Showing papers on "Ecosystem published in 1989"

Texture, climate, and cultivation effects on soil organic matter content in U.S. grassland soils

Abstract: Soil organic C content, a major source of system stability in agroecosystems, is controlled by many factors that have complex interactions. The purpose of this study was to evaluate the major controls over soil organic carbon content, and to predict regional patterns of carbon in range and cultivated soils. We obtained pedon and climate data for 500 rangeland and 300 cultivated soils in the U.S. Central Plains Grasslands, and statistically analyzed relationships between C and soil texture and climate. Regression models of the regional soils database indicated that organic C increased with precipitation and clay content, and decreased with temperature. Analysis of cultivated and rangeland soils indicated that C losses due to cultivation increased with precipitation, and that relative organic C losses are lowest in clay soils. Application of the regression models to a regional climate database showed potential soil organic matter losses to be highest in the northeastern section of the Central Plains Grasslands, decreasing generally from east to west. These statistical data analyses can be combined with more mechanistic models to evaluate controls of soil organic matter formation and turnover, and the implications for regional management. S ORGANIC MATTER is a major component of biogeochemical cycles of the major nutrient elements, and the quantity and quality of soil organic matter both reflect and control primary productivity. The amount of soil organic matter represents the balance of primary productivity and decomposition and as such is a sensitive and integrated measure of change in ecosystem function. Understanding the processes that control soil organic matter dynamics and their I.C. Burke, CM. Yonker, W.J. Parton, C.V. Cole and D.S. Schimel, Natural Resource Ecology Lab., Colorado State Univ., Fort Collins, CO 80523; and K. Flach, Agronomy Dep., Colorado State Univ., Fort Collins, CO 80523. Received 20 June 1988. 'Corresponding author. Published in Soil Sci. Soc. Am. J. 53:800-805 (1989). response to management is essential for informed use of agricultural land. Jenny (1980) describes four sets of state factors responsible for the formation of soil organic matter, and illustrates the influence of parent material, time, climate, and biota as individual controls over soil properties. Controls over soil organic matter properties may have complex interactions; separate analysis of such controls may limit useful predictions. Parton et al. (1988) illustrate the use of a mechanistic model in evaluating simultaneously changing controls. Although such models can be highly successful, field data are necessary to validate predictions across complex gradients. It is widely recognized that cultivation of grassland soils leads to depletion of soil organic matter (Alway, 1909; Russel, 1929; Hide and Metzger, 1939; Haas et al., 1957; and many others). Soil organic C losses of as much as 50% have been documented in the U.S. Central Plains Grasslands (Haas et al., 1957), with losses strongly dependent on management regime and regional location. The extent of soil organic matter depletion has been shown to depend upon the same variables as those controlling soil organic matter formation: climate (Haas et al., 1957; Honeycutt, 1986; Cole et al., 1989), soil texture (Tiessen et al., 1982; Schimel et al., 1985a), landscape position (Schimel et al., 1985a,b; Honeycutt, 1986; Yonker et al., 1988), and management regime (Janzen, 1987; Cole et al., 1988). An integrated assessment of soil organic matter losses across the U.S. Central Grasslands requires analysis of soils with varying temperature, precipitation, and soil physical properties. The objectives of this paper were threefold: (i) to establish quantitative relationships between native soil organic matter levels in the Central Plains Grasslands and key driving variables: precipitation, temperature, and soil texture; (ii) to develop predictions of cultivation induced soil organic carbon loss as a function BURKE ET AL.: TEXTURE, CLIMATE, AND CULTIVATION EFFECTS ON U.S. GRASSLAND SOILS 801 of climate and soil texture; and (iii) to use these predictions to map potential soil organic C depletion.

The Seasonal Dynamics of The Chesapeake Bay Ecosystem

Abstract: The full suite of carbon exchanges among the 36 most important components of the Chesapeake Bay mesohaline ecosystem is estimated to examine the seasonal trends in energy flow and the trophic dynamics of the ecosystem. The networks provide information on the rates of energy transfer between the trophic components in a system wherein autochthonous production is dominated by phytoplankton production. A key seasonal feature of the system is that the summer grazing of primary producers by zooplankton is greatly reduced due to top—down control of zooplankton by ctenophores and sea nettles. Some of the ungrazed phytoplankton is left to fuel the activities of the pelagic microbial community, and the remainder falls to the bottom where it augments the deposit—feeding assemblage of polychaetes, amphipods, and blue crabs. There is a dominant seasonal cycle in the activities of all subcommunities, which is greatest in the summer and least in the cold season. However, the overall topology of the ecosystem does not ap...

Environmental Factors and Ecological Processes in Boreal Forests

Abstract: This paper reports on the boreal forest, a broad, circumpolar mixture of cool coniferous and deciduous tree species which covers over 14.7 million km{sup 2}, or 11%, of the earth's terrestrial surface. At these latitudes, a strong correlation exists between the seasonal dynamics of atmospheric carbon dioxide and the seasonal dynamics of the greenness of the earth. A possible causal relation, in which the dynamics of the forests at these latitudes regulates the atmospheric carbon concentrations, appears to be consistent with the present-day understanding of ecological processes in these ecosystems. Along with its familiar role in plant photosynthesis, carbon dioxide is a greenhouse gas that markedly affects the heat budget of the earth. Thus the possibility that boreal forests may actively participate in the dynamics of atmospheric carbon dioxide is of considerable significance, especially since the climatic response to elevated atmospheric carbon dioxide concentrations seems to be strongly directed to the boreal forests of the world.

Magnification of secondary production by kelp detritus in coastal marine ecosystems

Abstract: Kelps are highly productive seaweeds found along most temperate latitude coastlines, but the fate and importance of kelp production to nearshore ecosystems are largely unknown. The trophic role of kelp-derived carbon in a wide range of marine organisms was assessed by a natural experiment. Growth rates of benthic suspension feeders were greatly increased in the presence of organic detritus (particulate and dissolved) originating from large benthic seaweeds (kelps). Stable carbon isotope analysis confirmed that kelp-derived carbon is found throughout the nearshore food web.

Ecosystem-level patterns of primary productivity and herbivory in terrestrial habitats

Abstract: ECOSYSTEMS are structurally organized as food webs within which energy is transmitted between trophic levels and dissipated into the environment. Energy flow between two trophic levels is given by the amount of production at the lower level and by the proportion of production that is consumed, assimilated and res-pired at the higher level. Considerable evidence indicates that food-web structure varies predictably in different habitats1–5, but much less is known about quantitative relationships among food web fluxes. Many of the energetic properties of herbivores in African game parks are associated with rainfall and, by inference, with net primary productivity6,7. Respiratory costs per unit produc-tion at the consumer trophic level are higher for homeotherms than for heterotherms8. Plant secondary chemicals affect herbivore dietary choices9,10 and the allocation of plant resources to those chemicals varies with resource availability11. How these phenomena are translated into ecosystem fluxes is unknown. We present evidence that herbivore biomass, consumption and produc-tivity are closely correlated with plant productivity, suggesting that the latter is a principal integrator and indicator of functional processes in food webs.

Microbial biomass acts as a source of plant nutrients in dry tropical forest and savanna

Abstract: MORE than half of all tropical soils are highly weathered, leached and impoverished, requiring the ecosystem to develop nutrient-conserving mechanisms1,2. Nutrient retention and withdrawal mechanisms are most effective in nutrient-poor systems3,4. Thus, although dry tropical forests and savanna have the potential capacity to grow at high rates5,6, this capacity is strictly limited by climate and nutrients. Our studies on these nutrient-poor ecosystems show that a reciprocal relationship exists between microbial biomass and plant growth rate. This suggests that microbial immobilization may be a main source of nutrients for the plants and may lead to nutrient conservation.

Stability of Periphyton and Macroinvertebrates to Disturbance by Flash Floods in a Desert Stream

Abstract: Resistance, resilience, and patterns of succession were evaluated for periphyton and macroinvertebrates of Sycamore Creek, Arizona, between 1984 and 1987. During this period, 35 flash-flood disturbances occurred, ranging in magnitude (peak discharge) from 0.2 m 3 /s to 58 m 3 /s; peak discharge of the largest flash floods exceeded base flow by 3-4 orders of magnitude. Macroinvertebrates and algal assemblages dominated by diatoms were more resistant, i.e., showed less change in response to spates, than macroalgae (filamentous Chlorophyta) and cyanobacterial mats, but resistance of all groups declined with increasing disturbance magnitude. Biota showed little resistance to events large enough to move substrata. Twenty sequences of postflood succession were analyzed to characterize resilience and patterns of recovery. Resilience was very high compared with other streams and other ecosystems, because of high rates of biotic production in this desert stream. Resilience of periphyton (as indic...

The changing solar ultraviolet climate and the ecological consequences for higher plants

Abstract: There is compelling evidence that a general erosion of the global ozone layer is occurring. Since ozone in the stratosphere absorbs much of the shortwave solar ultraviolet radiation (UV-B), diminished ozone means that more UV-B of a very specific wavelength composition will be received at the earth's surface. Evaluating the implications for vegetation involves consideration of the wavelength specificity of biological photochemical reactions and their sensitivity to the extant and future solar spectrum. Recent research suggests the occurrence of direct damaging reactions and of indirect morphological and chemical responses with implications at the community and ecosystem levels.

Effects of environmental stress on species rich assemblages

Abstract: Selye's widely used model of responses of individual organisms to a stressor (1973, American Science, 61: 692 699) is not appropriate for describing effects at the population or community level. At the ecosystem level a number of functional responses have been suggested by Rapport, Regier & Hutchinson (1985, American Naturalist, 125: 617–640) but detailed analysis shows that, in general, functional responses are not sensitive to the early detection of impending ecosystem damage. Three clear changes in community structure occur in response to stressors. These are reduction in diversity, retrogression to dominance by opportunist species and reduction in mean size of the dominating species. Statistically significant reductions in diversity occur rather late in the sequence of increased stressor impact. The first stages of impact are clearly shown by moderately common species yet most attention has concentrated on the common species. Species which dominate in heavily stressed habitats are often species complexes and the possible genetic mechanisms causing this are considered. Whilst changes in the mean size of the dominant organisms can be shown in experiments there is no clear evidence that recorded reductions in the size of North Atlantic and North Sea plankton are induced by man-made stressors.

The habitat diversity and fish reproductive function of floodplain ecosystems

Abstract: Fish reproduction in floodplain ecosystems, based on relative abundance and total biomass of 0+ juveniles, was studied using the synchronic approach to typological analysis in conjunction with Point Abundance Sampling by modified electrofishing. In 3 different flood plains of the Upper Rhone River, 1015 point samples yielding 4573 juveniles (0+) from 21 species were collected from 48 ecosystems of various geomorphological origin. The results demonstrate the lotic-to-lentic succession of floodplain ecosystems to be a series of non-sequential reproductive zones, with spawning conditions being reflected by the specific composition and guild structure of the YOY fish assemblages. The habitat diversity and the fish reproductive potential of floodplain ecosystems are strongly influenced by geomorphological origin and by past and present hydrological conditions. The YOY assemblages of autogenically driven ecosystems (usually of anastomose or meander origin) tend to differ both in composition and in quantity from those found in allogenically driven ecosystems (generally of braided origin). Ecosystems of intermediate character, and fish reproduction thereof, occur as the result of either ecosystem rejuvenation or senescence: autogenically driven ecosystems by allogenic mechanisms, or allogenically driven ecosystems by anthropic and/or autogenic mechanisms, respectively. Because of co-occurrence of ecosystems at similar and at different successional status, the flood plain as an entity is seen as ‘stable’ with respect to fish reproduction.

Algae of terrestrial habitats

Abstract: The paper summarizes the present knowledge on the most important terrestrial habitats for algae and deals with soil, lithophytic, cave, snow and ice, epiphytic, and epizooic algae. For each habitat the physical parameters of the environment, the characteristic vegetation, and the functioning of the ecosystem are detailed.

