ISSN:1354-1013    

DOI:10.1111/j.1365-2486.1996.tb00044.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

AbstractWe used the Terrestrial Ecosystem Model (TEM) to investigate how alternative input data sets of climate (temperature/precipitation), solar radiation, and soil texture affect estimates of net primary productivity (NPP) for the conterminous United States. At the continental resolution, the climates of Cramer and Leemans (C&L) and of the Vegetation/ Ecosystem Modelling and Analysis Project (VEMAP) represent cooler and drier conditions for the United States in comparison to the Legates and Willmott (L&W) climate, and cause 5.2% and 2.3% lower estimates of NPP. Solar radiation derived from C&L and given in VEMAP is 32% and 60% higher than the solar radiation data derived from Hahn cloudiness. These differences cause ∼ 8% and 10% lower NPP because of radiation‐induced water stress. In comparison to the FAO/CSRC soil texture, which represents most biomes with loam soils, the soil textures are finer (more silt and clay) in the Zobler and VEMAP data sets. The use of VEMAP soil textures instead of FAO/CSRC soil textures causes ∼ 3% higher NPP because enhanced volumetric soil moisture causes higher rates of nitrogen cycling, but use of the Zobler soil textures has little effect. In general, NPP estimates of TEM are more sensitive to alternative data sets at the biome and grid cell resolutions than at the continental resolution. At all spatial resolutions, the sensitivity of NPP estimates represents the impact of uncertainty among the alternative data sets we used in this study. The reduction of uncertainty in input data sets is required to improve the spatial resolution of NPP estimates by process‐based ecosystem models, and is especially important for improving assessments of the regional impacts of global

ISSN:1354-1013    

DOI:10.1111/j.1365-2486.1996.tb00045.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

AbstractWe measured rates of leaf senescence and leaf level gas exchange during autumnal senescence for seedlings of five temperate forest tree species under current and elevated atmospheric CO2concentrations and low‐ and high‐nutrient regimes. Relative indices of whole canopy carbon gain, water loss and water use efficiency through the senescent period were calculated based on a simple integrative model combining gas exchange per unit leaf area and standing canopy area per unit time. Seedlings grown under elevated [CO2] generally had smaller canopies than their current [CO2]‐grown counterparts throughout most of the senescent period. This was a result of smaller pre‐senescent canopies or accelerated rates of leaf drop. Leaf‐level photosynthetic rates were higher under elevated [CO2] for grey birch canopies and for low‐nutrient red maple and high‐nutrient ash canopies, but declined rapidly to values below those of their current [CO2] counterparts by midway through the senescent period. CO2enrichment reduced photosynthetic rates for the remaining species throughout some or all of the senescent period. As a result of smaller canopy sizes and reduced photosynthetic rates, elevated [CO2]‐grown seedlings had lower indices of whole canopy end‐of‐season carbon gain with few exceptions. Leaf level transpiration rates were highly variable during autumnal senescence and neither [CO2] nor nutrient regime had consistent effects on water loss per unit leaf area or integrated whole canopy water loss throughout the senescent period. Indices of whole canopy, end‐of‐season estimates of water use efficiency, however, were consistently lower under CO2enrichment, with few exceptions. These results suggest that whole canopy end‐of‐season gas exchange may be altered significantly in an elevated [CO2] world, resulting in reduced carbon gain and water use efficiency for many temperate forest tree seedlings. Seedling growth and survivorship, and ultimately temperate forest regeneration, could be reduced in CO2‐e

ISSN:1354-1013    

DOI:10.1111/j.1365-2486.1996.tb00046.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

AbstractAn experiment was carried out to determine the effects of elevated CO2, elevated temperatures, and altered water regimes in native shortgrass steppe. Intact soil cores dominated byBouteloua gracilis, a C4perennial grass, orPascopyrum smithii, a C3perennial grass, were placed in growth chambers with 350 or 700 μL L−1atmospheric CO2, and under either normal or elevated temperatures. The normal regime mimicked field patterns of diurnal and seasonal temperatures, and the high‐temperature regime was 4 °C warmer. Water was supplied at three different levels in a seasonal pattern similar to that observed in the field.Total biomass after two growing seasons was 19% greater under elevated CO2, with no significant difference between the C3and C4grass. The effect of elevated CO2on biomass was greatest at the intermediate water level. The positive effect of elevated CO2on shoot biomass was greater at normal temperatures inB. gracilis, and greater at elevated temperatures inP. smithii.Neither root‐to‐shoot ratio nor production of seed heads was affected by elevated CO2.Plant tissue N and soil inorganic N concentrations were lower under elevated Co2, but no more so in the C3than the C4plant. Elevated CO2appeared to increase plant N limitation, but there was no strong evidence for an increase in N limitation or a decrease in the size of the CO2effect from the first to the second growing season. Autumn samples of large roots plus crowns, the perennial organs, had 11% greater total N under elevated CO2, in spite of greater N l

ISSN:1354-1013    

DOI:10.1111/j.1365-2486.1996.tb00047.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

AbstractVegetation responses to high [CO2] include both direct photosynthetic effects and indirect effects associated with various plant and soil feedbacks. Synthesis of these direct and indirect effects requires ecosystem process models describing the cycling of carbon and essential mineral nutrients through plants and soils. Here we use the ecosystem model G'DAY to investigate responses to an instantaneous doubling of [CO2]. The analysis indicates that the magnitude and even direction of the growth response to high [CO2] can vary widely on different timescales, because responses on different timescales are determined by different ecosystem‐level feedbacks and hence by different sets of key model parameters. Of particular importance are parameters describing the flexibility of plant and soil nitrogen to carbon (N:C) ratios; large responses occur if N:C ratios decline significantly at high [CO2], with little or no response if N:C ratios are inflexible. According to G'DAY, the CO2‐response changes over time because responses on longer timescales are dictated by the N:C ratios of less rapidly cycled organic mat

ISSN:1354-1013    

DOI:10.1111/j.1365-2486.1996.tb00048.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

AbstractIn 1993 a project was set up at Upper Teesdale to investigate some of the effects of predicted increased temperatures and increased nutrient availability resulting from increased litter decomposition rates, and atmospheric deposition of nitrogen, on the interaction between bracken and heather, two species which cover large areas globally. 32 2 × 2 m plots were laid out on a hillside on, and just below, the interface between the two species. Half of the plots were placed on pure bracken, and the other half on the boundary. Within each vegetation type half of the plots were covered with open‐topped polythene tents to simulate climate warming, while the other plots remained open. The second treatment consisted of additional fertilizer at 50 kg N ha−1y−1simulating increased decomposition rates at higher temperatures and increased atmospheric nitrogen deposition. The two treatments and their controls were combined in a 2 × 2 factorial design, with two replicates in each of two blocks.Results obtained in the first two summers indicated that fronds emerged earlier in spring and senesced later in autumn inside the tents, effectively lengthening the growing season. Fronds were taller when growing in warmer conditions, and carried more pinnae. In addition, the frond density was higher inside the tents, and together these effects contributed to greatly enhanced vigour of bracken in some of the conditions predicted by General Circulation Models. It seems likely that bracken will be more competitive, and have the potential to further encroach into heather dominated areas under these con

ISSN:1354-1013    

DOI:10.1111/j.1365-2486.1996.tb00049.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

AbstractIn a laboratory experiment interaction effects of UV‐B and CO2on photosynthesis and growth of the mossHylocomium splendenswere studied. The plants were exposed to two CO2levels (350 ppm and 600 ppm) and three UV‐B levels (no UV‐B, ambient UV‐B and that corresponding to 20% ozone depletion) for 5 months. The effects were recorded by measuring the photosynthetic response and growth of the plants.There was a statistically significant change in photosynthetic efficiency and maximum photosynthetic rates due to time and to enhanced CO2concentration, whereas there was no effect due to UV‐B. There was a decreased growth due to both UV‐B and CO2and an interaction effect on growth (in length). The UV‐B dose corresponding to the ambient level had a larger reducing effect on growth than the highest UV‐B dose. This was a counter‐intuitive result and the following tentative interpretation was made: differences in the measured UV‐A/UV‐B/PAR ratios between the treatments could explain the result provided there was a non‐linear response to UV over the range of

ISSN:1354-1013    

DOI:10.1111/j.1365-2486.1996.tb00050.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

AbstractVisual observations of smoke dispersion in a wind tunnel and a computational fluid dynamics (CFD) model were used to evaluate methods of improving the performance of Free‐Air COT Enrichment (FACE) Systems for field studies of the effects of elevated CO2on vegetation. A special baffle, named an Enhanced Local Mixing (ELM) system, was observed to increase the turbulence and consequent dilution of fumigant gas in the atmosphere. Modelling results suggest that the ELM design reduces the spatial variation of fumigant gas concentration in small experimental plots. The potential reduction in CO2use and costs warrants further evaluation under Held condition

ISSN:1354-1013    

DOI:10.1111/j.1365-2486.1996.tb00051.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY