Elevated CO2 enhances water relations and productivity and affects gas exchange in C3 and C4 grasses of the Colorado shortgrass steppe.

Six open‐top chambers were installed on the shortgrass steppe in north‐eastern Colorado, USA from late March until mid‐October in 1997 and 1998 to evaluate how this grassland will be affected by rising atmospheric CO₂. Three chambers were maintained at current CO₂ concentration (ambient treatment), three at twice ambient CO₂, or approximately 720 μmol mol^?1 (elevated treatment), and three nonchambered plots served as controls. Above‐ground phytomass was measured in summer and autumn during each growing season, soil water was monitored weekly, and leaf photosynthesis, conductance and water potential were measured periodically on important C₃ and C₄ grasses. Mid‐season and seasonal above‐ground productivity were enhanced from 26 to 47% at elevated CO₂, with no differences in the relative responses of C₃/C₄ grasses or forbs. Annual above‐ground phytomass accrual was greater on plots which were defoliated once in mid‐summer compared to plots which were not defoliated during the growing season, but there was no interactive effect of defoliation and CO₂ on growth. Leaf photosynthesis was often greater in Pascopyrum smithii (C₃) and Bouteloua gracilis (C₄) plants in the elevated chambers, due in large part to higher soil water contents and leaf water potentials. Persistent downward photosynthetic acclimation in P. smithii leaves prevented large photosynthetic enhancement for elevated CO₂‐grown plants. Shoot N concentrations tended to be lower in grasses under elevated CO₂, but only Stipa comata (C₃) plants exhibited significant reductions in N under elevated compared to ambient CO₂ chambers. Despite chamber warming of 2.6 °C and apparent drier chamber conditions compared to unchambered controls, above‐ground production in all chambers was always greater than in unchambered plots. Collectively, these results suggest increased productivity of the shortgrass steppe in future warmer, CO₂ enriched environments.     

Multi-objective environment chamber system for studying plant responses to climate change

This paper describes the technical information and performance of a new multi-objective chamber system enabling the control of environmental variables (e.g., temperature, CO₂, air humidity, wind speed, and UV-B radiation) for understanding plant responses to climate change. Over a whole growing season, four different climate scenarios were evenly programmed into the system’s 16 chambers as ambient environment (AMB), elevated temperature (ET), elevated CO₂ concentration (EC) and elevated temperature and CO₂ concentration (ETC). Simultaneously, the chamber effects were assessed regarding the physiological responses and growth of a boreal perennial grass (reed canary grass, Phalaris arundinacea L.). During the growing season, the chamber system provided a wide variety of climatic conditions for air temperature (T _a), relative humidity (RH) and CO₂ concentration (C _a) in the AMB chambers following outside conditions. The target temperature (+3.5°C) was achieved to a good degree in the ET and ETC chambers, being on average 3.3°C and 3.7°C higher than ambient conditions, respectively. The target concentration of CO₂ (700 ppm) was also well achieved in the EC and ETC chambers, being on average 704 ppm and 703 ppm, respectively. The stable airflow condition inside all of the chambers provided a homogeneous distribution of gases and temperature. The decreases in RH and increases in vapour pressure deficit (VPD) in the elevated temperature chambers were also maintained at a low level. Chamber effects were observed, with some physiological and growth parameters of plants being significantly lower in the AMB chambers, compared to outside conditions. The plant growth was negatively affected by the reduced radiation inside the chambers.     

Effects of elevated atmospheric CO2 on root decomposition in a scrub oak ecosystem

The effects of elevated atmospheric CO₂ on fine root decomposition over a 828‐day period were investigated using open top chambers with both ambient and elevated (700 ppm) CO₂ treatments in an oak–palmetto scrub ecosystem at Kennedy Space Center, Florida. Carbon dioxide enrichment of the chambers began 15 May 1996. The experiment included roots grown in ambient and elevated carbon dioxide. Vertical litterbags installed in September 1996 in each elevated and ambient chamber incubated from December 1996 to December 1998 showed no significant treatment effect on fine root or rhizome mass loss. Initial fine root percentage mass loss varied from 10.3% to 13.5% after three months; 55.5% to 38.3% of original mass had been lost after 828 days. A period of nitrogen immobilization occurred in both fine roots and rhizomes in the elevated CO₂ incubation, which is a potential mechanism for nitrogen conservation for this system in an elevated CO₂ world .     

Multichannel automated chamber system for continuous monitoring of CO2 exchange between the agro-ecosystem or soil and the atmosphere

A multichannel automated chamber system was developed for continuous monitoring of CO₂ exchange at multiple points between agro-ecosystem or soil and atmosphere. This system consisted of an automated chamber subsystem with a CO₂ concentration analyzer and a data logging subsystem. Both subsystems were under the control of a programmable logic controller (PLC). The automated chamber subsystem contained 18 chambers (50 cm × 50 cm × 50 cm) and a compressor. The chamber lids were closed and can be automatically opened. During measurement, one of the 18 chambers was kept closed for three minutes for measuring and the other chambers were kept open to maintain the natural soil conditions to the maximum extent. Environmental variables were simultaneously measured using sensors and recorded by the data logger. The reliability of the multichannel automated chamber system was tested and the results showed that the turbulence of the fans had no significant effect on the CO₂ exchange. The changes in the air and the temperature of soil and soil moisture inside the chambers, caused by the enclosure of the chambers, were not significant. The net ecosystem CO₂ exchange for the wheat ecosystem was ?2.35 μmol·m^?2·s^>?1 and the soil respiration was 3.87 μmol·m^?2·s^>?1 in the wheat field, and 6.61 μmol·m^?2·s^>?1 in the apple orchard.     

Elevated atmospheric CO2 alters stomatal responses to variable sunlight in a C4 grass

Native tallgrass prairie in NE Kansas was exposed to elevated (twice ambient) or ambient atmospheric CO₂ levels in open-top chambers. Within chambers or in adjacent unchambered plots, the dominant C₄ grass, Andropogon gerardii, was subjected to fluctuations in sunlight similar to that produced by clouds or within canopy shading (full sun > 1500 μmol m⁻² s⁻¹ versus 350 μmol m⁻² s⁻¹ shade) and responses in gas exchange were measured. These field experiments demonstrated that stomatal conductance in A. gerardii achieved new steady state levels more rapidly after abrupt changes in sunlight at elevated CO₂ when compared to plants at ambient CO₂. This was due primarily to the 50% reduction in stomatal conductance at elevated CO₂, but was also a result of more rapid stomatal responses. Time constants describing stomatal responses were significantly reduced (29–33%) at elevated CO₂. As a result, water loss was decreased by as much as 57% (6.5% due to more rapid stomatal responses). Concurrent increases in leaf xylem pressure potential during periods of sunlight variability provided additional evidence that more rapid stomatal responses at elevated CO₂ enhanced plant water status. CO₂-induced alterations in the kinetics of stomatal responses to variable sunlight will likely enhance direct effects of elevated CO₂ on plant water relations in all ecosystems.     

Development of ectomycorrhizas in model beech–spruce ecosystems on siliceous and calcareous soil: a 4-year experiment with atmospheric CO2 enrichment and nitrogen fertilization

To study responses of forests to global change, model ecosystems consisting of beech and spruce trees were established in open top chambers. The ecosystems were exposed to four conditions for 4 years, each replicated four times: ambient and elevated CO₂, and low and high nitrogen input. At the end of the trial, the trees were 6–8 years old. Each chamber contained two separate compartments with siliceous and calcareous soil. Here, we focus on the development of ectomycorrhizas in the topsoil layer. Ectomycorrhizal fungal biomass associated with the fine roots, estimated by using ergosterol content as a marker, was much higher in the siliceous compared to the richer calcareous soil. Also, in root-free soil samples, the level of ergosterol, indicative of the extraradical mycelium of ectomycorrhizal fungi as well as the mycelium of other fungi, was about six times higher in the siliceous than in the calcareous soil. Conditions of elevated atmospheric CO₂ primarily affected ectomycorrhizas in the calcareous soil. Fungal biomass, calculated per soil volume of the top soil layer, increased significantly, as did the metabolic activity of the ectomycorrhizal fungi, measured as uptake of glucose and synthesis of trehalose. Conditions of nitrogen fertilization affected ectomycorrhizas in the siliceous, nutrient poor soil.     

