92 vascular plant species) than at the two perennial-flow sites (68 species). This greater year-round richness was a combination of multiple factors: greater light, space, and bare ground, a diverse soil seed bank (with the seed banks equally species-rich among hydrologic types), and moderately abundant precipitation and flooding sufficient to stimulate establishment of opportunistic species (mainly annuals) during the bimodal wet seasons. These results indicate that long-term patterns of site water availability, by influencing woody plant cover, mediate the diversity response to episodic water pulses in dryland rivers. The results also have implications for riparian conservation efforts, which to date have focused primarily on perennial stream reaches: ephemeral reaches of spatially intermittent rivers harbor many riparian plant species, and warrant conservation efforts, as well.     

亚高寒草甸植物群落种多度分布关系及相似性对氮磷添加的响应

以青藏高原亚高寒草甸为研究对象,采用随机区组设计,通过连续4a添加N、P,研究了不同施肥(N、P、N+P)处理下群落物种丰富度、种多度分布模式以及群落相似性的变化特征。结果显示:(1)N、N+P连续添加4年后,随N素添加水平的增加,草地植物群落物种丰富度逐渐降低(P0.001);种多度分布曲线的斜率逐渐增大;N+P添加处理对植物群落物种丰富度和种多度分布(SAD)曲线的影响较单独N添加处理更显著,如N15P15处理下群落物种丰富度的降幅最大,达对照群落的65.5%;(2)单一N或N+P处理中,不同添加量间的植被组成趋异,而相同添加量的植被组成趋同(stress level=0.152);(3)N、N+P添加引起刷状根的丛生型禾本科植物逐渐在植物群落中占据优势;(4)P素添加对群落物种丰富度、种多度分布曲线、群落相似性和不同生长型组成及比例的影响不显著;(5)植物生长型特征和N/P添加处理可解释56.97%植物群落的物种多度分布特征。这些结果表明:亚高寒草甸地区N添加引起植物群落组成的重新排序、优势种的变化、SAD曲线逐渐陡峭,群落的相似性增加;N富集时,添加P素会增加N素的利用效率,且群落结构受N、P供应水平的影响。     

Large-scale manipulation of plant litter and fertilizer in a managed successional temperate grassland

Plant litter may play an important role in herbaceous plant communities by limiting primary production and influencing plant species richness. However, it is not known how the effect of litter interacts with fertilization. We tested for the role of litter and fertilization in a large-scale experiment to investigate effects on diversity and biomass of plant species, growth forms, native vs. non-native groups, and abiotic ecosystem components (e.g., soil moisture, PAR). We manipulated plant litter (removed vs. left in situ) and nutrient availability (NPK-fertilized vs. unfertilized) for 4 years in 314-m² plots, replicated six times, in an old-field grassland. While many of our species-level results supported previously published studies and theory, our plant group results generally did not. Specifically, grass species richness and forb biomass was not affected by either fertilization or plant litter. Moreover, plant litter removal significantly increased non-native plant species richness. Relative to native plant species, all of our experimental manipulations significantly increased both the biomass and the species richness of non-native plant species. Thus, this grassland system was sensitive to management treatments through the facilitation of non-native plant species. We coupled biotic and abiotic components within a nonmetric multidimensional scaling (NMS) analysis to investigate treatment effects, which revealed that specific treatments altered ecosystem development. These results suggest that fertilization and plant litter may have larger impacts on plant communities and on ecosystem properties than previously understood, underscoring the need for larger-scale and longer-term experiments.     

Changes in alpine snowbed‐wetland vegetation over three decades in northern Norway

We have quantified floristic changes in alpine snowbeds and wetland vegetation during three decades and analyzed to what extent these changes are related to initial variations in snow cover duration and distance to groundwater level. Vascular plant species richness and total plant cover were estimated along three transects in northern Norway. Three different vegetation zones were identified along the original transects: relatively dry snowbeds, wet snowbeds and wetlands. The resampling shows major changes in species richness and plant cover. In general, there was a net immigration of species and 13 new species were found. Five rare species with initial low cover were lost. In the dry and wet snowbeds, species richness and total plant cover increased, mostly because of invasion by shrubs, graminoids and herbs. A general trend was that species indicating high soil moisture were strongly reduced. In the wetland zones there were no significant floristic changes but hygrophilous species had decreased and were replaced by graminoids and shrub species with lower water requirements. These floristic changes were significantly related to snow and soil moisture conditions, important factors for rate and direction of change. Contrasting vegetation responses within very short distances demonstrate the importance of detailed knowledge of the actual microhabitats when effects of climate change in alpine habitats are considered.     

Hydrologic effects on riparian vegetation in a boreal river: an experiment testing climate change predictions

Climate change is expected to alter the magnitude and variation of flow in streams and rivers, hence providing new conditions for riverine communities. We evaluated plant ecological responses to climate change by transplanting turfs of riparian vegetation to new elevations in the riparian zone, thus simulating expected changes in water‐level variation, and monitored the results over 6 years. Turfs moved to higher elevations decreased in biomass and increased in species richness, whereas turfs transplanted to lower elevations gained biomass but lost species. Transplanted plant communities responded slowly to the new hydrologic conditions. After 6 years, biomass of transplanted turfs was statistically indistinguishable from target level controls, but species richness and species composition of transplants were intermediate between original and target levels. By using projections of future stream flow according to IPCC climate change scenarios, we predict likely changes to riparian vegetation in boreal rivers. Climate‐driven hydrologic changes are predicted to result in narrower riparian zones along the studied Vindel River in northern Sweden towards the end of the 21st century. Present riparian plant communities are projected to be replaced by terrestrial communities at high elevations as a result of lower‐magnitude spring floods, and by amphibious or aquatic communities at low elevations as a result of higher autumn and winter flows. Changes to riparian vegetation may be larger in other boreal climate regions: snow melt fed spring floods are predicted to disappear in southern parts of the boreal zone, which would result in considerable loss of riparian habitat. Our study emphasizes the importance of long‐term ecological field experiments given that plant communities often respond slowly and in a nonlinear fashion to external pressures.     

