The greening of the Northern Great Plains and its biogeochemical precursors

Vegetation greenness has increased across much of the global land surface over recent decades. This trend is projected to continue—particularly in northern latitudes—but future greening may be constrained by nutrient availability needed for plant carbon (C) assimilation in response to CO₂ enrichment (eCO₂). eCO₂ impacts foliar chemistry and function, yet the relative strengths of these effects versus climate in driving patterns of vegetative greening remain uncertain. Here we combine satellite measurements of greening with a 135 year record of plant C and nitrogen (N) concentrations and stable isotope ratios (δ¹³C and δ¹⁵N) in the Northern Great Plains (NGP) of North America to examine N constraints on greening. We document significant greening over the past two decades with the highest proportional increases in net greening occurring in the dries and warmest areas. In contrast to the climate dependency of greening, we find spatially uniform increases in leaf‐level intercellular CO₂ and intrinsic water use efficiency that track rising atmospheric CO₂. Despite large spatial variation in greening, we find sustained and climate‐independent declines in foliar N over the last century. Parallel declines in foliar δ¹⁵N and increases in C:N ratios point to diminished N availability as the likely cause. The simultaneous increase in greening and decline in foliar N across our study area points to increased N use efficiency (NUE) over the last two decades. However, our results suggest that plant NUE responses are likely insufficient to sustain observed greening trends in NGP grasslands in the future.     

Shifting states,shifting services: Linking regime shifts to changes in ecosystem services of shallow lakes

1. Shallow lakes can shift between stable states as a result of anthropogenic or natural drivers. Four common stable states differ in dominant groups of primary producers: submerged, floating, or emergent macrophytes or phytoplankton. Shifts in primary producer dominance affect key supporting, provisioning, regulating, and cultural ecosystem services supplied by lakes. However, links between states and services are often neglected or unknown in lake management, resulting in conflicts and additional costs.
2. Here, we identify major shallow lake ecosystem services and their links to Sustainable Development Goals (SDGs), compare service provisioning among the four ecosystem states and discuss potential trade‐offs.
3. We identified 39 ecosystem services potentially provided by shallow lakes. Submerged macrophytes facilitate most of the supporting (86%) and cultural (63%) services, emergent macrophytes facilitate most regulating services (60%), and both emergent and floating macrophytes facilitate most provisioning services (63%). Phytoplankton dominance supports fewer ecosystem services, and contributes most to provisioning services (42%).
4. The shallow lake ecosystem services we identified could be linked to 10 different SDGs, notably zero hunger (SDG 2), clean water and sanitation (SDG 6), sustainable cities and communities (SDG 11), and climate action (SDG13).
5. We highlighted several trade‐offs (1) among ecosystem services, (2) within ecosystem services, and (3) between ecosystem services across ecosystems. These trade‐offs can have significant ecological and economic consequences that may be prevented by early identification in water quality management.
6. In conclusion, common stable states in shallow lakes provide a different and diverse set of ecosystem services with numerous links to the majority of SDGs. Conserving and restoring ecosystem states should account for potential trade‐offs between ecosystem services and preserving the natural value of shallow lakes.

     

Engineering and biological approaches to the restoration from eutrophication of shallow lakes in which aquatic plant communities are important components

Engineering approaches (nutrient removal, sediment pumping, hypolimnion oxygenation, alum treatments) may be most appropriate to deep lakes where the aim of restoration from eutrophication is simply to reduce the production and crop of one component, the phytoplankton. They do not always give the desired results because the nutrient loading may only be reduced to a limited extent. There are additional problems in shallow lakes where change of state between community dominance (aquatic plants versus plankton) is wanted. Each community has powerful buffering mechanisms and biomanipulation may be essential to switch one state to another even with considerable nutrient reduction. For the phytoplankton-dominated community the buffers include the advantages of early growth, lower diffusion pathways for CO₂, overhead shading, and an absence of large cladoceran grazers. This later is because open-water shallow environments provide no refuges against predation for the large Cladocera which are both the most efficient grazers and the most favoured prey for fish. Restoration of aquatic plants may then require provision of refuges for the grazers. Different sorts of refuge are discussed using case studies of Hoveton Great Broad and Cockshoot Broad in the Norfolk Broadland.     

