Carbon dioxide exchange in lichens: Estimation of internal thallus CO2 transport resistances

The gaseous exchange pathways of Sticla latifrons Rich. and Pseudocyphellaria amphisticta Kremp. were examined using both light and scanning electron microscopes. The size and frequency of the pores in the gas exchange structures (cyphellae and pseudocyphellae) and in the medulla were measured and from these CO₂ diffusion resistances were calculated. Pseudocyphellae were found to be smaller and more widely spaced than cyphellae, consequently the resistance of the pseudocyphellae, was much greater than that of the cyphellae. Medulla resistances were low in both lichens and are probably unimportant, even at high water contents. No evidence of hyphal swelling was found. Gas exchange structure resistances were more than five fold greater than medulla resistances. It is suggested that this arrangement of resistances may simultaneously encourage refixation of respired CO₂ and maintain a non desiccating environment for the lichen algae. The internal transport resistances calculated in this work approximate experimentally obtained values.     

Net ecosystem CO2 exchange in Mojave Desert shrublands during the eighth year of exposure to elevated CO2

Arid ecosystems, which occupy about 35% of the Earth's terrestrial surface area, are believed to be among the most responsive to elevated [CO₂]. Net ecosystem CO₂ exchange (NEE) was measured in the eighth year of CO₂ enrichment at the Nevada Desert Free‐Air CO₂ Enrichment (FACE) Facility between the months of December 2003–December 2004. On most dates mean daily NEE (24 h) (μmol CO₂ m^?2 s^?1) of ecosystems exposed to elevated atmospheric CO₂ were similar to those maintained at current ambient CO₂ levels. However, on sampling dates following rains, mean daily NEEs of ecosystems exposed to elevated [CO₂] averaged 23 to 56% lower than mean daily NEEs of ecosystems maintained at ambient [CO₂]. Mean daily NEE varied seasonally across both CO₂ treatments, increasing from about 0.1 μmol CO₂ m^?2 s^?1 in December to a maximum of 0.5–0.6 μmol CO₂ m^?2 s^?1 in early spring. Maximum NEE in ecosystems exposed to elevated CO₂ occurred 1 month earlier than it did in ecosystems exposed to ambient CO₂, with declines in both treatments to lowest seasonal levels by early October (0.09±0.03 μmol CO₂ m^?2 s^?1), but then increasing to near peak levels in late October (0.36±0.08 μmol CO₂ m^?2 s^?1), November (0.28±0.03 μmol CO₂ m^?2 s^?1), and December (0.54±0.06 μmol CO₂ m^?2 s^?1). Seasonal patterns of mean daily NEE primarily resulted from larger seasonal fluctuations in rates of daytime net ecosystem CO₂ uptake which were closely tied to plant community phenology and precipitation. Photosynthesis in the autotrophic crust community (lichens, mosses, and free‐living cyanobacteria) following rains were probably responsible for the high NEEs observed in January, February, and late October 2004 when vascular plant photosynthesis was low. Both CO₂ treatments were net CO₂ sinks in 2004, but exposure to elevated CO₂ reduced CO₂ sink strength by 30% (positive net ecosystem productivity=127±17 g C m^?2 yr^?1 ambient CO₂ and 90±11 g C m^?2 yr^?1 elevated CO₂, P=0.011). This level of net C uptake rivals or exceeds levels observed in some forested and grassland ecosystems. Thus, the decrease in C sequestration seen in our study under elevated CO₂– along with the extensive coverage of arid and semi‐arid ecosystems globally – points to a significant drop in global C sequestration potential in the next several decades because of responses of heretofore overlooked dryland ecosystems.     

CO2 uptake patterns depend on water current velocity and shoot morphology in submerged stream macrophytes

1. The influence of current velocity on the pattern of photosynthetic CO₂ uptake in three species of submerged stream macrophytes was described by analysing the grain density in autoradiographs of leaves exposed to ¹⁴CO₂. 2. In Elodea canadensis, the CO₂ uptake was approximately two‐fold higher near the leaf periphery compared with the midrib section at high current velocity, whereas at low current velocity the area of relatively high CO₂ uptake expanded from the leaf periphery towards the midrib and basal sections of the leaves. 3. In Potamogeton crispus and Callitriche stagnalis the CO₂ uptake was uniform throughout the leaves at low current velocity, whereas at high current velocity the CO₂ uptake appeared to increase randomly in some areas of the leaves. 4. The relationship between the photosynthetic CO₂ uptake pattern and the dynamics of flow surrounding submerged shoots at low and high current velocity is discussed in relation to shoot morphology. In E. canadensis, thick diffusive boundary layers may develop between leaves because of screening effects at high current velocity. Increased diffusion path for CO₂ may contribute to inhibitory effects on photosynthesis in this species.     

Forest and agricultural land-use-dependent CO2 exchange in Thuringia, Germany

Eddy covariance was used to measure the net CO₂ exchange (NEE) over ecosystems differing in land use (forest and agriculture) in Thuringia, Germany. Measurements were carried out at a managed, even‐aged European beech stand (Fagus sylvatica, 70–150 years old), an unmanaged, uneven‐aged mixed beech stand in a late stage of development (F. sylvatica, Fraxinus excelsior, Acer pseudoplantanus, and other hardwood trees, 0–250 years old), a managed young Norway spruce stand (Picea abies, 50 years old), and an agricultural field growing winter wheat in 2001, and potato in 2002. Large contrasts were found in NEE rates between the land uses of the ecosystems. The managed and unmanaged beech sites had very similar net CO₂ uptake rates (～?480 to ?500 g C m^?2 yr^?1). Main differences in seasonal NEE patterns between the beech sites were because of a later leaf emergence and higher maximum leaf area index at the unmanaged beech site, probably as a result of the species mix at the site. In contrast, the spruce stand had a higher CO₂ uptake in spring but substantially lower net CO₂ uptake in summer than the beech stands. This resulted in a near neutral annual NEE (?4 g C m^?2 yr^?1), mainly attributable to an ecosystem respiration rate almost twice as high as that of the beech stands, despite slightly lower temperatures, because of the higher elevation. Crops in the agricultural field had high CO₂ uptake rates, but growing season length was short compared with the forest ecosystems. Therefore, the agricultural land had low‐to‐moderate annual net CO₂ uptake (?34 to ?193 g C m^?2), but with annual harvest taken into account it will be a source of CO₂ (+97 to +386 g C m^?2). The annually changing patchwork of crops will have strong consequences on the regions' seasonal and annual carbon exchange. Thus, not only land use, but also land‐use history and site‐specific management decisions affect the large‐scale carbon balance.     

The effects of water table manipulation and elevated temperature on the net CO2 flux of wet sedge tundra ecosystems

In situ manipulations were conducted in a naturally drained lake on the arctic coastal plain near Prudhoe Bay, Alaska (70 °21.98′ N, 148 °33.72′ W) to assess the potential short-term effects of decreased water table and elevated temperature on net ecosystem CO₂ flux. The experiments were conducted over a 2-year period, and during that time, water table depth of drained plots was maintained on average 7 cm lower than the ambient water table, and surface temperatures of plots exposed to elevated temperature were increased on average 0.5 °C. Water table drainage, and to a lesser extent elevated temperature, resulted in significant increases in ecosystem respiration (ER) rates, and only small and variable changes in gross ecosystem productivity (GEP). As a result, drained plots were net sources of ≈ 40 gC m^–2 season^–1 over both years of manipulation, while control plots were net sinks of atmospheric CO₂ of about 10 gC m^–2 season^–1 (growing season length was an estimated 125 days). Control plots exposed to elevated temperatures accumulated slightly more carbon than control plots exposed to ambient temperatures. The direct effects of elevated temperature on net CO₂ flux, ER, and GEP were small, however, elevated temperature appeared to interact with drainage to exacerbate the amount of net carbon loss. These data suggest that many currently saturated or nearly saturated wet sedge ecosystems of the north slope of Alaska may become significant sources of CO₂ to the atmosphere if climate change predictions of increased evapotranspiration and reduced soil water status are realized. There is ample evidence that this may be already occurring in arctic Alaska, as a change in net carbon balance has been observed for both tussock and wet-sedge tundra ecosystems over the last 2–3 decades, which coincides with a recent increase in surface temperature and an associated decrease in soil water content. In contrast, if precipitation increases relatively more than evapotranspiration, then increases in soil moisture content will likely result in greater carbon accumulation.     

Carbon and water fluxes in a calcareous grassland under elevated CO2

1. As part of a long-term study of the effects of elevated CO₂ on biodiversity and ecosystem function in a calcareous grassland, we measured ecosystem carbon dioxide and water-vapour fluxes over 24-h periods during the 1994 and 1995 growing seasons. Data were used to derive CO₂ and H₂O gas-exchange response functions to quantum flux density (QFD).
2. The relative increase in net ecosystem CO₂ flux (NEC) owing to CO₂ enrichment increased as QFD rose. Daytime NEC at high QFD under elevated CO₂ increased by 25% to 60%, with the greatest increases in the spring and after mowing in June when above-ground biomass was lowest. There was much less stimulation of NEC in early June and again in October when the canopy was fully developed. Night-time NEC was not significantly altered under elevated CO₂.
3. Short-term reversal of CO₂ concentrations between treatments after two seasons of CO₂ exposure provided evidence for a 50% downward adjustment of NEC expressed per unit above-ground plant dry weight. However, when expressed on a land area basis, this difference disappeared because of a c. 20% increase in above-ground biomass under elevated CO₂.
4. Ecosystem evapotranspiration (ET) was not significantly altered by elevated CO₂ when averaged over all measurement dates and positions. However, ET was reduced 3–18% at high QFD in plots at the top of the slope at our study site. In summary, CO₂ enrichment resulted in a large stimulation of ecosystem CO₂ capture, especially during periods of a large demand of carbon in relationship to its supply, and resulted in a relatively small and variable effect on ecosystem water consumption.     

Water velocity, growth-form and diffusion resistances to photosynthetic CO2 uptake in aquatic bryophytes

Abstract. Boundary-layer resistances of aquatic bryophytes for CO₂ diffusion in water were estimated from wind tunnel measurements of evaporation of aniline in air, using the principle of dynamic similarity. The results indicated resistances at water velocities between 0.02 and 0.2 m s ⁻¹ ranging from about 35 to 5 s mm⁻¹ and 70 to 9 s mm⁻¹, respectively, for the mat-forming liverworts Nardia compressa and Scapania undulata , measured on a projected area (canopy) basis. Over a range of velocities from 0.01 to 0.2 m s⁻¹ the estimated CO₂ boundary-layer resistance of the streamer-like shoots of the moss Fontinalis antipyretica is between about 180 and 15 s mm⁻¹. Comparison with experiments on photosynthetic ¹⁴CO₂-uptake at a range of water velocities suggests that boundary-layer resistance limits photosynthesis at velocities below about 0.01 m s^−l in Fontinalis and below about 0.1 m s⁻¹ in the mat-forming species. It is suggested that high leaf-area index allows the mat growth form more effectively to exploit the low boundary-layer resistance at high velocities while remaining relatively invulnerable to drag. By contrast, the streamer form allows Fontinalis to maximize surface area under conditions where boundary-layer resistance is limiting.     

Gas exchange and photosynthetic acclimation over subambient to elevated CO2 in a C3–C4 grassland

Atmospheric CO₂ (C_a) has risen dramatically since preglacial times and is projected to double in the next century. As part of a 4‐year study, we examined leaf gas exchange and photosynthetic acclimation in C₃ and C₄ plants using unique chambers that maintained a continuous C_a gradient from 200 to 550 µmol mol^?1 in a natural grassland. Our goals were to characterize linear, nonlinear and threshold responses to increasing C_a from past to future C_a levels. Photosynthesis (A), stomatal conductance (g_s), leaf water‐use efficiency (A/g_s) and leaf N content were measured in three common species: Bothriochloa ischaemum, a C₄ perennial grass, Bromus japonicus, a C₃ annual grass, and Solanum dimidiatum, a C₃ perennial forb. Assimilation responses to internal CO₂ concentrations (A/C_i curves) and photosynthetically active radiation (A/PAR curves) were also assessed, and acclimation parameters estimated from these data. Photosynthesis increased linearly with C_a in all species (P < 0.05). S. dimidiatum and B. ischaemum had greater carboxylation rates for Rubisco and PEP carboxylase, respectively, at subambient than superambient C_a (P < 0.05). To our knowledge, this is the first published evidence of A up‐regulation at subambient C_a in the field. No species showed down‐regulation at superambient C_a. Stomatal conductance generally showed curvilinear decreases with C_a in the perennial species (P < 0.05), with steeper declines over subambient C_a than superambient, suggesting that plant water relations have already changed significantly with past C_a increases. Resource‐use efficiency (A/g_s and A/leaf N) in all species increased linearly with C_a. As both C₃ and C₄ plants had significant responses in A, g_s, A/g_s and A/leaf N to C_a enrichment, future C_a increases in this grassland may not favour C₃ species as much as originally thought. Non‐linear responses and acclimation to low C_a should be incorporated into mechanistic models to better predict the effects of past and present rising C_a on grassland ecosystems.     

Ecosystem CO2 exchange and plant biomass in the littoral zone of a boreal eutrophic lake

• 1 In order to study the dynamics of primary production and decomposition in the lake littoral, an interface zone between the pelagial, the catchment and the atmosphere, we measured ecosystem/atmosphere carbon dioxide (CO₂) exchange in the littoral zone of an eutrophic boreal lake in Finland during two open water periods (1998–1999). We reconstructed the seasonal net CO₂ exchange and identified the key factors controlling CO₂ dynamics. The seasonal net ecosystem exchange (NEE) was related to the amount of carbon accumulated in plant biomass.
• 2 In the continuously inundated zones, spatial and temporal variation in the density of aerial shoots controlled CO₂ fluxes, but seasonal net exchange was in most cases close to zero. The lower flooded zone had a net CO₂ uptake of 1.8–6.2 mol m^?2 per open water period, but the upper flooded zone with the highest photosynthetic capacity and above‐ground plant biomass, had a net CO₂ loss of 1.1–7.1 mol m^?2 per open water period as a result of the high respiration rate. The excess of respiration can be explained by decomposition of organic matter produced on site in previous years or leached from the catchment.
• 3 Our results from the two study years suggest that changes in phenology and water level were the prime cause of the large interannual difference in NEE in the littoral zone. Thus, the littoral is a dynamic buffer and source for the load of allochthonous and autochthonous carbon to small lakes.

     

Spatial variation in regional CO2 exchange for the Kuparuk River Basin, Alaska over the summer growing season

The spatial and temporal patterns in CO₂ flux for the Kuparuk River Basin, a 9200‐km² watershed located in NE Alaska were estimated using the Regional Arctic CO₂ Exchange Simulator (RACES) for the 1994–1995 growing seasons. RACES uses non‐linear models and a Geographical Information System database (GIS) consisting of the normalized difference vegetation index (NDVI) and dynamic temperature and radiation maps. The spatial and temporal patterns in the NDVI during both growing seasons suggest that ecosystem development occurred 2–4 weeks earlier and was relatively more rapid in the southern portion of the Kuparuk River Basin. Rates of gross primary production (GPP) and whole‐ecosystem respiration (R) were 2–4 fold higher in the southern basin than along the arctic coastal plain depending on time of year. The higher rate of GPP estimated for the southern basin was primarily due to higher NDVI values, while the higher R estimated for the southern basin was due in part to higher temperature and the NDVI. While GPP and R showed strong latitudinal trends, spatial and temporal trends in net ecosystem CO₂ exchange (NEE) were much more variable. Thus, while spatial trends in carbon gain (GPP) and loss (R) were highly correlated, small spatial and temporal differences in these large fluxes (GPP and/or R) lead to corresponding large spatial variations in the NEE.     

Consequences of wildfire on ecosystem CO2 and water vapour fluxes in the Great Basin

Increased fire frequency in the Great Basin of North America's intermountain West has led to large‐scale conversion of native sagebrush (Artemisia tridentata Nutt.) communities to postfire successional communities dominated by native and non‐native annual species during the last century. The consequences of this conversion for basic ecosystem functions, however, are poorly understood. We measured net ecosystem CO₂ exchange (NEE) and evapotranspiration (ET) during the first two dry years after wildfire using a 4‐m diameter (16.4 m³) translucent static chamber (dome), and found that both NEE and ET were higher in a postfire successional ecosystem (?0.9–2.6 µ mol CO₂ m^?2 s^?1 and 0.0–1.0 mmol H₂O m^?2 s^?2, respectively) than in an adjacent intact sagebrush ecosystem (?1.2–2.3 µ mol CO₂ m^?2 s^?1 and ?0.1–0.8 mmol H₂O m^?2 s^?2, respectively) during relatively moist periods. Higher NEE in the postfire ecosystem appears to be due to lower rates of above‐ground plant respiration while higher ET appears to be caused by higher surface soil temperatures and increased soil water recharge after rains. These patterns disappeared or were reversed, however, when the conditions were drier. Daily net ecosystem productivity (NEP; g C m^?2 d^?1), derived from multiple linear regressions of measured fluxes with continuously measured climate variables, was very small (close to zero) throughout most of the year. The wintertime was an exception in the intact sagebrush ecosystem with C losses exceeding C gains leading to negative NEP while C balance of the postfire ecosystem remained near zero. Taken together, our results indicate that wildfire‐induced conversion of native sagebrush steppe to ecosystems dominated by herbaceous annual species may have little effect on C balance during relatively dry years (except in winter months) but may stimulate water loss immediately following fires.     