Mineral nutrients in tropical forest and savanna ecosystems.

Abstract: Tropical forest nutrients, where do we stand? Soil characteristics and classification in relation to the mineral nutrition of tropical wooded ecosystems Pedalogical processes and nutrient supply from parent material in tropical soils Variations in soil nutrients in relation to soil moisture status in a tropical forested ecosystem Nitrification and denitrification in humid tropical ecosystems: potential controls on nitrogen retention The effect of humus acids and soil heating on the availability of phosphate in oxide-rich tropical soils Factors affecting nutrient cycling in tropical soils Mineral nutrients in some Botswana savannah types.

Nitrogen availability and nitrification during succession: Primary, secondary, and old-field seres

Abstract: Suggestions that nutrient cycles become more strongly regulated and that nitrification is progressively inhibited in the course of ecological succession have stimulated numerous field measurements. Results of these are inconsistent; in some cases nitrogen turnover and nitrification decrease during succession, while in others both increase substantially.

Control of Nitrogen Mineralization a Sagebrush Steppe Landscape

Abstract: Factors controlling N turnover in sagebrush ecosystems are separable into two groups. The first group consists of properties that exhibit strong spatial patterning at a landscape scale but are temporally static at time scales of years or tens of years. These static properties include plant species assemblages, total soil nutrient pools, and soil texture. A second group includes properties that vary across the landscape over shorter time scales, i.e., annually, seasonally, and diurnally. These dynamic properties include soil moisture, temperature, and amount of available nutrients. This paper evaluates the landscape vari- ability of properties in both of these groups, and examines the extent to which these factors control N turnover. Static ecosystem properties were entered into a principal components analysis resulting in four axes of landscape variability. A statistical analysis of the relationship of net N mineralization with the principal components and with soil temperature and moisture suggested that soil microclimate and organic matter quality both control in situ N turnover. Soil microclimate limited N mineralization to a short season in early spring and summer; only during this time did soil organic matter exert control. In landscape positions where soil organic matter pools were low, improved soil microclimate conditions did not increase N mineralization rates. A similar approach may be useful in evaluating control over eco- system processes in other systems that are characterized by strong seasonal and spatial variability.

Critical loads for nitrogen deposition on forest ecosystems

Abstract: Critical loads for N deposition are derived from an ecosystem's anion and cation balance assuming that the processes determining ecosystem stability are soil acidification and nitrate leaching. Depending on the deposition of S, the parent soil material, and the site quality critical N deposition rates will range between 20 to 200 mmol m−2 yr−1 (3 to 14 kg ha−1 yr−1) on silicate soils and reach 20 to 390 mmol m−2 yr−1 (3 to 48 kg ha−1) on calcareous soils.

Reconstruction of tree-line vegetation response to long-term climate change

Abstract: KNOWLEDGE of the vegetation response to climate change is necessary to assess and predict realistic ecosystem development in the anticipated, CO2-induced warmer world, particularly at high latitudes where greater warming is expected1–3. Reconstruction of vegetation development over the past 1,000 years may be helpful in this respect, because this period was characterized by contrasting climatic conditions4–9. Here we report the reconstruction of wind-exposed, tree-line vegetation associated with long-term climate change in northern Canada, using tree-ring and growth-form analyses of spruce subfossils. Three major types of growth form within the exposed, but stable, lichen–spruce community successively predominated in response to climate forcing: high krummholz (dwarf spruce, 2–3 m high) and high krummholz (AD 1435–1570, warm period) and low krummholz ( ≲50 cm) (little ice age to present: AD 1570 onwards, cold period and present climate, respectively). Whereas the expansion of a marginal lichen–spruce woodland climaxed during the late Middle Ages (AD 1435–1570), present development of a low-krummholz vegetation at these sites seems to be out of phase with the twentieth century warming. This suggests that ecosystem recovery to global warming is not straightforward, depending on the nature of vegetation structure present at the time climate change occurred. The implications of such ecosystem resilience for the detection and monitoring of the expected CO2-induced warming is discussed, particularly for the climate-sensitive arctic and subarctic regions.

