Showing papers on "Ecosystem published in 1998"

Mycorrhizal fungal diversity determines plant biodiversity, ecosystem variability and productivity

Abstract: The functioning and stability of terrestrial ecosystems are determined by plant biodiversity and species composition1,2,3,4,5 However, the ecological mechanisms by which plant biodiversity and species composition are regulated and maintained are not well understood These mechanisms need to be identified to ensure successful management for conservation and restoration of diverse natural ecosystems Here we show, by using two independent, but complementary, ecological experiments, that below-ground diversity of arbuscular mycorrhizal fungi (AMF) is a major factor contributing to the maintenance of plant biodiversity and to ecosystem functioning At low AMF diversity, the plant species composition and overall structure of microcosms that simulate European calcareous grassland fluctuate greatly when the AMF taxa that are present are changed Plant biodiversity, nutrient capture and productivity in macrocosms that simulate North American old-fields increase significantly with increasing AMF-species richness These results emphasize the need to protect AMF and to consider these fungi in future management practices in order to maintain diverse ecosystems Our results also show that microbial interactions can drive ecosystem functions such as plant biodiversity, productivity and variability

Nitrogen Saturation in Temperate Forest Ecosystems

Abstract: N itrogen emissions to the atmosphere due to human activity remain elevated in industrialized regions of the world and are accelerating in many developing regions (Galloway 1995). Although the deposition of sulfur has been reduced over much of the United States and Europe by aggressive environmental protection policies, current nitrogen deposition reduction targets in the US are modest. Nitrogen deposition remains relatively constant in the northeastern United States and is increasing in the Southeast and the West (Fenn et al. in press). The US acid deposition effects

The effects of air‐borne nitrogen pollutants on species diversity in natural and semi‐natural European vegetation

Abstract: Summary The effects of increased atmospheric nitrogen inputs, from both NOy and NHx, on diversity in various semi-natural and natural ecosystems are reviewed The severity of these impacts depends on abiotic conditions (eg buffering capacity, soil nutrient status and soil factors that influence the nitrification potential and nitrogen immobilization rate) in the particular system The sensitivity of fresh water ecosystems, wetlands and bogs, species-rich grasslands, heathlands and field layer of forests, all of which have conservational value, are discussed in detail The most important effects of nitrogen deposition are: (i) accumulation of nitrogenous compounds resulting in enhanced availability of nitrate or ammonium; (ii) soil-mediated effects of acidification; and (iii) increased susceptibility to secondary stress factors Long-term nitrogen enrichment has gradually increased the availability of nitrogen in several vegetation types, leading to competitive exclusion of characteristic species by more nitrophilic plants, especially under oligo- to mesotrophic soil conditions Soil acidification (with losses of buffering capacity and increased concentrations of toxic metals) is especially important after nitrification of ammonium in weakly buffered environments: acid-resistant plant species then become dominant at the expense of the often rare plants typical of intermediate pH The related change in the balance between ammonium and nitrate may also affect the performance of several species The susceptibility of plant species to secondary stress factors (pathogens; frost and drought) may be affected by air-borne nitrogen but data are only available for a few communities (eg dry heathlands) Most global biodiversity is contained within natural and semi-natural vegetation It is thus crucial to control emissions of nitrogenous compounds to the atmosphere, in order to reduce or prevent effects on diversity in these systems Most research has focused on acidification in forestry stands and lakes and on the effects on trees We highlight serious gaps in knowledge of other ecosystems

Accelerating invasion rate in a highly invaded estuary

Abstract: Biological invasions are a major global environmental and economic problem. Analysis of the San Francisco Bay and Delta ecosystem revealed a large number of exotic species that dominate many habitats in terms of number of species, number of individuals and biomass, and a high and accelerating rate of invasion. These factors suggest that this may be the most invaded estuary in the world. Possible causes include a large number and variety of transport vectors, a depauperate native biota, and extensive natural and anthropogenic disturbance.

Mesoscale Disturbance and Ecological Response to Decadal Climatic Variability in the American Southwest

Abstract: Climatic variables such as radiation, temperature and precipitation determine rates of ecosystem processes from net primary productivity to soil development. They predict a wide array of biogeographic phenomena, including soil carbon pools, vegetation physiognomy, species range, and plant and animal diversity. Climate also influences ecosystems indirectly by modulating the frequency, magnitude, and spatial scales of natural disturbances (Clark 1988; Overpeck et al. 1990; Swetnam 1993).

Carbon and carbonate metabolism in coastal aquatic ecosystems

Abstract: The coastal zone is where land, ocean, and atmosphere interact. It exhibits a wide diversity of geomorphological types and ecosystems, each one displaying great variability in terms of physical and biogeochemical forcings. Despite its relatively modest surface area, the coastal zone plays a considerable role in the biogeochemical cycles because it receives massive inputs of terrestrial organic matter and nutrients, is among the most geochemically and biologically active areas of the biosphere, and exchanges large amounts of matter and energy with the open ocean. Coastal ecosystems have therefore attracted much attention recently and are the focus of several current national and international research programs (e.g. LOICZ, ELOISE). The primary production, respiration, calcification, carbon burial and exchange with adjacent systems, including the atmosphere, are reviewed for the major coastal ecosystems (estuaries, macrophyte communities, mangroves, coral reefs, and the remaining continental shelf ). All ecosystems

Fire's effects on ecosystems

Abstract: FIRE DYNAMICS. Combustion Processes and Heat Transfer. Fuels and Fire Behavior. SOIL REPONSES. Soil Resource. Physical Soil System. Chemical Soil System. Biological Soil System. RESPONSES OF OTHER RESOURCES. Water. Vegetation. Wetlands and Riparian Ecosystems. Air. Cultural Resources. MANAGEMENT IMPLICATIONS. Economic Considerations. Fire in Ecosystem Management. Index.

Ant biodiversity and its relationship to ecosystem functioning: a review

Abstract: Ants are important components of ecosystems not only because they constitute a great part of the animal biomass but also because they act as ecosystem engineers. Ant biodiversity is incredibly high and these organisms are highly responsive to human impact, which obviously reduces its richness. However, it is not clear how such disturbance damages the maintenance of ant services to the ecosystem. Ants are important in below ground processes through the alteration of the physical and chemical environment and through their effects on plants, microorganisms, and other soil organisms. This review summarizes the information available on ant biodiversity patterns, how it can be quantified, and how biodiversity is affected by human impacts such as land use change, pollution, invasions, and climate change. The role of ants in ecosystems is discussed, mainly from the perspective of the effects of ground-dwelling ants on soil processes and function, emphasizing their role as ecosystem engineers. Some lines of research are suggested after demonstrating the gaps in our current information on ant-soil interactions.

Ungulate effects on the functional species composition of plant communities: herbivore selectivity and plant tolerance.

Abstract: Large mammalian herbivores not only depend on plant communities for their existence but cause major changes in plant community composition and structure, These changes have direct consequences for ecosystem processes, but recent studies of ungulate-ecosystem relations show widely divergent ungulate effects in different ecosystems. We reviewed studies of ungulate effects on plant community composition to gain insight into potential mechanisms of ungulate-induced changes in both community composition and ecosystem processes. Our analysis of these studies is based on the premise that the effect ungulates exert on plant communities depends on the balance between (1) feeding selectivity of herbivores (i.e., degree to which different plant species or ecotypes experience different levels of tissue loss), and (2) differences among plant species in their ability to recover from tissue loss. A large number of studies clearly show that selective ungulate herbivory leads to the dominance of unpalatable, chemically defended plant species in communities. However, many studies have also demonstrated that intensive long-term herbivory does not lead to the invasion of unpalatable species into the community, and can even increase the dominance of highly palatable species. Our review indicates that high levels of nutrient inputs or recycling and an intermittent temporal pattern of herbivory (often due to migration) are key factors increasing the regrowth capacity of palatable species and hence maintaining their dominance in plant communities supporting abundant herbivores. Key factors limiting ungulate foraging selectivity, again limiting herbivore-induced dominance of slow-growing, unpalatable species, include herding behavior, early growing season and postfire herbivory, asynchronous phenology of palatable versus unpalatable species, and low relative abundance of unpalatable species. Our review indicates differences among ecosystems in the role played by ungulate herbivory result from the relative strength of these factors enhancing plant tolerance to herbivory and limiting foraging selectivity. Anthropogenic changes in these factors (e.g., alteration of migration patterns) therefore have the potential to significantly alter the effects of ungulates on plant communities and ecosystem processes.

Nitrogen excess in North American ecosystems: Predisposing factors, ecosystem responses, and management strategies

Abstract: Most forests in North America remain nitrogen limited, although recent studies have identified forested areas that exhibit symptoms of N excess, analogous to overfertilization of arable land. Nitrogen excess in watersheds is detrimental because of disruptions in plant/soil nutrient relations, increased soil acidification and aluminum mobility, increased emissions of nitrogenous greenhouse gases from soil, reduced methane consumption in soil, decreased water quality, toxic effects on freshwater biota, and eutrophication of coastal marine waters. Elevated nitrate (NO3−) loss to groundwater or surface waters is the primary symptom of N excess. Additional symptoms include increasing N concentrations and higher N:nutrient ratios in foliage (i.e., N:Mg, N:P), foliar accumulation of amino acids or NO3−, and low soil C:N ratios. Recent nitrogen-fertilization studies in New England and Europe provide preliminary evidence that some forests receiving chronic N inputs may decline in productivity and experience greate...

Impacts of biological invasions on disturbance regimes

Abstract: Human management activities have altered the frequency and intensity of ecosystem disturbance often with enormous impacts on landscape structure and composition. One additional and under-appreciated way in which humans have altered disturbance regimes is through the introduction of invasive non-native species, themselves capable of modifying existing disturbance regimes or introducing entirely new disturbances. In many cases, modifications of disturbance regimes results in maintenance of ecosystems in a new or transitional state. There is now evidence that alteration of disturbance regime may be the most profound effect that a species or functional group can have on ecosystem structure and function.