Plant community responses to nitrogen addition and increased precipitation: the importance of water availability and species traits

Global nitrogen (N) enrichment and changing precipitation regimes are likely to alter plant community structure and composition, with consequent influences on biodiversity and ecosystem functioning. Responses of plant community structure and composition to N addition and increased precipitation were examined in a temperate steppe in northern China. Increased precipitation and N addition stimulated and suppressed community species richness, respectively, across 6 years (2005–2010) of the manipulative experiment. N addition and increased precipitation significantly altered plant community structure and composition at functional groups levels. The significant relationship between species richness and soil moisture (SM) suggests that plant community structure is mediated by water under changing environmental conditions. In addition, plant height played an important role in affecting the responses of plant communities to N addition, and the effects of increased precipitation on plant community were dependent on species rooting depth. Our results highlight the importance and complexity of both abiotic (SM) and biotic factors (species traits) in structuring plant community under changing environmental scenarios. These findings indicate that knowledge of species traits can contribute to mechanistic understanding and projection of vegetation dynamics in response to future environmental change.     

Effects of fire regime on plant species richness and composition differ among forest,woodland and heath vegetation

Question

Do the effects of fire regimes on plant species richness and composition differ among floristically similar vegetation types?

Location

Booderee National Park, south‐eastern Australia.

Methods

We completed floristic surveys of 87 sites in Sydney Coastal dry sclerophyll vegetation, where fire history records have been maintained for over 55 years. We tested for associations between different aspects of the recent fire history and plant species richness and composition, and whether these relationships were consistent among structurally defined forest, woodland and heath vegetation types.

Results

The relationship between fire regime variables and plant species richness and composition differed among vegetation types, despite the three vegetation types having similar species pools. Fire frequency was positively related to species richness in woodland, negatively related to species richness in heath, and unrelated to species richness in forest. These different relationships were explained by differences in the associations between fire history and species traits among vegetation types. The negative relationship between fire frequency and species richness in heath vegetation was underpinned by reduced occurrence of resprouting species at high fire frequency sites (more than four fires in 55 years). However, in forest and woodland vegetation, resprouting species were not negatively associated with fire frequency.

Conclusions

We hypothesize that differing relationships among vegetation types were underpinned by differences in fire behaviour, and/or biotic and abiotic conditions, leading to differences in plant species mortality and post‐fire recovery among vegetation types. Our findings suggest that even when there is a high proportion of shared species between vegetation types, fires can have very different effects on vegetation communities, depending on the structural vegetation type. Both research and management of fire regimes may therefore benefit from considering vegetation types as separate management units.     

施肥干扰对陕北黄土丘陵区三个典型群落结构组成的影响

在陕北黄土丘陵区,为了加快当地植被的恢复速度,结合当地条件,通过小区施肥实验与野外调查,研究了施肥对不同演替阶段的3个典型群落:猪毛蒿(Artemisia scoparia)、长芒草(Stipa bungeana)和铁杆蒿(Artemisia sacrorum)结构组成的影响。结果表明:(1)与对照相比,施肥后3个群落中物种的重要值发生改变。猪毛蒿群落中,1年生植物狗尾草(Setaria viridis)重要值减小,而多年生植物阿尔泰狗哇花(Heteropappus altaicus)、冰草(Agropyron cristatum)等重要值增大,成为群落中的主要物种;长芒草群落,与对照相比,施肥降低了优势种长芒草的重要性,而达乌里胡枝子(Lespedeza dahurica)的重要值明显增大;铁杆蒿群落则随着肥量增加,铁杆蒿的重要值表现为逐渐上升,与对照相比,群落中的伴生种二裂萎陵菜(Potentilla bifurca)、茭蒿(Artemisia giraldii)在处理区的平均重要值有所下降,而演替后期物种达乌里胡枝子的重要值增大。(2)与对照相比,通过施肥,除铁杆蒿群落高肥区密度外,3个群落的高度、生物量和密度均有显著提高,表现为高度和生物量在高肥区达最大,密度在中肥区达最高。3个群落相比,演替前期的猪毛蒿群落提高幅度最大,长芒草次之,铁杆蒿最小。(3)施肥改变了3个群落的物种多样性。群落类型不同,其变化趋势亦不同。猪毛蒿和铁杆蒿群落Patrick丰富度指数在中肥区达最高,随着施肥量的增加,Margalef(Ima)、Shannon-Wiener(Isw)和Pielou(J)指数产生不同程度的下降,而Berger-Parker优势度指数(I)逐渐增大;长芒草群落则随着施肥量的增加,Patrick指数、Ima、Isw和J均有不同程度的上升,而优势度指数I呈下降趋势。