Modeled interactive effects of precipitation, temperature, and [CO2] on ecosystem carbon and water dynamics in different climatic zones

Interactive effects of multiple global change factors on ecosystem processes are complex. It is relatively expensive to explore those interactions in manipulative experiments. We conducted a modeling analysis to identify potentially important interactions and to stimulate hypothesis formulation for experimental research. Four models were used to quantify interactive effects of climate warming (T), altered precipitation amounts [doubled (DP) and halved (HP)] and seasonality (SP, moving precipitation in July and August to January and February to create summer drought), and elevated [CO₂] (C) on net primary production (NPP), heterotrophic respiration (R_h), net ecosystem production (NEP), transpiration, and runoff. We examined those responses in seven ecosystems, including forests, grasslands, and heathlands in different climate zones. The modeling analysis showed that none of the three‐way interactions among T, C, and altered precipitation was substantial for either carbon or water processes, nor consistent among the seven ecosystems. However, two‐way interactive effects on NPP, R_h, and NEP were generally positive (i.e. amplification of one factor's effect by the other factor) between T and C or between T and DP. A negative interaction (i.e. depression of one factor's effect by the other factor) occurred for simulated NPP between T and HP. The interactive effects on runoff were positive between T and HP. Four pairs of two‐way interactive effects on plant transpiration were positive and two pairs negative. In addition, wet sites generally had smaller relative changes in NPP, R_h, runoff, and transpiration but larger absolute changes in NEP than dry sites in response to the treatments. The modeling results suggest new hypotheses to be tested in multifactor global change experiments. Likewise, more experimental evidence is needed for the further improvement of ecosystem models in order to adequately simulate complex interactive processes.     

Limitation of net CO2 assimilation rate by internal resistances to CO2 transfer in the leaves of two tree species (Fagus sylvatica L. and Castanea sativa Mill.)

Using a combination of gas-exchange and chlorophyll fluorescence measurements, low apparent CO₂/O₂ specificity factors (1300 mol mol_?1) were estimated for the leaves of two deciduous tree species (Fagus sylvatica and Castanea sativa). These low values contrasted with those estimated for two herbaceous species and were ascribed to a drop in the CO₂ mole fraction between the intercellular airspace (C_i) and the catalytic site of Rubisco (C_c) due to internal resistances to CO₂ transfer. C_c. was calculated assuming a specificity of Rubisco value of 2560 mol mol^?1. The drop between C_i and C_c was used to calculate the internal conductance for CO₂ (g_i). A good correlation between mean values of net CO₂ assimilation rate (A) and g_i was observed within a set of data obtained using 13 woody plant species, including our own data. We report that the relative limitation of A, which can be ascribed to internal resistances to CO₂ transfer, was 24–30%. High internal resistances to CO₂ transfer may explain the low apparent maximal rates of carboxylation and electron transport of some woody plant species calculated from A/C_i curves.     

RuBPCO kinetics and the mechanism of CO2 entry in C3 plants

Abstract. The CO₂ partial pressure in the chloroplasts of intact photosynthetic C₃ leaves is thought to be less than the intercellular CO₂ partial pressure. The intercellular CO₂ partial pressure can be calculated from CO₂ and H₂O gas exchange measurements, whereas the CO₂ partial pressure in the chloroplasts is unknown. The conductance of CO₂ from the intercellular space to the chloroplast stroma and the CO₂ partial pressure in the chloroplast stroma can be calculated if the properties of photosynthetic gas exchange are compared with the kinetics of the enzyme ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBPCO). A discrepancy between gas exchange and RuBPCO kinetics can be attributed to a deviation of CO₂ partial pressure in the chloroplast stroma from that calculated in the intercellular space. This paper is concerned with the following: estimation of the kinetic constants of RuBPCO and their comparison with the CO₂ compensation concentration; their comparison with differential uptake of ¹⁴CO₂ and ¹²CO₂; and their comparison with O₂ dependence of net CO₂ uptake of photosynthetic leaves. Discrepancy between RuBPCO kinetics and gas exchange was found at a temperature of 12.5 °C, a photosynthetic photon flux density (PPFD) of 550 μmol quanta m^?2 s^?1, and an ambient CO₂ partial pressure of 40 Pa. Consistency between RuBPCO kinetics and gas exchange was found if CO₂ partial pressure was decreased, temperature incresed and PPFD decreased. The results suggest that a discrepancy between RuBPCO kinetics and gas exchange is due to a diffusion resistance for CO₂ across the chloroplast envelope which decreases with increasing temperature. At low CO₂ partial pressure, the diffusion resistance appears to be counterbalanced by active CO₂ (or HCO₃) transport with high affinity and low maximum velocity. At low PPFD, CO₂ partial pressure in the chloroplast stroma appears to be in equilibrium with that in the intercellular space due to low CO₂ flux.