CO2 balance of boreal, temperate, and tropical forests derived from a global database

Terrestrial ecosystems sequester 2.1 Pg of atmospheric carbon annually. A large amount of the terrestrial sink is realized by forests. However, considerable uncertainties remain regarding the fate of this carbon over both short and long timescales. Relevant data to address these uncertainties are being collected at many sites around the world, but syntheses of these data are still sparse. To facilitate future synthesis activities, we have assembled a comprehensive global database for forest ecosystems, which includes carbon budget variables (fluxes and stocks), ecosystem traits (e.g. leaf area index, age), as well as ancillary site information such as management regime, climate, and soil characteristics. This publicly available database can be used to quantify global, regional or biome‐specific carbon budgets; to re‐examine established relationships; to test emerging hypotheses about ecosystem functioning [e.g. a constant net ecosystem production (NEP) to gross primary production (GPP) ratio]; and as benchmarks for model evaluations. In this paper, we present the first analysis of this database. We discuss the climatic influences on GPP, net primary production (NPP) and NEP and present the CO₂ balances for boreal, temperate, and tropical forest biomes based on micrometeorological, ecophysiological, and biometric flux and inventory estimates. Globally, GPP of forests benefited from higher temperatures and precipitation whereas NPP saturated above either a threshold of 1500 mm precipitation or a mean annual temperature of 10 °C. The global pattern in NEP was insensitive to climate and is hypothesized to be mainly determined by nonclimatic conditions such as successional stage, management, site history, and site disturbance. In all biomes, closing the CO₂ balance required the introduction of substantial biome‐specific closure terms. Nonclosure was taken as an indication that respiratory processes, advection, and non‐CO₂ carbon fluxes are not presently being adequately accounted for.     

C3 grasses have higher nutritional quality than C4 grasses under ambient and elevated atmospheric CO2

Grasses with the C₃ photosynthetic pathway are commonly considered to be more nutritious host plants than C₄ grasses, but the nutritional quality of C₃ grasses is also more greatly impacted by elevated atmospheric CO₂ than is that of C₄ grasses; C₃ grasses produce greater amounts of nonstructural carbohydrates and have greater declines in their nitrogen content than do C₄ grasses under elevated CO₂. Will C₃ grasses remain nutritionally superior to C₄ grasses under elevated CO₂ levels? We addressed this question by determining whether levels of protein in C₃ grasses decline to similar levels as in C₄ grasses, and whether total carbohydrate : protein ratios become similar in C₃ and C₄ grasses under elevated CO₂. In addition, we tested the hypothesis that, among the nonstructural carbohydrates in C₃ grasses, levels of fructan respond most strongly to elevated CO₂. Five C₃ and five C₄ grass species were grown from seed in outdoor open‐top chambers at ambient (370 ppm) or elevated (740 ppm) CO₂ for 2 months. As expected, a significant increase in sugars, starch and fructan in the C₃ grasses under elevated CO₂ was associated with a significant reduction in their protein levels, while protein levels in most C₄ grasses were little affected by elevated CO₂. However, this differential response of the two types of grasses was insufficient to reduce protein in C₃ grasses to the levels in C₄ grasses. Although levels of fructan in the C₃ grasses tripled under elevated CO₂, the amounts produced remained relatively low, both in absolute terms and as a fraction of the total nonstructural carbohydrates in the C₃ grasses. We conclude that C₃ grasses will generally remain more nutritious than C₄ grasses at elevated CO₂ concentrations, having higher levels of protein, nonstructural carbohydrates, and water, but lower levels of fiber and toughness, and lower total carbohydrate : protein ratios than C₄ grasses.     

