Showing papers on "Ecosystem published in 1991"

An Ecosystem Perspective of Riparian Zones

Abstract: R iparian zones are the interfaces between terrestrial and aquatic ecosystems. As ecotones, they encompass sharp gradients of environmental factors, ecological processes, and plant communities. Riparian zones are not easily delineated but are comprised of mosaics of landforms, communities, and environments within the larger landscape. We propose a conceptual model of riparian tones that integrates the physical processes that shape valleyfloor landscapes, the succession of terrestrial plant communities on these geomorphic surfaces, the formation of habitat, and the production of nutritional resources for aquatic ecosys-

Biogeochemical Diversity Along a Riverside Toposequence in Arctic Alaska

Abstract: Nitrogen and phosphorus pool sizes, distribution, and cycling rates were described and compared for six different ecosystem types occurring along a single topose- quence in northern Alaska. The toposequence was located on a series of old floodplains of the Sagavanirktok River, in the northern foothills of the Brooks Range. From tussock tundra in the uplands, the toposequence passed through a relatively dry hilltop heath zone, a hillslope shrub/lupine/Cassiope zone, a footslope Equisetum zone, a wet sedge tundra, and a riparian shrub zone. A late-melting snowbank covered the hillslope site in early June of each year, and the sites consistently varied in soil temperature, soil moisture, thaw depth, and the seasonal pattern of soil thaw. The standing stocks of N, P, and C in soils of these six ecosystem types varied dra- matically but not monotonically along the toposequence, as did the turnover rates of these elements. Several measures were used in comparisons of N and P availability, including soil solution concentrations, in situ accumulation on ion-exchange resins, and levels of KCl-extractable N and P. Annual rates of net N mineralization were assayed using a buried bag method, and ecosystem respiration was measured by trapping CO2 in soda lime (NaOH + Ca (OH)2). Soil P pools were characterized by sequential extraction methods into four major pools, including loosely bound P, Al- and Fe-bound P, primary mineral P, and organic P. Both N and P availability were low in all six ecosystems when compared with temperate forests or wetlands. Among ecosystems, however, there was considerable variation in the relative availability of N vs. P, and in the apparent relative importance of nitrate as a nitrogen source.

Nitrogen in Terrestrial Ecosystems: Questions of Productivity, Vegetational Changes, and Ecosystem Stability

Abstract: Nitrogen is a key element in ecosystem processes. Aspects of local and global changes in nitrogen in both undisturbed and disturbed conditions are discussed. Environmental changes caused by pollution from nitrogenous compounds and changes in landuse are also described. Organisms, plants, animals and microorganisms are all affecting nitrogen supply. Emphasis is placed on natural and anthropogenic transfer of nitrogen between ecosystems and also on the interaction of nitrogen with other bioelements.

Predicting ecosystem responses to elevated co2 concentrations

Abstract: One of the many changes occurring in the biosphere due to human activities is the increase in the carbon dioxide concentration in the atmosphere. This change is due both to the burning of fossil fuels and to deforestation. We do not know how these changes are affecting terrestrial ecosystems. This ignorance is partly because we have relatively poor records of the functional and structural response of any ecosystem through time. More studies are required to be able to accurately assess the effects of carbon dioxide.

A general biogeochemical model describing the responses of the C and N cycles in terrestrial ecosystems to changes in CO2, climate, and N deposition

Abstract: A model that simulates carbon (C) and nitrogen (N) cycles in terrestrial ecosystems is developed. The model is based on the principle that the responses of terrestrial ecosystems to changes in CO(2), climate, and N deposition will encompass enzymatic responses, shifts in tissue stoichiometry, changes in biomass allocation among plant tissues, altered rates of soil organic matter turnover and N mineralization, and ultimately a redistribution of C and N between vegetation and soils. The model is a highly aggregated, process-based, biogeochemical model designed to examine changes in the fluxes and allocation of C and N among foliage, fine roots, stems, and soils in response to changes in atmospheric CO(2) concentration, temperature, soil water, irradiance, and inorganic nitrogen inputs. We use the model to explore how changes in CO(2) concentration, temperature, and N inputs affect carbon storage in two ecosystems: arctic tundra and temperate hardwood forest. The qualitative responses of the two ecosystems were similar. Quantitative differences are attributed to the initial distribution of C and N between vegetation and soils, to the amounts of woody tissue in the two ecosystems, and to their relative degree of N limitation. We conclude with a critical analysis of the model's strengths and weaknesses, and discuss possible future directions.

Effects of Enhanced Solar Ultraviolet Radiation on Aquatic Ecosystems

Abstract: The photosynthetic production of organic biomass using solar energy is the almost exclusive source of energy for life on our planet. The amount of carbon in the form of its dioxide incorporated annually into organic molecules exceeds 100 gigatons which can be visualized by the load filling 10 coal trains spanning the distance from the earth to the moon (Hader et al., 1989). However, only about one third of this enormous production is accounted for by terrestrial plants — forests, savannas, crop plants etc. — while the majority is produced by the phytoplankton organisms (primary producers) in aquatic habitats, especially in the world oceans. The marine phytoplankton communities represent by far the largest ecosystem on earth (Schneider, 1989); therefore even a small percentage decrease in the populations would result in enormous losses in the biomass productivity of these organisms, which could have dramatic effects both for the intricate ecosystem itself and for humans, who depend on this system in many ways (Hader et al., 1989).

Ecological dynamics in the subarctic Pacific, a possibly iron-limited ecosystem

Abstract: It has been suggested that production in offshore waters of the subarctic Pacific is limited by availability of dissolved Fe. Although that is not yet adequately established, the functional consequences of the limitation (if it exists) can be characterized from the results of the Subarctic Pacific Ecosystem Research (SUPER) program. Fe limitation, or something like it, establishes a phytoplankton community dominated by very small cells. These plants are not limited by Fe availability. Rather, their production is limited by their stock and available illumination. Stock is set by microzooplankton grazers with rapid population growth rates and, thus, rapid response to increases in phytoplankton abundance. Micrograzers provide efficient recycling of nitrogen as NH,, and the ready availability of NH, sharply limits the annual utilization of NO,. Persistently high NO, concentrations result. Other possibly Fe-limited, oceanic ecosystems with persistently high, near-surface nutrients require similar, detailed analysis of ecosystem function.

Nutrient dynamics, bacterial populations, and mosquito productivity in tree hole ecosystems and microcosms'

Abstract: Water-filled treeholes provide an experimental setting for examining pro- cesses within an ecosystem, and influences of external factors on those processes. Using a limnological, experimental approach involving both natural tree holes and laboratory mi- crocosms of the tree hole ecosystem, we identified and studied interacting, biotic processes, including dynamics of bacterial populations and variation in concentration of inorganic nutrients in tree hole water, and density-dependent competition for food among larvae of the mosquito Aedes triseriatus. We characterized the influence of external factors (inputs of leaf detritus and stemflow) on those processes. Analyses of water samples over time showed that tree hole water was rich and dynamic in nutrients (nitrite, nitrate, ammonium, phosphate, and sulfate); ammonium was the dominant form ofinorganic nitrogen. Variation in nutrient concentrations in microcosms depended upon exogenous inputs (leaf detritus and stemflow water), dilution of nutrients by stemflow, nutrient cycling processes (nitri- fication, denitrification, and sulfate reduction), and ammonium excretion by mosquito larvae. The densities of bacteria in tree hole water, obtained using direct counts of DAPI- fluorochrome stained samples and epifluorescence microscopy, ranged from 2.0 x 106 to 6.0 x 107 cells/mL, and in microcosms from 4.6 x 105 to 2.6 x 108 cells/mL. Experi- mentation involving microcosms revealed that bacterial abundance was reduced by mos- quito feeding and stemflow flushing. Further experiments showed that stemflow flushing increased mosquito productivity from microcosms several-fold and released mosquitoes from density-dependent competition. This effect was likely related to nutrient input and the simultaneous removal of toxic metabolites owing to inputs of stemflow water. We conclude that disturbance by a physical factor, stemflow, has a major influence on the interactions of nutrient dynamics, bacterial populations, and mosquito productivity in temperate tree-hole ecosystems.

Factors controlling nitrogen cycling and nitrogen saturation in northern temperate forest ecosystems

Abstract: An analysis of the factors controlling rates of nitrogen cycling in northern temperate forest ecosystems is presented based on a quantitative analysis of an extensive data set for forests in Wisconsin and Massachusetts as those data are synthesized in a computer model (VEGIE) of organic matter and nutrient dynamics. The model is of the "lumped-parameter," nutrient-flux-density type, dealing with major components of forest ecosystems rather than stems or species. It deals explicitly with the interactions among light, water, and nutrient availability in determining transient and equilibrium rates of primary production and nutrient cycling. Data are presented for parameterizing the plant component of the system at either the species or community level. A major conclusion is that the ultimate control on equilibrium nitrogen-cycling rates resides not within the nitrogen cycle itself (for example in litter quality or net primary production [NPP] allocation patterns) but rather in ratios of resource-use efficiency by vegetation as compared with the ratios of resource availability. Litter quality and allocation patterns, along with rates of N deposition, do affect the rate at which a system approaches the equilibrium cycling rate. The model is used to explain observed variation in nitrogen-cycling rates among forest types, and to predict the timing and occurrence of "nitrogen saturation" (N availability in excess of biotic demand) as a function of nitrogen deposition rates and harvesting.

Carbon budgets of temperate forest ecosystems.

Abstract: A summary of carbon-budget data from 30 forest stands is presented together with information on both above- and belowground ecosystem components. Only 10 of the stands had complete biomass data, 21 had complete productivity data and six had heterotrophic, or autotrophic respiration data, or both. The most comprehensive stand-level data have been collected for Pinus spp., Pseudotsuga menziesii, Abies amabilis and Liriodendron tulipifera. Only incomplete carbon budgets are available for most ecosystems studied, because measurements have been limited to one or several ecosystem processes (e.g., decay, productivity, storage). Based on the few complete C-budget studies that have been published, it is concluded that tree species differ in annual turnover of tissues.

Inputs of Sediment and Carbon to an Estuarine Ecosystem: Influence of Land Use.

Abstract: Estuaries and coastal marine ecosystems receive large inputs of nutrients, organic carbon, and sediments from non-point-source runoff from terrestrial ecosystems. In the tidal, freshwater Hudson River estuary, such inputs are the major sources of organic carbon, driving ecosystem metabolism, and thus strongly influencing dissolved oxygen concentrations. We used a watershed simulation model (GWLF) to examine the controls on inputs of organic carbon and sediment to this estuary. The model provides estimates of water discharge, sediment inputs, and organic carbon inputs that agree reasonably well with independent estimates of these fluxes. Even though the watershed for the Hudson River estuary is dominated by forests, the model predicts that both sediment and organic carbon inputs come overwhelmingly from urban and suburban areas and from agricultural fields. Thus changes in land use within the Hudson River basin may be expected to alter inputs to the estuary, thereby altering its metabolism. Precipitation is important in controlling carbon fluxes to the estuary, and so climate change can be expected to alter estuarine metabolism. However, the day-to-day and seasonal patterns of precipitation appear more important than annual mean precipitation in con- trolling organic carbon fluxes.

Differences in processing dynamics of fresh and dried leaf litter in a stream ecosystem

Abstract: SUMMARY. 1. Although the bulk of litter input to stream ecosystems is in the form of fresh leaves, current understanding of organic matter processing is largely founded on experimental studies made with pre-dried leaves. This paradox points to the critical need for evaluating to what extent those experiments with dried leaves reflect natural litter decomposition. 2. The mass loss rates, patterns of mass loss, and chemical changes during processing of fresh leaf litter were compared with air-dried leaf litter in a stream ecosystem. 3. Although overall mass loss rates were similar between treatments (k= 0.0213 day−1 and 0.0206 day−1), fresh leaves lost mass at a constant rate, whereas the decay of dried leaves proceeded in two distinct phases. Soluble organic carbon, phosphorus, and potassium were rapidly leached from dried litter, but were largely retained in fresh material for more than a week. Kinetics of concentrations of cellulose and changes in amounts of lignin remaining per leaf pack revealed further differences in decomposition dynamics between treatments, apparently related, either directly or indirectly, to differences in leaching behaviour. 4. Dynamics of nitrogen and protein contents were similar between treatments, indicating that microbial colonization was not greatly delayed on fresh leaves. 5. It is concluded that the retention of labile carbon and nutrients in fresh leaf litter facilitates their utilization by leaf-associated micro-organisms and invertebrates, resulting in an increased importance of biotic processes relative to physical processes such as leaching. 6. At the ecosystem level, retention of carbon and nutrients in streams would be increased, allowing greater overall productivity. Conversely, the availability of labile organic carbon would be reduced in compartments such as the epilithon, fine sediments, and the water column.

Functional classification of soil fauna : a new approach

Abstract: The guild and functional group concepts are generally accepted as suitable units in ecosystem modelling, when interspecific relationship and energy or nutrient flows are studied.It is argued, howewer, that members of a feeding guild assemblage can have specific effects on ecosystem processes through microstratification, particularly in stratified habitats such as soils. A new concept, league, is introduced which follows a guild like classification, but assembles organisms by their exploitation or processing of more than one habitat resource in a homologuous manner.The concept is illustrated for soil fauna feeding habits and choice of microhabitat, and the classification is discussed in view of soil fauna mediated decomposition and nutrient cycling processes

Environmental regulation of nitrogen fixation in a high arctic lowland ecosystem

Abstract: This study examined spatial and temporal variation in cyanobacterial nitrogen fixation and the environmental regulation of this variation at Truelove Lowland, Devon Island, N.W.T. Acetylene reducti...

Plant-animal interactions and the structure and function of mangrove forest ecosystems*

Abstract: Trophic interactions involving plants and animals in tropical mangrove forests have important controlling influences on several population, community and ecosystem-level processes. Insect herbivores remove up to 35% of leaf area from some mangrove tree species and can cause the death of seedlings. Leaf chemistry and toughness and soil nutrient status all appear to be important in explaining the between- and among-species variance in leaf damage. Insects also attack and damage, mainly by boring, a large proportion of mangrove seeds. Shadehouse experiments have shown that such post-dispersal predation can have a significant effect on seedling survival, growth and biomass allocation to leaves, stems and roots. Sesarmid crabs are also responsible for severe post-dispersal seed predation. In field trials, crabs consumed more than 70% of the seeds of five tree species. For four of these five species there was an inverse relationship between seed predation rate and the dominance of conspecific adult trees, while the within-site distribution pattern of one tree species appears to be partially controlled by crabs. The same crab species also consume 30–80% (depending on forest type and intertidal elevation), of the annual litter fall in mangrove forests and, thus, have an important role in controlling the rate of remineralization of detritus within forests and the export of particulate matter from the forests to other nearshore habitats. The other major component of litter in the forests is wood, which is broken down relatively rapidly by teredinid molluscs (shipworms). More than 90% of the weight loss from decomposing trunks of Rhizophora species during the first four years of decay is through ingestion by teredinids. The annual turnover of dead wood mass in Rhizophora forests is equivalent to that of the processing of leaf detritus by crabs. Because of the relatively low species richness of trees and consumers in tropical mangrove forests, they are likely to serve as productive sites for further investigations of the influence of plant-animal interactions on the dynamics of tropical forests.

The Mosaic-Cycle Concept of Ecosystems — An Overview

Abstract: The mosaic-cycle concept was developed in 1938 by Aubreville, using as an example the pristine forests of what was then French West Africa. In the succeeding years the concept was seldom referred to, with a few rare exceptions such as Richards (1981), until it was revived and expanded by Remmert (1985, 1987, 1988a,b). It seems likely that the mosaic-cycle concept is valid for most, if not all ecosystems; the resulting conclusions and predictions are therefore of the greatest importance for ecosystem research. This can best be illustrated in the case of temperate forest ecosystems.

From Individual Plankton Cells To Pelagic Marine Ecosystems And To Global Biogeochemical Cycles

Abstract: It is often assumed that the biological CO2 pump in the oceans is essentially driven by the upward flux of NO3, so that the potential export of biogenic carbon from the upper ocean is stoichiometrically equivalent to N-derived phytoplankton new production. The steady-state/stoichiometry model does not consider that ecosystem dynamics have a significant influence on the export or sequestration of biogenic carbon. Contrary to this assumption, there are several cases where ecosystems do have a significant effect on the export and sequestration of biogenic carbon. These include the fixation of nitrogen gas in the upper layer of the ocean (phytoplankton and coral reefs), the export of carbon by organisms with high carbon content (thecate dinoflagellates, coccolithophores, foraminifers, pteropods), the production of long-lived dissolved organic matter, and the fact that the pathways of export may differ in length and complexity. This is considered within the context of a general typology of pelagic marine ecosystems, based on the various possible combinations linking standing stock to production of phytoplankton: (1) production and standing stock dominated by large cells (e.g. upwelling, ice-edge and episodic blooms); (2) production by small and large cells, standing stock dominated by large cells (e.g. exceptional blooms); (3) production and standing stock of small and large cells (e.g. spring bloom in the North Atlantic); (4) production by small and large cells, standing stock dominated by small cells (e.g. Alaskan Gyre in the North Pacific); (5) production and standing stock dominated by small cells (e.g. oligotrophic ocean). These five types of ecosystems correspond to different modes of phytoplankton production (as controlled by hydrodynamics) and different structures (as reflected in the standing stocks), which influence the pathways of carbon export and sequestration as well as the renewable marine resources.

Mineral cycling and epiphytic lichens: implications at the ecosystem level

Abstract: The nutrient contribution of lichens as litterfall in forests is discussed for a number of different ecosystems and it is hypothesized that lichens are important in capturing nutrients from wet deposition, occult precipitation, sedimentation, impaction and gaseous uptake. Most nutrients captured by these processes represent new nutrient inputs that would otherwise not be intercepted by the ecosystem. Part of these nutrients will be incorporated into lichen biomass and only become available upon death and decomposition, but a portion will be leached by precipitation and become deposited on the soil surface. Although quantifying nutrient sources, fluxes and pool sizes is a potentially complex task, we describe a simplified approach for determining whether lichens significantly affect the mineral cycling of a forest. Preliminary results for an oak woodland in California document that epiphytic lichens may reduce throughfall and alter throughfall chemistry.

Nitrogen mineralization, nitrification and denitrification in upland and wetland ecosystems.

Abstract: Nitrogen mineralization, nitrification, denitrification, and microbial biomass were evaluated in four representative ecosystems in east-central Minnesota. The study ecosystems included: old field, swamp forest, savanna, and upland pin oak forest. Due to a high regional water table and permeable soils, the upland and wetland ecosystems were separated by relatively short distances (2 to 5 m). Two randomly selected sites within each ecosystem were sampled for an entire growing season. Soil samples were collected at 5-week intervals to determine rates of N cycling processes and changes in microbial biomass. Mean daily N mineralization rates during five-week in situ soil incubations were significantly different among sampling dates and ecosystems. The highest annual rates were measured in the upland pin oak ecosystem (8.6 g N m−2 yr−1), and the lowest rates in the swamp forest (1.5 g N m−2 yr−1); nitrification followed an identical pattern. Denitrification was relatively high in the swamp forest during early spring (8040 μg N2O−N m−2 d−1) and late autumn (2525 μg N2O−N m−2 d−1); nitrification occurred at rates sufficient to sustain these losses. In the well-drained uplands, rates of denitrification were generally lower and equivalent to rates of atmospheric N inputs. Microbial C and N were consistently higher in the swamp forest than in the other ecosystems; both were positively correlated with average daily rates of N mineralization. In the subtle landscape of east-central Minnesota, rates of N cycling can differ by an order of magnitude across relatively short distances.

Dynamic Aquaria: Building Living Ecosystems

Abstract: Introduction. Physical Environment: The Envelope, Its Physical Parameters and Energy State. Substrate. Water Composition. Input of Solar Energy. Input of Organic Energy. Biochemical Environment: Metabolism. Organisms and Gas Exchange. The Primary Nutrients--Nitrogen, Phosphorus and Silica. Biomineralization & Calcification. Control of the Biochemical Environment. Biological Structure: Community Structure. Trophic Structure. Primary Producers. Herbivores. Carnivores. Plankton and Planktivores. Detritus and Detritivores. Symbionts and Other Feeders. Ecological Systems in Microcosms, Mesocosms, and Aquaria: Models of Coral Reef Ecosystems. A Subarctic Shore, Maine. Estuaries, Chesapeake, and Everglades. Freshwater. The Environment and Ecological Engineering: The Culturing of Ecosystems for the Aquarium Hobby. The Control of Human Wastewaters. Summary.

The Place of Humans in the State Factor Theory of Ecosystems and Their Soils

Abstract: Landscapes and their soils often are studied and described by the clorpt equation of the state factor theory. It links properties of animals, plants, and soils to the factors climate (cl), organism (o), topography (r), parent material (p) and time (t) or age of an ecosystem. Humans are implicitly co

Nitrogen deposition, distribution and cycling in a subalpine spruce-fir forest in the Adirondacks, New York, USA

Abstract: Nitrogen inputs, fluxes, internal generation and consumption, and outputs were monitored in a subalpine spruce-fir forest at approximately 1000-m elevation on Whiteface Mountain in the Adirondacks of New York, USA. Nitrogen in precipitation, cloudwater and dry deposition was collected on an event basis and quantified as an input. Throughfall, stemflow, litterfall and soil water were measured to determine fluxes within the forest. Nitrogen mineralization in the forest floor was estimated to determine internal sources of available N. Lower mineral horizon soil water was used to estimate output from the ecosystem. Vegetation and soil N pools were determined. During four years of continuous monitoring, an average of 16 kg N ha−1 yr−1 was delivered to the forest canopy as precipitation, cloudwater and dry deposition from the atmosphere. Approximately 30% of the input was retained by the canopy. Canopy retention is likely the result of both foliar uptake and immobilization by bark, foliage and microorganisms. Approximately 40 kg of N was made available within the forest floor from mineralization of organic matter. Virtually all the available ammonium (mineralized plus input from throughfall) is utilized in the forest floor, either by microorganisms or through uptake by vegetation. The most abundant N component of soil water solutions leaving the system was nitrate. Net ecosystem fluxes indicate accumulation of both ammonium and nitrate. There is a small net loss of organic N from the ecosystem. Some nitrate leaves the bottom of the B horizon throughout the year. Comparisons with other temperate coniferous sites and examination of the ecosystem N mass balance indicate that N use efficiency is less at our site, which suggests that the site is not severely limited by N.

Watersheds and Vegetation of the Greater Yellowstone Ecosystem

Abstract: This paper describes major watershed systems and broad patterns of vegetation within the Greater Yellowstone Ecosystem, explains the geographic links between these sys- tems, andproposes factors that could be used to measure the integrity (condition and naturalness) of watersheds and vegetation The Greater Yellowstone Ecosystem can be geo- graphically defined as the Yellowstone Plateau and eleva- tions in the 14 surrounding mountain ranges above the 2130-meter (7000-foot) contoul: certain lower elevations need to be included to account for some ecologicalprocesses. The Greater Yellowstone Ecosystem is the headwaters for three major continental-scale river systems: the Missouri- Mississippi, Snake-Columbia, and Green-Colorado. Features, processes, and materials of watersheds provide structure and key functions to ecosystems of Greater Yellowstone. Changes in elevation--and the accordant changes in precipitation, temperature, land forms, and the stream network-exert the strongest control on the distribution of plant species in the Greater Yellowstone Ecosystem. The condition and natural- ness of watmheds and vegetation remain to be quantified but both decline with distance away from the core of the Greater Yellowstone Ecosystem due to timber harvest, oil and gas exploration and development, mineral exploration and development, reservoir operations, flood control, agri- cultural development, and livestock grazing.

Nitrogen cycling in an acid forest ecosystem in the Netherlands under increased atmospheric nitrogen input The nitrogen budget and the effect of nitrogen transformations on the proton budget

Abstract: Within a long-term research project studying the biogeochemical budget of an oak-beech forest ecosystem in the eastern part of the Netherlands, the nitrogen transformations and solute fluxes were determined in order to trace the fate of atmospherically deposited NH4+ and to determine the contribution of nitrogen transformations to soil acidification.