Forest ecosystems : analysis at multiple scales

Abstract: 1. Forest Ecosystem Analysis at Multiple Time and Space Scales I. Introduction II. The Scientific Domain of Forest Ecosystem Analysis III. The Space/Time Domain of Ecosystem Analysis IV. Time and Space Scaling from the Stand/Seasonal Level V. Management Applications of Ecosystem Analysis VI. Related Textbooks VII. Web Site for Updated Materials Section I. Introduction to Analysis of Seasonal Cycles of Water, Carbon, and Minerals through Forest Stands 2. Water Cycles I. Introduction II. Heat and Water Vapor Transfer from Vegetation III. Water Flow through Trees IV. Water Storage and Losses from Snow V. Water Flow across and through Soil VI. Coupled Water Balance Models VII. Summary 3. Carbon Cycle I. Introduction II. Photosynthesis III. Autotrophic Respiration IV. Heterotrophic Respiration V. Modeling Photosynthesis and Respiration VI. Net Primary Production and Allocation VII. Comparison of Forest Ecosystem Models VIII. Summary 4. Mineral Cycles I. Introduction II. Plant Processes Affecting Nutrient Cycling III. Sources of Nutrients IV. Soil and Litter Processes V. Mass Balance and Models of Mineral Cycles VI. Summary Section II. Introduction to Temporal Scaling 5. Temporal Changes in Forest Structure and Function I. Introduction II. Structural Stages in Stand Development III. Functional Responses of Stands at Different Stages in Development IV. Looking Back in Time V. Ecosystem Models, Projections Forward in Time VI. Summary 6. Susceptibility and Response of Forests to Disturbance I. Introduction II. Biotic Factors III. Abiotic Factors IV. Summary Section III. Introduction to Spatial Scaling and Spatial/Temporal Modeling 7. Spatial Scaling Methods for Landscape and Regional Ecosystem Analysis I. Introduction II. Abiotic Site Variables III. Providing the Driving Variables, Climatology IV. Describing the Ecosystem V. Spatially Explicit Landscape Pattern Analysis VI. Data Layer Inconsistencies VII. Summary 8. Regional and Landscape Ecological Analysis I. Introduction II. Horizontal Connections: Biotic Analysis of Forest Patterns III. Vertical Connections: Forest-Atmosphere Interactions IV. Vertical and Horizontal Connections: Regional Biogeochemistry V. Summary 9. The Role of Forests in Global Ecology I. Introduction II. Global Forest Distribution III. Forest-Climate Interactions IV. Forests in the Global Carbon Cycle V. Forests and Biodiversity VI. Sustainability of Global Forests VII. Summary 10. Advances in Eddy-Flux Analyses, Remote Sensing, and Evidence of Climate Change I. Introduction II. Eddy-Covariance Fluxes III. New Remote Sensing of Forests IV. Climate Change and Forests Epilogue Bibliography Index

Effects of plant composition and diversity on nutrient cycling

Abstract: We evaluated the effects of plant functional group richness on seasonal patterns of soil nitrogen and phosphorus cycling, using serpentine grassland in south San Jose, California. We established experimental plots with four functional types of plants: early-season annual forbs (E), late-season annual forbs (L), nitrogen-fixers (N), and perennial bunchgrasses (P). These groups differ in several traits relevant to nutrient cycling, including phenology, rooting depth, root:shoot ratio, size, and leaf C:N content. Two or three species of each group were planted in single functional group (SFG) treatments, and in two-, three-, and four-way combinations of functional groups. We analyzed available nutrient pool sizes, microbial biomass nitrogen and phosphorus, microbial nitrogen immobilization, nitrification rates, and leaching losses. We used an index of “relative resource use” that incorporates the effects of plants on pool sizes of several depletable soil resources: inorganic nitrogen in all seasons, availabl...

Linking above-ground and below-ground interactions: How plant responses to foliar herbivory influence soil organisms

Abstract: Studies of the effects of above-ground herbivory on soil organisms and decomposer food webs, as well as the processes that they regulate, have largely concentrated on the effects of non-living inputs into the soil, such as dung, urine, body parts and litter. However, there is an increasing body of information which points to the importance of plant physiological responses to herbivory in regulating soil organisms and therefore, implicitly, key soil processes such as decomposition and nutrient mineralisation. In this review we identify the mechanisms by which foliar herbivory may indirectly affect the soil biota and associated below-ground processes through affecting plants, so as to better understand the nature of interactions which exist between above-ground and below-ground biota. We consider two broad pathways by which above-ground foliar herbivory may affect soil biotic communities. The first of these occurs through herbivore effects on patterns of root exudation and carbon allocation. These effects manifest themselves either as short-term changes in plant C allocation and root exudation or as long-term changes in root biomass and morphology. Evidence suggests that these mechanisms positively influence the size and activity of the soil biotic community and may alter the supply of nutrients in the rhizosphere for plant uptake and regrowth. The second of these involves herbivores influencing soil organisms through altering the quality of input of plant litter. Possible mechanisms by which this occurs are through herbivory enhancing nitrogen contents of root litter, through herbivory affecting production of secondary metabolites and concentrations of nutrients in foliage and thus in leaf litter and through selective foliar feeding causing shifts in plant community structure and thus the nature of litter input to the soil. While the effects of herbivory on soil organisms via plant responses may be extremely important, the directions of these effects are often unpredictable because several mechanisms are often involved and because of the inherently complex nature of soil food-web interactions; this creates obvious difficulties in developing general principles about how herbivory affects soil food-webs. Finally, it is apparent that very little is understood on how responses of soil organisms to herbivory affect those ecosystem-level processes regulated by the soil food-web (e.g. decomposition, nutrient mineralisation) and that such information is essential in developing a balanced understanding about how herbivory affects ecosystem function.

Dynamic responses of terrestrial ecosystem carbon cycling to global climate change

Abstract: Terrestrial ecosystems and the climate system are closely coupled, particularly by cycling of carbon between vegetation, soils and the atmosphere. It has been suggested1,2 that changes in climate and in atmospheric carbon dioxide concentrations have modified the carbon cycle so as to render terrestrial ecosystems as substantial carbon sinks3,4; but direct evidence for this is very limited5,6. Changes in ecosystem carbon stocks caused by shifts between stable climate states have been evaluated7,8, but the dynamic responses of ecosystem carbon fluxes to transient climate changes are still poorly understood. Here we use a terrestrial biogeochemical model9, forced by simulations of transient climate change with a general circulation model10, to quantify the dynamic variations in ecosystem carbon fluxes induced by transient changes in atmospheric CO2 and climate from 1861 to 2070. Wepredict that these changes increase global net ecosystem production significantly, but that this response will decline as the CO2 fertilization effect becomes saturated and is diminished by changes in climatic factors. Thus terrestrial ecosystem carbon fluxes both respond to and strongly influence the atmospheric CO2 increase and climate change.

Reversal of grazing impact on plant species richness in nutrient-poor vs. nutrient-rich ecosystems

Abstract: To test the hypothesis that the impacts of grazers on plant species richness reverse under contrasting nutrient richness, we analyzed unpublished and published data from lake, stream, marine, grassland, and forest ecosystems. We analyzed data from 30 studies providing 44 comparisons of plant species richness under low vs. high grazing pressure in enriched or nutrient-rich and non-enriched or nutrient-poor ecosystems. All 19 comparisons from non-enriched or nutrient-poor ecosystems exhibited significantly lower species richness under high grazing than under low grazing. In contrast, 14 of 25 comparisons from enriched or nutrient-rich ecosystems showed significantly higher species richness under high grazing than under low grazing. However, nine of these 25 comparisons showed no significant impact of grazers on species richness, while two comparisons showed declines in species richness under high grazing. Based on all the comparisons, plant species richness decreases with high grazing in nutrient-poor ecosystems, while it increases with high grazing in nutrient-rich ecosystems. Although nutrient-rich ecosystems seemed to produce more variable responses to grazers than did nutrient-poor ecosystems, in rare cases high grazing produced a decline in species richness in nutrient-rich environments. We suggest that species richness declines with high grazing in nutrient-poor ecosystems because a limitation of available resources prevents regrowth of species after grazing, which may not be the case in nutrient-rich ecosystems. It is also possible that an increase in species richness under high grazing in nutrient-rich ecosystems may be due to an increase in the dominance of inedible species. Our observation of a grazer reversal of plant species richness under contrasting nutrient richness may have important implications for management of species diversity.

Effects of invasive, non-indigenous plant species on ecosystem processes: lessons from Florida.

Abstract: Individual plant species that modify ecosystem properties have traditionally been thought to be uncommon in natural systems. I hypothesize that many invasive non- indigenous species do alter these properties at several scales. The non-indigenous plant species in Florida considered the most invasive by the Florida Exotic Pest Plant Council are examined for this capability through review of the available literature. Out of 31 species total, 12-20 (39-64%) potentially alter the ecosystem properties of geomorphology, hy- drology, biogeochemistry, and disturbance. When population-level properties that indicate superior competitive ability of the invading species are examined, 13-24 (42-77%) of the species are included, with the majority of species showing traits capable of modifying natural systems at both ecosystem and community/population scales. This review suggests that ecosystem alteration may be relatively common among invasive non-indigenous spe- cies. However, much of the current information is anecdotal. Empirical studies directly examining the effects of species on ecosystem and smaller-scale processes are necessary, and highly invasive species may be particularly appropriate for such research. Further, as non-indigenous species homogenize the global flora, they may also homogenize the local flora by increasing the representation of ruderal species. Where ecosystem processes have been altered, site restoration likely will require both control of the invader(s) and recovery of processes.

