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  • 1
    Call number: PIK N 071-99-0174
    Type of Medium: Monograph available for loan
    Pages: 276 p.
    ISSN: 0003-4312
    Series Statement: Annales des Science Forestries
    Location: A 18 - must be ordered
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  • 2
    Call number: AWI G3-96-0166
    In: Ecological studies, 120
    Description / Table of Contents: The discovery of large petroleum reserves in northern Alaska prompted the US National Research Council to recommend priorities for ecological research on disturbance effects in the Arctic. Subsequently, this led to the implementation of a field study by the Department of Energy in a small watershed on the North Slope of Alaska. This volume describes results by a research team charged with seeking answers to a number of questions related to disturbance in tundra regions: will short-term disturbances have long-term ecological consequences? Will localized effects be transferred to adjacent systems, e.g., from terrestrial to aquatic? Is it possible to extrapolate understanding of impacts from one landscape to another? The results reported in this volume are an important contribution towards the goal of implementing ecosystem-based management in arctic tundra landscapes. Landscape function and disturbance in Arctic Tundra covers a broad array of topics, from ecosystem physiology to landscape modeling. It is an important source for researchers and students interested in arctic ecology, as well as for environmental managers concerned with practical issues of disturbance.
    Type of Medium: Monograph available for loan
    Pages: XX, 437 Seiten , Illustrationen , 24 cm
    ISBN: 3-540-59263-6
    Series Statement: Ecological Studies 120
    Language: English
    Note: Contents: I INTRODUCTION. - 1 Ecosystem Response, Resistance, Resilience, and Recovery in Arctic Landscapes: Introduction / J. F. Reynolds and J. D. Tenhunen. - 1.1 Introduction. - 1.2 NRC Committee Report. - 1.3 The R4D Program. - 1.3.1 Objectives and Conceptual Framework. - 1.3.2 Program Implementation. - 1.3.3 Landscape Function. - 1.4 Summary. - References. - 2 Integrated Ecosystem Research in Northern Alaska, 1947-1994 / G. R. Shaver. - 2.1 Introduction. - 2.2 Early Days at NARL. - 2.3 The U.S. Tundra Biome Program. - 2.4 The Meade River RATE Program. - 2.5 Eagle Creek and Eagle Summit. - 2.6 The Arctic LTER Program at Toolik Lake. - 2.7 Other Studies In Alaska and Elsewhere. - 2.8 Summary and Prospects. - References. - 3 Disturbance and Recovery of Arctic Alaskan Vegetation / D. A. Walker. - 3.1 Introduction. - 3.2 Disturbance and Recovery. - 3.3Typical Disturbance and Recovery Patterns. - 3.3.1 Small Disturbed Patches. - 3.3.2 Contaminants. - 3.3.2.1 Hydrocarbon Spills. - 3.3.2.2 Seawater and Reserve-Pit Spills. - 3.3.3 Fire. - 3.3.4 Transportation Corridors. - 3.3.4.1 Bulldozed Tundra and Related Disturbances. - 3.3.4.2 Off-Road Vehicle Trails. - 3.3.4.2.1 Summer Travel. - 3.3.4.2.2 Winter Travel. - 3.3.4.3 Permanent Roads and Pads. - 3.3.4.4 Gravel Mines. - 3.3.4.5 Native Species in Revegetation of Gravel Pads and Mines. - 3.3.4.6 Road Dust. - 3.3.4.7 Roadside Impoundments. - 3.3.5 Cumulative Impacts. - 3.4 Conclusions. - References. - 4 Terrain and Vegetation of the Imnavait Creek Watershed / D. A. Walker and M. D. Walker. - 4.1 Introduction. - 4.2 Terrain. - 4.2.1 Glacial Deposits. - 4.2.2 Retransported Hillslope Deposits. - 4.2.3 Colluvial Basin Deposits. - 4.2.4 Floodplain Deposits. - 4.3 Vegetation. - 4.3.1 Flora. - 4.3.2 Vegetation Types. - 4.3.2.1 Lichen-Covered Rocks. - 4.3.2.2 Dry Heath. - 4.3.2.2.1 Exposed Sites. - 4.3.2.2.2 Snowbeds. - 4.3.2.3 Tussock Tundra. - 4.3.2.4 Riparian Areas. - 4.3.2.5 Mires. - 4.3.2.6 Beaded Ponds. - 4.4 West-Facing Toposequence. - 4.5 Terrain Sensitivity to Disturbance. - 4.6 Conclusions. - Appendix A. List of Plants for Imnavait Creek, Alaska. - References. - 5 Vegetation Structure and Aboveground Carbon and Nutrient Pools in the Imnavait Creek Watershed / S. C. Hahn, S. F. Oberbauer, R. Gebauer, N. E. Grulke, O. L. Lange, and J. D. Tenhunen. - 5.1 ntroduction. - 5.2 Description of Vegetation. - 5.3 Sampling Methods. - 5.3.1 Cover. - 5.3.2 Biomass and Nutrient Pools. - 5.4 Cover. - 5.5 Aboveground Biomass. - 5.5.1 Live Biomass. - 5.5.2 Photosynthetic Biomass. - 5.5.3 Lichen Biomass. - 5.5.4 Organic Litter. - 5.5.5 Watershed Patterns. - 5.6 Nutrient Pools. - 5.6.1 N and P in Heath Cryptogams. - 5.6.2 N and P in Communities. - 5.7 Discussion and Conclusions. - References. - II PHYSICAL ENVIRONMENT, HYDROLOGY, and TRANSPORT. - 6 Energy Balance and Hydrological Processes in an Arctic Watershed / L. Hinzmann, D. L. Kane, C. S. Benson, and K. R. Everett. - 6.1 Introduction. - 6.2 Radiation and Thermal Regimes. - 6.2.1 Surface Energy Balance. - 6.2.2 Snow Cover and Soil Thermal Regime. - 6.3 Hydrological Processes. - 6.3.1 Snowmelt. - 6.3.2 Plot and Basin Water Balance. - 6.3.3 Runoff and Basin Discharge. - 6.3.4 Precipitation, Evaporation, and Evapotranspiration. - 6.4 Energy Balance and Hydrology Models. - 6.4.1 Simulation of the Thermal Regime. - 6.4.2 Simulation of Snowmelt. - 6.4.3 Simulation of Catchment Runoff. - 6.5 Conclusions. - References. - 7 Shortwave Reflectance Properties of Arctic Tundra Landscapes / A. S. Hope and D. A. Stow. - 7.1 Introduction. - 7.2 Shortwave Reflectance Studies in Arctic Environments. - 7.2.1 Environmental Considerations. - 7.2.2 Radiometric Data. - 7.2.3 Image Data. - 7.3 Spectral Reflectance. - 7.3.1 Aboveground Biomass. - 7.3.2 Vegetation Composition. - 7.3.3 Landscape Patterns. - 7.3.4 Effects of Dust Deposition. - 7.4 Albedo. - 7.4.1 Undisturbed Tussock Tundra. - 7.4.2 Effects of Dust Deposition. - 7.5 Conclusions. - References. - 8 Isotopic Tracers for Investigating Hydrological Processes / L. W. Cooper, I. L. Larsen, C. Solis, J. M. Grebmeier, C. R. Olsen, D. K. Solomon, and R. B. Cook. - 8.1 Introduction. - 8.1.1 Units. - 8.1.2 Conservative vs Nonconservative Isotopes. - 8.2 Nonconservative Tracers. - 8.3 Sulfur-35. - 8.4 Oxygen-18. - 8.4.1 Oxygen-18 Content of Snowpack. - 8.4.2 Oxygen-18 Content of Imnavait Creek. - 8.4.3 Oxygen-18 Content of Soil Moisture. - 8.4.4 Covariance of Oxygen-18 and Deuterium in Watershed Compartments. - 8.4.5 Covariance of Oxygen-18 and Deuterium in Plant Water. - 8.5 Long-Lived Radioisotopes: Lead-210 and Cesium-137. - 8.5.1 Distribution of 137Cs on Tundra and in Lake Sediments. - 8.5.2 Cycling of 137Cs in Annual Berries. - 8.5.3 Distribution of 210Pb in Tundra. - 8.6 Conclusions. - References. - III NUTRIENT AND CARBON FLUXES. - 9 Surface Water Chemistry and Hydrology of a Small Arctic Drainage Basin / K. R. Everett, D. L. Kane, and L. D. Hinzman. - 9.1 Introduction. - 9.2 Watershed Instrumentation. - 9.3 Snowmelt Period. - 9.3.1 Snowmelt Hydrology. - 9.3.2 Snowmelt Chemistry . - 9.3.2.1 Overland Flow. - 9.3.2.2 Water Track Flow. - 9.3.2.3 Imnavait Creek Flow. - 9.4 Post Snowmelt Period. - 9.4.1 Atmospheric Inputs. - 9.4.1.1 Rainfall. - 9.4.1.2 Dry Deposition. - 9.4.1.3 Rime. - 9.4.2 Water Chemistry. - 9.4.2.1 Overland Flow. - 9.4.2.2 Active Layer Flow. - 9.4.2.3 Imnavait Creek Flow. - 9.5 Conclusions. - References. - 10 Nutrient Availability and Uptake by Tundra Plants / J. P. Schimel, K. Kielland, and F. S. Chapin III. - 10.1 Introduction. - 10.2 Controls on Mineralization and Nutrient Supply. - 10.2.1 Patterns of Nutrient Supply