Resource availability hypothesis of plant antiherbivore defence tested in a South African savanna ecosystem

Abstract: THERE is considerable variation in the levels of herbivory experienced by woody species in ecosystems as diverse as arctic shrub tundra, boreal forests, tropical rain forests and African savannas1. An important objective of evolutionary ecology is the formulation of a hypothesis that can explain this variation. One such hypothesis predicts that mature leaves of inherently slowly growing species adapted to resource-limited habitats2,3 such as infertile soil4 are, because of more effective chemical defences, less preferred as a food source by herbivores than mature leaves of inherently rapidly growing species adapted to productive habitats2,3,5-8. In agreement with this prediction, we found positive correlations between the inherent growth rates of nine South African woody species and kudu and impala preferences for their mature leaves. Thus, the resource availability hypothesis of plant antiherbivore defence2,3,5-8 could explain why large African herbivores feed less on woody vegetation of dystrophic savanna-woodlands than on woody vegetation of eutrophic savannas9.

Lindeman's Contradiction and the Trophic Structure of Ecosystems

Abstract: R. L. Lindeman's empirically based generalizations that both progressive efficiencies and percent of energy lost due to respiration increase with trophic level appeared contradictory, because he linked trophic levels in series to form a simple trophic chain. This model equates the energy not respired by the populations aggregated to form a trophic level to the productivity of the populations assigned to the next trophic level. This is clearly not the case for Cedar Bog Lake and ecosystems generally, because of nonpredatory losses and energy gains from other than the previous step in the trophic chain, e.g., bacterivory, detritivory, and feeding on organisms at more than one trophic level. Network analyses of energy-flow models make possible an objective and unamibiguous definition of trophic level that does not suffer this contradiction. Trophic guild is proposed as an alternative to trophic level when what is meant is an aggregation of species with similar trophic resources.

Impact of fire in the eucalypt forest ecosystem of southern Western Australia: a critical review

Abstract: Summary This paper is a comprehensive review of the effects of fire on soils, nutrient cycling, micro-organisms, vascular flora, soil and litter and other invertebrates, reptiles, amphibians, birds, mammals and exotic plant species in jarrah and karri forest of Western Australia. Pre-European fire regimes are also briefly reviewed, and the evidence suggests a mosaic burning pattern of frequent fires under relatively mild conditions. The evidence suggests that soil properties and nutrient cycling are neither dramatically nor permanently affected by low intensity fire. The overall response of the flora and fauna is: (1) reduction in density and sometimes number of species immediately after fire; (2) recovery in density and number of species after fire, usually from propagules within burnt areas; (3) transient changes in relative density of species after fire. Rate of recovery of fauna depends largely on that of the vascular flora. Each species has a well-defined response to fire. There is very little inform...

Methane emissions from the Florida Everglades: Patterns of variability in a regional wetland ecosystem

Abstract: Natural wetlands are presumed to be major sources of atmospheric methane, but current estimates of the global wetland emission vary by almost a factor of 20. Estimates of global source strengths are based on extrapolation of in situ flux measurements to large areas occupied by broad classes of wetland environments, and recent efforts at refinement of these estimates have concentrated on improving inventories of the global distribution of major wetland types. An additional potential source of uncertainty which has not been quantified is regional scale variability in emission rates within the major wetland types. We conducted an experiment which examined the spatial variability of methane flux within a large regional wetland system, the Florida Everglades. We also investigated the association of flux variability with relevant surface characteristics such as soil thickness, water depth, soil temperature, and vegetative community distribution. Unit area methane flux to the atmosphere from water-saturated Everglades environments, measured in situ, varied over more than an order of magnitude (4.2 to 81.9 mg CH4/m2/d), depending on which habitat component of the ecosystem was sampled. Observed physical characteristics of the surface (water and soil depth, soil temperature) were not quantitatively associated with the variability in flux rates. However, the distribution of vegetative community types provided an empirical indicator of flux, permitting an inventory of emissions to be based on mapping of regional vegetation patterns. Use of high-resolution, orbital remote sensing data helped reduce uncertainty in the emission inventory of the Everglades by directing in situ sampling efforts to important habitat types and by providing a means for calculating area-weighted mean flux for the system as a whole. The results indicated that spatial variability in flux within a major wetland ecosystem can introduce significant uncertainty in extrapolations to larger areas, even if the extent of the major ecosystem itself is well known. The results also suggested that the response of total ecosystem flux to changing water level is not a linear function of flooded area, but is damped, with regional flux at lowered water levels decreasing proportionally less than flooded area. Both sources of variability can be addressed by the combination of remote sensing and in situ techniques we have employed in the Everglades.