Resistance to drought affects persistence of alternative regimes in shallow lakes of the Boreal Plains (Alberta,Canada)

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Land management and climate change determine second‐generation bioenergy potential of the US Northern Great Plains

Bioenergy with carbon capture and storage (BECCS) has been proposed as a potential climate mitigation strategy raising concerns over trade‐offs with existing ecosystem services. We evaluate the feasibility of BECCS in the Upper Missouri River Basin (UMRB), a landscape with diverse land use, ownership, and bioenergy potential. We develop land‐use change scenarios and a switchgrass (Panicum virgatum L.) crop functional type to use in a land‐surface model to simulate second‐generation bioenergy production. By the end of this century, average annual switchgrass production over the UMRB ranges from 60 to 210 Tg dry mass/year and is dependent on the Representative Concentration Pathway for greenhouse gas emissions and on land‐use change assumptions. Under our simple phase‐in assumptions this results in a cumulative total production of 2,000–6,000 Tg C over the study period with the upper estimates only possible in the absence of climate change. Switchgrass yields decreased as average CO₂ concentrations and temperatures increased, suggesting the effect of elevated atmospheric CO₂ was small because of its C4 photosynthetic pathway. By the end of the 21st century, the potential energy stored annually in harvested switchgrass averaged between 1 and 4 EJ/year assuming perfect conversion efficiency, or an annual electrical generation capacity of 7,000–28,000 MW assuming current bioenergy efficiency rates. Trade‐offs between bioenergy and ecosystem services were identified, including cumulative direct losses of 1,000–2,600 Tg C stored in natural ecosystems from land‐use change by 2090. Total cumulative losses of ecosystem carbon stocks were higher than the potential ~300 Tg C in fossil fuel emissions from the single largest power plant in the region over the same time period, and equivalent to potential carbon removal from the atmosphere from using biofuels grown in the same region. Numerous trade‐offs from BECCS expansion in the UMRB must be balanced against the potential benefits of a carbon‐negative energy system.     

The Influence of Climate, Soils, Weather, and Land Use on Primary Production and Biomass Seasonality in the US Great Plains

Identifying the conditions and mechanisms that control ecosystem processes, such as net primary production, is a central goal of ecosystem ecology. Ideas have ranged from single limiting-resource theories to colimitation by nutrients and climate, to simulation models with edaphic, climatic, and competitive controls. Although some investigators have begun to consider the influence of land-use practices, especially cropping, few studies have quantified the impact of cropping at large scales relative to other known controls over ecosystem processes. We used a 9-year record of productivity, biomass seasonality, climate, weather, soil conditions, and cropping in the US Great Plains to quantify the controls over spatial and temporal patterns of net primary production and to estimate sensitivity to specific driving variables. We considered climate, soil conditions, and long-term average cropping as controls over spatial patterns, while weather and interannual cropping variations were used as controls over temporal variability. We found that variation in primary production is primarily spatial, whereas variation in seasonality is more evenly split between spatial and temporal components. Our statistical (multiple linear regression) models explained more of the variation in the amount of primary production than in its seasonality, and more of the spatial than the temporal patterns. Our results indicate that although climate is the most important variable for explaining spatial patterns, cropping explains a substantial amount of the residual variability. Soil texture and depth contributed very little to our models of spatial variability. Weather and cropping deviation both made modest contributions to the models of temporal variability. These results suggest that the controls over seasonality and temporal variation are not well understood. Our sensitivity analysis indicates that production is more sensitive to climate than to weather and that it is very sensitive to cropping intensity. In addition to identifying potential gaps in out knowledge, these results provide insight into the probable long- and short-term ecosystem response to changes in climate, weather, and cropping.     

Eutrophication control strategies for three shallow Vecht lakes in the province of North Holland

Like many shallow surface waters in the Netherlands the North Holland Vecht lakes, formerly known for their rich variety of flora and fauna, now face a serious eutrophication problem. Nutrient enrichment has been mainly in the form of (treated) wastewater discharges, and the continuing ingress of nutrient-loaded water from the river Vecht. Yet, this water has to be supplied in order to compensate for water shortages resulting from (i) changes in the groundwater flow pattern due to reclamation of the deep polder Horstermeer, (ii) extensive groundwater extraction in the Gooi hills, and (iii) extensive drainage for agricultural purposes.The present policy of eutrophication abatement and restoration of the Ankeveen and Kortenhoef lakes ecosystems is focused on eliminating wastewater discharges and Vecht water supply. It also allows for additional dredging measures. Because of the un-suitable major ion composition of the Vecht, the aim is to compensate for this water supply by (i) partial restoration of the original groundwater flow from the Gooi hills and (ii) periphere additional supply with fresh seepage water from the skirts of the Horstermeer polder. However, uncertainty exists about the amounts of water needed.Water balances and phosphorus budgets have been established to ascertain the water demands of the lakes and to gain a detailed insight into the nutrient fluxes through the lakes. A groundwater flow model is used to assess the beneficial effects of the proposed measures.The results obtained, question the current unilateral restoration objectives. Calculations reveal that, both in the present situation and after (total) reduction of groundwater extractions in the future, the available quantity of fresh seepage water from the skirts of the polder Horstermeer is not sufficient to replace the inlet from the river Vecht into the Kortenhoef lakes. Additional supply options are available but the ones favoured from an ecological viewpoint are either the most expensive or less favoured from a social point of view. Although the sediments of the lakes appear to be a major source of eutrophication, the possibility of dredging the lakes will be considered only after reviewing results of a pilot-dredging project in the Hollands Ankeveen lakes in 1991.     

The restoration of shallow eutrophic lakes,and the role of northern pike,aquatic vegetation and nutrient concentration

The feasibility of biomanipulation is related particularly to reducing the production and recruitment of planktivorous fish stocks. For assessing the level of predation needed to suppress planktivorous fish stocks, the relation between fish P and B, on one hand, and nutrient concentration on the other were analyzed. The carrying capacity of shallow lakes in terms of biomass is related to the total phosphorus concentration and nature of the lake bottom substrate. The production of planktivorous fish was 60–80% of the maximum carrying capacity. It is argued that aquatic vegetation and northern pike are effective tools to maintain water quality, but these are limited by the maximum nutrient concentration aquatic vegetation can sustain.     

The effects of land use upon water chemistry,particularly nutrient enrichment,in shallow lowland lakes: comparative studies of three lochs in Scotland

Three shallow, lowland lochs (lakes) in the Tayside region of Scotland, experiencing the same climatic regime, were found to be dimictic lakes showing similar clinograde oxygen distributions in summer. Land use differences in their catchments were shown to result in estimated total nutrient surface loadings from 0.3 to 32 g m^–2 a^–1 phosphorus and from 4 to 240 g m^–2 a^–1 nitrogen. The major ions in the lochs were calcium and carbonate, but with elevated sulphate levels in all three lochs and an increase in sodium, chloride and sulphate in Forfar loch, which was affected by sewage effluent. Conductivity and total alkalinity showed marked increases with greater intensity of land use, from 64 to 439 µS cm^–1 and 0.5–3 meq l^–1 Maximum winter loch concentrations of soluble reactive phosphorus ranged from 60 to just under 5 000 mg m^–3 and of inorganic nitrogen from 500 to 10500 mg m^–3. Maximum chlorophyll a ranged from 20 to 250 mg m^–3 and comparisons indicated that above winter levels of 5000 mg m^–3 N and 500 mg m^–3 P, the nutrient-chlorophyll relationships did not hold. Predictions of nutrient input, from land use categories and soil losses of N and P derived from other north temperate areas, were shown to be comparable with inputs calculated from loch measurements. Models predicting loch concentrations of phosphorus from inputs were comparable with measured concentrations, but predictions of chlorophyll and transparency became less accurate with higher nutrient levels. The lochs were mesotrophic (the Lowes), eutrophic (Balgavies) and hypertrophic (Forfar) under the several classification systems used. The implications of their nutrient status for lake management are discussed and the value of studying this unique lake series in a similar physical environment but with considerable chemical differences is considered.     

Effects of sequential depositional basins on lake response to urban and agricultural pollution: a palaeoecological analysis of the Qu'Appelle Valley, Saskatchewan, Canada

1. Palaeolimnological analyses of fossil diatoms and pigments were conducted in four lakes of the Qu'Appelle Valley, Saskatchewan, Canada, to quantify the effect of upstream depositional basins on lake response to urban and agricultural human activities. Pasqua, Echo, Mission and Katepwa lakes exhibit similar modern limnological characteristics, lie sequentially downstream from urban point sources of growth‐limiting nitrogen (N), yet drain similarly large areas of farmland (38–40 × 10³ km²). 2. Analyses indicated that all lakes were naturally productive, contained eutrophic diatoms (i.e. Stephanodiscus niagarae, S. hantzchii, S. parvus and Aulacoseira granulata), and supported blooms of colonial (as myxoxanthophyll) and potentially toxic N‐fixing cyanobacteria (aphanizophyll), even prior to the onset of European settlement (ca. 1890) and urban development (ca. 1930). 3. The onset of agricultural practices ca. 1890 had only modest effects on algal communities in the Qu'Appelle lakes, with subtle increases in eutrophic diatom species (Pasqua, Mission and Katepwa lakes) and 25–50% increases in pigment‐inferred algal abundance (Echo, Mission and Katepwa lakes). 4. Despite naturally high production, total algal abundance (β‐carotene) in upstream Pasqua Lake increased by more than 350% after intense urbanization beginning ca. 1930, while eutrophic diatoms became more common and cyanobacteria populations increased ten‐fold. Principal components analysis (PCA) explained 64% of diatom variance, and identified three eras corresponding to baseline, pre‐agricultural communities (1776–1890), an era of high production (ca. 1925–1960) and recent variable community composition following tertiary treatment of urban sewage (ca. 1977–1990). 5. Analyses of three downstream lakes demonstrated that urban impacts following 1930 remained evident in fossil profiles of β‐carotene and myxoxanthophyll, but that large blooms of N‐fixing cyanobacteria were restricted to the past 25 years at downstream Mission and Katepwa lakes. Similarly, PCA showed that fossil diatom assemblages exhibited little directional variation until the 1970s. 6. Together, these analyses support the hypothesis that upstream lakes were effective at reducing the impacts of point‐source urban nutrients on downstream lakes. In contrast, diffuse agricultural activities had only limited impacts on water quality and these were less well ameliorated by upstream basins.     

氮、磷对热带浅水湖泊惠州西湖蓝藻的控制

湖泊富营养化常导致蓝藻生物量的增加,水质恶化.于2011年2月至12月对热带浅水湖泊惠州西湖六个湖区的蓝藻群落结构进行研究,以了解其时空变化特征及主要影响因素.结果表明,以沉水植物为优势的元妙观湖区与南南湖蓝藻无明显的优势种;平湖蓝藻优势种为银灰平裂藻(Merismopedia glauca)和湖丝藻(Limnothrix sp.),南丰湖、北丰湖和北南湖的主要优势种均为银灰平裂藻(Merismopedia glauca).平湖、南丰湖、北丰湖和北南湖蓝藻丰度及生物量存在显著的季节变化.相关分析显示惠州西湖夏季蓝藻生物量受氮、磷盐控制.冬季温度的影响,蓝藻生物量与氮、磷的相关性不显著.     

Community composition and migration chronology of shorebirds using the saline lakes of the Southern Great Plains, USA

ABSTRACT.   Shorebirds migrating through the Southern Great Plains (SGP), USA, use freshwater playas and saline lakes as stopovers. The importance of playas is well documented, but the role of saline lakes is not clearly understood. During 2002 and 2003, we conducted surveys to determine the extent to which the saline lakes serve as stopovers. Twenty-eight species were recorded, and total seasonal abundance ranged from 6779 to 29,924 birds. Potential shorebird abundance for extant saline lakes was estimated at 37,000–71,000 shorebirds annually. American Avocets ( Recurvirostra americana ), Western Sandpipers ( Calidris mauri ), Baird's Sandpipers ( C. bairdi ), Least Sandpipers ( C. minutilla ), Snowy Plovers ( Charadrius alexandrinus ), Killdeer ( Charadrius vociferus ), and Wilson's Phalaropes ( Phalaropus tricolor ) were the most abundant species. Community composition of shorebirds differed between saline lakes and regional freshwater playas. Peak spring abundance was generally in April, whereas summer/fall migration was more protracted and shorebird abundance peaked during 6–8 weeks in August and September. Migration chronologies differed among morphologically similar species, and among representative species from different guilds. Such patterns of temporal separation permit partitioning of resources by shorebirds migrating through the SGP. The saline lakes of the SGP should be regarded as stopover sites of regional and international value. To ensure that saline lakes function as stopovers and to help maintain those unique communities that inhabit them, conservation of saline lakes should focus on preserving spring flows and conserving water.     

Hierarchical regulation of pelagic invertebrates in lakes of the northern Great Plains: a novel model for interdecadal effects of future climate change on lakes

Endorheic lakes of the northern Great Plains encompass a wide range of environmental parameters (e.g., salinity, pH, DOC, Ca, nutrients, depth) that vary 1000‐fold among sites and through the past 2000 years due to variation in basin hydrology and evaporative forcing. However, while many environmental parameters are known to individually influence zooplankton diversity and taxonomic composition, relatively little is known of the hierarchical relationships among potential controls or of how regulatory mechanisms may change in response to climate variation on diverse scales. To address these issues, we surveyed 70 lakes within a 100 000 km² prairie region to simulate the magnitude of environmental change expected to occur over 100–1000 years and to quantify the unique and interactive effects of diverse environmental parameters in regulating pelagic invertebrate community structure at that scale. Multivariate analyses showed that salinity was the principal correlate of changes in invertebrate composition among lakes, with a sequential loss of taxa between salinities of 4 and 50 g total dissolved solids L^?1 until one to two species predominated in highly saline systems. In contrast, changes in the concentrations of Ca²⁺ and other mineral nutrients exerted secondary controls of invertebrate assemblages independent of salinity, whereas lake depth provided a tertiary regulatory mechanism structuring species composition. In contrast to these large‐scale hierarchical patterns, seasonal surveys (May, July, September) of a subset of 21 lakes in each of 2003–2005 revealed that annual meteorological variation had no measurable effect on pelagic invertebrates, despite large differences in temperature, precipitation, and evaporation arising from regional droughts. Together these findings show that pelagic invertebrate communities in saline lakes are resilient to interannual variability in climate, but suggest that lakes of the northern Great Plains may provide a sensitive model to forecast centennial effects of future climate change.     

Northern pike (Esox lucius L.) and aquatic vegetation,tools in the management of fisheries and water quality in shallow waters

Species and size composition of fish communities in shallow stagnant waters appear to be associated with the type, abundance and pattern of the vegetation. Man-induced impacts as eutrophication, and suppression of vegetation for reasons of water quantity management or angling pleasure may induce irreversible changes in the aquatic ecosystem. Water quality management should aim at restoring former pike habitat. Submerged weeds are important to that effect. These habitats are characterized by relatively low densities of fish stocks. Stocking of bream and carp interferes strongly with these objectives. With respect to fish stocks, interests of water quality and fishery management may be opposed.     

The eutrophication of shallow coastal lakes in Southwest England — understanding and recommendations for restoration,based on palaeolimnology,historical records,and the modelling of changing phosphorus loads

Palaeolimnological studies of sediments from Slapton Ley and Loe Pool, two coastal freshwater lakes in Southwest England, show that in the period since 1945, they have been eutrophicated by nutrient inputs from intensification of agriculture, but also from sewage effluent. Two simple models have been used to identify the main sources of catchment outputs, and in the case of Slapton Ley, to evaluate historical changes in land use, and their likely effect on lake trophic status.Restoration strategies may also be evaluated using the same models. They suggest that in order to reduce loads on either lake to within OECD permissible limits, not only will all sewage inputs need to be prevented, and non-phosphate detergents used, but also losses from agricultural land must be reduced. This could take the form of the keeping of fewer cattle (the main source of organic nitrogen and phosphorus in both catchments), or the zoning of the respective catchments so that steep slopes close to riparian zones are not used, as at present, for the grazing of livestock.A better option, however, would appear to be the establishment along most of the rivers draining into these lakes, of buffer strips of woodland at least 15 m wide. According to the models, this measure, along with treatment or diversion of sewage effluent, would reduce phosphorus loads upon the lakes to within acceptable limits.     

Plant community responses to bison reintroduction on the Northern Great Plains,United States: a test of the keystone species concept

Keystone species restoration, or the restoration of species whose effect on an ecosystem is much greater than their abundance would suggest, is a central justification for many wildlife reintroduction projects globally. Following restoration, plains bison (Bison bison L.) have been identified as a keystone species in the tallgrass prairie ecoregion, but we know of no research to document similar effects in the mixed‐grass prairie where restoration efforts are ongoing. This study addresses whether Northern Great Plains (NGP) mixed‐grass prairie plant communities exhibit traits consistent with four central keystone effects documented for bison in the tallgrass prairie. We collected species composition, diversity, abundance, bare ground cover, and plant height data in three treatments: where livestock (Bos taurus L.) continuously grazed, livestock were removed for 10 years, and bison have been introduced and resident for 10 years. We observed mixed support for bison acting as keystone species in this system. Supporting the keystone role of bison, we observed higher species richness and compositional heterogeneity (β‐diversity) in the bison treatment than either the livestock retention or livestock removal treatments. However, we observed comparable forb, bare ground, and plant height heterogeneity between bison‐restored sites and sites where livestock were retained, contradicting reported keystone effects in other systems. Our results suggest that after 10 years of being restored, bison partially fulfill their role as a keystone species in the mixed‐grass prairie, and we encourage continued long‐term data collection to evaluate their influence in the NGP.     

厦门后溪水质与流域景观特征沿城乡梯度的变化分析

基于河流水质观测数据和Landsat 8影像数据,分析厦门后溪2013至2017年干季和湿季的水质时空变化特征,研究沿城乡梯度河流水质变化与流域景观特征的关系。结果表明:后溪上游(饮用水水源地水体)和下游(景观水体)溶解氧(DO)分别符合《地表水环境质量标准(GB3838—2002)》Ⅱ类标准和Ⅴ类标准,但是总氮(TN)和总磷(TP)有不同程度的超标,其中TN的超标比例较高。后溪上游土地利用类型中70%以上为林地,具有涵养水源、保护水质的作用;下游建设用地和耕地比重增加,TN、TP和叶绿素a (Chl a)显著高于上游。耕地和建设用地是提高TN、TP和Chl a等水质参数的主要土地利用类型,斑块密度、香农多样性指数和源汇景观指数与各水质指标也显著相关。冗余分析表明,流域景观特征可解释70%以上水质变化,对TN和TP的影响在湿季更大,而对Chl a的影响在干季较大。土地利用组成、配置、距离、高程和坡度对流域水质均有较大影响,其中土地利用组成对Chl a的影响较大,而景观指数则对TN和TP的影响较大。因此,在流域尺度加强土地利用的规划与综合管理,通过减少和控制地表径流面源污染的途径在一定程度上可降低人类活动对水质的不良影响。     

Fossil diatom inferred reconstruction of the pH history of two acidic,clear water lakes from insular Newfoundland,Canada

Palaeoecological research has been used to evaluate the impact of acidic deposition on lakes in insular Newfoundland. Terrestrial organic deposits in the region have considerable influence on freshwaters and have placed constrains on interpretation of the degree of anthropogenic acidification. In this paper, a region-specific calibration equation unique to clear water lakes (colour values 15) is developed from diatom assemblages in surface sediments from 22 lakes. The inferred pH history of two acidic, clear water lakes is then developed with a view to eliminating the influence of organic acidity from the interpretation of historical acid-base chemistry. The pH histories of the two lakes suggest modest declines (0.3 to 0.4 unit) in the most recent strata (i.e. since the 1930's). Both lakes demonstrate an increase in inferred pH in the surface horizon, which is consistent with declines in acidic deposition in the region since the mid 1970's. The magnitude and timing of the pH trends in these two lakes is common to those previously developed from more highly coloured lakes. The similar magnitude and onset of pH declines in lakes with varying amounts of organic influence provides no palaeolimnological evidence to suggest a contribution to, or modification of, lake acidification by organic acids.     

Holocene vegetation and climate change on the Colorado Great Plains,USA, and the invasion of Colorado piñon (Pinus edulis)

Aim To reconstruct the last c. 7000 years of vegetation and climate change in an unusual region of modern Great Plains grassland and scarp woodland in south‐east Colorado (USA), and to determine the late Holocene biogeography of Colorado piñon (Pinus edulis) at its easternmost extent, using a series of radiocarbon‐dated packrat (Neotoma sp.) middens. Location The West Carrizo Canyon drains the Chaquaqua Plateau, a plateau that projects into the western extent of the southern Great Plains grasslands in south‐eastern Colorado, USA. Elevations of the study sites are 1448 to 1525 m a.s.l. Today the plateau is mostly Juniperus scopulorum–P. edulis woodland. Methods Plant macrofossils and pollen assemblages were analysed from 11 ¹⁴C‐dated packrat middens. Ages ranged from 5990 yr bp (6839 cal. yr bp ) to 280 yr bp (485 cal. yr bp ). Results The results presented here provide information on the establishment and expansion of Juniperus–P. edulis woodland at its eastern limits. The analysis of both plant macrofossils and pollen from the 11 middens documents changes in plant communities over the last 7000 years, and the establishment of P. edulis at its easternmost limit. Though very minor amounts of P. edulis pollen occur as early as the middle Holocene, plant macrofossils were only recovered in middens dating after c. 480 cal. yr bp . Main conclusions Originally, midden research suggested a late glacial refuge to the north‐east of the Carrizo Canyon site, and a middle Holocene expansion of P. edulis. Results reported here are consistent with a late Holocene expansion, here at its eastern limits, but noted elsewhere at its northern and north‐eastern limits. In general, this late Holocene expansion is consistent with pollen data from sediments in Colorado and New Mexico, and suggests that P. edulis is still expanding its range at its present extremes. This has implications for further extension of its range due to changing climatic conditions in the future.     

鱼类生物群落对太子河流域土地利用、河岸带栖息地质量的响应

河流生态系统的退化是多空间尺度环境因子作用的结果。探讨不同尺度环境因子及水生生物之间的作用关系,识别影响水生生物群落完整性的尺度问题,是有效开展水生生物保护的基础。基于2009年对太子河流域15个样点的鱼类、河岸带栖息地质量评价,结合遥感影像解译的太子河流域土地利用情况(包括流域尺度和河段尺度),研究鱼类完整性指数(F-IBI)与两种尺度土地利用、栖息地质量参数之间的关系。结果表明太子河上游地区河岸栖息地质量较好,下游地区由于农业用地、城镇用地比例的增加河岸栖息地质量明显下降。F-IBI与自然用地比例呈正相关,与农业、城镇用地比例呈负相关。农业用地对F-IBI的影响体现在流域尺度,而城镇用地在两种尺度上都存在显著影响。相比于农业用地,城镇用地相同比例的增加会导致F-IBI更快的下降。底质、水质状况、人类活动强度是显著影响F-IBI的栖息地质量评价参数。3项参数均随农业和城镇用地比例增加而降低,农业用地主要在流域尺度上对3项参数产生影响,城镇用地主要影响底质和水质状况2项参数,而在两种尺度上的影响相差不大。