Integration of CO2 flux and remotely-sensed data for primary production and ecosystem respiration analyses in the Northern Great Plains: potential for quantitative spatial extrapolation

Aim Extrapolation of tower CO₂ fluxes will be greatly facilitated if robust relationships between flux components and remotely sensed factors are established. Long‐term measurements at five Northern Great Plains locations were used to obtain relationships between CO₂ fluxes and photosynthetically active radiation (Q), other on‐site factors, and Normalized Difference Vegetation Index (NDVI) from the SPOT VEGETATION data set. Location CO₂ flux data from the following stations and years were analysed: Lethbridge, Alberta 1998–2001; Fort Peck, MT 2000, 2002; Miles City, MT 2000–01; Mandan, ND 1999–2001; and Cheyenne, WY 1997–98. Results Analyses based on light‐response functions allowed partitioning net CO₂ flux (F) into gross primary productivity (P_g) and ecosystem respiration (R_e). Weekly averages of daytime respiration, γ_day, estimated from light responses were closely correlated with weekly averages of measured night‐time respiration, γ_night (R² 0.64 to 0.95). Daytime respiration tended to be higher than night‐time respiration, and regressions of γ_day on γ_night for all sites were different from 1 : 1 relationships. Over 13 site‐years, gross primary production varied from 459 to 2491 g CO₂ m^?2 year^?1, ecosystem respiration from 996 to 1881 g CO₂ m^?2 year^?1, and net ecosystem exchange from ?537 (source) to +610 g CO₂ m^?2 year^?1 (sink). Maximum daily ecological light‐use efficiencies, ?_d,max = P_g/Q, were in the range 0.014 to 0.032 mol CO₂ (mol incident quanta)^?1. Main conclusions Ten‐day average P_g was significantly more highly correlated with NDVI than 10‐day average daytime flux, P_d (R² = 0.46 to 0.77 for P_g‐NDVI and 0.05 to 0.58 for P_d‐NDVI relationships). Ten‐day average R_e was also positively correlated with NDVI, with R² values from 0.57 to 0.77. Patterns of the relationships of P_g and R_e with NDVI and other factors indicate possibilities for establishing multivariate functions allowing scaling‐up local fluxes to larger areas using GIS data, temporal NDVI, and other factors.     

Scaling up evolutionary responses to elevated CO2: lessons from Arabidopsis

Results from norm of reaction studies and selection experiments indicate that elevated CO₂ will act as a selective agent on natural plant populations, especially for C₃ species that are most sensitive to changes in atmospheric CO₂ concentration. Evolutionary responses to CO₂ may alter plant physiology, development rate, growth, and reproduction in ways that cannot be predicted from single generation studies. Moreover, ecological and evolutionary changes in plant communities will have a range of consequences at higher spatial scales and may cause substantial deviations from ecosystem level predictions based on short‐term responses to elevated CO₂. Therefore, steps need to be taken to identify the plant traits that are most likely to evolve at elevated CO₂, and to understand how these changes may affect net primary productivity within ecosystems. These processes may range in scale from molecular and physiological changes that occur among genotypes at the individual and population levels, to changes in community‐ and ecosystem‐level productivity that result from the integrative effects of different plant species evolving simultaneously. In this review, we (1) synthesize recent studies investigating the role of atmospheric CO₂ as a selective agent on plants, (2) discuss possible control points during plant development that may change in response to selection at elevated CO₂ with an emphasis at the primary molecular level, and (3) provide a quantitative framework for scaling the evolutionary effects of CO₂ on plants in order to determine changes in community and ecosystem productivity. Furthermore, this review points out that studies integrating the effects of plant evolution in response to elevated CO₂ are lacking, and therefore more attention needs be devoted to this issue among the global change research community.     

The turnover of carbon pools contributing to soil CO2 and soil respiration in a temperate forest exposed to elevated CO2 concentration

Soil carbon is returned to the atmosphere through the process of soil respiration, which represents one of the largest fluxes in the terrestrial C cycle. The effects of climate change on the components of soil respiration can affect the sink or source capacity of ecosystems for atmospheric carbon, but no current techniques can unambiguously separate soil respiration into its components. Long‐term free air CO₂ enrichment (FACE) experiments provide a unique opportunity to study soil C dynamics because the CO₂ used for fumigation has a distinct isotopic signature and serves as a continuous label at the ecosystem level. We used the ¹³C tracer at the Duke Forest FACE site to follow the disappearance of C fixed before fumigation began in 1996 (pretreatment C) from soil CO₂ and soil‐respired CO₂, as an index of belowground C dynamics during the first 8 years of the experiment. The decay of pretreatment C as detected in the isotopic composition of soil‐respired CO₂ and soil CO₂ at 15, 30, 70, and 200 cm soil depth was best described by a model having one to three exponential pools within the soil system. The majority of soil‐respired CO₂ (71%) originated in soil C pools with a turnover time of about 35 days. About 55%, 50%, and 68% of soil CO₂ at 15, 30, and 70 cm, respectively, originated in soil pools with turnover times of less than 1 year. The rest of soil CO₂ and soil‐respired CO₂ originated in soil pools that turn over at decadal time scales. Our results suggest that a large fraction of the C returned to the atmosphere through soil respiration results from dynamic soil C pools that cannot be easily detected in traditionally defined soil organic matter standing stocks. Fast oxidation of labile C substrates may prevent increases in soil C accumulation in forests exposed to elevated [CO₂] and may consequently result in shorter ecosystem C residence times.     

The response of soil CO2 flux to changes in atmospheric CO2, nitrogen supply and plant diversity

We measured soil CO₂ flux over 19 sampling periods that spanned two growing seasons in a grassland Free Air Carbon dioxide Enrichment (FACE) experiment that factorially manipulated three major anthropogenic global changes: atmospheric carbon dioxide (CO₂) concentration, nitrogen (N) supply, and plant species richness. On average, over two growing seasons, elevated atmospheric CO₂ and N fertilization increased soil CO₂ flux by 0.57 µmol m^?2 s^?1 (13% increase) and 0.37 µmol m^?2 s^?1 (8% increase) above average control soil CO₂ flux, respectively. Decreases in planted diversity from 16 to 9, 4 and 1 species decreased soil CO₂ flux by 0.23, 0.41 and 1.09 µmol m^?2 s^?1 (5%, 8% and 21% decreases), respectively. There were no statistically significant pairwise interactions among the three treatments. During 19 sampling periods that spanned two growing seasons, elevated atmospheric CO₂ increased soil CO₂ flux most when soil moisture was low and soils were warm. Effects on soil CO₂ flux due to fertilization with N and decreases in diversity were greatest at the times of the year when soils were warm, although there were no significant correlations between these effects and soil moisture. Of the treatments, only the N and diversity treatments were correlated over time; neither were correlated with the CO₂ effect. Models of soil CO₂ flux will need to incorporate ecosystem CO₂ and N availability, as well as ecosystem plant diversity, and incorporate different environmental factors when determining the magnitude of the CO₂, N and diversity effects on soil CO₂ flux.     

Increasing CO2 from subambient to elevated concentrations increases grassland respiration per unit of net carbon fixation

Respiration (carbon efflux) by terrestrial ecosystems is a major component of the global carbon (C) cycle, but the response of C efflux to atmospheric CO₂ enrichment remains uncertain. Respiration may respond directly to an increase in the availability of C substrates at high CO₂, but also may be affected indirectly by a CO₂‐mediated alteration in the amount by which respiration changes per unit of change in temperature or C uptake (sensitivity of respiration to temperature or C uptake). We measured CO₂ fluxes continuously during the final 2 years of a 4‐year experiment on C₃/C₄ grassland that was exposed to a 200–560 μmol mol^?1 CO₂ gradient. Flux measurements were used to determine whether CO₂ treatment affected nighttime respiration rates and the response of ecosystem respiration to seasonal changes in net C uptake and air temperature. Increasing CO₂ from subambient to elevated concentrations stimulated grassland respiration at night by increasing the net amount of C fixed during daylight and by increasing either the sensitivity of C efflux to daily changes in C fixation or the respiration rate in the absence of C uptake (basal ecosystem respiration rate). These latter two changes contributed to a 30–47% increase in the ratio of nighttime respiration to daytime net C influx as CO₂ increased from subamient to elevated concentrations. Daily changes in net C uptake were highly correlated with variation in temperature, meaning that the shared contribution of C uptake and temperature in explaining variance in respiration rates was large. Statistically controlling for collinearity between temperature and C uptake reduced the effect of a given change in C influx on respiration. Conversely, CO₂ treatment did not affect the response of grassland respiration to seasonal variation in temperature. Elevating CO₂ concentration increased grassland respiration rates by increasing both net C input and respiration per unit of C input. A better understanding of how C efflux varies with substrate supply thus may be required to accurately assess the C balance of terrestrial ecosystems.     

Salinity and sea level mediate elevated CO2 effects on C3–C4 plant interactions and tissue nitrogen in a Chesapeake Bay tidal wetland

Elevated atmospheric carbon dioxide concentrations ([CO₂]) generally increase plant photosynthesis in C₃ species, but not in C₄ species, and reduce stomatal conductance in both C₃ and C₄ plants. In addition, tissue nitrogen concentration ([N]) often fails to keep pace with enhanced carbon gain under elevated CO₂, particularly in C₃ species. While these responses are well documented in many species, implications for plant growth and nutrient cycling in native ecosystems are not clear. Here we present data on 18 years of measurement of above and belowground biomass, tissue [N] and total standing crop of N for a Scirpus olneyi‐dominated (C₃ sedge) community, a Spartina patens‐dominated (C₄ grass) community and a C₃–C₄‐mixed species community exposed to ambient and elevated (ambient +340 ppm) atmospheric [CO₂] in natural salinity and sea level conditions of a Chesapeake Bay wetland. Increased biomass production (shoots plus roots) under elevated [CO₂] in the S. olneyi‐dominated community was sustained throughout the study, averaging approximately 35%, while no significant effect of elevated [CO₂] was found for total biomass in the C₄‐dominated community. We found a significant decline in C₄ biomass (correlated with rising sea level) and a concomitant increase in C₃ biomass in the mixed community. This shift from C₄ to C₃ was accelerated by the elevated [CO₂] treatment. The elevated [CO₂] stimulation of total biomass accumulation was greatest during rainy, low salinity years: the average increase above the ambient treatment during the three wettest years (1994, 1996, 2003) was 2.9 t ha⁻¹ but in the three driest years (1995, 1999, 2002), it was 1.2 t ha⁻¹. Elevated [CO₂] depressed tissue [N] in both species, but especially in the S. olneyi where the relative depression was positively correlated with salinity and negatively related with the relative enhancement of total biomass production. Thus, the greatest amount of carbon was added to the S. olneyi‐dominated community during years when shoot [N] was reduced the most, suggesting that the availability of N was not the most or even the main limitation to elevated [CO₂] stimulation of carbon accumulation in this ecosystem.     

Growth at elevated CO2: photosynthetic responses mediated through Rubisco

Abstract. The global uptake of CO₂ in photosynthesis is about 120 gigatons (Gt) of carbon per year. Virtually all passes through one enzyme, ribulose bisphosphate carboxylase/oxygenase (rubisco), which initiates both the photosynthetic carbon reduction, and photorespiratory carbon oxidation, cycles. Both CO₂ and O₂ are substrates; CO₂ also activates the enzyme. In C₃ plants, rubisco has a low catalytic activity, operates below its K_m (CO₂), and is inhibited by O₂. Consequently, increases in the CO₂/O₂ ratio stimulate C₃ photosynthesis and inhibit photorespiration. CO₂ enrichment usually enhances the productivity of C₃ plants, but the effect is marginal in C₄ species. It also causes acclimation in various ways: anatomically, morphologically, physiologically or biochemically. So, CO₂ exerts secondary effects in growth regulation, probably at the molecular level, that are not predictable from its primary biochemical role in carboxylation. After an initial increase with CO₂ enrichment, net photosynthesis often declines. This is a common acclimation phenomenon, less so in field studies, that is ultimately mediated by a decline in rubisco activity, though the RuBP/P_i-regeneration capacities of the plant may play a role. The decline is due to decreased rubisco protein, activation state, and/or specific activity, and it maintains the rubisco fixation and RuBP/P_i regeneration capacities in balance. Carbohydrate accumulation is sometimes associated with reduced net photosynthesis, possibly causing feedback inhibition of the RuBP/P_iregeneration capacities, or chloroplast disruption. As exemplified by field-grown soybeans and salt marsh species, a reduction in net photosynthesis and rubisco activity is not inevitable under CO₂ enrichment. Strong sinks or rapid translocation may avoid such acclimation responses. Over geological time, aquatic autotrophs and terrestrial C₄ and CAM plants have genetically adapted to a decline in the external CO₂/O₂ ratio, by the development of mechanisms to concentrate CO₂ internally; thus circumventing O₂ inhibition of rubisco. Here rubisco affinity for CO₂ is less, but its catalytic activity is greater, a situation compatible with a high-CO₂ internal environment. In aquatic autotrophs, the CO₂ concentrating mechanisms acclimate to the external CO₂, being suppressed at high-CO₂. It is unclear, whether a doubling in atmospheric CO₂ will be sufficient to cause a de-adaptive trend in the rubisco kinetics of future C₃ plants, producing higher catalytic activities.     

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.     

The use of CO2 flux measurements in models of the global terrestrial carbon budget

CO₂ flux measurements give access to two critical terms of the carbon budget of terrestrial ecosystems, the gross primary productivity (GPP) and the net ecosystem productivity (NEP). CO₂ fluxes measured by micrometeorological methods have spatial and temporal characteristics that make them potentially useful in modelling the global terrestrial carbon budget. The first use is in parameterizing ecosystem physiological processes. We present an example, based on parameterizing the mean light response of GPP. This parameterization can be used in diagnostic, satellite-based GPP models. A global application leads to realistic estimates of global GPP. The second use is in testing the seasonality of fluxes predicted by global models. Our example of this use tests two global GPP models. One is a diagnostic, satellite-based model, and one is a prognostic, process-based model. Despite the limitations of the models, both agree reasonably well with the measurements. The agreements and disagreements are useful in addressing the problems of available input datasets and representation of processes, in global models. Long-term CO₂ flux measurements give access to key variables of terrestrial vegetation models and therefore offer exciting perspectives.     

Elevated atmospheric CO2 increases fine root production, respiration, rhizosphere respiration and soil CO2 efflux in Scots pine seedlings

In this study, we investigated the impact of elevated atmospheric CO₂ (ambient + 350 μmol mol^–1) on fine root production and respiration in Scots pine (Pinus sylvestris L.) seedlings. After six months exposure to elevated CO₂, root production measured by root in-growth bags, showed significant increases in mean total root length and biomass, which were more than 100% greater compared to the ambient treatment. This increased root length may have lead to a more intensive soil exploration. Chemical analysis of the roots showed that the roots in the elevated treatment accumulated more starch and had a lower C/N-ratio. Specific root respiration rates were significantly higher in the elevated treatment and this was probably attributed to increased nitrogen concentrations in the roots. Rhizospheric respiration and soil CO₂ efflux were also enhanced in the elevated treatment. These results clearly indicate that under elevated atmospheric CO₂ root production and development in Scots pine seedlings is altered and respiratory carbon losses through the root system are increased.     

Inversion of net ecosystem CO2 flux measurements for estimation of canopy PAR absorption

The fractional absorption of photosynthetically active radiation (f_PAR) is frequently a key variable in models describing terrestrial ecosystem–atmosphere interactions, carbon uptake, growth and biogeochemistry. We present a novel approach to the estimation of the fraction of incident photosynthetically active radiation absorbed by the photosynthetic components of a plant canopy (f_Chl). The method uses micrometeorological measurements of CO₂ flux and incident radiation to estimate light response parameters from which canopy structure is deduced. Data from two Ameriflux sites in Oklahoma, a tallgrass prairie site and a wheat site, are used to derive 7‐day moving average estimates of f_Chl during three years (1997–1999). The inverse estimates are compared to long‐term field measurements of PAR absorption. Good correlations are obtained when the field‐measured f_PAR is scaled by an estimate of the green fraction of total leaf area, although the inverse technique tends to be lower in value than the field measurements. The inverse estimates of f_Chl using CO₂ flux measurements are different from measurements of f_PAR that might be made by other, more direct, techniques. However, because the inverse estimates are based on observed canopy CO₂ uptake, they might be considered more biologically relevant than direct measurements that are affected by non‐physiologically active components of the canopy. With the increasing number of eddy covariance sites around the world the technique provides the opportunity to examine seasonal and inter‐annual variation in canopy structure and light harvesting capacity at individual sites. Furthermore, the inverse f_Chl provide a new source of data for development and testing of f_PAR retrieval using remote sensing. New remote sensing algorithms, or adjustments to existing algorithms, might thus become better conditioned to ‘biologically significant’ light absorption than currently possible.