Effects of elevated atmospheric CO2 on fine root length and distribution in an oak-palmetto scrub ecosystem in central Florida

Atmospheric CO₂ concentration is rising and it has been suggested that a portion of the additional carbon is being sequestered in terrestrial vegetation and much of that in below-ground structures. The objective of the present study was to quantify the effects of elevated atmospheric CO₂ on fine root length and distribution with depth with minirhizotrons in an open-top chamber experiment in an oak-palmetto scrub ecosystem at Kennedy Space Centre, Florida, USA. Observations were made five times over a period of one and a half years in three ambient chambers (350 p.p.m. CO₂), three CO₂ enriched chambers (700 p.p.m. CO₂), and three unchambered plots. Greater root length densities were produced in the elevated CO₂ chambers (14.2 mm cm^?2) compared to the ambient chambers (8.7 mm cm^?2). More roots may presumably lead to more efficient acquisition of resources. Fine root abundance varied significantly with soil depth, and there appeared to be enhanced proliferation of fine roots near the surface (0–12 cm) and at greater depth (49–61 cm) in the elevated CO₂ chambers. The vertical root distribution pattern may be a response to availability of nutrients and water. More studies are needed to determine if increased root length under CO₂ enriched conditions actually results in greater sequestering of carbon below ground.     

Root dynamics in a semi-natural grassland in relation to atmospheric carbon dioxide enrichment,soil water and shoot biomass

Plant responses to increasing atmospheric CO₂ concentrations have been studied intensively. However, the effects of elevated CO₂ on root dynamics, which is important for global carbon budgets as well as for nutrient cycling in ecosystems, has received much less attention. We used minirhizotrons inside open-top chambers to study the effects of elevated atmospheric carbon dioxide concentration on root dynamics in a nutrient-poor semi-natural grassland in central Sweden. We conducted our investigation over three consecutive growing seasons during which three treatments were applied at the site: Elevated (≈ 700 μmol mol^-1) and ambient (≈ 360 μmol mol^-1) chamber levels of CO₂ and a control, without a chamber. During 1997, a summer with two dry periods, the elevated treatment compared with ambient had 25% greater mean root counts, 65% greater above-ground biomass and 15% greater soil moisture. The chambers seemed responsible for changes in root dynamics, whereas the elevated CO₂ treatment in general increased the absolute sum of root counts compared with the ambient chamber. In 1998, a wet growing season, there were no significant differences in shoot biomass or root dynamics and both chamber treatments had lower soil moisture than the control. We found that as seasonal dryness increased, the ratio of elevated – ambient shoot biomass production increased while the root to shoot ratio decreased. We conclude that this grasslands response to elevated CO₂ is dependent on seasonal weather conditions and that CO₂ enrichment will most significantly increase production in such a grassland when under water stress. This revised version was published online in June 2006 with corrections to the Cover Date.     

Soil CO2 efflux in a beech forest: comparison of two closed dynamic systems

The aim of this study was to understand why two closed dynamic systems with a very similar design gave large differences in soil CO₂ efflux measurements (PP systems and LI-COR). Both in the field (forest beech stand) and in the laboratory, the PPsystems gave higher estimations of soil CO₂ efflux than the LI-COR system (ranging from 30% to 50%). The difference in wind speed occurring within the soil respiration chambers (0.9 m s⁻¹ within the SRC-1 and 0.4 m s⁻¹ within the LI-6000-09 chambers) may account for the discrepancy between the two systems. An excessive air movement inside the respiration chamber is thought to disrupt the high laminar boundary layer over the forest floor. This would promote an exhaust of the CO₂ accumulated into the upper soil layers into the chamber and a lateral diffusion of CO₂ in the soil towards the respiration chamber. The discrepancy between the two systems was reduced (i) by decreasing fan speed within the SRC-1, (ii) by increasing wind speed over the soil surface outside the respiration chamber, or (iii) by using an artificial soil design without high CO₂ concentration in soil pores. We show that wind speed is an important component of soil CO₂ diffusion which must be taken into account when measuring soil CO₂ efflux, even on very fine textured soil like silt-loam soil. Proper measurement can be achieved by maintaining wind speed inside the chamber below 0.4 m s⁻¹ since low wind speed conditions predominate under forest canopies. However, more accurate measurements will be obtained by regulating wind speeds within the chamber at a velocity representative of the wind speed recorded simultaneously at the floor surface. This revised version was published online in June 2006 with corrections to the Cover Date.     

A Direct Confirmation of the Standard Method of Estimating Intercellular Partial Pressure of CO(2)

The partial pressure of CO₂ inside leaves of several species was measured directly. Small gas exchange chambers were clamped above and below the same section of an amphistomatous leaf. A flowing gas stream through one chamber allowed normal CO₂ and water vapor exchange. The other chamber was in a closed circuit consisting of the chamber, an infrared gas analyzer, and a peristaltic pump. The CO₂ in the closed system rapidly reached a steady pressure which it is believed was identical to the CO₂ pressure inside the leaf, because there was no flux of CO₂ across the epidermis. This measured partial pressure was in close agreement with that estimated from a consideration of the fluxes of CO₂ and vapor at the other surface.     

Elevated atmospheric CO2 lowers herbivore abundance, but increases leaf abscission rates

Increased levels of atmospheric carbon dioxide (CO₂) are likely to affect the trophic relationships that exist between plants, their herbivores and the herbivores' natural enemies. This study takes advantage of an open‐top CO₂ fertilization experiment in a Florida scrub oak community at Kennedy Space Center, Florida, consisting of eight chambers supplied with ambient CO₂ (360 ppm) and eight chambers supplied with elevated CO₂ (710 ppm). We examined the effects of elevated CO₂ on herbivore densities and levels of leaf consumption, rates of herbivore attack by natural enemies and effects on leaf abscission. Cumulative levels of herbivores and herbivore damage were significantly lower in elevated CO₂ than in ambient CO₂. This may be because leaf nitrogen levels are lower in elevated CO₂. More herbivores die of host plant‐induced death in elevated CO₂ than in ambient CO₂. Attack rates of herbivores by parasitoids are also higher in elevated CO₂, possibly because herbivores need to feed for a longer time in order to accrue sufficient nitrogen (N), thus exposing themselves longer to natural enemies. Insect herbivores cause an increase in abscission rates of leaves throughout the year. Because of the lower insect density in elevated CO₂, we thought, abscission rates would be lower in these chambers. However, abscission rates were significantly higher in elevated CO₂. Thus, the direct effects of elevated CO₂ on abscission are greater than the indirect effects on abscission mediated via lower insect densities. A consequence of increased leaf abscission in elevated CO₂ is that nutrient deposition rates to the soil surface are accelerated.     

Elevated atmospheric CO2 and feedback between carbon and nitrogen cycles

We tested a conceptual model describing the influence of elevated atmospheric CO₂ on plant production, soil microorganisms, and the cycling of C and N in the plant-soil system. Our model is based on the observation that in nutrient-poor soils, plants (C₃) grown in an elevated CO₂ atmosphere often increase production and allocation to belowground structures. We predicted that greater belowground C inputs at elevated CO₂ should elicit an increase in soil microbial biomass and increased rates of organic matter turnover and nitrogen availability. We measured photosynthesis, biomass production, and C allocation of Populus grandidentata Michx. grown in nutrient-poor soil for one field season at ambient and twice-ambient (i.e., elevated) atmospheric CO₂ concentrations. Plants were grown in a sandy subsurface soil i) at ambient CO₂ with no open top chamber, ii) at ambient CO₂ in an open top chamber, and iii) at twice-ambient CO₂ in an open top chamber. Plants were fertilized with 4.5 g N m⁻² over a 47 d period midway through the growing season. Following 152 d of growth, we quantified microbial biomass and the availabilities of C and N in rhizosphere and bulk soil. We tested for a significant CO₂ effect on plant growth and soil C and N dynamics by comparing the means of the chambered ambient and chambered elevated CO₂ treatments. Rates of photosynthesis in plants grown at elevated CO₂ were significantly greater than those measured under ambient conditions. The number of roots, root length, and root length increment were also substantially greater at elevated CO₂. Total and belowground biomass were significantly greater at elevated CO₂. Under N-limited conditions, plants allocated 50–70% of their biomass to roots. Labile C in the rhizosphere of elevated-grown plants was significantly greater than that measured in the ambient treatments; there were no significant differences between labile C pools in the bulk soil of ambient and elevated-grown plants. Microbial biomass C was significantly greater in the rhizosphere and bulk soil of plants grown at elevated CO₂ compared to that in the ambient treatment. Moreover, a short-term laboratory assay of N mineralization indicated that N availability was significantly greater in the bulk soil of the elevated-grown plants. Our results suggest that elevated atmospheric CO₂ concentrations can have a positive feedback effect on soil C and N dynamics producing greater N availability. Experiments conducted for longer periods of time will be necessary to test the potential for negative feedback due to altered leaf litter chemistry. ei]{gnH}{fnLambers} ei]{gnA C}{fnBorstlap}     

Does elevated atmospheric CO2 concentrations affect wood decomposition?

This study was conducted to test the hypothesis that wood tissues generated under elevated atmospheric [CO₂] have lower quality and subsequent reduced decomposition rates. Chemical composition and subsequent field decomposition rates were studied for beech (Fagus sylvatica L.) twigs grown under ambient and elevated [CO₂] in open top chambers. Elevated [CO₂] significantly affected the chemical composition of beech twigs, which had 38% lower N and 12% lower lignin concentrations than twigs grown under ambient [CO₂]. The strong decrease in N concentration resulted in a significant increase in the C/N and lignin/N ratios of the beech wood grown at elevated [CO₂]. However, the elevated [CO₂] treatment did not reduce the decomposition rates of twigs, neither were the dynamics of N and lignin in the decomposing beech wood affected by the [CO₂] treatment, despite initial changes in N and lignin concentrations between the ambient and elevated [CO₂] beech wood. This revised version was published online in June 2006 with corrections to the Cover Date.     

An improved hexagon open‐top chamber system for stable diurnal and nocturnal warming and atmospheric carbon dioxide enrichment

The use of solar passive hexagonal open‐top chambers (POTCs) is a viable method for experimentally manipulating daytime air temperatures in low‐stature plant communities at high latitudes. Here we describe a new hexagon POTC‐based system that uses thermal inertia to increase overnight temperatures and variable chamber height to reduce overheating in summer. Field data collected in tall temperate grasslands show that the presence of thermal mass raised minimum and mean nighttime air temperatures by up to 1.5 °C while lowering chamber height, along with thermal mass, limited the development of extreme daytime chamber temperatures in summer. We also demonstrate that, by using a simple, inexpensive twin carbon dioxide (CO₂) injection system regulated by an infrared gas monitor, it is possible to generate targeted and stable atmospheric CO₂ enrichment within these chambers. These innovations significantly improve the conventional hexagon POTC design and represent a low‐cost method for assessing the effects of warming and CO₂ enrichment on low‐stature vegetation in low latitude environments.     

Mesocosms Mimic Natural Meadows as regards Greenhouse Gas Fluxes and Potential Activities of Nitrifying and Denitrifying Bacteria

The objective of this study was to determine whether a planted mesocosm mimics a natural habitat in terms of N₂O and CH₄ fluxes, soil characteristics and potential nitrification and denitrification activities. We compared mesocosms in unchambered open-field plots and in open-top chambers with nonfiltered ambient air with three natural meadows that had similar soil characteristics and species composition. The N₂O fluxes in the mesocosms were very similar to the fluxes in the three natural meadows. There were no marked differences in potential nitrification and denitrification activities between the mesocosms and the natural meadows, either. Only the CH₄ fluxes differed slightly between the mesocosms and some of the natural meadows. Therefore, it seems that the mesocosms compared rather well to natural habitats. The open-top chambers modified only the soil water content, the values being higher in the unchambered plots than in the chambered plots. These results thus suggest that the open-top chamber experiment enables estimates of greenhouse gas and potential activities of nitrifying and denitrifying bacteria in unfertilized Finnish meadows, in spite of the chamber effects on the soil water content.     

Plant Nitrogen Dynamics in Shortgrass Steppe under Elevated Atmospheric Carbon Dioxide

The direct and indirect effects of increasing levels of atmospheric carbon dioxide (CO₂) on plant nitrogen (N) content were studied in a shortgrass steppe ecosystem in northeastern Colorado, USA. Beginning in 1997 nine experimental plots were established: three open-top chambers with ambient CO₂ levels (approximately 365 mol mol^–1), three open-top chambers with twice-ambient CO₂ levels (approximately 720 mol mol^–1), and three unchambered control plots. After 3 years of growing-season CO₂ treatment, the aboveground N concentration of plants grown under elevated atmospheric CO₂ decreased, and the carbon–nitrogen (C:N) ratio increased. At the same time, increased aboveground biomass production under elevated atmospheric CO₂ conditions increased the net transfer of N out of the soil of elevated-CO₂ plots. Aboveground biomass production after simulated herbivory was also greater under elevated CO₂ compared to ambient CO₂. Surprisingly, no significant changes in belowground plant tissue N content were detected in response to elevated CO₂. Measurements of individual species at peak standing phytomass showed significant effects of CO₂ treatment on aboveground plant tissue N concentration and significant differences between species in N concentration, suggesting that changes in species composition under elevated CO₂ will contribute to overall changes in nutrient cycling. Changes in plant N content, driven by changes in aboveground plant N concentration, could have important consequences for biogeochemical cycling rates and the long-term productivity of the shortgrass steppe as atmospheric CO₂ concentrations increase.     

The effect of CO2 enrichment on leaf photosynthetic rates and instantaneous water use efficiency of Andropogon gerardii in the tallgrass prairie

Open-top chambers were used to study the effects of CO₂ enrichment on leaf-level photosynthetic rates of the C₄ grass Andropogon gerardii in the native tallgrass prairie ecosystem near Manhattan, Kansas. Measurements were made during a year with abundant rainfall (1993) and a year with below-normal rainfall (1994). Treatments included: No chamber, ambient CO₂ (A); chamber with ambient CO₂ (CA); and chamber with twice-ambient CO₂ (CE). Measurements of photosynthesis were made at 2-hour intervals, or at midday, on cloudless days throughout the growing season using an open-flow gas-exchange system. No significant differences in midday rates of photosynthesis or in daily carbon accumulation as a result of CO₂ enrichment were found in the year with abundant precipitation. In the dry year, midday rates of photosynthesis were significantly higher in the CE treatment than in the CA or A treatments throughout the season. Estimates of daily carbon accumulation also indicated that CO₂ enrichment allowed plants to maximize carbon acquisition on a diurnal basis. The increased carbon accumulation was accounted for by greater rates of photosynthesis in the CE plots during midday. During the wet year, CO₂ enrichment decreased stomatal conductance, which allowed plants to decrease transpiration while still photosynthesizing at rates similar to plants in ambient conditions. During the dry year, CO₂ enrichment allowed plants to maintain photosynthetic rates even though stomatal conductance and transpiration had been reduced in all treatments due to stress. Estimates of instantaneous water-use efficiency were reduced under CO₂ enrichment for both years. This revised version was published online in June 2006 with corrections to the Cover Date.     

Soil respiration in a poor upland site of Scots pine stand subjected to elevated temperatures and atmospheric carbon concentration

Soil respiration rates under elevated temperature and atmospheric CO₂ concentrations were studied in eastern Finland (62° 47N, 30° 58E, 144 m.a.s.1.) around naturally regenerated 20 – 30 years old Scots pine trees, enclosed in open top chambers. The production of CO₂ varied spatially and temporally, but clearly followed the changes in temperature measured at the soil surface. However, soil respiration in the open control was higher than that in chambers; i.e. the chamber itself changed the conditions by increasing the temperature, altering the movement of water, and thereby soil moisture. Nevertheless, an elevation in the concentration of atmospheric CO₂ raised soil respiration and brought it nearer to the level in the open control. An increase in temperature seemed to inhibit this rise, possibly because of an imbalance between temperature and moisture.     

Net Ecosystem Exchange of Carbon dioxide in a Temperate Poor Fen: a Comparison of Automated and Manual Chamber Techniques

We used five analytical approaches to compare net ecosystem exchange (NEE) of carbon dioxide (CO₂) from automated and manual static chambers in a peatland, and found the methods comparable. Once per week we sampled manually from 10 collars with a closed chamber system using a LiCor 6200 portable photosynthesis system, and simulated four photosynthetically active radiation (PAR) levels using shrouds. Ten automated chambers sampled CO₂ flux every 3 h with a LiCor 6252 infrared gas analyzer. Results of the five comparisons showed (1) NEE measurements made from May to August, 2001 by the manual and automated chambers had similar ranges: −10.8 to 12.7 μmol CO₂ m⁻² s⁻¹ and −17.2 to 13.1 μmol CO₂ m⁻² s⁻¹, respectively. (2) When sorted into four PAR regimes and adjusted for temperature (respiration was measured under different temperature regimes), mean NEE did not differ significantly between the chambers (p < 0.05). (3) Chambers were not significantly different in regression of ln( − respiration) on temperature. (4) But differences were found in the PAR vs. NEE relationship with manual chambers providing higher maximum gross photosynthesis estimates (GP_max), and slower uptake of CO₂ at low PAR (α) even after temperature adjustment. (5) Due to the high variability in chamber characteristics, we developed an equation that includes foliar biomass, water table, temperature, and PAR, to more directly compare automated and manual NEE. Comparing fitted parameters did not identify new differences between the chambers. These complementary chamber techniques offer a unique opportunity to assess the variability and uncertainty in CO₂ flux measurements.     

Effects of elevated CO2 and temperature on the leaf chemistry of birch Betula pendula (Roth) and the feeding behaviour of the weevil Phyllobius maculicornis

Abstract 1 The effect of elevated CO₂ and temperature on the foliar chemistry Betula pendula Roth and the feeding performance of polyphagous weevils Phyllobius maculicornis Germ. was studied. Birch seedlings were grown during one growing season in chamber‐less field conditions and in closed‐top chambers exposed to four different treatments: ambient CO₂ (350 p.p.m) and temperature, elevated atmospheric CO₂ (700 p.p.m) and ambient temperature, elevated temperature +3 °C above ambient) and ambient CO₂, and a combination of elevated CO₂ and temperature. 2 In leaves under CO₂ enrichment, the concentration of nitrogen and some flavonol glycosides significantly decreased, whereas the concentration of total phenolics, condensed tannins and (+)‐catechin significantly increased. The total concentration of cinnamoylquinic acids was significantly increased by CO₂ and decreased by temperature. The concentration of salidroside increased under elevated temperature. 3 Weevil‐feeding experiments were carried out in a five‐choice arrangement, one leaf from each of the five treatments (chamber‐less field controls and four different treatments in chambers) being placed in random order in a plastic box. The weevils preferred the leaves grown under elevated CO₂, which had low nitrogen, high phenolics and the highest condensed tannin concentrations. Whether the reason for this trend is due to the stimulating effect of condensed tannins and/or a change in other secondary compounds, remains unknown. The weevils did not show any obviously different response in feeding performance to temperature and the combination of elevated CO₂ and temperature.