Strontium isotope studies of atmospheric inputs to forested watersheds in New Mexico

Abstract: Stable isotopes of strontium provide a unique quantification of ecosystem processes because organisms do not differentiate between them. For landscapes with contrasting geologies, these isotopes can identify atmospheric source material from local weathered material. This study quantified the input of strontium, distribution within the ecosystem, canopy capture versus leaf leachate, canopy loss, and Sr increment in biomass from an atmospheric origin. Forest ecosystems were studied along an elevational gradient in New Mexico. Spruce forests had a much greater capacity for capturing atmospheric Sr than aspen forests; however, aspen contained more total atmospheric Sr in their tissues because of greater uptake rates and the ability to utilize atmospheric deposited Sr before the initiation of the aspen forest. This technique has excellent capabilities for estimating the relative importance of atmospheric and weathering inputs to certain ecosystems.

Symptoms of pathology in the Gulf of Bothnia (Baltic Sea): ecosystem response to stress from human activity

Abstract: An extensive review of the data now accumulated on the response of the Gulf of Bothnia ecosystem to stress from human activity provides abundant evidence for incipient ecosystem pathology. Signs of ‘ecosystem distress' appear at local, coastal and basin-wide scales. Symptoms include early signs of eutrophication in local and coastal waters, formation of local abiotic zones, reduction in species diversity, reduction in genetic diversity (particularly in salmonids), reduced size of biota, increased dominance by opportunistic species, increased disease prevalence and bio-accumulation of toxic substances (e.g. PCBs, DDT, heavy metals). Pathology may propagate between local, coastal and basin-wide regions by several different pathways. Despite reductions in loadings of some toxic substances in recent years (e.g. PCBs, DDT), stress from human activities on the Gulf of Bothnia continues to impact the ecosystem at all spatial scales.

Phytoplankton species and abundance in response to eutrophication in coastal marine mesocosms

Abstract: In this paper we investigate the impact of nutrients on the taxonomic category or species and abundance of phytoplankton in model ecosystems and find that while diatom abundances increase with nutrient treatments small forms do not

A Seasonal Analysis of the Southern North Sea Ecosystem

Abstract: We developed an input—output model of the Southern North Sea ecosystem in order to determine the relationships between 10 biotic and abiotic stock levels and to investigate how their direct and indirect dependency changed through the typical year. Ecologists now examine stomach contents of top carnivores, for example, to evaluate their direct connection to the food source. Our method allows ecologists to extend their understanding of connection in an ecosystem. The method captures not only the food source connection but also those species that contribute to the food source, those at the next level that contribute to those sources, and so on, until all the exchanges have been assigned without ambiguity. The method even captures the indirect connection that a species may have to itself. A small change in the activity level of any species in an ecosystem affects all the rest; we can show the effect of that change on each species in the system. We followed the fate of absorbed solar energy to trace the connections between the elements of this ecosystem, although any external input (e.g., phosphorus, nitrogen) could have been used, depending on the objectives of the analysis. We traced only physical exchanges in the sample ecosystem, but measurable nonphysical exchanges, such as pollinator time in a terrestrial ecosystem, could be included. A sensitivity analysis showed which of the direct and indirect interconnecting flows is most affected by the rate of fish removal. We also used the model to show where specific management intervention (e.g., "fertilization") would be the most efficient in allowing stable system responses to fishing rate increases.