in the Soil. - 10.2.2 Patterns of Mineralization. - 10.2.3 Controls on N and P Mineralization. - 10.2.4 Controls on Decomposition and Mineralization. - 10.2.4.1 Temperature. - 10.2.4.1.1 Enzyme Activities. - 10.2.4.1.2 Microbial Activity at Low Temperatures. - 10.2.4.1.3 Freeze-Thaw Events. - 10.2.4.2 Effects of Low Oxygen on Microbial Activity and Mineralization. - 10.2.4.3 Substrate Quality. - 10.3 Fate of Available Nutrients. - 10.3.1 Microbial Nutrient Uptake and Competition with Plants. - 10.3.2 Plant Uptake. - 10.3.2.1 Soil Factors Controlling Nutrient Absorption. - 10.3.2.2 Rooting Strategies. - 10.3.2.3 Uptake Characteristics of Tundra Plants. - 10.3.2.4 Retranslocation vs Current Uptake. - 10.4 Disturbances. - 10.4.1 Vehicle Tracks. - 10.4.2 Road Dust. - 10.4.3 Gray Water. - 10.4.4 Climate Change. - References. - 11 Landscape Patterns of Carbon Dioxide Exchange in Tundra Ecosytems / S. F. Oberbauer, W. Cheng, C. T. Gillespie, B. Ostendorf, A. Sala, R. Gebauer, R. A. Virginia, and J. D. Tenhunen. - 11.1 Introduction. - 11.2 Methods. - 11.2.1 Community Types. - 11.2.2 Leaf Photosynthesis. - 11.2.3 Ecosystem Efflux. - 11.2.4 Ecosystem Net CO2 Exchange. - 11.3 CO2 Uptake. - 11.3.1 Factors Affecting CO2 Uptake. - 11.3.1.1 Light. - 11.3.1.2 Temperature. - 11.3.1.3 Phenology. - 11.3.1.4 Water Availability. - 11.3.1.5 Nutrition. - 11.3.2 Landscape Patterns in Leaf Photosynthesis. - 11.4 CO2 Efflux. - 11.4.1 Factors Affecting CO2 Efflux. - 11.4.1.1 Live Plant Biomass. - 11.4.1.2 Soil Quality. - 11.4.1.3 Thaw Depth and Depth to Water Table. - 11.4.1.4 Soil Moisture. - 11.4.1.5 Soil Temperature. - 11.4.2 Landscape Patterns of CO2 Efflux. - 11.4.3 Daily and Seasonal Patterns of CO2 Efflux. - 11.4.4 Dust Deposition Effects on CO2 Efflux. - 11.5 Landscape Patterns in Net CO2 Exchange. - 11.6 Conclusions. - References. - 12 Control of Tundra Methane Emission by Microbial Oxidation / S. C. Whalen, W. S. Reeburgh, and C. E. Reimers. - 12.1 Introduction. - 12.2 Sampling Procedure. - 12.3 Results and Discussion. - 12.3.1 Methane Flux and Environmental Variables in Tundra and Taiga. - 12.3.2 Physiology, Controls, and Potential for Microbial CH4 Oxidation. - 12.3.3 Methane Oxidation by Tundra Soils in a Warmer Climate. - 12.4 Conclusions. - References. - 13 Dynamics of Dissolved and Particulate Car
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  • 3
    ISSN: 1365-2486
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology , Energy, Environment Protection, Nuclear Power Engineering , Geography
    Notes: Eddy covariance and sapflow data from three Mediterranean ecosystems were analysed via top-down approaches in conjunction with a mechanistic ecosystem gas-exchange model to test current assumptions about drought effects on ecosystem respiration and canopy CO2/H2O exchange. The three sites include two nearly monospecific Quercus ilex L. forests – one on karstic limestone (Puéchabon), the other on fluvial sand with access to ground water (Castelporziano) – and a typical mixed macchia on limestone (Arca di Noè). Estimates of ecosystem respiration were derived from light response curves of net ecosystem CO2 exchange. Subsequently, values of ecosystem gross carbon uptake were computed from eddy covariance CO2 fluxes and estimates of ecosystem respiration as a function of soil temperature and moisture. Bulk canopy conductance was calculated by inversion of the Penman-Monteith equation. In a top-down analysis, it was shown that all three sites exhibit similar behaviour in terms of their overall response to drought. In contrast to common assumptions, at all sites ecosystem respiration revealed a decreasing temperature sensitivity (Q10) in response to drought. Soil temperature and soil water content explained 70–80% of the seasonal variability of ecosystem respiration. During the drought, light-saturated ecosystem gross carbon uptake and day-time averaged canopy conductance declined by up to 90%. These changes were closely related to soil water content. Ecosystem water-use efficiency of gross carbon uptake decreased during the drought, regardless whether evapotranspiration from eddy covariance or transpiration from sapflow had been used for the calculation. We evidence that this clearly contrasts current models of canopy function which predict increasing ecosystem water-use efficiency (WUE) during the drought. Four potential explanations to those results were identified (patchy stomatal closure, changes in physiological capacities of photosynthesis, decreases in mesophyll conductance for CO2, and photoinhibition), which will be tested in a forthcoming paper. It is suggested to incorporate the new findings into current biogeochemical models after further testing as this will improve estimates of climate change effects on (semi)arid ecosystems' carbon balances.
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  • 4
    ISSN: 1365-2486
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology , Energy, Environment Protection, Nuclear Power Engineering , Geography
    Notes: Several studies have shown multiple confounding factors influencing soil respiration in the field, which often hampers a correct separation and interpretation of the different environmental effects on respiration. Here, we present a controlled laboratory experiment on undisturbed organic and mineral soil cores separating the effects of temperature, drying–rewetting and decomposition dynamics on soil respiration. Specifically, we address the following questions:〈list xml:id="l1" style="custom"〉1Is the temperature sensitivity of soil respiration (Q10) dependent on soil moisture or soil organic matter age (incubation time) and does it differ for organic and mineral soil as suggested by recent field studies.2How much do organic and mineral soil layers contribute to total soil respiration?3Is there potential to improve soil flux models of soil introducing a multilayer source model for soil respiration?Eight organic soil and eight mineral soil cores were taken from a Norway spruce (Picea abies) stand in southern Germany, and incubated for 90 days in a climate chamber with a diurnal temperature regime between 7 and 23°C. Half of the samples were rewetted daily, while the other half were left to dry and rewetted thereafter. Soil respiration was measured with a continuously operating open dynamic soil respiration chamber system. The Q10 was stable at around 2.7, independent of soil horizon and incubation time, decreasing only slightly when the soil dried. We suggest that recent findings of the Q10 dependency on several factors are emergent properties at the ecosystem level, that should be analysed further e.g. with regard to rhizosphere effects. Most of the soil CO2 efflux was released from the organic samples. Initially, it averaged 4.0 μmol m−2 s−1 and declined to 1.8 μmol m−2 s−1 at the end of the experiment. In terms of the third question, we show that models using only one temperature as predictor of soil respiration fail to explain more than 80% of the diurnal variability, are biased with a hysteresis effect, and slightly underestimate the temperature sensitivity of respiration. In contrast, consistently more than 95% of the diurnal variability is explained by a dual-source model, with one CO2 source related to the surface temperature and another CO2 source related to the central temperature, highlighting the role of soil surface processes for ecosystem carbon balances.
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  • 5
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Physiologia plantarum 70 (1987), S. 0 
    ISSN: 1399-3054
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Gas exchange characteristics of leaves of Vitis vinifera L. cvs Tinta Amarela and Periquita, two grapevine cultivars grown in distinct climatic regions of Portugal, were studied under natural and controlled conditions. Daily time courses of gas exchange were measured on both a hot, sunny day and a cooler, partly cloudy day. Responses of net photosynthesis to irradiance and internal partial pressure of CO2, were also obtained. A strong correlation between net photosynthesis (PN) and leaf conductance (gs) was found during the diurnal time courses of gas exchange, as well as a relatively constant internal partial pressure of CO2 (Pi), even under non-steady-state conditions. On the cloudless day, both PN and gs were lower in the afternoon than in the morning, despite similar conditions of leaf temperature, air to leaf water vapor deficit and irradiance. The response curves of net photosynthesis to internal CO2 showed linearity up to pi values of 50 Pa, possibly indicating a substantial excess of photosynthetic capacity. When measured at low partial pressures of O2 (1 kPa), PN became inhibited at high CO2 levels. Inhibition of PN at high CO2 was absent under normal levels of O2 (21 kPa). Significant differences in gas exchange characteristics were found between the two cultivars, with T. Amarela having higher rates under similar measurement conditions. In particular, the superior performance of T. Amarela at high temperatures may represent adaptation to the warmer conditions at its place of origin.
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  • 6
    ISSN: 1432-1939
    Keywords: Anaerobic ; Biomass partitioning ; Eriophorum ; Growth ; Nutrient stress
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract In arctic tundra soil, oxygen depletion associated with soil flooding may control plant growth either directly through anoxia or indirectly through effects on nutrient availability. This study was designed to evaluate whether plant growth and physiology of two arctic sedge species are more strongly controlled by the direct or indirect effects of decreased soil aeration. Eriophorum angustifolium and E. vaginatum, which originate from flooded and well-drained habitats, respectively, were grown in an in situ transplant garden at two levels of soil oxygen, nitrogen, and phosphorus availability over two growing seasons. In both species, N addition had a stronger effect on growth and biomass allocation than P addition or soil oxygen depletion. Net photosynthesis and carbohydrate concentrations were relatively insensitive to changes in these factors. Biomass reallocated from shoots to below-ground parts in response to limited N supply was equally divided between roots (nutrient acquisition) and perennating rhizomes (storage tissue formation) in E. angustifolium. E. Vaginatum only increased its allocation to rhizomes. In the flood-tolerant E. angustifolium, growth was improved by soil anoxia and biomass allocation among plant parts was not significantly affected. Contrary to our initial hypothesis, whole-plant growth in E. vaginatum improved in flooded soils; however, it only did so when N availability was high. Under low N availability growth in flooded soils was reduced by 20% compared to growth in the aerobic environment. Reduced biomass allocation to rhizomes and thus to storage potential under anaerobic conditions may reduce long-term survival of E. vaginatum in flooded habitats.
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  • 7
    ISSN: 1432-1939
    Keywords: Canopy structure ; Coordination ; Nitrogen allocation ; Optimization ; Photosynthesis
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract It has long been observed that leaf nitrogen concentrations decline with depth in closed canopies in a number of plant communities. This phenomenon is generally believed to be related to a changing radiation environment and it has been suggested by some researchers that plants allocate nitrogen in order to optimize total whole canopy photosynthesis. Although optimization theory has been successfully utilized to describe a variety of physiological and ecological phenomena, it has some shortcomings that are subject to criticism (e.g., time constraints, oversimplifications, lack of insights, etc.). In this paper we present an alternative to the optimization theory of plant canopy nitrogen distribution, which we term coordination theory. We hypothesize that plants allocate nitrogen to maintain a balance between two processes, each of which is dependent on leaf nitrogen content and each of which potentially limits photosynthesis. These two processes are defined as Wc, the Rubiscolimited rate of carboxylation, and Wj, the electron transport-limited rate of carboxylation. We suggest that plants allocate nitrogen differentially to, leaves in different canopy layers in such a way that Wc and Wj remain roughly balanced. In this scheme, the driving force for the allocation of nitrogen within a canopy is the difference between the leaf nitrogen content that is required to bring Wc and Wj into balance and the current nitrogen content. We show that the daily carbon assimilation of a canopy with a nitrogen distribution resulting from this internal coordination of Wc and Wj is very similar to that obtained using optimization theory.
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  • 8
    ISSN: 1432-1939
    Keywords: Growth ; Photosynthesis ; Resprouts ; Water relations ; Sclerophyll shrubs and trees
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Resprout and mature plant shoot growth, leaf water status and gas exchange behavior, tissue nutrient content, flowering, and production were studied for co-occurring shallow-rooted (Arbutus unedo L.) and deeprooted (Quercus ilex L.) Mediterranean tree species at the Collserola Natural Park in Northeast Spain Resprouts showed higher growth rates than mature plant shoots. During fall, no differences in eco-physiological performance of leaves were found, but mobilization of carbohydrates from burls strongly stimulated growth of fall resprouts compared to spring resprouts, despite low exposed leaf area of the fall shoots. During summer drought, resprouts exhibited improved water status and carbon fixation compared to mature plant shoots. Shoot growth of Q. ilex was apparently extended due to deep rooting so that initial slower growth during spring and early summer as compared to A. unedo was compensated. Tissue nutrient contents varied only slightly and are postulated to be of minor importance in controlling rate of shoot growth, perhaps due to the relatively fertile soil of the site. Fall flowering appeared to inhibit fall shoot growth in A. unedo, but did not occur in Q. ilex. The results demonstrate that comparative examinations utilizing vegetation elements with differing morphological and physiological adaptations can be used to analyze relatively complex phenomena related to resprouting behavior. The studies provide an important multi-dimensional background framework for further studies of resprouting in the European Mediterranean region.
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  • 9
    ISSN: 1432-2285
    Keywords: Quercus ilex ; Canopy structure ; Leaf area index ; Water deficit ; Mediterranean sclerophylls
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Abstract Spatial and temporal changes in canopy structure were studied in 1988 and 1989 in a Mediterranean Quercus ilex forest in north-eastern Spain. Due to differences in precipitation patterns the 1989 growing season was drier than the 1988 growing season. Sampling was conducted in parallel at two sites which represent endpoints along a slope gradient within a watershed (ridge top at 975 m, and valley bottom at 700 m). At both sites, similar inter-annual changes in canopy structure were observed in response to differences in water availability. Samples harvested in the upper 50 cm of the canopy during 1989 exhibited a decrease in both average leaf size and the ratio of young to old leaf and stem biomass relative to samples obtained in 1988. At the whole canopy level, a decrease in leaf production efficiency and an increase in the stem to leaf biomass ratio was observed in 1989. Temporal changes in canopy leaf area index (LAI) were not statistically significant. Average LAI values of Q. ilex at the two sites were not significantly different despite differences in tree stature and density (4.6 m2 m−2 at the ridge top, and 5.3 m2 m−2 at the valley bottom). Vertical distribution of leaves and stems within the canopy was very similar at the two locations, with more than 60% of the total LAI in the uppermost metre of the canopy. The possible significance of such an LAI distribution on the canopy carbon budget is discussed.
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  • 10
    ISSN: 1573-5036
    Keywords: anaerobic ; N availability ; permafrost ; tundra ; tussock
    Source: Springer Online Journal Archives 1860-2000
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Abstract The seasonal change in soil oxygen availability was determined in several habitats along a topographic moisture gradient in an arctic watershed. The effect of changes in soil aeration on soil chemical and plant properties was examined by comparison of the driest (tussocks) and wettest (wet sedge tundra) sites along this gradient. Spatial variability and seasonal change in soil oxygen availability was closely linked to the hydrologic regime and the thickness of the organic soil horizon. The greatest extension of the aerobic soil layer was found beneath well-drained tussocks, while less than 10% of the unfrozen soil layer is aerated in flooded wet sedge tundra. Intertussock areas and watertracks (channels of water drainage) have intermediate levels of aeration. In tussock tundra, soil oxygen diffusion is restricted in the mineral soil layer below the organic horizon due to reduced pore space. Organic matter constituents and their change with depth were similar beneath tussocks and in wet sedge tundra, indicating that factors other than soil aeration (e.g. low soil temperatures, short growing season) are the primary controls on decomposition in these two arctic tundra systems. NH4 +, the dominant form of inorganic N, was more available in wet sedge tundra than in tussock tundra. At both sites, extractable and soil solution NO3 - concentrations increased 4 to 8 fold in the second part of the growing season, indicating increased nitrifier activity with improved soil oxygen availability. Although soils thawed as deep as 60 cm, approx. 90% of the root biomass was concentrated within 20 cm of the surface. Despite the anaerobic soil environment in wet sedge tundra, the dominant species there, Eriophorum angustifolium, reached slightly greater rooting depths than E. vaginatum, whose roots grow in the elevated, aerobic portion of tussocks. E. angustifolium had a root porosity of 31%, within the range found for wetland species, while roots of E. vaginatum had a porosity close to 12%. Rhizome porosity were low in both species (11%).
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