Interannual variability in carbon dioxide fluxes and flux–climate relationships on grazed and ungrazed northern mixed-grass prairie

The annual carbon (C) budget of grasslands is highly dynamic, dependent on grazing history and on effects of interannual variability (IAV) in climate on carbon dioxide (CO₂) fluxes. Variability in climatic drivers may directly affect fluxes, but also may indirectly affect fluxes by altering the response of the biota to the environment, an effect termed ‘functional change’. We measured net ecosystem exchange of CO₂ (NEE) and its diurnal components, daytime ecosystem CO₂ exchange (P_D) and night‐time respiration (R_E), on grazed and ungrazed mixed‐grass prairie in North Dakota, USA, for five growing seasons. Our primary objective was to determine how climatic anomalies influence variability in CO₂ exchange. We used regression analysis to distinguish direct effects of IAV in climate on fluxes from functional change. Functional change was quantified as the improvement in regression on fitting a model in which slopes of flux–climate relationships vary among years rather than remain invariant. Functional change and direct effects of climatic variation together explained about 20% of variance in weekly means of NEE, P_D, and R_E. Functional change accounted for more than twice the variance in fluxes of direct effects of climatic variability. Grazing did not consistently influence the contribution of functional change to flux variability, but altered which environmental variable best explained year‐to‐year differences in flux–climate slopes, reduced IAV in seasonal means of fluxes, lessened the strength of flux–climate correlations, and increased NEE by reducing R_E relatively more than P_D. Most of these trends are consistent with the interpretation that grazing reduced the influence of plants on ecosystem fluxes. Because relationships between weekly values of fluxes and climatic regulators changed annually, year‐to‐year differences in the C balance of these ecosystems cannot be predicted from knowledge of IAV in climate alone.     

Seasonal controls on interannual variability in carbon dioxide exchange of a near-end-of rotation Douglas-fir stand in the Pacific Northwest, 1997–2006

This study analyzes 9 years of eddy‐covariance (EC) data carried out in a Pacific Northwest Douglas‐fir (Pseudotsuga menzesii) forest (58‐year old in 2007) on the east coast of Vancouver Island, Canada, and characterizes the seasonal and interannual variability in net ecosystem productivity (NEP), gross primary productivity (GPP), and ecosystem respiration (R_e) and primary climatic controls on these fluxes. The annual values (± SD) of NEP, GPP and R_e were 357 ± 51, 2124 ± 125, and 1767 ± 146 g C m^?2 yr^?1, respectively, with ranges of 267–410, 1592–2338, and 1642–2071 g C m^?2 yr^?1, respectively. Spring to early summer (March–June) accounted for more than 80% of annual NEP while late spring to early autumn (May–August) was mainly responsible for its interannual variability (～80%). The major drivers of interannual variability in annual carbon (C) fluxes were annual and spring mean air temperatures (T_a) and water deficiency during late summer and autumn (July–October) when this Douglas‐fir forest growth was often water‐limited. Photosynthetically active radiation (Q), and the combination of Q and soil water content (θ) explained 85% and 91% of the variance of monthly GPP, respectively; and 91% and 96% of the variance of monthly R_e was explained by T_a and the combination of T_a and θ, respectively. Annual net C sequestration was high during optimally warm and normal precipitation years, but low in unusually warm or severely dry years. Excluding 1998 and 1999, the 2 years strongly affected by an El Niño/La Niña cycle, annual NEP significantly decreased with increasing annual mean T_a. Annual NEP will likely decrease whereas both annual GPP and R_e will likely increase if the future climate at the site follows a trend similar to that of the past 40 years.     

Latitudinal patterns of magnitude and interannual variability in net ecosystem exchange regulated by biological and environmental variables

Over the last two and half decades, strong evidence showed that the terrestrial ecosystems are acting as a net sink for atmospheric carbon. However the spatial and temporal patterns of variation in the sink are not well known. In this study, we examined latitudinal patterns of interannual variability (IAV) in net ecosystem exchange (NEE) of CO₂ based on 163 site-years of eddy covariance data, from 39 northern-hemisphere research sites located at latitudes ranging from ∼29°N to ∼64°N. We computed the standard deviation of annual NEE integrals at individual sites to represent absolute interannual variability (AIAV), and the corresponding coefficient of variation as a measure of relative interannual variability (RIAV). Our results showed decreased trends of annual NEE with increasing latitude for both deciduous broadleaf forests and evergreen needleleaf forests. Gross primary production (GPP) explained a significant proportion of the spatial variation of NEE across evergreen needleleaf forests, whereas, across deciduous broadleaf forests, it is ecosystem respiration (Re). In addition, AIAV in GPP and Re increased significantly with latitude in deciduous broadleaf forests, but AIAV in GPP decreased significantly with latitude in evergreen needleleaf forests. Furthermore, RIAV in NEE, GPP, and Re appeared to increase significantly with latitude in deciduous broadleaf forests, but not in evergreen needleleaf forests. Correlation analyses showed air temperature was the primary environmental factor that determined RIAV of NEE in deciduous broadleaf forest across the North American sites, and none of the chosen climatic factors could explain RIAV of NEE in evergreen needleleaf forests. Mean annual NEE significantly increased with latitude in grasslands. Precipitation was dominant environmental factor for the spatial variation of magnitude and IAV in GPP and Re in grasslands.     

Impacts of Hurricane Frances on Florida scrub-oak ecosystem processes: defoliation, net CO2 exchange and interactions with elevated CO2

Hurricane disturbances have profound impacts on ecosystem structure and function, yet their effects on ecosystem CO₂ exchange have not been reported. In September 2004, our research site on a fire‐regenerated scrub‐oak ecosystem in central Florida was struck by Hurricane Frances with sustained winds of 113 km h⁻¹ and wind gusts as high as 152 km h⁻¹. We quantified the hurricane damage on this ecosystem resulting from defoliation: we measured net ecosystem CO₂ exchange, the damage and recovery of leaf area, and determined whether growth in elevated carbon dioxide concentration in the atmosphere (C_a) altered this disturbance. The hurricane decreased leaf area index (LAI) by 21%, which was equal to 60% of seasonal variation in canopy growth during the previous 3 years, but stem damage was negligible. The reduction in LAI led to a 22% decline in gross primary production (GPP) and a 25% decline in ecosystem respiration (R_e). The compensatory declines in GPP and R_e resulted in no significant change in net ecosystem production (NEP). Refoliation began within a month after the hurricane, although this period was out of phase with the regular foliation period, and recovered 20% of the defoliation loss within 2.5 months. Full recovery of LAI, ecosystem CO₂ assimilation, and ecosystem respiration did not occur until the next growing season. Plants exposed to elevated C_a did not sustain greater damage, nor did they recover faster than plants grown under ambient C_a. Thus, our results indicate that hurricanes capable of causing significant defoliation with negligible damage to stems have negligible effects on NEP under current or future CO₂‐enriched environment.     

Seasonal and interannual variation in carbon dioxide exchange and carbon balance in a northern temperate grassland

Net ecosystem carbon dioxide (CO₂) exchange (NEE) was measured in a northern temperate grassland near Lethbridge, Alberta, Canada for three growing seasons using the eddy covariance technique. The study objectives were to document how NEE and its major component processes—gross photosynthesis (GPP) and total ecosystem respiration (TER)—vary seasonally and interannually, and to examine how environmental and physiological factors influence the annual C budget. The greatest difference among the three study years was the amount of precipitation received. The annual precipitation for 1998 (481.7 mm) was significantly above the 1971–2000 mean (± SD, 377.9 ± 97.0 mm) for Lethbridge, whereas 1999 (341.3 mm) was close to average, and 2000 (275.5 mm) was significantly below average. The high precipitation and soil moisture in 1998 allowed a much higher GPP and an extended period of net carbon gain relative to 1999 and 2000. In 1998, the peak NEE was a gain of 5 g C m^?2 d^?1 (day 173). Peak NEE was lower and also occurred earlier in the year on days 161 (3.2 g C m^?2 d^?1) and 141 (2.4 g C m^?2 d^?1) in 1999 and 2000, respectively. Change in soil moisture was the most important ecological factor controlling C gain in this grassland ecosystem. Soil moisture content was positively correlated with leaf area index (LAI). Gross photosynthesis was strongly correlated with changes in both LAI and canopy nitrogen (N) content. Maximum GPP (A_max: value calculated from a rectangular hyperbola fitted to the relationship between GPP and incident photosynthetic photon flux density (PPFD)) was 27.5, 12.9 and 8.6 µmol m^?2 s^?1 during 1998, 1999 and 2000, respectively. The apparent quantum yield also differed among years at the time of peak photosynthetic activity, with calculated values of 0.0254, 0.018 and 0.018 during 1998, 1999 and 2000, respectively. The ecosystem accumulated a total of 111.9 g C m^?2 from the time the eddy covariance measurements were initiated in June 1998 until the end of December 2000, with most of that C gained during 1998. There was a net uptake of almost 21 g C m^?2 in 1999, whereas a net loss of 18 g C m^?2 was observed in 2000. The net uptake of C during 1999 was the combined result of slightly higher GPP (287.2 vs. 272.3 g C m^?2 year^?1) and lower TER (266.6 vs. 290.4 g C m^?2 year^?1) than occurred in 2000.     

Diurnal, seasonal and annual variation in the net ecosystem CO2 exchange of a desert shrub community (Sarcocaulescent) in Baja California, Mexico

Estimates of net ecosystem exchange (NEE) of CO₂ have been measured on a variety of ecosystems world wide including grasslands, savannahs, boreal, pine, deciduous, Mediterranean and tropical rain forests as well as arctic tundra. While there have been numerous comparisons between net primary productivity of arid and semiarid grasslands and shrublands, notably lacking are estimates of NEE with a few exceptions. The objective of this study was to characterize the seasonal and annual carbon flux of a desert shrub ecosystem using the eddy covariance technique to determine the sensitivity of the system to the timing and varying amounts of precipitation. Measurements began in July of 2001, a year with 339 mm of rainfall, considerably above the long‐term average of 174 mm and preceded by 2 years of below average rainfall (50–62 mm). Over the 2 complete years of measurements, precipitation was 147 and 197 mm in 2002 and 2003, respectively. In all years, the majority of the precipitation fell between August and September. The site was a sink of ?39 g C m^?2 yr^?1 in 2002 with a relatively strong uptake in the early part of the year and reduced uptake after the suboptimal rainfall in September. This contrasts with 2003 when the ecosystem took up ?52 g C m^?2 yr^?1 concentrated in the fall after significant rain in August and September. Likely, extremely low rainfall years would result in a carbon loss while a strengthening of the typical winter secondary peak in precipitation (notably absent in the 2 years of measurements) may extend uptake into the spring resulting in more carbon accumulation. The system appears to be buffered against variations in annual rainfall attributed to water storage in the stems and roots.     

CO2 supersaturation along the aquatic conduit in Swedish watersheds as constrained by terrestrial respiration,aquatic respiration and weathering

We tested the hypothesis that CO₂ supersaturation along the aquatic conduit over Sweden can be explained by processes other than aquatic respiration. A first generalized‐additive model (GAM) analysis evaluating the relationships between single water chemistry variables and pCO₂ in lakes and streams revealed that water chemistry variables typical for groundwater input, e.g., dissolved silicate (DSi) and Mg²⁺ had explanatory power similar to total organic carbon (TOC). Further GAM analyses on various lake size classes and stream orders corroborated the slightly higher explanatory power for DSi in lakes and Mg²⁺ for streams compared with TOC. Both DSi and TOC explained 22–46% of the pCO₂ variability in various lake classes (0.01–>100 km²) and Mg²⁺ and TOC explained 11–41% of the pCO₂ variability in the various stream orders. This suggests that aquatic pCO₂ has a strong groundwater signature. Terrestrial respiration is a significant source of the observed supersaturation and we may assume that both terrestrial respiration and aquatic respiration contributed equally to pCO₂ efflux. pCO₂ and TOC concentrations decreased with lake size suggesting that the longer water residence time allow greater equilibration of CO₂ with the atmosphere and in‐lake mineralization of TOC. For streams, we observed a decreasing trend in pCO₂ with stream orders between 3 and 6. We calculated the total CO₂ efflux from all Swedish lakes and streams to be 2.58 Tg C yr^?1. Our analyses also demonstrated that 0.70 Tg C yr^?1 are exported to the ocean by Swedish watersheds as HCO₃^? and CO₃^2? of which about 0.56 Tg C yr^?1 is also a residual from terrestrial respiration and constitute a long‐term sink for atmospheric CO₂. Taking all dissolved inorganic carbon (DIC) fluxes along the aquatic conduit into account will lower the estimated net ecosystem C exchange (NEE) by 2.02 Tg C yr^?1, which corresponds to 10% of the NEE in Sweden.     

Diurnal, seasonal and annual variation in net ecosystem CO2 exchange of an alpine shrubland on Qinghai-Tibetan plateau

Thus far, grassland ecosystem research has mainly been focused on low‐lying grassland areas, whereas research on high‐altitude grassland areas, especially on the carbon budget of remote areas like the Qinghai‐Tibetan plateau is insufficient. To address this issue, flux of CO₂ were measured over an alpine shrubland ecosystem (37°36′N, 101°18′E; 325 above sea level [a. s. l.]) on the Qinghai‐Tibetan Plateau, China, for 2 years (2003 and 2004) with the eddy covariance method. The vegetation is dominated by formation Potentilla fruticosa L. The soil is Mol–Cryic Cambisols. To interpret the biotic and abiotic factors that modulate CO₂ flux over the course of a year we decomposed net ecosystem CO₂ exchange (NEE) into its constituent components, and ecosystem respiration (R_eco). Results showed that seasonal trends of annual total biomass and NEE followed closely the change in leaf area index. Integrated NEE were ?58.5 and ?75.5 g C m^?2, respectively, for the 2003 and 2004 years. Carbon uptake was mainly attributed from June, July, August, and September of the growing season. In July, NEE reached seasonal peaks of similar magnitude (4–5 g C m^?2 day^?1) each of the 2 years. Also, the integrated night‐time NEE reached comparable peak values (1.5–2 g C m^?2 day^?1) in the 2 years of study. Despite the large difference in time between carbon uptake and release (carbon uptake time < release time), the alpine shrubland was carbon sink. This is probably because the ecosystem respiration at our site was confined significantly by low temperature and small biomass and large day/night temperature difference and usually soil moisture was not limiting factor for carbon uptake. In general, R_eco was an exponential function of soil temperature, but with season‐dependent values of Q₁₀. The temperature‐dependent respiration model failed immediately after rain events, when large pulses of R_eco were observed. Thus, for this alpine shrubland in Qinghai‐Tibetan plateau, the timing of rain events had more impact than the total amount of precipitation on ecosystem R_eco and NEE.     

A long-term record of carbon exchange in a boreal black spruce forest: means, responses to interannual variability, and decadal trends

We present a decadal (1994–2004) record of carbon dioxide flux in a 160‐year‐old black spruce forest/veneer bog complex in central Manitoba, Canada. The ecosystem shifted from a source (+41 g C m⁻², 1995) to a sink (−21 g C m⁻², 2004) of CO₂ over the decade, with an average net carbon balance near zero. Annual mean temperatures increased 1–2° during the period, consistent with the decadal trend across the North American boreal biome. We found that ecosystem carbon exchange responded strongly to air temperature, moisture status, potential evapotranspiration, and summertime solar radiation. The seasonal cycle of ecosystem respiration significantly lagged that of photosynthesis, limited by the rate of soil thaw and the slow drainage of the soil column. Factors acting over long time scales, especially water table depth, strongly influenced the carbon budget on annual time scales. Net uptake was enhanced and respiration inhibited by multiple years of rainfall in excess of evaporative demand. Contrary to expectations, we observed no correlation between longer growing seasons and net uptake, possibly because of offsetting increases in ecosystem respiration. The results indicate that the interactions between soil thaw and water table depth provide critical controls on carbon exchange in boreal forests underlain by peat, on seasonal to decadal time scales, and these factors must be simulated in terrestrial biosphere models to predict response of these regions to future climate.     

Antecedent moisture and seasonal precipitation influence the response of canopy-scale carbon and water exchange to rainfall pulses in a semi-arid grassland

The influences of prior monsoon-season drought (PMSD) and the seasonal timing of episodic rainfall ('pulses') on carbon and water exchange in water-limited ecosystems are poorly quantified. *In the present study, we estimated net ecosystem exchange of CO(2) (NEE) and evapotranspiration (ET) before, and for 15 d following, experimental irrigation in a semi-arid grassland during June and August 2003. Rainout shelters near Tucson, Arizona, USA, were positioned on contrasting soils (clay and sand) and planted with native (Heteropogon contortus) or non-native invasive (Eragrostis lehmanniana) C4 bunchgrasses. Plots received increased ('wet') or decreased ('dry') monsoon-season (July-September) rainfall during 2002 and 2003. Following a June 2003 39-mm pulse, species treatments had similar NEE and ET dynamics including 15-d integrated NEE (NEE(pulse)). Contrary to predictions, PMSD increased net C uptake during June in plots of both species. Greater flux rates after an August 2003 39-mm pulse reflected biotic activity associated with the North American Monsoon. Furthermore, August NEE(pulse) and ecosystem pulse-use efficiency (PUE(e) = NEE(pulse)/ET(pulse)) was greatest in Heteropogon plots. PMSD and rainfall seasonal timing may interact with bunchgrass invasions to alter NEE and ET dynamics with consequences for PUE(e) in water-limited 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.

     

Seasonal variability in the effect of elevated CO2 on ecosystem leaf area index in a scrub-oak ecosystem

We report effects of elevated atmospheric CO₂ concentration (C_a) 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 C_a, 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 C_a. 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 C_a. In addition, increased LAI decreased potential reductions in canopy transpiration from decreases in stomatal conductance in elevated C_a. These findings have important implications for biogeochemical cycles of C, N and H₂O in woody ecosystems regenerating from disturbance in elevated C_a.     

Importance of changing CO2, temperature, precipitation, and ozone on carbon and water cycles of an upland-oak forest: incorporating experimental results into model simulations

Observed responses of upland‐oak vegetation of the eastern deciduous hardwood forest to changing CO₂, temperature, precipitation and tropospheric ozone (O₃) were derived from field studies and interpreted with a stand‐level model for an 11‐year range of environmental variation upon which scenarios of future environmental change were imposed. Scenarios for the year 2100 included elevated [CO₂] and [O₃] (+385 ppm and +20 ppb, respectively), warming (+4°C), and increased winter precipitation (+20% November–March). Simulations were run with and without adjustments for experimentally observed physiological and biomass adjustments. Initial simplistic model runs for single‐factor changes in CO₂ and temperature predicted substantial increases (+191% or 508 g C m^?2 yr^?1) or decreases (?206% or ?549 g C m^?2 yr^?1), respectively, in mean annual net ecosystem carbon exchange (NEE_a≈266±23 g C m^?2 yr^?1 from 1993 to 2003). Conversely, single‐factor changes in precipitation or O₃ had comparatively small effects on NEE_a (0% and ?35%, respectively). The combined influence of all four environmental changes yielded a 29% reduction in mean annual NEE_a. These results suggested that future CO₂‐induced enhancements of gross photosynthesis would be largely offset by temperature‐induced increases in respiration, exacerbation of water deficits, and O₃‐induced reductions in photosynthesis. However, when experimentally observed physiological adjustments were included in the simulations (e.g. acclimation of leaf respiration to warming), the combined influence of the year 2100 scenario resulted in a 20% increase in NEE_a not a decrease. Consistent with the annual model's predictions, simulations with a forest succession model run for gradually changing conditions from 2000 to 2100 indicated an 11% increase in stand wood biomass in the future compared with current conditions. These model‐based analyses identify critical areas of uncertainty for multivariate predictions of future ecosystem response, and underscore the importance of long term field experiments for the evaluation of acclimation and growth under complex environmental scenarios.     

On the relationship between water table depth and water vapor and carbon dioxide fluxes in a minerotrophic fen

The focus of this study is the relationship between water table depth (WTD) and water vapor [evapotranspiration (ET)] and carbon dioxide [CO₂; net ecosystem exchange (NEE)] fluxes in a fen in western Canada. We analyzed hydrological and eddy covariance measurements from four snow‐free periods (2003–2006) with contrasting meteorological conditions to establish the link between daily WTD and ET and gross ecosystem CO₂ exchange (GEE) and ecosystem respiration (R_eco; NEE=R_eco?GEE), respectively: 2003 was warm and dry, 2004 was cool and wet, and 2005 and 2006 were both wet. In 2003, the water table (WT) was below the ground surface. In 2004, the WT rose above the ground surface, and in 2005 and 2006, the WT stayed well above the ground surface. There were no significant differences in total ET (～316 mm period^?1), but total NEE was significantly different (2003: 8 g C m^?2 period^?1; 2004: ?139 g C m^?2 period^?1; 2005: ?163 g C m^?2 period^?1; 2006: ?195 g C m^?2 period^?1), mostly due to differences in total GEE (2003: 327 g C m^?2 period^?1; 2004: 513 g C m^?2 period^?1; 2005: 411 g C m^?2 period^?1; 2006: 556 g C m^?2 period^?1). Variation in ET is mostly explained by radiation (67%), and the contribution of WTD is only minor (33%). WTD controls the compensating contributions of different land surface components, resulting in similar total ET regardless of the hydrological conditions. WTD and temperature each contribute about half to the explained variation in GEE up to a threshold ponding depth, below which temperature alone is the key explanatory variable. WTD is only of minor importance for the variation in R_eco, which is mainly controlled by temperature. Our study implies that future peatland modeling efforts explicitly consider topographic and hydrogeological influences on WTD.     

Warmer and drier conditions stimulate respiration more than photosynthesis in a boreal peatland ecosystem: Analysis of automatic chambers and eddy covariance measurements

Continuous half‐hourly net CO₂ exchange measurements were made using nine automatic chambers in a treed fen in northern Alberta, Canada from June–October in 2005 and from May–October in 2006. The 2006 growing season was warmer and drier than in 2005. The average chamber respiration rates normalized to 10 °C were much higher in 2006 than in 2005, while calculations of the temperature sensitivity (Q₁₀) values were similar in the two years. Daytime average respiration values were lower than the corresponding, temperature‐corrected respiration rates calculated from night‐time chamber measurements. From June to September, the season‐integrated estimates of chamber photosynthesis and respiration were 384 and 590 g C m^?2, respectively in 2006, an increase of 100 and 203 g C m^?2 over the corresponding values in 2005. The season‐integrated photosynthesis and respiration rates obtained using the eddy covariance technique, which included trees and a tall shrub not present in the chambers, were 720 and 513 g C m^?2, respectively, in 2006, an increase of 50 and 125 g C m^?2 over the corresponding values in 2005. While both photosynthesis and respiration rates were higher in the warmer and drier conditions of 2006, the increase in respiration was more than twice the increase in photosynthesis.     

Interannual variation and climatic regulation of the CO2 emission from large boreal lakes

We studied the interannual variation of surface water partial pressure of CO₂ (pCO₂) and the CO₂ emissions from the 37 large Finnish lakes linking them to the water quality, catchment and climate attributes in 1996–2001. The lake water CO₂ was measured three times a year in the study lakes in 1998 and 1999 and for the rest of the years the CO₂ was modeled by measured alkalinity. The median annual CO₂ emission to the atmosphere ranged between 1.49 and 2.29 mol m^?2 a^?1. The annual CO₂ emission followed closely the annual precipitation pattern with the highest emission during the years when the precipitation was highest (r²=0.81–0.97, P<0.05). There was a strong negative correlation (r²=0.50–0.82, P<0.001) between O₂ and CO₂ saturation in the lake water during stratification suggesting effective decomposition of organic matter in the lakes. Furthermore, total phosphorus and the proportion of agricultural land in the catchment had significant positive correlations with CO₂ saturation.     

Functional diversity of photosynthesis during drought in a model tropical rainforest – the contributions of leaf area, photosynthetic electron transport and stomatal conductance to reduction in net ecosystem carbon exchange

The tropical rainforest mesocosm within the Biosphere 2 Laboratory, a model system of some 110 species developed over 12 years under controlled environmental conditions, has been subjected to a series of comparable drought experiments during 2000–2002. In each study, the mesocosm was subjected to a 4–6 week drought, with well‐defined rainfall events before and after the treatment. Ecosystem CO₂ uptake rate (A_eco) declined 32% in response to the drought, with changes occurring within days and being reversible within weeks, even though the deeper soil layers did not become significantly drier and leaf‐level water status of most large trees was not greatly affected. The reduced A_eco during the drought reflected both morphological and physiological responses. It is estimated that the drought‐induced 32% reduction of A_eco has three principal components: (1) leaf fall increased two‐fold whereas leaf expansion growth of some canopy dominants declined to 60%, leading to a 10% decrease in foliage coverage of the canopy. This might be the main reason for the persistent reduction of A_eco after rewatering. (2) The maximum photosynthetic electron transport rate at high light intensities in remaining leaves was reduced to 71% for three of the four species measured, even though no chronic photo‐inhibition occurred. (3) Stomata closed, leading to a reduced ecosystem water conductance to water vapour (33% of pre‐drought values), which not only reduced ecosystem carbon uptake rate, but may also have implications for water and energy budgets of tropical ecosystems. Additionally, individual rainforest trees responded differently, expressing different levels of stress and stress avoiding mechanisms. This functional diversity renders the individual response heterogeneous and has fundamental implications to scale leaf level responses to ecosystem dynamics.     

CO2 exchange of CAM exhibiting succulents in the southern Namib desert in relation to microclimate and water stress

The responses of CO₂ exchange and overnight malate accumulation of leaf and stem succulent CAM-plants to water stress and the particular climatic conditions of fog and föhn in the southern Namib desert have been investigated. In most of the investigated CAM plants a long term water stress gradually attenuated any uptake of external CO₂ and led to CO₂ release throughout day and night. No CAM-idling was observed. Rainfall or irrigation immediately restored daytime CO₂ uptake while the recovery of the nocturnal CO₂ uptake was delayed. Dawn peak of photosynthesis was only found in well watered plants but was markedly reduced by the short term water stress of a föhn-storm. Morning fog with its higher diffuse light intensity compared with clear days increased photosynthetic CO₂ uptake considerably. Even in well watered plants nocturnal CO₂ uptake and malate accumulation were strongly affected by föhn indicating that the water vapour pressure deficit during the night determines the degree of acidification.     

Ecosystem responses to water and nitrogen amendment in a California grassland

The world's ecosystems are experiencing simultaneous changes in the supply of multiple limiting resources. Two of these, water and nitrogen (N) can strongly limit grassland production and can affect community composition and biogeochemical cycles in different ways. Grassland ecosystems in California may be particularly vulnerable to current and predicted changes in precipitation and N deposition, and ecosystem responses to potential interactive effects of water and N are not well understood. Here, we show strong colimitation of plant production resulting from factorial addition of water and N. In addition, water and N addition in combination led to increased dominance of the two most abundant grass species, while N addition regardless of water availability led to decreased species diversity. Late season carbon (C) flux response to water addition depended on N. Only plots that received additional water, but not N, still showed net ecosystem C uptake at the end of the experiment. Our results suggest that grassland ecosystem response to N deposition will be strongly dependent on future precipitation patterns.     

Strong seasonal disequilibrium measured between the oxygen isotope signals of leaf and soil CO2 exchange

The oxygen isotope composition (δ¹⁸O) of atmospheric CO₂ is among a very limited number of tools available to constrain estimates of the biospheric gross CO₂ fluxes, photosynthesis and respiration at large scales. However, the accuracy of the partitioning strongly depends on the extent of isotopic disequilibrium between the signals carried by these two gross fluxes. Chamber‐based field measurements of total CO₂ and CO¹⁸O fluxes from foliage and soil can help evaluate and refine our models of isotopic fractionation by plants and soils and validate the extent and pattern of isotopic disequilibrium within terrestrial ecosystems. Owing to sampling limitations in the past, such measurements have been very rare and covered only a few days. In this study, we coupled automated branch and soil chambers with tuneable diode laser absorption spectroscopy techniques to continuously capture the δ¹⁸O signals of foliage and soil CO₂ exchange in a Pinus pinaster Aït forest in France. Over the growing season, we observed a seasonally persistent isotopic disequilibrium between the δ¹⁸O signatures of net CO₂ fluxes from leaves and soils, except during rain events when the isotopic imbalance became temporarily weaker. Variations in the δ¹⁸O of CO₂ exchanged between leaves, soil and the atmosphere were well explained by theory describing changes in the oxygen isotope composition of ecosystem water pools in response to changes in leaf transpiration and soil evaporation.