摘要:

Abstract1 Carbon dioxide and water vapour fluxes were measured for 55 days by eddy covariance over an undisturbed tropical rain forest in Rondonia, Brazil. Profiles of CO2inside the canopy were also measured.2 During the night, CO2concentration frequently built up to 500 ppm throughout the canopy as a result of low rates of exchange with the atmosphere. In the early morning hours, ventilation of the canopy occurred.3 Ecosystem gas exchange was calculated from a knowledge of fluxes above the canopy and changes of CO2stored inside the canopy. Typically, uptake by the canopy was 15 μmol m−2s−1in bright sunlight and dark respiration was 6‐7 μmol m−2s−1The quantum requirement at low irradiance was: 40 mol photons per mol of CO2.4 Bulk stomatal conductance of the ecosystem was maximal in the early morning (0.4‐1.0 mol m−2s−1) and declined over the course of the day as leaf‐to‐air vapour

ISSN:1354-1013    

DOI:10.1111/j.1365-2486.1995.tb00001.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractThe impact of climate change and increasing atmospheric CO2was modelled for 31 temperate and tropical grassland sites, using the CENTURY model. Climate change increased net primary production, except in cold desert steppe regions, and CO2increased production everywhere. Climate change caused soil carbon to decrease overall, with a loss of 4 Pg from global grasslands after 50 years. Combined climate change and elevated CO2increased production and reduced global grassland C losses to 2 Pg, with tropical savannas becoming small sinks for soil C. Detection of statistically significant change in plant production would require a 16% change in measured plant production because of high year to year variability in plant production. Most of the predicted changes in plant production are less than 10%.

ISSN:1354-1013    

DOI:10.1111/j.1365-2486.1995.tb00002.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

Abstract1 Theory suggests that any given rise in temperature resulting from climate change will have its greatest effect on high Arctic ecosystems where growing seasons are short and temperatures low.2 A small temperature rise, similar to that predicted for the middle of the next century, has profound effects on a population of the high Arctic, Dryas‐feeding aphidAcyrthosiphon svalbardicumon Spitsbergen (Strathdeeet al.1993a).3 Here comparative experiments on a closely related Dryas‐feeding species,A. brevicorne, at two contrasting sub‐Arctic sites are described. Together with the results from Spitsbergen these sites represent two colder sites (high Arctic and upland sub‐Arctic) and one warmer site (lowland sub‐Arctic).4 Differential responses in aphid population density and overwintering egg production to temperature elevation support the hypothesis that the ecological effects are greatest at sites with the most severe climates; however, there is no similar gradient in advancement of host plant pheno

ISSN:1354-1013    

DOI:10.1111/j.1365-2486.1995.tb00003.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractThis paper considers the use of passive greenhouse apparatus in field experiments investigating the biological consequences of climate change. The literature contains many accounts of such experiments claiming relevance of greenhouse treatment effects to global change scenarios. However, inadequacies in microclimate monitoring, together with incomplete understanding of greenhouse modes of action, cast doubt upon such claims. Here, treatment effects upon temperature (magnitude, range, variation, rates of change), moisture (humidity, precipitation, soil water content), light (intensity, spectral distribution), gas composition, snow cover, and wind speed are reviewed in the context of Intergovernmental Panel on Climate Change (IPCC) predictions. It is revealed that greenhouses modify each of these potentially limiting factors in a complex and interactive manner, but that the relationship between this modification and forecast conditions of climate change is poor. Interpretation of biological responses, and their extrapolation to predictive models, is thus unreliable. In order that future greenhouse experiments may overcome criticisms of artefact and lack of rigour, two amendments to methodology are proposed: (1) objective‐orientated design of greenhouse apparatus (2) multiple controls addressing individual environmental factors. The importance of a priori testing of microclimate treatment effects is stresse

ISSN:1354-1013    

DOI:10.1111/j.1365-2486.1995.tb00004.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractMixtures and monocultures of wheat (Triticum aestivum) and wild oat (Avena fatua), a common weedy competitor of wheat, were exposed to enhanced solar UV‐B radiation simulating a 20% reduction in stratospheric ozone to assess the timing and seasonal development of the UV‐B effects on light competition in these species. Results from two years of field study revealed that UV‐B enhancement had no detectable effect on the magnitude or timing of seedling emergence in either species. End‐of‐season measurements showed significant UV‐B inhibition of leaf insertion height in wild oat in mixture and monoculture in the second year (irrigated year) but not in the first year (drought year). Leaf insertion height of wheat was not affected by UV‐B in either year. The UV‐B treatment had no detectable effect on monoculture or total (combined species) mixture LAI but did significantly increase (5–7%) the fractional contribution of wheat to the mixture LAI after four weeks of growth in both years. In addition, the UV‐B treatment had subtle effects on LAI height profiles with early season mixtures showing significant reductions in wild oat LAI in lower canopy layers in both years while midseason Year 2 mixtures showed significant reductions in wild oat LAI in upper canopy layers. The changes in canopy structure were found to significantly increase (6–7%) the proportional simulated clear sky canopy photosynthesis and light interception of wheat in mixture. These findings, and others, indicate that the effects of UV‐B enhancement on competition are realized very early in canopy development and provide additional support for the hypothesis that UV‐B enhancement may shift the balance of competition between these species indirectly by altering competiti

ISSN:1354-1013    

DOI:10.1111/j.1365-2486.1995.tb00005.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractFew studies have investigated the effects of elevated CO2on the physiology of symbiotic N2‐fixing trees. Tree species grown in low N soils at elevated CO2generally show a decline in photosynthetic capacity over time relative to ambient CO2controls. This negative adjustment may be due to a reallocation of leaf N away from the photosynthetic apparatus, allowing for more efficient use of limiting N. We investigated the effect of twice ambient CO2on net CO2assimilation (A), photosynthetic capacity, leaf dark respiration, and leaf N content of N2‐fixingAlnus glutinosa(black alder) grown in field open top chambers in a low N soil for 160 d.At growth CO2, A was always greater in elevated compared to ambient CO2plants. Late season A vs. internal leaf p(CO2) response curves indicated no negative adjustment of photosynthesis in elevated CO2plants. Rather, elevated CO2plants had 16% greater maximum rate of CO2fixation by Rubisco. Leaf dark respiration was greater at elevated CO2on an area basis, but unaffected by CO2on a mass or N basis. In elevated CO2plants, leaf N content (μg N cm−2) increased 50% between Julian Date 208 and 264. Leaf N content showed little seasonal change in ambient CO2plants. A single point acetylene reduction assay of detached, nodulated root segments indicated a 46% increase in specific nitrogenase activity in elevated compared to ambient CO2plants. Our results suggest that N2‐fixing trees will be able to maintain high A with minimal negative adjustment of photosynthetic capacity following prolonged exposure to elevated CO2on N‐

ISSN:1354-1013    

DOI:10.1111/j.1365-2486.1995.tb00006.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractModels of global change must come to incorporate changes in terrestrial vegetation. Here we choose a 1‐ year meshing (coupling) period to link a global climate model to a well‐known biophysical representation of the continental surface by means of eleven vegetation functional types. This coupled model is used to answer two questions:Can a ‘standard’ GCM ‘cope' with sudden switches in continental characteristics?’ andDoes the climate ‘care’ about the changing underlying vegetation? We find affirmative answers to both questions. Our results also suggest that those content to generate vegetationpost factofrom climate output have inc

ISSN:1354-1013    

DOI:10.1111/j.1365-2486.1995.tb00007.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractThe terrestrial biosphere plays an important role in the global carbon cycle. In the 1994 Intergovernmental Panel Assessment on Climate Change (IPCC), an effort was made to improve the quantification of terrestrial exchanges and potential feedbacks from climate, changing CO2, and other factors; this paper presents the key results from that assessment, together with expanded discussion. The carbon cycle is the fluxes of carbon among four main reservoirs: fossil carbon, the atmosphere, the oceans, and the terrestrial biosphere. Emissions of fossil carbon during the 1980s averaged 5.5 Gt y−1. During the same period, the atmosphere gained 3.2 Gt C y−1and the oceans are believed to have absorbed 2.0 Gt C y−1. The regrowing forests of the Northern Hemisphere may have absorbed 0.5 Gt C y−1during this period. Meanwhile, tropical deforestation is thought to have released an average 1.6 Gt C y−1over the 1980s. While the fluxes among the four pools should balance, the average 198Ds values lead to a ‘missing sink’ of 1.4 Gt C y−1Several processes, including forest regrowth, CO2fertilization of plant growth (c.1.0 Gt C y−1), N deposition (c.0.6 Gt C y−1), and their interactions, may account for the budget imbalance. However, it remains difficult to quantify the influences of these separate but interactive processes. Uncertainties in the individual numbers are large, and are themselves poorly quantified. This paper presents detail beyond the IPCC assessment on procedures used to approximate the flux uncertainties.Lack of knowledge about positive and negative feedbacks from the biosphere is a major limiting factor to credible simulations of future atmospheric CO2concentrations. Analyses of the atmospheric gradients of CO2and13CO2concentrations provide increasingly strong evidence for terrestrial sinks, potentially distributed between Northern Hemisphere and tropical regions, but conclusive detection in direct biomass and soil measurements remains elusive.Current regional‐to‐global terrestrial ecosystem models with coupled carbon and nitrogen cycles represent the effects of CO2fertilization differently, but all suggest longterm responses to CO2that are substantially smaller than potential leaf‐ or laboratory whole plant‐level responses. Analyses of emissions and biogeochemical fluxes consistent with eventual stabilization of atmospheric CO2concentrations are sensitive to the way in which biospheric feedbacks are modeled byc.15%. Decisions about land use can have effects of 100s of Gt C over the next few centuries, with similarly significant effects on the atmosphere.Critical areas for future research are continued measurements and analyses of atmospheric data (CO2and13CO2) to serve as large‐scale constraints, process studies of the scaling from the photosynthetic response to CO2to whole‐ecosystem carbon storage, and rigorous quantification of the effects of chan

ISSN:1354-1013    

DOI:10.1111/j.1365-2486.1995.tb00008.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY