Net Ecosystem Carbon Dioxide Exchange Research Articles

Net ecosystem carbon dioxide exchange of dedicated bioenergy feedstocks: Switchgrass and high biomass sorghum

As switchgrass (Panicum virgatum L.) and high biomass sorghum (Sorghum bicolor L. Moench) acreages are expanding for cellulosic biofuels, it is critical to improve understanding of carbon dynamics of these two potential bioenergy crops. Eddy flux measurements from co-located switchgrass and high biomass sorghum fields during the 2012 and 2013 growing seasons were analyzed to quantify and compare net ecosystem CO2 exchange (NEE) between two species. Monthly ensemble averaged NEE reached seasonal peak values of −36.9±1.78 and −35.9±2.32μmolm−2s−1 in switchgrass and sorghum, respectively. Similar magnitudes of NEE (−10 to −11gCm−2d−1), gross primary production (GPP, 19–20gCm−2d−1) and ecosystem respiration (ER, 10–12gCm−2d−1) were observed in both ecosystems. Similarly, carbon fluxes of both ecosystems had similar response to air temperature and vapor pressure deficit (VPD). Carbon fluxes exhibited an optimum temperature of slightly over 30°C and decreased markedly beyond 35°C. The NEE decreased markedly at higher VPD (>3kPa) because of the stomatal closure control of photosynthesis. The switchgrass field was a larger carbon sink, with a cumulative seasonal carbon uptake of −406±24 to −490±59gCm−2 compared to −261±48 to −330±45gCm−2 by the sorghum field. The switchgrass stand was a net carbon sink for four to five months (April/May–August), while sorghum appeared to be a net carbon sink for only three months (June–August). Our results imply that the difference in carbon sink strength between the two species was driven mainly by the length of the growing season.

Can boreal peatlands with pools be net sinks for CO2?

Peatland open-water pools, a common feature on temperate to subarctic peatlands, are sources of carbon (C) to the atmosphere but their contribution to the net ecosystem carbon dioxide exchange (NEE-CO2) is poorly known; there is a question as to whether peatlands with pools are smaller sinks of atmospheric C, or even C-neutral, compared to other peatlands. We present growing season NEE-CO2 measurements using the eddy covariance technique in a peatland with pools. We found the maximum photosynthetic uptake and ecosystem respiration rates at 10 °C to be in the lower range of the published data. The lower total vegetation biomass, due to the presence of pools, reduced CO2 uptake during day and the autotrophic component of ecosystem respiration. The low CO2 uptake combined with reduced CO2 loss resulted in the site being a net sink for CO2 of a similar magnitude as other northern peatlands despite the inclusion of pools.

Ecosystem carbon exchange in response to locust outbreaks in a temperate steppe.

It is predicted that locust outbreaks will occur more frequently under future climate change scenarios, with consequent effects on ecological goods and services. A field manipulative experiment was conducted to examine the responses of gross ecosystem productivity (GEP), net ecosystem carbon dioxide (CO2) exchange (NEE), ecosystem respiration (ER), and soil respiration (SR) to locust outbreaks in a temperate steppe of northern China from 2010 to 2011. Two processes related to locust outbreaks, natural locust feeding and carcass deposition, were mimicked by clipping 80 % of aboveground biomass and adding locust carcasses, respectively. Ecosystem carbon (C) exchange (i.e., GEP, NEE, ER, and SR) was suppressed by locust feeding in 2010, but stimulated by locust carcass deposition in both years (except SR in 2011). Experimental locust outbreaks (i.e., clipping plus locust carcass addition) decreased GEP and NEE in 2010 whereas they increased GEP, NEE, and ER in 2011, leading to neutral changes in GEP, NEE, and SR across the 2 years. The responses of ecosystem C exchange could have been due to the changes in soil ammonium nitrogen, community cover, and aboveground net primary productivity. Our findings of the transient and neutral changes in ecosystem C cycling under locust outbreaks highlight the importance of resistance, resilience, and stability of the temperate steppe in maintaining reliable ecosystem services, and facilitate the projections of ecosystem functioning in response to natural disturbance and climate change.

Carbon dioxide exchange between an alpine steppe ecosystem and the atmosphere on the Nam Co area of the Tibetan Plateau

The net ecosystem carbon dioxide (CO2) exchange (NEE) between the atmosphere and an alpine steppe ecosystem was measured by the Eddy covariance (EC) method on the Nam Co area of the Tibetan Plateau during the growing season in 2008 and 2009. The diurnal amplitude of NEE varied substantially during the growing seasons. The maximum CO2 uptake rates were 3.74 and 3.44μmol CO2m−2s−1 in August 2008 and 2009, respectively. The peak daily CO2 uptake was observed on 16 July (DOY 198, 1.19gCm−2day−1) 2008, which occurred one month earlier than that in 2009 (DOY 228, 1.12gCm−2day−1). This indicated that the alpine steppe had lower carbon sequestration potential comparing with other grasslands. The daily NEE responded to AGB in a linear manner (NEE=(−0.0073±0.0027)×AGB)+(0.0533±0.2049), R2=0.52). Fifty-two percent of the NEE variance could be explained by the variance of AGB. The amount and pattern of precipitation was significantly different between the two growing seasons. In 2008, the steppe received 493mm from May to September. In contrast, the value was 327mm in 2009 and more than 50% of this precipitation was received in August. The alpine steppe ecosystem became a weak carbon source in June 2009 (0.48gCm−2) due to relatively low soil volumetric water content (VWC) and the relatively high temperature. The cumulative NEE for the growing season from May to September was 63.17 and 35.51gCm−2 in 2008 and 2009, respectively. Variations in soil moisture had important effect on carbon exchange in this alpine steppe ecosystem. Both the photosynthesis and respiration were active under high soil moisture content, and suppressed during periods of water shortage.

SEASONAL DYNAMICS IN CARBON DIOXIDE FLUX ES OF THE HERBACEOUS LAYER OF A MOIST KENYAN SAVANNAH

Strong seasonal variability in African carbon source/sink relationship is considered the continent's most significant contribution to the global carbon cycle. Alternating dry and wet periods dictate ecosystem carbon exchange and productivity of tropical African savannah. We examined the seasonal and daily trends in ecosystem CO2 exchange in the herbaceous layer of a humid Kenyan savannah devoid of grazing. Microclimate, soil moisture, soil and tissue nitrogen, aboveground biomass and carbon dioxide fluxes were measured. The ecosystem carbon dioxide fluxes followed the seasonal rainfall pattern and were strongly correlated to soil water content. Peak mean carbon dioxide fluxes were 3.21±0.99, 8.21±1.02 and 4.67±1.06 µmolm -2 s -1 during drought and -13.86±1.48, 15.03±0.98, and 27.73±1.47 µmolm -2 s -1 during wet periods for net ecosystem carbon dioxide exchange, ecosystem respiration and gross primary production respectively. At daily scale, net ecosystem carbon dioxide exchange increased with increasing photosynthetic photon flux density under wet but decreased with increasing vapour pressure deficit under dry conditions. Ecosystem respiration increased with increasing soil temperature during wet but decreased with increasing soil temperature during drought. Our results point to the overriding role of soil moisture in the ecosystem carbon dioxide exchange processes of this

黄河三角洲芦苇湿地生态系统碳、水热通量特征研究

PDF HTML阅读 XML下载 导出引用 引用提醒 黄河三角洲芦苇湿地生态系统碳、水热通量特征 DOI: 10.5846/stxb201212051750 作者: 作者单位: 中国科学院大学生命科学学院,中国科学院大学生命科学学院,中国科学院大学生命科学学院,中国科学院大学生命科学学院,中国科学院大学生命科学学院,中国科学院大学生命科学学院,中国农业科学院农业环境与可持续发展研究所 作者简介: 通讯作者: 中图分类号: 基金项目: 国家自然科学基金项目（31170459） Carbon,water and heat fluxes of a reed (Phragmites australis) wetland in the Yellow River Delta, China Author: Affiliation: University of Chinese Academy of Sciences,,University of Chinese Academy of Sciences,,,, Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:利用涡度相关法对黄河三角洲芦苇湿地生态系统进行了连续两年的通量观测，对2009-2010 年生长季芦苇湿地的净生态系统碳交换量（NEE），感热通量（Hs）和潜热通量（LE）数据进行了分析。结果表明，在日尺度上，芦苇湿地NEE日变化特征表现为两个CO2吸收高峰，分别出现在11：00和16：00左右，其特点是在午间出现了碳交换通量的降低。CO2吸收的日最大值在两个生长季出现的时间有所不同，分别出现在2009 年7 月（-0.30 mg CO2 m-2s-1）和2010 年6 月（-0.37 mg CO2 m-2s-1）。CO2排放的日最大值两个生长季均出现在9 月，分别为0.19和0.25 mg CO2 m-2s-1。Hs和LE的日动态均为单峰型，极值都出现在中午前后，生长季生态系统的能量消耗主要以潜热为主，且在日尺度上，热通量和NEE有显著的负相关关系。在季节尺度上，芦苇湿地生长季具有明显的碳汇功能，2009 年生长季生态系统白天固定354.63 g CO2/m2，同时期夜间释放159.24 g CO2/m2，净CO2吸收量为-195.39 g CO2/m2。2009 年整个生长季生态系统总初级生产力（GPP）为-651.13 g CO2/m2，生态系统呼吸（Re）为455.74 g CO2/m2，系统表现为碳汇。路径分析表明：光合有效辐射（PAR）显著影响NEE的日动态（R2=0.46-0.84），而NEE的季节动态主要受土壤温度的影响，降水和PAR的影响次之。 Abstract:Wetlands are regarded as one of the largest ‘unknowns’ regarding future carbon (C) dynamics and greenhouse gas fluxes in the context of global change and climate policy-making. To understand the dynamics of carbon cycling of wetland ecosystem, we used an eddy covariance technique to measure the net ecosystem carbon dioxide (CO2) exchange (NEE, positive or negative values of NEE represent net losses or gains of C, respectively, for the ecosystem), sensible heat flux (Hs) and latent heat flux (LE) between vegetation and the atmosphere at a reed wetland ecosystem in the Yellow River Delta during the periods of two growing seasons in 2009 and 2010. The total amount of rainfall in 2009 (571.4 mm) was higher than the annual average (551 mm). In contrast, precipitation in 2010 (523.5 mm) was significantly lower than average. The results from 2-year eddy tower observations showed that there was a dual peak in diurnal pattern of NEE fluxes for the reed wetland, which occurred at about 11:00 and 16:00, respectively. There were two different temporal patterns for the maximum diurnal uptake values of CO2 in 2 years. The maximum diurnal uptake values of CO2 were-0.30 mg CO2 m-2s-1(July, 2009) and-0.37 mg CO2 m-2s-1(June, 2010), respectively. The maximum diurnal emitting values of CO2 were 0.19 and 0.25 mg CO2 m-2s-1, respectively, and both occurred in September for 2 years. The diurnal patterns of Hs and LE were both single peaks, and their peak values both occurred at noon. The maximum latent heat flux was higher than the sensible heat flux, and the latent heat flux was the primary consumption component for net radiation during both two years. In diurnal scale, the heat fluxes were strongly negatively correlated to NEE fluxes (R2≥0.5, P < 0.0001). On the seasonal scale, the reed wetland was a strong C sink during the growing season. In 2009, the wetland ecosystem fixed 354.63 g CO2 m-2 in daytime of the whole growing season, and meanwhile it released 159.24 g CO2/m2 in nighttime. Approximate 651.13 g CO2/m2 was fixed by gross primary production (GPP), and 455.74 g CO2/m2 were released as ecosystem respiration (Re), which resulted in a strong sink of atmospheric CO2 with-195.39 g CO2/m2 sequestered in 2009 growing season based on the observed data. Path analysis results showed that the fluctuation of diurnal NEE fluxes was closely related to the photosynthetic active radiation (PAR) (R2=0.46-0.84)。However, soil temperature had the greatest effect on seasonal dynamics of ecosystem CO2 exchange during the growing seasons at the study site, higher than the contributions of the other environmental factors such as precipitation and PAR. Our results strongly suggested that the combination of temperature, precipitation and PAR, as well as phonological stage of vegetation, control the C dynamics of reed wetland ecosystem. Therefore, an accurate representation of these parameters is extremely valuable for developing accurate and predictive wetland C cycle models and for the success of forecasting carbon budgets of reed wetlands. 参考文献 相似文献 引证文献

Water use efficiency of annual‐dominated and bunchgrass‐dominated savanna intercanopy space

ABSTRACTIn semi‐arid savannas, dominance of intercanopy space by annual or perennial grasses may alter partitioning of ecosystem water and carbon fluxes and affect ecosystem water use efficiency (WUEe), the ratio of net ecosystem carbon dioxide exchange (NEE) to evapotranspiration (ET). To establish if these contrasting growth habits changed controls to WUEe, we tracked volumetric soil moisture (θ25cm), ET and transpiration (T), NEE and its constituent ecosystem respiration (Reco) and gross ecosystem photosynthesis (GEP) fluxes, and community water use efficiency (WUEc = GEP : T) in annual‐dominated and bunchgrass‐dominated plots in a southern Arizona, United States, savanna. Annual and bunchgrass plots had similar θ25cm, ET, and T, suggesting the similarity in ET was due to higher soil evaporation in annual plots. Seasonal NEE was delayed and lower in annual plots compared with that in bunchgrass plots, owing to higher Reco in annual plots. Transpiration, GEP, and Reco in both vegetation types increased following late‐season rain, indicating similar late‐season phenological constraint. WUEe was lower in annual plots, but with similar WUEc between plot types. These results suggest that differences in annual plant biomass allocation and plot‐level leaf area distribution increased proportional soil evaporation and aboveground Reco contributions, reducing plot‐level WUEe, not lowering plant WUE typical of arid‐land annuals. Lower plot‐level WUEe suggests that any increase in annual plant dominance would increase interannual variation of productivity in savanna intercanopy spaces, which could enhance the negative effects of predicted higher temperatures, greater aridity, and larger and more widely spaced storms on arid‐land watershed processes. Published 2013. This article is a U.S. Government work and is in the public domain in the USA.

Impacts of exceptional and extreme inter-annual climatic events on the net ecosystem carbon dioxide exchange of a Sitka spruce forest

Climatic parameters such as temperature and precipitation are key drivers of the global carbon cycle and both inter-annual climatic variability and extreme climatic events can influence carbon sequestration and greenhouse gas mitigation within terrestrial ecosystems. The impacts of the timing, intensity, frequency and duration of both exceptional and extreme inter-annual climatic events, characterised by major differences in rainfall and temperature, on a Sitka spruce forest in the Irish midlands were assessed using eddy covariance-based measurements of net ecosystem carbon dioxide exchange (NEE). Annual precipitation in 2009 (1156mm) represented an extremely wet year characterised by extreme precipitation episodes in July (155mm) and November (220mm), however the differences in precipitation were not uniformly distributed throughout the year with above average rainfall occurring during the growing season (April–August). The mean annual air temperature in 2010 (8.2°C) was 1.7°C lower than the 30 year mean and represented an extremely cold year, and extreme low temperature events, characterised by a number of extended sub-zero temperature periods, were recorded in December of 2009 and 2010 where the mean monthly temperatures were 0.7°C and −0.7°C, respectively. These climatic differences resulted in a 1.07tCha−1yr−1 difference between the annual estimates of NEE in 2009 (−8.14±1.94tCha−1yr−1) and 2010 (−9.21±0.99tCha−1yr−1), respectively. The measured NEE in 2009 and 2010 represented a 5.6% decrease and a 6.9% increase relative to the long-term mean measured at this site (2002–2008; −8.62±1.39tCha−1yr−1). The components of NEE, gross primary productivity (GPP) and ecosystem respiration (Reco), also showed differences between years. In 2009, GPP was ∼19% lower when compared to 2010, due to a reduction in net stand carbon assimilation at higher irradiances during the growing season. This was most likely due to the development of low oxygen conditions in the rooting zone during periods of higher soil water availability, which may have limited photosynthetic carbon assimilation through a reduction in stomatal conductance. The extended sub-zero temperatures experienced during the winter of 2010 had a greater impact on GPP, relative to Reco, resulting in a net loss of carbon during these periods. Variations in GPP were, however, positively correlated with Reco in both years. NEE was correlated with temperature in all years, with a slope (negative) of 0.2 to 0.3gCm−2d−1 per degree increase in air temperature. At temperatures below ∼0°C the forest was a carbon source, whilst there was a progressive increase in sink capacity as temperatures increased up to ∼20°C. The observed decrease in NEE was dependent on both the duration of exposure and the extent to which the temperature was reduced below zero. This information indicates that while temperature remains the key driver of NEE in this forest ecosystem, the observed reduction in NEE during periods of high water availability in 2009 suggests that under future climatic scenarios the magnitude and timing of extreme precipitation events may also have a significant impact on annual carbon gain.

Seasonal variability in net ecosystem carbon dioxide exchange over a young Switchgrass stand

AbstractUnderstanding carbon dynamics of switchgrass ecosystems is crucial as switchgrass (Panicum virgatum L.) acreage is expanding for cellulosic biofuels. We used eddy covariance system and examined seasonal changes in net ecosystem CO2 exchange (NEE) and its components – gross ecosystem photosynthesis (GEP) and ecosystem respiration (ER) – in response to controlling factors during the second (2011) and third (2012) years of stand establishment in the southern Great Plains of the United States (Chickasha, OK). Larger vapor pressure deficit (VPD &gt; 3 kPa) limited photosynthesis and caused asymmetrical diurnal NEE cycles (substantially higher NEE in the morning hours than in the afternoon at equal light levels). Consequently, rectangular hyperbolic light–response curve (NEE partitioning algorithm) consistently failed to provide good fits at high VPD. Modified rectangular hyperbolic light–VPD response model accounted for the limitation of VPD on photosynthesis and improved the model performance significantly. The maximum monthly average NEE reached up to −33.02 ± 1.96 μmol CO2 m−2 s−1 and the highest daily integrated NEE was −35.89 g CO2 m−2 during peak growth. Although large differences in cumulative seasonal GEP and ER were observed between two seasons, total seasonal ER accounted for about 75% of GEP regardless of the growing season lengths and differences in aboveground biomass production. It suggests that net ecosystem carbon uptake increases with increasing GEP. The ecosystem was a net sink of CO2 during 5–6 months and total seasonal uptakes were −1128 ± 130 and −1796 ± 217 g CO2 m−2 in 2011 and 2012, respectively. In conclusion, our findings suggest that the annual carbon status of a switchgrass ecosystem can be a small sink to small source in this region if carbon loss from biomass harvesting is considered. However, year‐round measurements over several years are required to assess a long‐term source‐sink status of the ecosystem.

Sensitivity of Temperate Desert Steppe Carbon Exchange to Seasonal Droughts and Precipitation Variations in Inner Mongolia, China

Arid grassland ecosystems have significant interannual variation in carbon exchange; however, it is unclear how environmental factors influence carbon exchange in different hydrological years. In this study, the eddy covariance technique was used to investigate the seasonal and interannual variability of CO2 flux over a temperate desert steppe in Inner Mongolia, China from 2008 to 2010. The amounts and times of precipitation varied significantly throughout the study period. The precipitation in 2009 (186.4 mm) was close to the long-term average (183.9±47.6 mm), while the precipitation in 2008 (136.3 mm) and 2010 (141.3 mm) was approximately a quarter below the long-term average. The temperate desert steppe showed carbon neutrality for atmospheric CO2 throughout the study period, with a net ecosystem carbon dioxide exchange (NEE) of −7.2, −22.9, and 26.0 g C m−2 yr−1 in 2008, 2009, and 2010, not significantly different from zero. The ecosystem gained more carbon in 2009 compared to other two relatively dry years, while there was significant difference in carbon uptake between 2008 and 2010, although both years recorded similar annual precipitation. The results suggest that summer precipitation is a key factor determining annual NEE. The apparent quantum yield and saturation value of NEE (NEEsat) and the temperature sensitivity coefficient of ecosystem respiration (Reco) exhibited significant variations. The values of NEEsat were −2.6, −2.9, and −1.4 µmol CO2 m−2 s−1 in 2008, 2009, and 2010, respectively. Drought suppressed both the gross primary production (GPP) and Reco, and the drought sensitivity of GPP was greater than that of Reco. The soil water content sensitivity of GPP was high during the dry year of 2008 with limited soil moisture availability. Our results suggest the carbon balance of this temperate desert steppe was not only sensitive to total annual precipitation, but also to its seasonal distribution.

Remote sensing-based estimation of gross primary production in a subalpine grassland

Abstract. This study investigates the performances in a terrestrial ecosystem of gross primary production (GPP) estimation of a suite of spectral vegetation indexes (VIs) that can be computed from currently orbiting platforms. Vegetation indexes were computed from near-surface field spectroscopy measurements collected using an automatic system designed for high temporal frequency acquisition of spectral measurements in the visible near-infrared region. Spectral observations were collected for two consecutive years in Italy in a subalpine grassland equipped with an eddy covariance (EC) flux tower that provides continuous measurements of net ecosystem carbon dioxide (CO2) exchange (NEE) and the derived GPP. Different VIs were calculated based on ESA-MERIS and NASA-MODIS spectral bands and correlated with biophysical (Leaf area index, LAI; fraction of photosynthetically active radiation intercepted by green vegetation, fIPARg), biochemical (chlorophyll concentration) and ecophysiological (green light-use efficiency, LUEg) canopy variables. In this study, the normalized difference vegetation index (NDVI) was the index best correlated with LAI and fIPARg (r = 0.90 and 0.95, respectively), the MERIS terrestrial chlorophyll index (MTCI) with leaf chlorophyll content (r = 0.91) and the photochemical reflectance index (PRI551), computed as (R531-R551)/(R531+R551) with LUEg (r = 0.64). Subsequently, these VIs were used to estimate GPP using different modelling solutions based on Monteith's light-use efficiency model describing the GPP as driven by the photosynthetically active radiation absorbed by green vegetation (APARg) and by the efficiency (ε) with which plants use the absorbed radiation to fix carbon via photosynthesis. Results show that GPP can be successfully modelled with a combination of VIs and meteorological data or VIs only. Vegetation indexes designed to be more sensitive to chlorophyll content explained most of the variability in GPP in the ecosystem investigated, characterised by a strong seasonal dynamic of GPP. Accuracy in GPP estimation slightly improves when taking into account high frequency modulations of GPP driven by incident PAR or modelling LUEg with the PRI in model formulation. Similar results were obtained for both measured daily VIs and VIs obtained as 16-day composite time series and then downscaled from the compositing period to daily scale (resampled data). However, the use of resampled data rather than measured daily input data decreases the accuracy of the total GPP estimation on an annual basis.

Cool-season whole-plant gas exchange of exotic and native semiarid bunchgrasses

The success of invasive aridland plants may depend on their utilization of precipitation not fully exploited by native species, which could lead to seasonally altered ecosystem carbon and water fluxes. We measured volumetric soil water across 25-cm profiles (θ25cm) and springtime whole-plant water- and carbon-fluxes of the exotic Lehmann lovegrass (Eragrostis lehmanniana) and a native bunchgrass, bush muhly (Muhlenbergia porteri), following typical (55 mm in 2009) and El Niño-enhanced accumulations (154 mm in 2010) in a SE Arizona savanna. Across both years, θ25cm was higher under lovegrass plots, with similar evapotranspiration (ET) between lovegrass and bush muhly plots. However, in 2010 transpiration (T) was higher in bush muhly than lovegrass, implying higher soil evaporation in lovegrass plots maintained similar ET. Net ecosystem carbon dioxide exchange (NEE) was similar between lovegrass and bush muhly plots in 2009, but was more negative in bush muhly plots following El Niño, indicating greater CO2 assimilation. Ecosystem respiration (R eco) and gross ecosystem photosynthesis (GEP) were similar between lovegrass and bush muhly plots in 2009, but were higher in bush muhly plots in 2010. As a result, lovegrass plots reduced ecosystem water-use efficiency (WUEe = NEE/ET), while bush muhly WUEe remained constant between 2009 and 2010. Concurrent whole-plant WUE (WUEp = GEP/T) did not change in lovegrass plots, but increased in bush muhly plots between these years. We concluded that cool-season precipitation use is not a component of Lehmann lovegrass invasive success, but that the change in ET partitioning and attendant shifts in cool-season WUEe may increase interannual variation in ecosystem water- and carbon-exchange dynamics in the water-limited systems it dominates.

Commonalities of carbon dioxide exchange in semiarid regions with monsoon and Mediterranean climates

Comparing biosphere–atmosphere carbon exchange across monsoon (warm-season rainfall) and Mediterranean (cool-season rainfall) regimes can yield information about the interaction between energy and water limitation. Using data collected from eddy covariance towers over grass and shrub ecosystems in Arizona, USA and Almeria, Spain, we used net ecosystem carbon dioxide exchange (NEE), gross ecosystem production (GEP), and other meteorological variables to examine the effects of the different precipitation seasonality. Considerable crossover behavior occurred between the two rainfall regimes. As expected in these usually water-limited ecosystems, precipitation magnitude and timing were the dominant drivers of carbon exchange, but temperature and/or light also played an important role in regulating GEP and NEE at all sites. If significant rainfall occurred in the winter at the Arizona sites, their behavior was characteristically Mediterranean whereby the carbon flux responses were delayed till springtime. Likewise, the Spanish Mediterranean sites showed immediate pulse-like responses to rainfall events in non-winter periods. The observed site differences were likely due to differences in vegetation, soils, and climatology. Together, these results support a more unified conceptual model for which processes governing carbon cycling in semiarid ecosystems need not differ between warm-season and cool-season rainfall regimes.

Resource availability mediates the importance of priority effects in plant community assembly and ecosystem function

Assembly history, including the order in which species arrive into a community, can influence long‐term community structure; however we know less about how timing of species arrival may alter assembly especially under varying resource conditions. To explore how the timing of species arrival interacts with resource availability to alter community assembly, we constructed experimental plant communities and manipulated the interval between plantings of groups of seedlings (0, 5, 10, 15 or 20 days) at low and high levels of soil nutrient supply. To see if community changes influenced ecosystem‐scale processes, we measured parameters across the plant–soil continuum (e.g. plant biomass and net ecosystem carbon dioxide exchange).We found that the timing of species arrival had a large impact on community assembly, but the size of the effect depended on soil fertility. As planting interval increased, plant communities diverged further from the control, but the divergence was stronger at high than at low nutrient supply. Our data suggest that at high nutrient supply, early‐planted species preempted light resources more quickly, thus preventing the successful establishment of later arriving species even at short planting intervals. Finally, we found that assembly related divergence in plant communities scaled to impact ecosystem‐level characteristics such as green leaf chemistry, but had little effect on total community biomass and net ecosystem exchange of CO2 and water vapor. Our data indicate that the effect of a stochastic factor, here the timing of species arrival on community composition, depends on the resource level under which the community assembles.

Agricultural encroachment: implications for carbon sequestration in tropical African wetlands

AbstractTropical wetlands have been shown to exhibit high rates of net primary productivity and may therefore play an important role in global climate change mitigation through carbon assimilation and sequestration. Many permanently flooded areas of tropical East Africa are dominated by the highly productive C4 emergent macrophyte sedge, Cyperus papyrus L. (papyrus). However, increasing population densities around wetland margins in East Africa are reducing the extent of papyrus coverage due to the planting of subsistence crops such as Colocasia esculenta (cocoyam). In this paper, we assess the impact of this land use change on the carbon cycle and in particular the impacts of land conversion on net ecosystem carbon dioxide exchange. Eddy covariance techniques were used, on a campaign basis, to measure fluxes of carbon dioxide over both papyrus and cocoyam dominated wetlands located on the Ugandan shore of Lake Victoria. Peak rates of net photosynthetic CO2 assimilation, derived from monthly diurnal averages of net ecosystem exchange, of 28–35 μmol CO2 m−2 s−1 and 15–20 μmol CO2 m−2 s−1 were recorded in the papyrus and cocoyam wetlands, respectively, whereas night‐time respiratory losses ranged between 10 and 15 μmol CO2 m−2 s−1 at the papyrus wetland and 5–10 μmol CO2 m−2 s−1 at the cocoyam site. The integration of the flux data suggests that papyrus wetlands have the potential to act as a sink for significant amounts of carbon, in the region of 10 t C ha−1 yr−1. The cocoyam vegetation assimilated ~7 t C ha−1 yr−1 but when carbon exports from crop biomass removal were accounted for these wetlands represent a significant net loss of carbon of similar magnitude. The development of sustainable wetland management strategies are therefore required to promote the dual wetland function of crop production and the mitigation of greenhouse gas emissions especially under future climate change scenarios.

