bluuebunny/crossref_metadata_2025 huggingface dataset:
Nonlinearity of photosynthetic responses to growth in rising atmospheric CO2: an experimental and modelling study
Authors: Luo YiqI. , Sims Daniel A. , Griffin Kevin L.
Published: March 2003

Nonlinear responses of photosynthesis to the CO2 concentration at which plants were grown (Cg) have been often reported in the literature. This study was designed to develop mechanistic understanding of the nonlinear responses with both experimental and modelling approaches. Soybean (Glycine max) was grown in five levels of Cg (280, 350, 525, 700, 1000 ppm) with either a high or low rate of nitrogen fertilization. When the rate of nitrogen fertilization was high, the photosynthetic rate measured at Cg was highest in plants from the 700 ppm CO2 treatment. When the rate of nitrogen fertilization was low, little variation was observed in the photosynthetic rates of plants from the different treatments measured at their respective Cg. Measurements of CO2‐induced changes in mass‐based leaf nitrogen concentration (nm, an index of changes in biochemical processes) and leaf mass per unit area (h, an index of morphological properties) were used in a model and indicate that the nonlinearity of photosynthetic responses to Cg is largely determined by relative changes in photosynthetic sensitivity, biochemical downregulation, and morphological upregulation. In order to further understand the nonlinear responses, we compiled data from the literature on CO2‐induced changes in nm and h. These compiled data indicate that h generally increases and nm usually decreases with increasing Cg, but that the trajectories and magnitudes of the changes in h and nm vary with species and growth environments. Integration of these variables (nm and h) into a biochemically based model of photosynthesis enabled us to predict diverse responses of photosynthesis to Cg. Thus a general mechanism is suggested for the highly variable, nonlinear responses of photosynthesis to Cg reported in the literature.
Evidence from rain‐use efficiencies does not indicate extensive Sahelian desertification
Authors: Prince S. D. , De Colstoun E. Brown , Kravitz L. L.
Published: March 2003

Desertification is regarded as one of the major global environmental problems of the 20th century and the African Sahel is often quoted as the most seriously affected region. Previous attempts to map the occurrence and severity of desertification in the Sahel have been unsatisfactory, mainly because of the lack of any readily measured, objective indicators. We explore here the properties of the ratio of net primary production (NPP) to precipitation – the rain‐use efficiency (RUE) – calculated from remotely sensed vegetation indices and rain gauge data. Negative deviations from the normal range of RUE values are shown to be an indicator of desertification. Observations of NPP of the entire Sahel were possible using satellite platforms for the period 1982–90, including the 1984 drought. The results suggest that NPP was remarkably resilient, a fact that was reflected in only little variation in the RUE during the period of study. Thus, in much of the region, NPP seems to be in step with rainfall, recovering rapidly following drought and not supporting the fears of widespread, subcontinental scale desertification taking place in the 9‐year period that is studied. In fact the results show a small but systematic increase in RUE for the Sahel as a whole from 1982 to 1990, although some areas contained within the region did have persistently low values.
Effect of climate change on mast‐seeding species: frequency of mass flowering and escape from specialist insect seed predators
Authors: MCKONE MARK. J. , KELLY DAVE. , LEE WILLIAM G.
Published: March 2003

Global surface temperatures are expected to increase by several degrees in the next century, with potentially large but poorly understood impacts on ecological interactions. Here we propose potential effects of increased temperatures on ecologically dominant New Zealand grasses (Chionochloa spp.) that mass flower and mast seed. Twenty‐two years’ data from five masting Chionochloa species in New Zealand showed that the cue for heavy flowering was unusually high temperature in the summer of the year before flowering. Attack by predispersal insect seed predators was much reduced in mast years, apparently because predator populations were satiated. Increased temperatures would greatly decrease interannual variation in Chionochloa flowering, allowing seed predator populations to increase and potentially to devastate the seed crop annually. Similar responses are likely in masting species worldwide. This previously unrecognized effect of global warming could have widespread impacts on temperate ecosystems.
Litter decomposition rates in Canadian forests
Authors: Moore T. R. , Trofymow J. A. , Taylor B. , Prescott C. , Camiré C. , Duschene L. , Fyles J. , Kozak L. , Kranabetter M. , Morrison I. , Siltanen M. , Smith S. , Titus B. , Visser S. , Wein R. , Zoltai S.
Published: March 2003

The effect of litter quality and climate on the rate of decomposition of plant tissues was examined by the measurement of mass remaining after 3 years’ exposure of 11 litter types placed at 18 forest sites across Canada. Amongst sites, mass remaining was strongly related to mean annual temperature and precipitation and amongst litter types the ratio of Klason lignin to nitrogen in the initial tissue was the most important litter quality variable. When combined into a multiple regression, mean annual temperature, mean annual precipitation and Klason lignin:nitrogen ratio explained 73% of the variance in mass remaining for all sites and tissues. Using three doubled CO2 GCM climate change scenarios for four Canadian regions, these relationships were used to predict increases in decomposition rate of 4–7% of contemporary rates (based on mass remaining after 3 years), because of increased temperature and precipitation. This increase may be partially offset by evidence that plants growing under elevated atmospheric CO2 concentrations produce litter with high lignin:nitrogen ratios which slows the rate of decomposition, but this change will be small compared to the increased rate of decomposition derived from climatic changes.
Hydrology, water availability and tundra ecosystem function in a changing climate: the need for a closer integration of ideas?
Authors: Hodkinson I. D. , Webb N. R. , Bale J. S. , Block W.
Published: March 2003

Hydrologists and ecologists studying tundra ecosystems have worked largely independently, with little cross‐fertilization between disciplines. Their disciplines are, however, inextricably linked by a need to understand the dynamics and significance of the common substance water, in its liquid, solid and gaseous state within tundra environments. The impacts of predicted long‐term changes in climate have particularly important consequences for the functioning of tundra systems and there is a pressing need to initiate studies that integrate hydrological and ecological methodologies and concepts. Our paper attempts to summarize existing information on the role of water within tundra ecosystems, to emphasize the fundamental links between the biotic and the physico/chemical environments and to suggest how a closer integration of ideas might be achieved. Given the breadth of the subject matter the paper is intended to be illustrative rather than comprehensive. The paper examines the physical impacts of water in its various states on the tundra environment, emphasizing in particular the causes of spatial variation in water availability to living organisms. The significance of water is discussed for a range of organism groups, including plants, invertebrates and microorganisms and its pivotal role in ecosystem function and disturbance stressed. The need to develop integrated hydological/ecological models for tundra systems on different spatial scales is emphasized.
Seasonal changes in the response of winter wheat to elevated atmospheric CO2 concentration grown in Open‐Top Chambers and field tracking enclosures
Authors: Dijkstra Paul , Schapendonk AD. H. M. C. , Groenwold KO. , Jansen Marinus , Van De Geijn Siebe C.
Published: March 2003

Winter wheat was grown at ambient and elevated (ambient plus 350 μL L–1) CO2 concentrations in open top chambers and in field‐tracking sun‐lit climatized enclosures (elevated is 718 μL L–1). There was no significant effect of CO2 concentration on sheath, leaf and root biomass and leaf area in the early spring (January to April). 24‐h canopy CO2 exchange rate (CCER) was not significantly affected either. However, elevated CO2 concentration increased CCER at midday, decreased evapotranspiration rate and increased instantaneous water‐use‐efficiency during early spring. Leaf, sheath and root nitrogen concentration per unit dry weight decreased and nonstructural carbohydrate concentration increased under elevated CO2, and N‐uptake per unit ground area decreased significantly (– 22%) towards the end of this period.

These results contrast with results from the final harvest, when grain yield and biomass were increased by 19% under elevated CO2. N concentration per dry weight was reduced by 5%, but N‐uptake per unit ground area was significantly higher (+ 11%) for the elevated CO2 treatment. 24‐h and midday‐CCER increased significantly more in late spring (period of 21 April to 30 May) (respectively by + 40% and 53%) than in the early spring (respectively 5% and 19%) in response to elevated CO2. Midday evapotranspiration rate was reduced less by elevated CO2 in the late spring (– 13%) than in early spring (– 21%). The CO2 response of midday and 24‐h CCER decreased again (+ 27% and + 23% resp.) towards the end of the growing season.

We conclude that the low response to CO2 concentration during the early spring was associated with a growth‐restriction, caused by low temperature and irradiance levels. The reduction of nitrogen concentration, the increase of nonstructural carbohydrate, and the lower evapotranspiration indicated that CO2 did have an effect towards the end of early spring, but not on biomass accumulation. Regression analysis showed that both irradiance and temperature affected the response to CO2.
Elevated UV‐B radiation effects on experimental grassland communities
Authors: Norton Lisa. R. , Mcleod Andrew R. , Greenslade Peter D. , Firbank Les. G. , Watkinson Andrew R.
Published: March 2003

Experimental grassland communities (turves) were exposed to supplemental levels of UV‐B radiation (280–315 nm) at an outdoor facility, under treatment arrays of cellulose diacetate‐filtered fluorescent lamps which also produce UV‐A radiation (315–400 nm). Control treatments consisted of arrays of polyester‐filtered lamps, which allowed for exposure to UV‐A radiation alone, and arrays of unenergized lamps allowing for exposure to ambient levels of solar radiation.
Carbon partitioning and rhizosphere C‐flow in Lolium perenne as affected by CO2 concentration, irradiance and below‐ground conditions
Authors: Paterson EriC. , Hodge Angela , Thornton Barry , Millard Peter , Killham KeN.
Published: March 2003

Plant responses to increasing atmospheric CO2 concentrations have received considerable interest. However, major uncertainties in relation to interactive effects of CO2 with above‐ and below‐ground conditions remain. This microcosm study investigated the impacts of CO2 concentration on plant growth, dry matter partitioning and rhizodeposition as affected by: (i) photon flux density (PFD), and (ii) growth matrix. Plants were grown in a sandy loam soil for 28 d under two photon flux densities: 350 (low PFD) and 1000 μmol m–2 s–1 (high PFD) and two CO2 concentrations: 450 (low CO2) and 720 μmol mol–1 (high CO2). Partitioning of recent assimilate amongst plant and rhizosphere C‐pools was determined by use of 14CO2 pulse‐labelling. In treatments with high PFD and/or high CO2, significant (P < 0.05) increases in dry matter production were found in comparison with the low PFD/low CO2 treatment. In addition, significant (P < 0.05) reductions in shoot %N and SLA were found in treatments imposing high PFD and/or high CO2. Root weight ratio (RWR) was unaffected by CO2 concentration, however, partitioning of 14C to below ground pools was significantly (P < 0.05) increased. In a separate study, L. perenne was grown for 28 d in microcosms percolated with nutrient solution, in either a sterile sand matrix or nonsterile soil, under high or low CO2. Dry matter production was significantly (P < 0.01) increased for both sand and soil grown seedlings. Dry matter partitioning was affected by matrix type. 14C‐allocation below ground was increased for sand grown plants. Rhizodeposition was affected by CO2 concentration for growth in each matrix, but was increased for plants grown in the soil matrix, and decreased for those in sand. The results illustrate that plant responses to CO2 are potentially affected by (i) PFD, and (ii) by feedbacks from the growth matrix. Such feedbacks are discussed in relation to soil nutrient status and interactions with the rhizosphere microbial biomass.
Toward an allocation scheme for global terrestrial carbon models
Authors: Friedlingstein P. , Joel G. , Field C. B. , Fung I. Y.
Published: March 2003

The distribution of assimilated carbon among the plant parts has a profound effect on plant growth, and at a larger scale, on terrestrial biogeochemistry. Although important progress has been made in modelling photosynthesis, less effort has been spent on understanding the carbon allocation, especially at large spatial scales. Whereas several individual‐level models of plant growth include an allocation scheme, most global terrestrial models still assume constant allocation of net primary production (NPP) among plant parts, without any environmental coupling. Here, we use the CASA biosphere model as a platform for exploring a new global allocation scheme that estimates allocation of photosynthesis products among leaves, stems, and roots depending on resource availability. The philosophy underlying the model is that allocation patterns result from evolved responses that adjust carbon investments to facilitate capture of the most limiting resources, i.e. light, water, and mineral nitrogen. In addition, we allow allocation of NPP to vary in response to changes in atmospheric CO2. The relative magnitudes of changes in NPP and resource‐use efficiency control the response of root:shoot allocation. For ambient CO2, the model produces realistic changes in above‐ground allocation along productivity gradients. In comparison to the CASA standard estimate using fixed allocation ratios, the new allocation scheme tends to favour root allocation, leading to a 10% lower global biomass. Elevated CO2, which alters the balance between growth and available resources, generally leads to reduced water stress and consequently, decreased root:shoot ratio. The major exception is forest ecosystems, where increased nitrogen stress induces a larger root allocation.
A moderate increase in the annual CH4 efflux by raised CO2 or NH4NO3 supply in a boreal oligotrophic mire
Authors: Saarnio Sanna , Saarinen T.I.M.O. , Vasander Harri , Silvola Jouko
Published: March 2003

Natural wetlands release about 20% of global emissions of CH4, an effective greenhouse gas contributing to the total radiative forcing. Thus, changes in the carbon cycle in wetlands could have significant impacts on climate. The effect of raised supply of CO2 or NH4NO3 on the annual CH4 efflux from the lawn of a boreal oligotrophic mire was investigated over two years. Ten study plots were enclosed with mini‐FACE rings, five vented with CO2‐enriched air and the other five with ambient air. In addition, five plots were sprayed with NH4NO3 so that the cumulative addition of N was 3 g m−2 y−1; and five plots were controls. The CO2 enrichment (target concentration 560 ppmv) increased CH4 efflux about 30–40%, but half of this increase seemed to be caused by the air‐blowing system. The increasing atmospheric concentration of CO2 would promote CH4 release in boreal mires, but the increase in CH4 efflux would be clearly smaller than that reported in studies made in temperate or subtropical temperature conditions. Addition of N enhanced the annual release of CH4 only slightly. At least over the short‐term, the increase in N deposition would have little effect on CH4 effluxes. The increase in CH4 release would probably increase radiative forcing and thus accelerate climate change. However, CH4 effluxes are only a small part in the whole matter balance in mires and thus further studies are needed to define the net effects of raised supply of CO2 or N for carbon accumulation, trace gas fluxes and radiative forcing.
Links between large‐scale anomalies, rainfall and wine quality in the Iberian Peninsula during the last three decades
Authors: Rodó Xavier , Comín F. A.
Published: March 2003

Recent strong El Niño‐Southern Oscillation (ENSO) signals have been identified in precipitation records from the Iberian Peninsula. Interannual association with ENSO accounts for more than half the total annual variance in selected stations of the south‐east, with ENSO leading rainfall by one year. In contrast, association with the North Atlantic Oscillation (NAO) at the Westernmost stations is much lower (25%). The potential of simple linear models is tested in the ENSO‐sensitive area, suggesting high capability of the Southern Oscillation Index (SOI) for predicting interannual rainfall fluctuations (mainly droughts and floods). Wine quality is associated with several factors, e.g. grape variety, soil type and processing, which can be considered invariable, mainly due to the strict regulations imposed by the quality regulating councils. Climate, however, has a great influence on resulting wine quality, and represents the most important source of variability at both short (day‐to‐day) and long (interannual) time scales.

