Showing papers on "Ecosystem published in 1996"

Productivity and sustainability influenced by biodiversity in grassland ecosystems

Abstract: THE functioning and sustainability of ecosystems may depend on their biological diversity1–8. Elton's9 hypothesis that more diverse ecosystems are more stable has received much attention1,3,6,7,10–14, but Darwin's proposal6,15 that more diverse plant communities are more productive, and the related conjectures4,5,16,17 that they have lower nutrient losses and more sustainable soils, are less well studied4–6,8,17,18. Here we use a well-replicated field experiment, in which species diversity was directly controlled, to show that ecosystem productivity in 147 grassland plots increased significantly with plant biodiversity. Moreover, the main limiting nutrient, soil mineral nitrogen, was utilized more completely when there was a greater diversity of species, leading to lower leaching loss of nitrogen from these ecosystems. Similarly, in nearby native grassland, plant productivity and soil nitrogen utilization increased with increasing plant species richness. This supports the diversity–productivity and diversity–sustainability hypotheses. Our results demonstrate that the loss of species threatens ecosystem functioning and sustainability.

Long-Term Effects of Acid Rain: Response and Recovery of a Forest Ecosystem

Abstract: Long-term data from the Hubbard Brook Experimental Forest, New Hampshire, suggest that although changes in stream pH have been relatively small, large quantities of calcium and magnesium have been lost from the soil complex and exported by drainage water because of inputs of acid rain and declines in atmospheric deposition of base cations. As a result, the recovery of soil and streamwater chemistry in response to any decreases in acid deposition will be delayed significantly.

Exchange of Carbon Dioxide by a Deciduous Forest: Response to Interannual Climate Variability

Abstract: The annual net uptake of CO2 by a deciduous forest in New England varied from 1.4 to 2.8 metric tons of carbon per hectare between 1991 and 1995. Carbon sequestration was higher than average in 1991 because of increased photosynthesis and in 1995 because of decreased respiration. Interannual shifts in photosynthesis were associated with the timing of leaf expansion and senescence. Shifts in annual respiration were associated with anomalies in soil temperature, deep snow in winter, and drought in summer. If this ecosystem is typical of northern biomes, interannual climate variations on seasonal time scales may modify annual CO2 exchange in the Northern Hemisphere by 1 gigaton of carbon or more each year.

A general protocol for restoration of regulated rivers

Abstract: Large catchment basins may be viewed as ecosystems in which natural and cultural attributes interact. Contemporary river ecology emphasizes the four-dimensional nature of the river continuum and the propensity for riverine biodiversity and bioproduction to be largely controlled by habitat maintenance processes, such as cut and fill alluviation mediated by catchment water yield. Stream regulation reduces annual flow amplitude, increases baseflow variation and changes temperature, mass transport and other important biophysical patterns and attributes. As a result, ecological connectivity between upstream and downstream reaches and between channels, ground waters and floodplains may be severed. Native biodiversity and bioproduction usually are reduced or changed and non-native biota proliferate. Regulated rivers regain normative attributes as distance from the dam increases and in relation to the mode of dam operation. Therefore, dam operations can be used to restructure altered temperature and flow regimes which, coupled with pollution abatement and management of non-native biota, enables natural processes to restore damaged habitats along the river’s course. The expectation is recovery of depressed populations of native species. The protocol requires: restoring peak flows needed to reconnect and periodically reconfigure channel and floodplain habitats; stabilizing baseflows to revitalize food-webs in shallow water habitats; reconstituting seasonal temperature patterns (e.g. by construction of depth selective withdrawal systems on storage dams); maximizing dam passage to allow recovery of fish metapopulation structure; instituting a management belief system that relies upon natural habitat restoration and maintenance, as opposed to artificial propagation, installation of artificial instream structures (river engineering) and predator control; and, practising adaptive ecosystem management. Our restoration protocol should be viewed as an hypothesis derived from the principles of river ecology. Although restoration to aboriginal state is not expected, nor necessarily desired, recovering some large portion of the lost capacity to sustain native biodiversity and bioproduction is possible by management for processes that maintain normative habitat conditions. The cost may be less than expected because the river can do most of the work.

Serengeti II : dynamics, management, and conservation of an ecosystem

Abstract: The aim of this text is to provide an up-to-date understanding of the Serengeti-Mara ecosystem in East Africa, home to one of the largest and most diverse populations of animals in the world. Building on the groundwork laid by "Serengeti: Dynamics of an Ecosystem", published in 1979 by the University of Chicago Press, this work integrates studies of the ecosystem at every level, from the plants at the bottom of the visible food chain to the many species of herbivores and predators, as well as the system as a whole. Drawing on data from long-term studies, the contributors examine the processes that have produced the Serengeti's biological diversity, with its species-species and species-environment interactions. The book also discusses computer modelling as a tool for exploring these interactions, employing it to test and anticipate the effects of social, political and economic changes on the entire ecosystem and on particular species, with the aim of assisting the development of future conservation and management strategies.

Plants in Changing Environments: Linking Physiological, Population, and Community Ecology

Abstract: 1. Introduction and background 2. Plant strategies and successional change: a resource-response perspective 3. Community composition and trends of dominance and diversity in recovering ecosystems 4. The environment of successional plants: disentangling causes and consequences 5. Recruitment in successional habitats: general trends and specific differences 6. How do plants interact with each other? 7. Plant/plant interactions and ecosystem recovery 8. Competition and the evolution of response breadths and niches 9. Ecological and genetic variation in early successional plants 10. Coping with a variable environment: habitat selection, response flexibility: tracking, acclimation, and plasticity 11. Physiological trends of plant in recovering ecosystems 12. Crossing the scales: can we predict community composition from individual species response? 13. From fields to forests: forest dynamics and regeneration in a changing environment 14. Succession and global change: the implications of migration, extinction, and adaptation References Index.

Physiological and Growth Responses of Arctic Plants to a Field Experiment Simulating Climatic Change

Abstract: Field manipulations of light, temperature, nutrients, and length of growing season in directions simulating global environmental change altered biomass of the four most abundant vascular plant species in tussock tundra of northern Alaska. These species are Betula nana, Ledum palustre, Vaccinium vitis-idaea, and Eriophorum vaginatum. Biomass response reflected changes in both growth and mortality, with growth being stimulated by treatments that enhanced biomass,a and mortality being enhanced by all treatments (except in Vaccinium). Those species with highest leaf and stem turnover (the graminoid and deciduous shrub) initially showed large positive responses to nutrient addition. By contrast, slow-turnover evergeen species showed little initial change in production in response to our manipulations, and their long-term biomass responses were in the opposite direction to those of the responsive species. Short-term measurement of leaf expansion, photosynthesis, and phosphate uptake showed little correlation with net production of biomass change in response to manipulations because of compensatory mechanisms at levels of growth and allocation. Changes in nutrient distribution among species accounted for many of the long-term changes in biomass and productivity. Processes that are readily integrated at annual time steps (e.g., shoot growth, shoot mortality, allocation) were more useful than instantaneous physiological measurements in predicting decadalmore » vegetation changes because (1) compensating responses among physiological processes buffer plant responses at progressively longer time scales, (2) species interactions in the community buffer ecosystem processes such as productivity and nutrient cycling from changes in growth of individual species, and (3) different time lags between physiological, demographic, and ecosystem processes complicate modeling of long-term responses from short-term mechanism. 76 refs., 11 figs., 5 tabs.« less

Dams and downstream aquatic biodiversity: Potential food web consequences of hydrologic and geomorphic change

Abstract: / Responses of rivers and river ecosystems to dams are complex and varied, as they depend on local sediment supplies, geomorphic constraints, climate, dam structure and operation, and key attributes of the biota. Therefore, "one-size-fits-all" prescriptions cannot substitute for local knowledge in developing prescriptions for dam structure and operation to protect local biodiversity. One general principle is self-evident: that biodiversity is best protected in rivers where physical regimes are the most natural. A sufficiently natural regime of flow variation is particularly crucial for river biota and food webs. We review our research and that of others to illustrate the ecological importance of alternating periods of low and high flow, of periodic bed scour, and of floodplain inundation and dewatering. These fluctuations regulate both the life cycles of river biota and species interactions in the food webs that sustain them. Even if the focus of biodiversity conservation efforts is on a target species rather than whole ecosystems, a food web perspective is necessary, because populations of any species depend critically on how their resources, prey, and potential predators also respond to environmental change. In regulated rivers, managers must determine how the frequency, magnitude, and timing of hydrologic events interact to constrain or support species and food webs. Simple ecological modeling, tailored to local systems, may provide a framework and some insight into explaining ecosystem response to dams and should give direction to mitigation efforts.KEY WORDS: Dams; Food webs; Hydrologic disturbance; Predator-prey dynamics; Succession

A Chemoautotrophically Based Cave Ecosystem

Abstract: Microbial mats discovered in a ground-water ecosystem in southern Romania contain chemoautotrophic bacteria that fix inorganic carbon, using hydrogen sulfide as an energy source. Analysis of stable carbon and nitrogen isotopes showed that this chemoautotrophic production is the food base for 48 species of cave-adapted terrestrial and aquatic invertebrates, 33 of which are endemic to this ecosystem. This is the only cave ecosystem known to be supported by in situ autotrophic production, and it contains the only terrestrial community known to be chemoautotrophically based.

Nitrogen Mineralization in Temperate Agricultural Soils: Processes and Measurement

Abstract: Publisher Summary Soils form a major repository of nitrogen (N) within both natural and agricultural terrestrial ecosystems, containing, on a global basis, an estimated 2.4 x 10 11 tons of N. The soil receives N inputs through fertilizer additions and from the atmosphere in precipitation and dry deposition or via biological fixation; inputs are also made in plant and animal residues. N is removed in the harvested crop and is lost by leaching and surface run-off of soluble forms, by gaseous transfer as N gas and N oxides (during nitrification and denitrification processes), and by ammonia volatilization. In some circumstances, erosion may also be important. In addition to these interactions with the total ecosystem, internal cycles also operate within the soil, so that even if gains and losses are in balance, then N still continues to cycle in the soil. This chapter describes the current understanding of the conceptual basis of the processes involved in mineralization, relationships among the processes and other factors, and how their effects can be determined practically. The aim is to present this in a way that is relevant to current and future agricultural development and to environmental issues.

Process modeling of controls on nitrogen trace gas emissions from soils worldwide

Abstract: We report on an ecosystem modeling approach that integrates global satellite, climate, vegetation, and soil data sets to (1) examine conceptual controls on nitrogen trace gas (NO, N2O, and N2) emissions from soils and (2) identify weaknesses in our bases of knowledge and data for these fluxes. Nitrous and nitric oxide emissions from well-drained soils were estimated by using an expanded version of the Carnegie-Ames-Stanford (CASA) Biosphere model, a coupled ecosystem production and soil carbon-nitrogen model on a 1° global grid. We estimate monthly production of NO, N2O, and N2 based on predicted rates of gross N mineralization, together with an index of transient water-filled pore space in soils. Analyses of model performance along selected climate gradients support the hypothesis that low temperature restricts predicted N mineralization and trace gas emission rates in moist northern temperate and boreal forest ecosystems, whereas in tropical zones, seasonal patterns in N mineralization result in emission peaks for N2O that coincide with wetting and high soil moisture content. The model predicts the annual N2O:NO flux ratio at a mean value of 1.2 in wet tropical forests, decreasing to around 0.6 in the seasonally dry savannas. Global emission estimates at the soil surface are 6.1 Tg N and 9.7 Tg N yr−1 for N2O and NO, respectively. Tropical dry forests and savannas are identified by using this formulation as important source areas for nitrogen trace gas emissions. Because humans continue to alter these ecosystems extensively for agricultural uses, our results suggest that more study is needed in seasonally dry ecosystems of the tropics in order to understand the global impacts of land use change on soil sources for N2O and NO.

Theoretical Ecosystem Ecology: Understanding Element Cycles

Abstract: Preface List of important symbols Part I. Prelude: 1. Introduction 2. Element cycling 3. Part II. The Soil: 3. Theory for homogeneous substrates 4. Theory for heterogeneous substrates 5. Carbon and nitrogen - applications 6. Carbon, nitrogen, phosphorus and sulphur - applications 7. Interactions with abiotic factors Part III. The Plant: 8. Theory for plant growth 9. Plant growth - applications and extensions Part IV. The Ecosystem: 10. Elements of an ecosystem theory 11. Ecosystems - applications 12. Quality - bridge between plant and soil Epilogue Appendices References Solutions to selected problems Index.

Atmosphere-wetland carbon exchanges: Scale dependency of CO2 and CH4 exchange on the developmental topography of a peatland

Abstract: Measurement of the spatial variability of CH4 emissions and net CO2 ecosystem exchange were made in a boreal peatland in northern Sweden in the summers of 1992 and 1993. Variability was monitored at the microscale (hummocks and hollows), mesoscale (ridges, lawns and pools), and macroscale (landforms) to assess the role of peatland topography on the magnitude and variability of the fluxes. The general trend is for topographically lower areas, such as hollows, pools, or peatland margins, to have higher CH4 emissions and lower CO2 uptake than the adjacent topographically higher areas such as hummocks, ridges, and plateaus. However, the greatest difference occurs at the microtopographic scale because the maximum differences in water table position and temperature occur at the microscale. The CH4 flux at the margins of the peatland was three to four times greater than that observed at the central plateau sites. Net CO2 uptake was also greatest at the margin sites. A combined meso-macro topographic model (MMF) was used to estimate the total peatland CH4 and CO2 exchange. The model indicates that a failure to measure the exchange of carbon from peatland pools can result in a large overestimate of the total peatland NEE and therefore carbon accumulation rates.

Contributions of fine root production and turnover to the carbon and nitrogen cycling in taiga forests of the Alaskan interior

Abstract: Fine root production and turnover were studied in hardwood and coniferous taiga forests using three methods. (1) Using soil cores, fine root production ranged from 1574 ± 76 kg•ha−1•year−1 in the u...

Principles of ecosystem sustainability

Abstract: Many natural ecosystems are self-sustaining, maintaining a characteristic mosaic of vegetation types for hundreds to thousands of years. In this article we present a new framework for defining the conditions that sustain natural ecosystems and apply these principles to sus- tainability of managed ecosystems. A sustainable ecosystem is one that, over the normal cycle of disturbance events, maintains its characteristic diversity of major functional groups, produc- tivity, and rates of biogeochemical cycling. These traits are determined by a set of four "interac- tive controls" (climate, soil resource supply, major functional groups of organisms, and distur- bance regime) that both govern and respond to ecosystem processes. Ecosystems cannot be sustained unless the interactive controls oscillate within stable bounds. This occurs when nega- tive feedbacks constrain changes in these controls. For example, negative feedbacks associated with food availability and predation often constrain changes in the population size of a species. Linkages among ecosystems in a landscape can contribute to sustainability by creating or ex- tending the feedback network beyond a single patch. The sustainability of managed systems can be increased by maintaining interactive controls so that they form negative feedbacks within ecosystems and by using laws and regulations to create negative feedbacks between ecosystems and human activities, such as between ocean ecosystems and marine fisheries. Degraded ecosys- tems can be restored through practices that enhance positive feedbacks to bring the ecosystem to a state where the interactive controls are commensurate with desired ecosystem characteris- tics. The possible combinations of interactive controls that govern ecosystem traits are limited by the environment, constraining the extent to which ecosystems can be managed sustainably for human purposes.

Linking planktonic diatoms and climate change in the large lakes of the Yellowstone ecosystem using resource theory

Abstract: Resource-based physiology of the eight important planktonic diatom species in the large lakes of the Yellowstone region can be used to explain their relative abundances and seasonal changes. The diatoms are ranked along resource ratio gradients according to their relative abilities to grow under limitation by Si, N, P, and light. Hypotheses based on resource physiology can be integrated with observations on seasonal changes in diatom assemblages to explain the present distributions of diatoms and to test the causal factors proposed to explain diatom distributions over the Holocene. Knowledge of the limnology of these lakes and process-oriented physiology provide the basis for a more detailed interpretation of the paleorecord and a firmer basis for landscape-level transfer functions for fine-scale climate reconstruction.

Climate and nitrogen controls on the geography and timescales of terrestrial biogeochemical cycling

Abstract: We used the terrestrial ecosystem model “Century” to evaluate the relative roles of water and nitrogen limitation of net primary productivity, spatially and in response to climate variability. Within ecology, there has been considerable confusion and controversy over the large-scale significance of limitation of net primary production (NPP) by nutrients versus biophysical quantities (e.g., heat, water, and sunlight) with considerable evidence supporting both views. The Century model, run to a quasi-steady state condition, predicts “equilibration” of water with nutrient limitation, because carbon fixation and nitrogen fluxes (inputs and losses) are controlled by water fluxes, and the capture of nitrogen into organic matter is governed by carbon fixation. Patterns in the coupled water, nitrogen, and carbon cycles are modified substantially by ecosystem type or species-specific controls over resource use efficiency (water and nitrogen used per unit NPP), detrital chemistry, and soil water holding capacity. We also examined the coupling between water and nutrients during several temperature perturbation experiments. Model experiments forced by satellite-observed temperatures suggest that climate anomalies can result in significant changes to terrestrial carbon dynamics. The cooling associated with the Mount Pinatubo eruption aerosol injection may have transiently increased terrestrial carbon storage. However, because processes in the water, carbon, and nitrogen cycles have different response times, model behavior during the return to steady state following perturbation was complex and extended for decades after 1- to 5-year perturbations. Thus consequences of climate anomalies are influenced by the climatic conditions of the preceding years, and climate-carbon correlations may not be simple to interpret.

A pelagic ecosystem model calibrated with BATS data

Abstract: Mechanistic models of ocean ecosystem dynamics are of fundamental importance to understanding and predicting the role of marine ecosystems in the oceanic uptake of carbon. In this paper, a new pelagic ecosystem model that is descended from the model of Fasham et al. (Journal of Marine Research, 99 (1990) 591–639) (FDM model) is presented. During model development, the FDM model was first simplified to reduce the number of variables unconstrained by data and to reduce the number of parameters to be estimated. Many alternative simplified model formulations were tested in an attempt to fit 1988–1991 Bermuda Atlantic Time-series Study (BATS) data. The model presented here incorporates the changes found to be important. (i) A feature of the FDM physics that gives rise to a troublesome fall bloom was replaced. (ii) A biodiversity effect was added: the addition of larger algal and detrital size classes as phytoplankton and detrital biomasses increase. (iii) A phytoplankton physiological effect was also added: the adjustment of the chlorophyll-to-nitrogen ratio by phytoplankton in response to light and nutrient availabilities. The new model has only four state variables and a total of 11 biological parameters; yet it fits the average annual cycle in BATS data better than the FDM model. The new model also responds reasonably well to interannual variability in physical forcing. Based on the justification for changes (i)--(iii) from empirical studies and the success of this simple model at fitting BATS data, it is argued that these changes may be generally important. It is also shown that two alternative assumptions about ammonium concentrations lead to very different model calibrations, emphasizing the need for time series data on ammonium.

Is Total Primary Production in Shallow Coastal Marine Waters Stimulated by Nitrogen Loading

Abstract: The global nitrogen cycle has been extensively modified by human activity to the extent that more N is fixed annually by human-driven than by natural processes (Vitousek 1994). This alteration influences production and species composition of terrestrial ecosystems (Tilman 1987) and contributes to acidification and forest dieback (Schulze 1989). The influence of nitrogen on eutrophication of coastal marine ecosystems is even stronger with profound effects on plant communities and food webs (Nixon et al. 1986). The total primary production of marine plants in coastal areas (as organic carbon or dry matter produced annually per unit of surface area within the ecosystem) is generally assumed to increase with increasing loading of nutrients from land (e.g. Boynton et al. 1982, Nixon et al. 1986, Paerl 1993). The effects of widespread eutrophication, such as oxygen deficiency and mass mortality of benthic invertebrates and fish, have alarmed the public and are considered to emerge from the enhanced oxygen consumption required to mineralize the increasing amounts of organic matter produced within the ecosystem. Much of our current analysis and understanding of energy flow and carbon and oxygen dynamics in the coastal marine environment presuppose that anthropogenic loading and total primary production are directly positively related. In the following we will argue, however, that the total primary production of shallow coastal areas, i.e. the combined production of microand macroscopic plants living in the water and at the bottom, does not change systematically by nutrient enrichment. Our analysis is based on a comparison of published rates of total primary production and nitrogen loading from land using the same approach as Boynton et al. (1982) and Nixon et al. (1986) to analyse the dependence of phytoplankton production on nitrogen loading. The data originate from different temperate coastal ecosystems with very different morphometry and hydraulics. The quality of the data (in terms of suitability of methods and number of measurements) also varies considerably among the systems but does not systematically influence the patterns reported. We have tried to ensure a fair representation of systems which are most likely to respond to enhanced nitrogen loading by including relatively deep (mean depth down to 40 m) and clearly phytoplankton dominated ecosystems and by omitting data we suspect overestimate benthic primary production. Thereby, we obtain a balanced analysis of our hypothesis.

Global change: effects on coniferous forests and grasslands.

Abstract: The Carbon Budget of Boreal Forests: Reducing the Uncertainty Carbon Budget: Temperate Coniferous Forests The Carbon Budget of Tropical Savannahs, Woodlands and Grasslands Carbon Budgets of Temperate Grasslands and the Effects of Global Change Photosynthesis, Rising Atmosphere CO2 Concentration and Climate Change Rising Atmospheric CO2 and Plant Respiration Carbon and Nutrient Allocation in Terrestrial Ecosystems Elevated CO2, Litter Quality and Decomposition Ecosystem Physiology - Soil Organic Matter Global Grassland Ecosystem Modelling: Development and Test of Ecosystem Models for Grassland Systems Impact of Climate and Atmospheric CO2 Changes on Grasslands of the World Comparing Models of Ecosystem Function for Temperate Coniferous Forests I - Model Description and Validation Comparing Models of Ecosystem Function for Temperate Conifer Forests II - Simulations of the Effects of Climate Change Global Climate Change and Carbon Cycling in Grasslands and Conifer Forests Prediction of Global Biome Distribution Using Bioclimatic Equilibrium Models

Ecosystem responses to changes in plant functional type composition: An example from the Patagonian steppe

Abstract: . Grass cover along a grazing intensity gradient in Patagonia decreases, whereas bare soil and shrub cover increases. Our objective was to study the effect of a change in the dominant plant functional type on soil water balance, primary production, herbivore biomass, roughness, and albedo. Using a soil water balance model, we found increases in evaporation and deep drainage, and a decrease in total transpiration along the grazing intensity gradient. Above-ground primary production, estimated from transpiration, decreased along the grazing intensity gradient because shrubs did not fully compensate for the decrease in grass production. Using a statistical model, we calculated herbivore biomass from estimates of above-ground primary production. Estimated herbivore biomass was lowest in the shrub-dominated extreme of the grazing gradient. Roughness increased from the grass-dominated to the shrub-dominated community. Albedo had a maximum at an intermediate position along the gradient. Our results suggest that changes in plant functional type composition, independent of changes in biomass, affect ecosystem functioning and the exchange of energy and material with the atmosphere. Grasses and shrubs proved to be appropriate plant functional types to link structure and function of ecosystems.

Biodiversity and ecosystem processes in tropical forests

Abstract: 1 Introduction.- References.- 2 Plant Species Diversity and Ecosystem Functioning in Tropical Forests.- 2.1 Introduction.- 2.2 The Dependence of Ecosystem Processes on Species Diversity.- 2.3 Plant Species Richness in Tropical Forests.- 2.4 The Primary Productivity of Tropical Forests.- 2.5 The Stability of Tropical Forests.- 2.6 Conclusions.- References.- 3 Consumer Diversity and Secondary Production.- 3.1 Introduction.- 3.2 Secondary Production and Biodiversity.- 3.3 Evolutionary Effects of Consumers on Ecosystem Properties.- 3.4 Ecological Effects of Consumers on Ecosystem Properties.- 3.4.1 Influence of Consumers on Plant Productivity.- 3.4.2 Influences of Consumers on Plant Diversity.- 3.5 Conclusion.- References.- 4 Biodiversity and Biogeochemical Cycles.- 4.1 Introduction.- 4.1.1 Definitions and Concepts.- 4.2 Species Richness and Biogeochemical Cycling.- 4.3 Functional Diversity and Biogeochemistry.- 4.3.1 The Atmospheric-Terrestrial Interface.- 4.3.2 The Biotic Interface.- 4.3.3 The Plant-Soil Interface.- 4.3.4 The Terrestrial-Hydrologic Interface.- 4.4 Evidence from Experimental Studies.- 4.4.1 Plantations versus Natural Forests.- 4.4.2 Experimental Manipulation of Species Composition.- 4.5 Conclusions.- References.- 5 Microbial Diversity and Tropical Forest Functioning.- 5.1 Introduction.- 5.2 The Knowledge Base.- 5.3 Food Chains.- 5.4 Pathogens.- 5.4.1 Control of Herbivores by Pathogens.- 5.4.2 Pathogens as a Source of Distribution.- 5.4.3 Effect of Pathogens on Patterns of Tree Dispersion.- 5.5 Microbial Contributions to Global Biogeochemistry.- 5.5.1 Atmospheric CO2.- 5.5.2 Methane.- 5.5.3 Nitrous Oxide.- 5.5.4 Rock Weathering.- 5. 6 Nutrient Cycling.- 5.6.1 Litter Decomposition and Soil Fertility.- 5.6.2 Symbiotic Nitrogen Fixation Associated with Plant Roots.- 5.6.3 Effects of Microbial Epiphylls and Epiphytes on Nutrient Fluxes.- 5.6.4 Mycorrhizae and Nutrient Uptake.- 5.7 Plant Endophytes.- 5.8 Threats to the Microbiota and the Processes They Mediate.- 5.8.1 Effects of Forest Fragmentation on Plant Symbioses.- 5.8.2 Effects of Forest Fragmentation on Cord-Forming Fungi.- 5.8.3 Effects of Acid Precipitation on Ectomycorrhizae.- 5.8.4 Effects of Air Pollution and Climate Change on Epiphyte Nitrogen Fixation.- 5.8.5 Are Decomposers Redundant in a Heterogeneous Environment?.- 5.9 Conclusions.- References.- 6 Plant Life-Forms and Tropical Ecosystem Functioning.- 6.1 Introduction.- 6.1.1 Functional Significance of Life-Forms.- 6.1.2 Assessing the Consequences of Life-Form Diversity.- 6.2 Classification.- 6.2.1 Stature.- 6.2.2 Longevity.- 6.3 Biogeographical Patterns.- 6.4 Environmental Correlates of Life-Form Diversity.- 6.4.1 Rainfall.- 6.4.2 Altitude.- 6.4.3 Soil Fertility.- 6.5 Episodic Impacts on Life-form Diversity.- 6.5.1 Wind.- 6.5.2 Fire.- 6.5.3 Animals.- 6.5.4 Climate change.- 6.6 Life-Forms and Succession.- 6. 7 Implications of Loss of Life-Forms.- 6.8 Conclusions.- References.- 7 Functional Group Diversity and Recovery from Disturbance.- 7.1 Introduction.- 7.2 Functional Groups Affecting Tropical Forest Dynamics.- 7.2.1 Pioneer Herbs and Shrubs.- 7.2.2 Large-Leaved Understory Herbs and Shrubs.- 7.2.3 Small-Leaved Understory Herbs and Shrubs.- 7.2.4 Pioneer Trees.- 7.2.5 Understory Treelets.- 7.2.6 Emergent and Canopy Trees.- 7.2.7 Canopy Palms.- 7.2.8 Canopy Legumes.- 7.2.9 Vines and Lianas.- 7.2.10 Epiphytes.- 7.2.11 Seed Dispersers.- 7.2.12 Herbivorous Insects and Pathogens.- 7.2.13 Decomposers.- 7.2.14 Mycorrhizal Fungi.- 7.2.15 Soil-Churning Animals.- 7.3 Functional Groups and Natural Disturbance Processes in Tropical Moist Forests.- 7.4 Anthropogenic Disturbances to Tropical Forests.- 7.4.1 Functional Groups Affect Successional Patterns.- 7.4.2 Causes of Depauperate Regeneration Pools.- 7.5 Functional Groups in Tropical Dry Forests.- 7.6 Redundancy within Functional Groups.- 7.7 Conclusions.- References.- 8 Species Richness and Resistance to Invasion.- 8.1 Diversity vs. Stability.- 8.2 Global Patterns.- 8.3 Intentional Introductions.- 8.4 Invasions into Undisturbed Tropical Forests.- 8.5 Speculations.- References.- 9 The Role of Biodiversity in Tropical Managed Ecosystems.- 9.1 Introduction.- 9.2 Examples of Tropical Managed Ecosystems.- 9.2.1 Managed Forests.- 9.2.2 Home Gardens.- 9.2.3 Swidden Agriculture.- 9.2.4 Intensive Annual and Perennial Crops.- 9.2.5 Traditional Rice Systems.- 9.3 Plant Diversity and Primary Productivity.- 9.3.1 Comparisons Between Natural and Managed Ecosystems.- 9.3.2 Productivity of Diverse Cropping Systems.- 9.3.3 Stability of Diverse Cropping Systems.- 9.4 Plant Diversity and Primary Consumers.- 9.5 Plant Diversity and Secondary Consumers.- 9.5.1 Ants in Diverse Cropping Systems.- 9.6 Conclusions.- References.- 10 Synthesis.- 10.1 Introduction.- 10.2 Environmental Gradients.- 10.2.1 Moisture.- 10.2.2 Fertility.- 10.2.3 Elevation.- 10.3 Biodiversity and Functioning of Tropical Forests.- 10.4 Energy Flow.- 10.4.1 Carbon Allocation and Consumption.- 10.4.2 Animal-Animal Interactions.- 10.4.3 Detritus-Detritivores.- 10.5 Materials Processing.- 10.5.1 Atmosphere-Organism.- 10.5.2 Biotic Interface.- 10.5.3 Plant-Soil.- 10.5.4 Atmosphere-Soil.- 10.5.5 Soil-Water Table.- 10.6 Functional Properties over Longer Temporal Scales.- 10.6.1 Provision and Maintenance of Structure.- 10.6.2 Resistance to Invasions.- 10.7 Functional Properties Over Larger Spatial Scales.- 10.7.1 Movement of Materials by Physical Agents.- 10.7.2 Movement of Materials and Energy by Animals.- 10.8 Biodiversity and Responses to Disturbances.- 10.9 Research Agenda.- 10.10 Conclusions.- References.- Species Index.- Topical Index.

Metabolism and organic carbon fluxes in the tidal freshwater Hudson River

Abstract: We summarize rates of metabolism and major sources and sinks of organic carbon in the 148-k long, tidally influenced, freshwater Hudson River. The river is strongly heterotrophic, with respiration exceeding gross primary production (GPP). The P:R ration averages 0.57 (defined as the ratio of GPP to total ecosystem respiration) if only the aquatic portion of the ecosystem is considered and 0.70 if the emergent marshes are also included. Gross primary production (GPP) by photoplankton averages approximately 300 g C m−2 yr−1 and is an order of magnitude greater than that by submersed macrophytes. However, the river is deep, well mixed, and turbid, and phytoplankton spend a majority of their time in the dark. As a result, respiration by living phytoplankton is extremely high and net primary production (NPP) by phytoplankton is estimated to be only some 6% of GPP. NPP by phytoplankton and submersed macrophytes are roughly equal (approximately 20 g C m−2 yr−1 each) when averaged over the river. Emergent marshes are quite productive, but probably less than 16 g C m−2 yr−1 enters the aquatic portion of the ecosystem from these marshes. Heterotrophic respiration and secondary production in the river are driven primarily by allochthonous inputs of organic matter from terrestrial sources. Rates of metabolism vary along the river, with depth being a critical controlling factor. The P:R ratio for the aquatic portion of the ecosystem varies from 1 in the mid-river to 0.2 in the deeper waters. NPP is actually negative in the downstream waters where average depths are greater since phytoplankton respiration exceeds GPP there; the positive rates of NPP occurring upriver support a downstream advection of phytoplankton to the deeper waters where this C is largely respired away by the algae themselves. This autotrophic respiration contributes significantly to oxygen depletion in the deeper waters of the Hudson. The tidally influenced freshwater Hudson largely fits the patterns predicted by the river continuum model for larger rivers. However, we suggest that the continuum model needs to more clearly distinguish between GPP and NPP and should include the importance of autotrophic respiration by phytoplankton that are advected along a river. The organic carbon budget for the tidally influenced freshwater Hudson is balanced to within a few percent. Respiration (54%) and downstream advection into the saline estuary (41%) are the major losses of organic carbon from the ecosystem. Allochthonous inputs from nonpoint sources on land (61%) and GPP by phytoplankton (28%) are the major sources to the system. Agricultural erosion is the major source of allochthonous inputs. Since agricultural land use increased dramatically in the last century, and has fallen in this century, the carbon cycle of the tidally influenced freshwater Hudson River has probably changed markedly over time. Before human disturbance, the Hudson was probably a less heterotrophic system and may even have been autotrophic, with gross primary production exceeding ecosystem respiration.

Model of Transient Changes in Arctic and Boreal Vegetation in Response to Climate and Land Use Change

Abstract: One of the greatest challenges in global-change research is to predict the future distribution of vegetation. Most models of vegetation change predict either the response of a patch of present vegetation to climatic change or the future equilibrium distribution of vegetation based on the present relationship between climate and vegetation. Here we present a model that is, to our knowledge, the first model of ecosystem change in response to transient changes in climate, disturbance regime, and recruitment over the next 50-500 yr. The frame-based model uses quantitative and qualitative variables to de- velop scenarios of vegetation change from arctic tundra to boreal forest in response to global changes in climate (as predicted by general circulation models (GCMs)), fire, and land use. Seed availability, tree growth rate, and probability of fire were the model param- eters that most strongly influenced the balance between tundra and boreal forest in tran- sitional climates. The rate of climatic warming strongly affected the time lag between the onset of climate change and the simulated ecosystem response but had relatively little effect on the rate or pattern of ecosystem change. The model calculated that, with a gradual ramped change of 3?C in the next century (corresponding to average rate of warming predicted by GCMs), any change from tundra to forest would take 150 yr, consistent with pollen records. The model suggested that tundra would first be invaded by conifer forests, but that the proportion of broad-leaved deciduous forest would increase, reflecting increased fire frequency, as climatic warming continued. The change in fire frequency was determined more strongly by climatically driven changes in vegetation than by direct climatic effects on fire probability. The pattern of climatic warming was more important than the rate of warming or change in precipitation in determining the rate of conversion from tundra to forest. Increased climatic variability promoted ecosystem change, particularly when oscil- lations were long relative to the time required for tree maturation. Management policies related to logging and moose-predator control affected vegetation as much or more than did changes in climate and must be included in future scenarios of global changes in ecosystem distribution. We suggest that frame-based models provide a critical link between patch and equilibrium models in predicting ecosystem change in response to transient changes in climate over the coming decades to centuries.