Biodiversity of Collembola and their functional role in the ecosystem

Abstract: More than 6500 species of Collembola are known from throughout the world and these are only a small part of the still undescribed species. There are many checklists and catalogues of Collembola for smaller territories and entire continents. Biogeographical analyses have been made for some genera and smaller territories. The most serious problems for a global biogeographical analysis is the lack of enough records from immense territories of all continents. Local biodiversity of Collembola can be very high, reaching over 100 species in small mountain ranges. Sampling methods do not impede documenting biodiversity on a global scale. Collembola have well differentiated ecomorphological life-forms and feeding guilds which enable the functional role that Collembola play in ecosystems to be recognised in some degree. Collembola play an important role in plant litter decomposition processes and in forming soil microstructure. They are hosts of many parasitic Protozoa, Nematoda, Trematoda and pathogenic bacteria and in turn are attacked by different predators. They utilise as food Protozoa, Nematoda, Rotatoria, Enchytraeidae, invertebrate carrion, bacteria, fungi, algae, plant litter, live plant tissues, and some plant pathogens. Soil acidification, nitrogen supply, global climate change and intensive farming have greatly impacted collembolan diversity.

Sliding baselines, ghosts, and reduced expectations in kelp forest communities

Abstract: The detection of trends in ecosystems depends upon (1) a good description of the foundation or benchmark against which changes are measured and (2) a distinction between natural and anthropogenic changes. Patterns and mechanisms observed over 25 years in a large kelp forest suggest that definition of a meaningful benchmark is impossible, because many of the large animals have been gone for years to decades, and kelps are sensitive to large-scale, low-frequency El Nifio-Southern Oscillation events and longer term regime shifts. A shift in the oceanographic climate has significantly reduced the average size and carrying capacity of the dominant plant. The animals that have been functionally removed from the community include sea otters, black sea bass, yellowtail, white sea bass, and abalones. Other species are still present, but fisheries have had huge effects on the abundances, size-frequencies, and/or spatial distributions of sheephead, kelp bass, rays, flatfish, rock fish, spiny lobsters, and red sea urchins. Now even sea cucumbers, crabs, and small snails are subject to unregulated fishing. The plants continue to exist without a hint of the effects of the loss of so much animal biomass. Furthermore, most of the megafauna have been removed with very little documentation or historical understanding of what the natural community was like. Thus, our ability to separate anthropogenic impacts from the "natural" dynamics of the system is severely compromised. We discuss the importance of both an ecosystem focus on productivity and careful monitoring of as many populations as possible. In addition, we show that this community is not tightly integrated with mutual dependencies; hence, many species can be removed without much affecting the rest of the ecosystem.

Biodiversity and ecosystem functioning: A mechanistic model

Abstract: Recent experiments have provided some evidence that loss of biodiversity may impair the functioning and sustainability of ecosystems. However, we still lack adequate theories and models to provide robust generalizations, predictions, and interpretations for such results. Here I present a mechanistic model of a spatially structured ecosystem in which plants compete for a limiting soil nutrient. This model shows that plant species richness does not necessarily enhance ecosystem processes, but it identifies two types of factors that could generate such an effect: (i) complementarity among species in the space they occupy below ground and (ii) positive correlation between mean resource-use intensity and diversity. In both cases, the model predicts that plant biomass, primary productivity, and nutrient retention all increase with diversity, similar to results reported in recent field experiments. These two factors, however, have different implications for the understanding of the relationship between biodiversity and ecosystem functioning. The model also shows that the effect of species richness on productivity or other ecosystem processes is masked by the effects of physical environmental parameters on these processes. Therefore, comparisons among sites cannot reveal it, unless abiotic conditions are very tightly controlled. Identifying and separating out the mechanisms behind ecosystem responses to biodiversity should become the focus of future experiments.

A spatial assessment of hydrologic alteration within a river network

Abstract: Maintaining natural hydrologic variability is essential in conserving native riverine biota and river ecosystem integrity. Hydrologic variation plays a major role in structuring the biotic diversity within river ecosystems as it controls key habitat conditions within the river channel, the floodplain, and hyporheic (stream-influenced ground water) zones. Alterations in streamflow regimes may modify many of these habitat attributes and impair ecosystem connectivity. We demonstrate use of the ‘Range of Variability Approach’ for assessing hydrologic alteration at available streamgauge sites throughout a river basin. We then illustrate a technique for spatially mapping the degree of hydrologic alteration for river reaches at and between streamgauge sites. Such maps can be used to assess the loss of natural hydrologic variation at a river basin scale, thereby facilitating river restoration planning. © 1998 John Wiley & Sons, Ltd.

Sense of place: An elusive concept that is finding a home in ecosystem management

Abstract: One of the great and largely unmet challenges associated with ecosystem management is treating people as a rightful part of ecosystems. In many ecosystem models, despite occasional rhetoric to the contrary, there is still a tendency to treat people as autonomous individual agents outside the ecosystem, at best a source of values to be incorporated into decisions, at worst agents of catastrophic disturbances of an otherwise smoothly running system. Many scholars have made suggestions for bringing social concepts and variables into ecosystem models and assessments (Driver et al. 1996; Force and Machlis 1997). Far fewer have demonstrated how day-to-day land management might change when people are recognized as part of the ecosystem.

Effect of interannual climate variability on carbon storage in Amazonian ecosystems

Abstract: The Amazon Basin contains almost one-half of the world's undisturbed tropical evergreen forest as well as large areas of tropical savanna1,2. The forests account for about 10 per cent of the world's terrestrial primary productivity and for a similar fraction of the carbon stored in land ecosystems2,3, and short-term field measurements4 suggest that these ecosystems are globally important carbon sinks. But tropical land ecosystems have experienced substantial interannual climate variability owing to frequent El Nino episodes in recent decades5. Of particular importance to climate change policy is how such climate variations, coupled with increases in atmospheric CO2 concentration, affect terrestrial carbon storage6,7,8. Previous model analyses have demonstrated the importance of temperature in controlling carbon storage9,10. Here we use a transient process-based biogeochemical model of terrestrial ecosystems3,11 to investigate interannual variations of carbon storage in undisturbed Amazonian ecosystems in response to climate variability and increasing atmospheric CO2 concentration during the period 1980 to 1994. In El Nino years, which bring hot, dry weather to much of the Amazon region, the ecosystems act as a source of carbon to the atmosphere (up to 0.2 petagrams of carbon in 1987 and 1992). In other years, these ecosystems act as a carbon sink (up to 0.7 Pg C in 1981 and 1993). These fluxes are large; they compare to a 0.3 Pg C per year source to the atmosphere associated with deforestation inthe Amazon Basin in the early 1990s12. Soil moisture, which is affected by both precipitation and temperature, and which affects both plant and soil processes, appears to be an important control on carbon storage.

Grassland dynamics : long-term ecological research in tallgrass prairie

Abstract: I. INTRODUCTION II. PHYSICAL ENVIRONMENT III. TERRESTRIAL POPULATIONS AND COMMUNITIES IV. HYDROLOGY AND AQUATIC ECOLOGY V. ECOSYSTEM AND LANDSCAPE-LEVEL ANALYSIS VI. TOWARD THE FUTURE

Effects on aquatic ecosystems

Abstract: Regarding the effects of UV-B radiation on aquatic ecosystems, recent scientific and public interest has focused on marine primary producers and on the aquatic web, which has resulted in a multitude of studies indicating mostly detrimental effects of UV-B radiation on aquatic organisms. The interest has expanded to include ecologically significant groups and major biomass producers using mesocosm studies, emphasizing species interactions. This paper assesses the effects of UV-B radiation on dissolved organic matter, decomposers, primary and secondary producers, and briefly summarizes recent studies in freshwater and marine systems. Dissolved organic carbon (DOC) and paniculate organic carbon (POC) are degradation products of living organisms. These substances are of importance in the cycling of carbon in aquatic ecosystems. UV-B radiation has been found to break down high-molecular-weight substances and make them available to bacterial degradation. In addition, DOC is responsible for short-wavelength absorption in the water column. Especially in coastal areas and freshwater ecosystems, penetration of solar radiation is limited by high concentrations of dissolved and particulate matter. On the other hand, climate warming and acidification result in faster degradation of these substances and thus enhance the penetration of UV radiation into the water column. Several research groups have investigated light penetration into the water column. Past studies on UV penetration into the water column were based on temporally and spatially scattered measurements. The process of spectral attenuation of radiant energy in natural waters is well understood and straightforward to model. Less known is the spatial and temporal variability of in-water optical properties influencing UV attenuation and there are few long-term observations. In Europe, this deficiency of measurements is being corrected by a project involving the development of a monitoring system (ELDONET) for solar radiation using three-channel dosimeters (UV-A, UV-B, PAR) that are being installed from Abisko (North Sweden, 68 °N, 19 °E) to Tenerife (Canary Islands, 27 °N, 17 °W). Some of the instruments have been installed in the water column (North Sea, Baltic Sea, Kattegat, East and Western Mediterranean, North Atlantic), establishing the first network of underwater dosimeters for continuous monitoring. Bacteria play a vital role in mineralization of organic matter and provide a trophic link to higher organisms. New techniques have substantially changed our perception of the role of bacteria in aquatic ecosystems over the recent past and bacterioplankton productivity is far greater than previously thought, having high division and turnover rates. It has been shown that bacterioplankton play a central role in the carbon flux in aquatic ecosystems by taking up DOC and remineralizing the carbon. Bacterioplankton are more prone to UV-B stress than larger eukaryotic organisms and, based on one study, produce about double the amount of cyclobutane dimers. Recently, the mechanism of nitrogen fixation by cyanobacteria has been shown to be affected by UV-B stress. Wetlands constitute important ecosystems both in the tropics and at temperate latitudes. In these areas, cyanobacteria form major constituents in microbial mats. The organisms optimize their position in the community by vertical migration in the mat, which is controlled by both visible and UV-B radiation. Cyanobacteria are also important in tropical and sub-tropical rice paddy fields, where they contribute significantly to the availability of nitrogen. Solar UV radiation affects growth, development and several physiological responses of these organisms. On a global basis, phytoplankton are the most important biomass producers in aquatic ecosystems. The organisms populate the top layers of the oceans and freshwater habitats where they receive sufficient solar radiation for photosynthetic processes. New research strengthens previous evidence that solar UV affects growth and reproduction, photosynthetic energy-harvesting enzymes and other cellular proteins, as well as photosynthetic pigment contents. The uptake of ammonium and nitrate is affected by solar radiation in phytoplankton, as well as in macroalgae. Damage to phytoplankton at the molecular, cellular, population and community levels has been demonstrated. In contrast, at the ecosystem level there are few convincing data with respect to the effects of ozone-related UV-B increases and considerable uncertainty remains. Following UV-B irradiation, shifts in phytoplankton community structure have been demonstrated, which may have consequences for the food web. Macroalgae and seagrasses are important biomass producers in aquatic ecosystems (but considerably smaller than phytoplankton). In contrast to phytoplankton, most of these organisms are sessile and can thus not avoid exposure to solar radiation at their growth site. Recent investigations showed a pronounced sensitivity to solar UV-B radiation, and effects have been found throughout the top 10–15 m of the water column. Photoinhibition can be quantified by oxygen exchange or by PAM (pulse amplitude modulated) fluorescence. Surface-adapted macroalgae, such as several brown and green algae, show a maximum of oxygen production at or close to the surface; whereas algae adapted to lower irradiances usually thrive best when exposed deeper in the water column. Mechanisms of protection and repair are being investigated. UV effects on aquatic animals are of increased interest. Evidence for UV effects has been demonstrated in Zooplankton activity. Other UV-B-sensitive aquatic organisms include sea urchins, corals and amphibians. Solar UV radiation has been known to affect corals directly. In addition, photosynthesis in their symbiotic algae is impaired, resulting in reduced organic carbon supply. Amphibian populations are in serious decline in many areas of the world, and scientists are seeking explanations for this phenomenon. Most amphibian population declines are probably due to habitat destruction or habitat alteration. Some declines are probably the result of natural population fluctuations. Other explanations for the population declines and reductions in range include disease, pollution, atmospheric changes and introduced competitors and predators. UV-B radiation is one agent that may act in conjunction with other stresses to affect amphibian populations adversely. The succession of algal communities is controlled by a complex array of external conditions, stress factors and interspecies influences. Freshwater ecosystems have a high turnover and the success of an individual species is difficult to predict, but the development of general patterns of community structure follows defined routes. There is a strong predictive relationship between DOC concentration and the depth to which UV radiation penetrates in lakes. Since DOC varies widely, freshwater systems display a wide range of sensitivity to UV penetration. In these systems, increased solar UV-B radiation is an additional stress factor that may change species composition and biomass productivity. The Arctic aquatic ecosystem is one of the most productive ecosystems on earth and is a source of fish and crustaceans for human consumption. Both endemic and migratory species breed and reproduce in this ocean in spring and early summer, at a time when recorded increases in UV-B radiation are maximal. Productivity in the Arctic ocean has been reported to be higher and more heterogeneous than in the Antarctic ocean. In the Bering Sea, the sea-edge communities contribute about 40–50% of the total productivity. Because of the shallow water and the prominent stratification of the water layer, the phytoplankton are more exposed and affected by solar UV-B radiation. In addition, many economically important fish (e.g., herring, pollock, cod and salmon) spawn in shallow waters where they are exposed to increased solar UV-B radiation. Many of the eggs and early larval stages are found at or near the surface. Consequently, reduced productivity of fish and other marine crops is possible but has not been demonstrated. There is increased consensus, covering a wide range of aquatic ecosystems, that environmental UV-B, independent of ozone-related increases, is an important ecological stress that influences the growth, survival and distribution of phytoplankton. Polar ecosystems, where ozone-related UV-B increases are the greatest and which are globally significant ecosystems, are of particular concern. However, these ecosystems are characterized by large spatial and temporal variability, which makes it difficult to separate out UV-B-specific effects on single species or whole phytoplankton communities. There is clear evidence for short-term effects. In one study a 4–23% photoinhibition of photosystem II activity was measured under the ozone hole. However, extrapolation of short-term effects to long-term ecological consequences requires various complex effects to be accounted for and quantitative evaluation remains uncertain.

The response of tundra plant biomass, aboveground production, nitrogen, and co2 flux to experimental warming

Abstract: We manipulated air temperature in tussock tundra near Toolik Lake, Alaska, and determined the consequences for total plant biomass, aboveground net primary production (ANPP), ecosystem nitrogen (N) pools and N uptake, and ecosystem CO2 flux. After 3.5 growing seasons, in situ plastic greenhouses that raised air temperature during the growing season had little effect on total biomass, N content, or growing-season N uptake of the major plant and soil pools. Similarly, vascular ANPP and net ecosystem CO2 exchange did not change with warming, although net primary production of mosses decreased with warming. Such general lack of response supports the hypothesis that productivity in tundra is constrained by the indirect effects of cold temperatures (e.g., low nutrient availability or short growing-season length) rather than by cold growing-season temperatures per se. Despite no effect on net ecosystem CO2 flux, air warming stimulated early-season gross photosynthesis (GP) and ecosystem respiration (ER) througho...

Stream amphibians as indicators of ecosystem stress: a case study from California's redwoods

Abstract: Road construction of the Redwood National Park highway bypass resulted in a large accidental infusion of fine sediments into pristine streams in Prairie Creek State Park, California, during an October 1989 storm event. This incident provided a natural experiment where we could measure, compare, and evaluate native stream amphibian densities as indicators of stream ecosystem stress. We employed a habitat-based, stratified sampling design to assess the impacts of these sediments on the densities of aquatic amphibians in five impacted streams by comparing them with densities in five adjacent, unimpacted (control) streams. Three species were sampled in numbers sufficient to be informative: tailed frogs (Ascaphus truei, larvae), Pacific giant salamanders (Dicamptodon tenebrosus, paedomorphs and larvae), and southern torrent salamanders (Rhyacotriton variegatus, adults and larvae). Densities of amphibians were significantly lower in the streams impacted by sediment. While sediment effects were species specific,...

Polyphenols as regulators of plant-litter-soil interactions in northern California’s pygmy forest: A positive feedback?

Abstract: The convergent evolution of polyphenol-rich plant communities has occurred on highly acidic and infertile soils throughout the world. The pygmy forest in coastal northern California is an example of an ecosystem on an extremely infertile soil that has exceptionally high concentrations of polyphenols. Many ‘negative feedbacks’ have been identified whereby plants degrade fertile soils through production of polyphenol-rich litter, sequestering soil nutrients into unavailable form and creating unfavorable conditions for seed germination, root growth, and nutrient uptake. But in the context of plant-litter-soil interactions in ecosystems adapted to soils that are inherently acidic and infertile (such as the pygmy forest), there are also many ‘positive feedbacks’ that result from polyphenol production. By inhibiting decomposition, polyphenols regulate the formation of a mor-humus litter layer, conserving nutrients and creating a more favorable medium for root growth. Polyphenols shift the dominant pathway of nitrogen cycling from mineral to organic forms to minimize potential N losses from the ecosystem and maximize litter-N recovery by mycorrhizal symbionts. Polyphenol complexation of Al, Mn and Fe reduce potential Al toxicity and P fixation in soil. Polyphenols regulate organic matter dynamics, leading to the accumulation of organic matter with cation exchange capacity to minimize leaching of nutrient cations. Humic substances derived from polyphenolic precursors coat rhizosphere soil surfaces, improving physical and chemical conditions for root growth and nutrient cycling. Although their long-accepted adaptive value for antiherbivore defense is now in doubt, polyphenol alteration of soil conditions and regulation of nutrient cycling illustrate how fitness can be influenced by the ‘extended’ phenotype in plant-litter-soil interactions.

The implications of predicted climate change for insect pests in the UK, with emphasis on non‐indigenous species

Abstract: Recent estimates for global warming predict increases in global mean surface air temperatures (relative to 1990) of between 1 and 3.5 °C, by 2100. The impact of such changes on agricultural systems in mid- to high-latitude regions are predicted to be less severe than in low-latitude regions, and possibly even beneficial, although the influence of pests and diseases is rarely taken into account. Most studies have concluded that insect pests will generally become more abundant as temperatures increase, through a number of inter-related processes, including range extensions and phenological changes, as well as increased rates of population development, growth, migration and over-wintering. A gradual, continuing rise in atmospheric CO2 will affect pest species directly (i.e. the CO2 fertilization effect) and indirectly (via interactions with other environmental variables). However, individual species responses to elevated CO2 vary: consumption rates of insect herbivores generally increase, but this does not necessarily compensate fully for reduced leaf nitrogen. The consequent effects on performance are strongly mediated via the host species. Some recent experiments under elevated CO2 have suggested that aphids may become more serious pests, although other studies have discerned no significant effects on sap-feeding homopterans. However, few, if any of these experiments have fully considered the effects on pest population dynamics. Climate change is also considered from the perspective of changes in the distribution and abundance of species and communities. Marked changes in the distribution of well-documented species – including Odonata, Orthoptera and Lepidoptera – in north-western Europe, in response to unusually hot summers, provide useful indications of the potential effects of climate change. Migrant pests are expected to respond more quickly to climate change than plants, and may be able to colonize newly available crops/habitats. Range expansions, and the removal of edge effects, could result in the increased abundance of species presently near the northern limits of their ranges in the UK. However, barriers to range expansions, or shifts, may include biotic (competition, predation, parasitism and disease), as well as abiotic, factors. Climatic phenomena, ecosystem processes and human activities are interactive and interdependent, making long-term predictions extremely tenuous. Nevertheless, it appears prudent to prepare for the possibility of increases in the diversity and abundance of pest species in the UK, in the context of climate change.