Over the last 30 years, high‐quality harvests in the five main wine regions in Spain, show a high probability (P < 0.0002) of being associated with an El Niño event occurring the same year or the year before. NAO influence is not significant during the same period. Thus, apart from considering the role of local climatic conditions in certain regions, which favour the production of excellent wines, larger‐scale climatic phenomena appear responsible for the year‐to‐year variations in quality.
Elevated atmospheric CO2 and increased nitrogen deposition: effects on C and N metabolism and growth of the peat moss Sphagnum recurvum P. Beauv. var. mucronatum (Russ.) Warnst
Authors: Van Der Heijden Edwin , Verbeek Steven K. , Kuiper Pieter J. C.
Published: March 2003

Sphagnum bogs play an important role when considering the impacts of global change on global carbon and nitrogen cycles. Sphagnum recurvum P. Beauv. var. mucronatum (Russ.) was grown at 360 (ambient) and 700 μL L−1 (elevated) atmospheric [CO2] in combination with different nitrogen deposition rates (6, 15, 23 g N m−2 y−1), in a short‐ and long‐term growth chamber experiment.

After 6 months, elevated atmospheric [CO2] in combination with the lowest nitrogen deposition rate, increased plant dry mass by 17%. In combination with a high nitrogen deposition rate, biomass production was not significantly stimulated.

At the start of the experiment, photosynthesis was stimulated by elevated atmospheric [CO2], but it was downregulated to control levels after three days of exposure. Elevated [CO2] substantially reduced dark respiration, which resulted in a continuous increase in soluble sugar content in capitula. Differences in growth response among different nitrogen and CO2 treatments could not be related to measured carbon exchange rates, which was mainly due to interference of microbial respiration.

Doubling atmospheric [CO2] reduced total nitrogen content in capitula but not in stems at all nitrogen deposition rates. Reduction in total nitrogen content coincided with a decrease in amino acids, but soluble protein levels remained unaffected. Thus, elevated [CO2] induced a substantial shift in the partitioning of nitrogen compounds in capitula. Soluble sugar concentration was negatively correlated with total nitrogen content, which implies that the reduction in amino acid content in capitula, exposed to elevated [CO2], might be caused by the accumulation of soluble sugars.

Growth was not stimulated by increased nitrogen deposition. High nitrogen deposition, resulting in a capitulum nitrogen content in excess of 15 mg g−1 dw, was detrimental to photosynthesis, reduced water content and induced necrosis. We propose a capitulum nitrogen content of 15 mg g−1 dw as a possible bioindicator for the detection of nitrogen pollution stress in oligo‐mesotrophic peat bog ecosystems.

At the lowest nitrogen deposition level, nitrogen recovery was higher than 100%, which indicates substantial dry deposition and/or gaseous nitrogen fixation by bacteria, associated with Sphagnum. Increasing nitrogen deposition rates decreased nitrogen recovery percentages, which indicates reduced efficiency of nitrogen fixation.
Mechanisms and kinetics of nitric and nitrous oxide production during nitrification in agricultural soil
Authors: Venterea Rodney T. , Rolston Dennis E.
Published: March 2003

Laboratory experiments were conducted with three California agricultural soils to examine substrate and process controls over temporal variability of NO and N2O production during nitrification, and to quantify the kinetics of HNO2‐mediated chemical reactions. Gross NO production rates were highly correlated (r2 = 0.93–0.97) with calculated concentrations of HNO2, which were shown to originate from autotrophic microbial oxidation of

NH4 + to
NO2 − Production of NO was not correlated with
NH4 + or NO3–, or with the overall nitrification rate. Distinct periods of high NO2– accumulation occurred below critical pH values in each soil, apparently due to inhibition of microbial NO2– oxidation. Data suggest that even during periods of relatively low NO2– accumulation and rapid overall nitrification, HNO2‐mediated reactions may have been the primary source of NO. Rate coefficients (kPNO) relating NO production to HNO2 concentrations were determined for sterile (λ‐irradiated) soils, and were similar to kPNO values in 2 of 3 nonsterile soils undergoing nitrification. Production of N2O was correlated with HNO2 (r2 = 0.88–0.99) in sterile soils, and with NO2– and NO3– (R2 = 0.72–0.91) in nonsterile soils. Experiments using 15N confirmed that dissimilatory NO3– reduction contributed to N2O production even under primarily aerobic conditions. Sterile kPNO and kPN2O values were correlated (r2 = 0.90 and 0.82) with soil organic matter content. Overall, the results demonstrate that both steps of the nitrification sequence, together with abiotic reactions involving NO2–/HNO2 need to be considered in developing improved models of NO and N2O emissions from soils.
Infection with the parasitic angiosperm Striga hermonthica influences the response of the C3 cereal Oryza sativa to elevated CO2
Authors: Watling Jennifer R. , Press Malcolm C.
Published: March 2003

Upland rice (Oryza sativa L.) was grown at both ambient (350 μmol mol−1) and elevated (700 μmol mol−1) CO2 in either the presence or absence of the root hemi‐parasitic angiosperm Striga hermonthica (Del) Benth. Elevated CO2 alleviated the impact of the parasite on host growth: biomass of infected rice grown at ambient CO2 was 35% that of uninfected, control plants, while at elevated CO2, biomass of infected plants was 73% that of controls. This amelioration occurred despite the fact that O. sativa grown at elevated CO2 supported both greater numbers and a higher biomass of parasites per host than plants grown at ambient CO2. The impact of infection on host leaf area, leaf mass, root mass and reproductive tissue mass was significantly lower in plants grown at elevated as compared with ambient CO2. There were significant CO2 and Striga effects on photosynthetic metabolism and instantaneous water‐use efficiency of O. sativa. The response of photosynthesis to internal [CO2] (A/Ci curves) indicated that, at 45 days after sowing (DAS), prior to emergence of the parasites, uninfected plants grown at elevated CO2 had significantly lower CO2 saturated rates of photosynthesis, carboxylation efficiencies and ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco; EC 4.1.1.39) contents than uninfected, ambient CO2‐grown O. sativa. In contrast, infection with S. hermonthica prevented down‐regulation of photosynthesis in O. sativa grown at elevated CO2, but had no impact on photosynthesis of hosts grown at ambient CO2. At 76 DAS (after parasites had emerged), however, infected plants grown at both elevated and ambient CO2 had lower carboxylation efficiencies and Rubisco contents than uninfected O. sativa grown at ambient CO2. The reductions in carboxylation efficiency (and Rubisco content) were accompanied by similar reductions in nitrogen concentration of O. sativa leaves, both before and after parasite emergence. There were no significant CO2 or infection effects on the concentrations of soluble sugars in leaves of O. sativa, but starch concentration was significantly lower in infected plants at both CO2 concentrations. These results demonstrate that elevated CO2 concentrations can alleviate the impact of infection with Striga on the growth of C3 hosts such as rice and also that infection can delay the onset of photosynthetic down‐regulation in rice grown at elevated CO2.
A mechanistic evaluation of photosynthetic acclimation at elevated CO2
Authors: Rogers Alistair , Humphries Steven W.
Published: March 2003

Plants grown at elevated pCO2 often fail to sustain the initial stimulation of net CO2 uptake rate (A). This reduced, acclimated, stimulation of A often occurs concomitantly with a reduction in the maximum carboxylation velocity (Vc,max) of Rubisco. To investigate this relationship we used the Farquhar model of C3 photosynthesis to predict the minimum Vc,max capable of supporting the acclimated stimulation in A observed at elevated pCO2. For a wide range of species grown at elevated pCO2 under contrasting conditions we found a strong correlation between observed and predicted values of Vc,max. This exercise mechanistically and quantitatively demonstrated that the observed acclimated stimulation of A and the simultaneous decrease in Vc,max observed at elevated pCO2 is mechanistically consistent. With the exception of plants grown at a high elevated pCO2 (> 90 Pa), which show evidence of an excess investment in Rubisco, the failure to maintain the initial stimulation of A is almost entirely attributable to the decrease in Vc,max and investment in Rubisco is coupled to requirements.
Terrestrial organic carbon storage in a British moorland
Authors: Garnett MarK. H. , Ineson Philip , Stevenson Anthony C. , Howard David C.
Published: March 2003

Accurate estimates for the size of terrestrial organic carbon (C) stores are needed to determine their importance in regulating atmospheric CO2 concentrations. The C stored in vegetation and soil components of a British moorland was evaluated in order to: (i) investigate the importance of these ecosystems for C storage and (ii) test the accuracy of the United Kingdom's terrestrial C inventory. The area of vegetation and soil types was determined using existing digitized maps and a Geographical Information System (GIS). The importance of evaluating C storage using 2D area projections, as opposed to true surface areas, was investigated and found to be largely insignificant. Vegetation C storage was estimated from published results of productivity studies at the site supplemented by field sampling to evaluate soil C storage. Vegetation was found to be much less important for C storage than soil, with peat soils, particularly Blanket bog, containing the greatest amounts of C. Whilst the total amount of C in vegetation was similar to the UK national C inventory's estimate for the same area, the national inventory estimate for soil C was over three times higher than the value derived in the current study. Because the UK's C inventory can be considered relatively accurate compared to many others, the results imply that current estimates for soil C storage, at national and global scales, should be treated with caution.
Interspecies variation in nitrogen uptake kinetic responses of temperate forest species to elevated CO2: potential causes and consequences
Authors: Zerihun Ayalsew , Bassirirad Hormoz
Published: March 2003

Despite the recognition that the capacity to acquire N is critical in plant response to CO2 enrichment, there is little information on how elevated CO2 affects root N uptake kinetics. The few available data indicate a highly variable pattern of response to elevated CO2, but it is presently unclear if the observed inconsistencies are caused by differences in experimental protocols or by true species differences. Furthermore, if there are interspecific variations in N uptake responses to elevated CO2, it is not clear whether these are associated with different functional groups. Accordingly, we examined intact root‐system NH4+ and NO3– uptake kinetic responses to elevated CO2 in seedlings of six temperate forest tree species, representing (i) fast‐ vs. slow‐growers and (ii) broad‐leaves vs. conifers, that were cultured and assayed in otherwise similar conditions. In general, the species tested had a higher uptake capacity (Vmax) for NH4+ than for NO3–. Species substantially differed in their NO3– and NH4+ uptake capacities, but the interspecific differences were markedly greater for NO3– than NH4+ uptake. Elevated CO2 had a species‐dependent effect on root uptake capacity for NH4+ ranging from an increase of 215% in Acer negundo L. to a decrease of about 40% in Quercus macrocarpa Michx. In contrast, NO3– uptake capacity responded little to CO2 in all the species except A. negundo in which it was significantly down‐regulated at elevated CO2. Across species, the capacity for NH4+ uptake was positively correlated with the relative growth rate (RGR) of species; however, the CO2 effect on NH4+ uptake capacity could not be explained by changes in RGR. The observed variation in NH4+ uptake response to elevated CO2 was also inconsistent with life‐form differences. Other possible mechanisms that may explain why elevated CO2 elicits a species‐specific response in root N uptake kinetics are discussed. Despite the fact that the exact mechanism(s) for such interspecific variation remains unresolved, these differences may have a significant implication for competitive interactions and community responses to elevated CO2 environment. We suggest that differential species responses in nutrient uptake capacity could be one potential mechanism for the CO2‐induced shifts in net primary productivity and species composition that have been observed in experimental communities exposed to elevated levels of CO2.
Assessing forest growth across southwestern Oregon under a range of current and future global change scenarios using a process model, 3‐PG
Authors: Coops N. C. , Waring R. H.
Published: March 2003

With improvements in mapping regional distributions of vegetation using satellite‐derived information, there is an increasing interest in the assessment of current limitations on forest growth and in making projections of how productivity may be altered in response to changing climatic conditions and management policies. We utilised a simplified physiologically based process model (3‐PG) across a 54 000 km2 mountainous region of southwestern Oregon, USA, to evaluate the degree to which maximum periodic mean annual increment (PAI) of forests could be predicted at a set of 448 forest inventory plots. The survey data were pooled into six broad forest types (coastal rain forest, interior coast range forest, mixed conifer, dry‐site Douglas‐fir, subalpine forest, and pine forest) and compared to the 3‐PG predictions at a spatial resolution of 1 km2. We found good agreement (r2 = 0.84) between mean PAI values of forest productivity for the six forest types with those obtained from field surveys. With confidence at this broader level of integration, we then ran model simulations to evaluate the constraints imposed by (i) soil fertility under current climatic conditions, (ii) the effect of doubling monthly precipitation across the region, and (iii) a widely used climatic change scenario that involves modifications in monthly mean temperatures and precipitation, as well as a doubling in atmospheric CO2 concentrations. These analyses showed that optimum soil fertility would more than double growth, with the greatest response in the subalpine type and the least increase in the coastal rain forests. Doubling the precipitation increased productivity in the pine type (> 50%) with reduced responses elsewhere. The climate change scenario with doubled atmospheric CO2 increased growth by 50% on average across all forest types, primarily as a result of a projected 33% increase in photosynthetic capacity. This modelling exercise indicates that, at a regional scale, a general relationship exists between simulated maximum leaf area index and maximum aboveground growth, supporting the contention that satellite‐derived estimates of leaf area index may be good measures of the potential productivity of temperate evergreen forests.
Raised atmospheric CO2 levels and increased N deposition cause shifts in plant species composition and production in Sphagnum bogs
Authors: Berendse Frank , Van Breemen Nico , Rydin HÅkan , Buttler Alexandre , Heijmans Monique , Hoosbeek Marcel R. , Lee John A. , Mitchell Edward , Saarinen Timo , Vasander Harri , Wallén Bo
Published: March 2003

Part of the missing sink in the global CO2 budget has been attributed to the positive effects of CO2 fertilization and N deposition on carbon sequestration in Northern Hemisphere terrestrial ecosystems. The genus Sphagnum is one of the most important groups of plant species sequestrating carbon in temperate and northern bog ecosystems, because of the low decomposability of the dead material it produces. The effects of raised CO2 and increased atmospheric N deposition on growth of Sphagnum and other plants were studied in bogs at four sites across Western Europe. Contrary to expectations, elevated CO2 did not significantly affect Sphagnum biomass growth. Increased N deposition reduced Sphagnum mass growth, because it increased the cover of vascular plants and the tall moss Polytrichum strictum. Such changes in plant species composition may decrease carbon sequestration in Sphagnum‐dominated bog ecosystems.
Evapotranspiration and soil water content in a scrub‐oak woodland under carbon dioxide enrichment
Authors: Hungate B. A. , Reichstein M. , Dijkstra P. , Johnson D. , Hymus G. , Tenhunen J. D. , Hinkle C.R. , Drake B. G.
Published: March 2003

Leaf conductance often decreases in response to elevated atmospheric CO2 concentration (Ca) potentially leading to changes in hydrology. We describe the hydrological responses of Florida scrub oak to elevated Ca during an eight‐month period two years after Ca manipulation began. Whole‐chamber gas exchange measurements revealed a consistent reduction in evapotranspiration in response to elevated Ca, despite an increase in leaf area index (LAI). Elevated Ca also increased surface soil water content, but xylem water deuterium measurements show that the dominant oaks in this system take up most of their water from the water table (which occurs at a depth of 1.5–3 m), suggesting that the water savings in elevated Ca in this system are primarily manifested as reduced water uptake at depth. Extrapolating these results to larger areas requires considering a number of processes that operate on scales beyond these accessible in this field experiment. Nevertheless, these results demonstrate the potential for reduced evapotranspiration and associated changes in hydrology in ecosystems dominated by woody vegetation in response to elevated Ca.
Marine eutrophication and benthos: the need for new approaches and concepts
Authors: Grall Jacques , Chauvaud Laurent
Published: March 2003

In this review, using examples drawn from field observations or experimental studies, our goals are (i) to briefly summarize the major changes, in terms of species composition and functional structure, occurring in phyto and zoobenthic communities in relation to nutrient enrichment of the ecosystems; particular interest is given to the major abiotic and biotic factors occurring during the eutrophication process, (ii) to discuss the direct and indirect influences of benthic organisms on eutrophication and whether the latter can be controlled or favoured by benthos; most benthic species play a major role in the process of benthic nutrient regeneration, affecting primary production by supplying nutrients directly and enhancing rates of pelagic recycling; experimental studies have shown that the impact of benthic fauna on benthic–pelagic coupling and nutrient release is considerable. Thus, once the eutrophication process is engaged—that is, high organic matter sedimentation—it may be indirectly favoured by benthic organisms; benthos should always be considered in eutrophication studies, (iii) to evaluate the limits of our observations and data, highlighting the strong need for integrated studies leading to new concepts. Coastal ecosystems and benthic communities are potentially impacted by numerous human activities (demersal fishing, toxic contaminants, aquaculture…); in order to design strategies of ecosystem restoration or rehabilitation, we have to better understand coastal eutrophication and develop tools for quantifying the impacts; in order to achieve this goal, some possible directions proposed are: integrated studies leading to new concepts, model development based on these concepts and finally comparison of various ecosystems on a global scale.
Spatio‐temporal variability and environmental controls of methane fluxes at the forest–tundra ecotone in the Fennoscandian mountains
Authors: Sjögersten Sofie , Wookey Philip A.
Published: March 2003

We report on temporal and spatial variability in net methane (CH4) fluxes measured during the thaw period of 1999 and 2000 at three study sites along a c. 8° latitudinal gradient in the Fennoscandian mountain range and across the mountain birch‐tundra ecotone. All of the sites studied here were underlain by well‐drained mesic soils. In addition, we conducted warming experiments in the field to simulate future climate change. Our results show significant CH4 uptake at mesic sites spanning the forest‐tundra ecotone: on average 0.031 and 0.0065 mg CH4 m−2 h−1 during the 1999 and 2000 thaw periods, respectively, in Abisko (Sweden), and 0.019 and 0.032 mg CH4 m−2 h−1 during 2000 in Dovrefjell and Joatka (Norway), respectively. These values were both temporally and spatially highly variable, and multiple regression analysis of data from Abisko showed no consistent relationship with soil‐moisture status and temperature. Also, there was no consistent difference in CH4 fluxes between forest and tundra plots; our data, therefore, provide no support for the hypothesis that conversion of tundra to mountain birch forest, or vice versa, would result in a systematic change in the magnitude or direction of net CH4 fluxes in this region. Experimental warming treatments were associated with a 2.4 °C increase in soil temperatures (5 cm depth) in 1999 in Abisko, but no consistent soil warming was noted at any of the three field locations during 2000. In spite of this, there were significant treatment effects, principally early during the thaw period, with increased CH4 uptake compared with control (ambient) plots. These results suggest that direct effects of air warming on vegetation processes (e.g. transpiration, root exudation and nutrient assimilation) can influence CH4 fluxes even in predominantly methanotrophic environments. We conclude that net CH4 oxidation is significant in these cold, mesic soils and could be strengthened in an environmental change scenario involving a combination of (i) an increase in the length of the thaw period and (ii) increased mean temperatures during this period in combination with decreased soil‐moisture content.
Seasonal variability in the effect of elevated CO2 on ecosystem leaf area index in a scrub‐oak ecosystem
Authors: Hymus Graham J. , Pontailler Jean‐Yves , Li Jiahong , Stiling Peter , Hinkle C. Ross , Drake Bert G.
Published: March 2003

We report effects of elevated atmospheric CO2 concentration (Ca) on leaf area index (LAI) of a Florida scrub‐oak ecosystem, which had regenerated after fire for between three and five years in open‐top chambers (OTCs) and was yet to reach canopy closure. LAI was measured using four nondestructive methods, calibrated and tested in experiments performed in calibration plots near the OTCs. The four methods were: PAR transmission through the canopy, normalized difference vegetation index (NDVI), hemispherical photography, and allometric relationships between plant stem diameter and plant leaf area. Calibration experiments showed: (1) Leaf area index could be accurately determined from either PAR transmission through the canopy or hemispherical photography. For LAI determined from PAR transmission through the canopy, ecosystem light extinction coefficient (k) varied with season and was best described as a function of PAR transmission through the canopy. (2) A negative exponential function described the relationship between NDVI and LAI; (3) Allometric relationships overestimated LAI. Throughout the two years of this study, LAI was always higher in elevated Ca, rising from, 20% during winter, to 55% during summer. This seasonality was driven by a more rapid development of leaf area during the spring and a relatively greater loss of leaf area during the winter, in elevated Ca. For this scrub‐oak ecosystem prior to canopy closure, increased leaf area was an indirect mechanism by which ecosystem C uptake and canopy N content were increased in elevated Ca. In addition, increased LAI decreased potential reductions in canopy transpiration from decreases in stomatal conductance in elevated Ca. These findings have important implications for biogeochemical cycles of C, N and H2O in woody ecosystems regenerating from disturbance in elevated Ca.
Decomposition of secondary compounds from needle litter of Scots pine grown under elevated CO2and O3
Authors: Kainulainen Pirjo , Holopainen Toini , Holopainen Jarmo K.
Published: March 2003

Elevated atmospheric carbon dioxide (CO2) and ozone (O3) concentrations have both been shown to affect plant tissue quality, which in turn could affect litter decomposition and carbon (C) and nutrient cycling. In order to evaluate effects of climate change on litter chemistry, needle litter was collected from Scots pine (Pinus sylvestrisL.) saplings exposed to elevated CO2or O3concentration and their combination over three growing seasons in open‐top chambers. The decomposition of needle litter was followed for 19 months in a pine forest. During decomposition, needle samples for secondary compound analysis were collected and the mass loss of needles was followed. Main nutrients and total phenolics were analysed from litter in the beginning and at the end of the experiment. After 19‐month decomposition, the accumulated mass loss was about 34%; however, no significant differences were found in decomposition rates of needle litter between various treatments. Concentrations of total monoterpenes were about 4%, total resin acids 21% and total phenolics 14% of the initial concentrations in litter after 19‐month decomposition. In the beginning of litter decomposition, concentrations of individual monoterpenes –α‐pinene and β‐pinene – were significantly higher in needle litter grown under elevated CO2. However, concentrations of total monoterpenes during the whole decomposition period were not significantly affected by CO2or O3treatments. Concentrations of some individual and total resin acids were higher in needle litter grown under elevated CO2or O3than under ambient air. There were no significant differences in concentrations of total phenolics as well as nitrogen (N) and the main nutrient concentrations between treatments during decomposition. High concentrations of monoterpenes and resin acids in needles might slightly delay C recycling in forest soils. It is concluded that elevated CO2and O3concentrations do not have remarkable impacts on litter decomposition processes in Scots pine forests.
Direct and indirect effects of elevated CO2 on transpiration from Quercus myrtifolia in a scrub‐oak ecosystem
Authors: Li J.‐H. , Dugas W. A. , Hymus G. J. , Johnson D. P. , Hinkle C. R. , Drake B. G. , Hungate B. A.
Published: March 2003

Elevated atmospheric carbon dioxide (Ca) usually reduces stomatal conductance, but the effects on plant transpiration in the field are not well understood. Using constant‐power sap flow gauges, we measured transpiration from Quercus myrtifolia Willd., the dominant species of the Florida scrub‐oak ecosystem, which had been exposed in situ to elevated Ca (350 µmol mol−1 above ambient) in open‐top chambers since May 1996. Elevated Ca reduced average transpiration per unit leaf area by 37%, 48% and 49% in March, May and October 2000, respectively. Temporarily reversing the Ca treatments showed that at least part of the reduction in transpiration was an immediate, reversible response to elevated Ca. However, there was also an apparent indirect effect of Ca on transpiration: when transpiration in all plants was measured under common Ca, transpiration in elevated Ca‐grown plants was lower than that in plants grown in normal ambient Ca. Results from measurements of stomatal conductance (gs), leaf area index (LAI), canopy light interception and correlation between light and gs indicated that the direct, reversible Ca effect on transpiration was due to changes in gs caused by Ca, and the indirect effect was caused mainly by greater self‐shading resulting from enhanced LAI, not from stomatal acclimation. By reducing light penetration through the canopy, the enhanced self‐shading at elevated Ca decreased stomatal conductance and transpiration of leaves at the middle and bottom of canopy. This self‐shading mechanism is likely to be important in ecosystems where LAI increases in response to elevated Ca.
Cross validation of open‐top chamber and eddy covariance measurements of ecosystem CO2 exchange in a Florida scrub‐oak ecosystem
Authors: Dore Sabina , Hymus Graham J. , Johnson David P. , Hinkle C. R. , Valentini Riccardo , Drake Bert G.
Published: March 2003

Simultaneous measurements of net ecosystem CO2 exchange (NEE) were made in a Florida scrub‐oak ecosystem in August 1997 and then every month between April 2000 to July 2001, using open top chambers (NEEO) and eddy covariance (NEEE). This study provided a cross validation of these two different techniques for measuring NEE. Unique characteristics of the comparison were that the measurements were made simultaneously, in the same stand, with large replicated chambers enclosing a representative portion of the ecosystem (75 m2, compared to approximately 1–2 ha measured by the eddy covariance system). The value of the comparison was greatest at night, when the microclimate was minimally affected by the chambers. For six of the 12 measurement periods, night NEEO was not significantly different to night NEEE, and for the other periods the maximum difference was 1.1 µmol m−2s−1, with an average of 0.72 ± 0.09 µmol m−2s−1. The comparison was more difficult during the photoperiod, because of differences between the microclimate inside and outside the chambers. During the photoperiod, air temperature (Tair) and air vapour pressure deficits (VPD) became progressively higher inside the chambers until mid‐afternoon. In the morning NEEO was higher than NEEE by about 26%, consistent with increased temperature inside the chambers. Over the mid‐day period and the afternoon, NEEO was 8% higher that NEEE, regardless of the large differences in microclimate. This study demonstrates both the uses and difficulties associated with attempting to cross validate NEE measurements made in chambers and using eddy covariance. The exercise was most useful at night when the chamber had a minimal effect on microclimate, and when the measurement of NEE is most difficult.