Biophysical regulation of net ecosystem carbon dioxide exchange over a temperate desert steppe in Inner Mongolia, China

Measurements of net ecosystem carbon dioxide (CO 2) exchange (NEE) were made, using eddy covariance, to investigate the biophysical regulation of a temperate desert steppe characterized drought in Inner Mongolia, China during 2008. The half-hourly maximum and minimum NEE were −3.07 and 0.85 μmol CO 2 m −2 s −1 (negative values denoting net carbon uptake). The maximum daily NEE was −6.0 g CO 2 m −2 day −1. On an annual basis, integrated NEE was −7.2 g C m −2 y −1, indicating a weak carbon sink. The light response curves of NEE showed a rather low apparent quantum yield ( α) and saturation value of NEE (NEE sat). Moreover, α and NEE sat varied with canopy development, soil water content (SWC), air temperature ( T a), and vapor pressure deficit (VPD). Piecewise regression results suggested that the optimal SWC, T a, and VPD for half-hourly daytime NEE were 12.6%, 24.3 °C, and 1.7 kPa, respectively. The apparent temperature sensitivity of ecosystem respiration was 1.6 for the entire growing season, and it was significantly controlled by soil moisture. During the growing season, leaf area index explained about 26% of the variation in daily NEE. Overall, NEE was strongly suppressed by water stress and this was the dominant biophysical regulator in the desert steppe.

Soil moisture and ecosystem function responses of desert grassland varying in vegetative cover to a saturating precipitation pulse

ABSTRACTA critical linkage between hydrological and ecological processes is plant cover, yet the ecosystem‐level responses of aridland systems of varying plant cover to extreme precipitation events, predicted to increase in the future, have not been well studied. We tracked volumetric soil water (θ15 cm) across the entire summer monsoon season, and ecosystem evapotranspiration (ET), net ecosystem carbon dioxide exchange (NEE), ecosystem respiration (Reco), gross ecosystem photosynthesis (GEP), and leaf‐level net carbon assimilation (Anet) and stomatal conductance to water vapour (gs) for 30 days in high‐cover (ca. 50%) and low‐cover (ca. 23%) plots in response to a runoff generating experimental rainfall. For 35 days following the pulse, θ15 cm was 2·5% higher in high‐cover plots compared to low‐cover values, with identical soil drying rates between cover conditions. After θ15 cm converged to similar values between cover conditions, dry‐down rates were longer in low‐cover plots. ET and gs did not differ between plots. For 7 days after the pulse, slower Anet development in some grass species compared to rapid development by the canopy dominant, Lehmann's lovegrass, may have resulted in similar GEP between low‐ and high‐cover plots. After this, GEP and Reco were higher and increased in parallel in high‐cover plots. In low‐cover plots, Reco levelled at + 21 days, resulting in similar NEE to high‐cover plots, even though GEP was lower. These findings suggest that low biomass in low‐cover areas may constrain Reco and lead to more evenly distributed productivity across semiarid grasslands following large, infrequent precipitation events. Published in 2011. This article is a US Government work and is in the public domain in the USA.

Carbon dioxide exchange of a pepperweed (Lepidium latifoliumL.) infestation: How do flowering and mowing affect canopy photosynthesis and autotrophic respiration?

[1] The net ecosystem carbon dioxide (CO2) exchange of invasive plant infestations, such as perennial pepperweed (Lepidium latifolium L.), is not well understood. A characteristic feature of pepperweed's phenological cycle is its small white flowers during secondary inflorescence. Pepperweed flowering causes uniform reflectance over the visible range of the electromagnetic spectrum, thus decreasing the amount of energy absorbed by the canopy and available for photosynthesis. Little is known about how pepperweed flowering and control measures such as mowing affect canopy photosynthesis and autotrophic respiration (FAR) and thus ecosystem respiration. To examine this question, we analyzed CO2 flux measurements made with eddy covariance over a pepperweed infestation in California, covering three growing seasons. Unmowed pepperweed caused the site to be almost CO2 neutral (2007: −28 g C m−2 period−1) or a net source (2009: 129 g C m−2 period−1), mostly because of reduced maximum photosynthetic capacity by 13 (2007) and 17 μmol m−2 s−1 (2009) due to flowering during the plant's prime photosynthetic period. Reference FAR at 10°C was reduced by 2 μmol m−2 s−1 in 2007 and 2009. Mowing during early flowering reversed the attenuating effects of pepperweed flowering, causing the site to act as a net CO2 sink (2008: −174 g C m−2 period−1) mainly due to prolonged photosynthetic CO2 uptake over the plant's early vegetative growth phase. Our results highlight the tight link between pepperweed's prominent key phenological phase and applied control measures, which together exert dominant control over the infestation's CO2 source-sink strength.

Carbon flux dynamics and its environmental controlling factors in a desert steppe

Net ecosystem carbon dioxide exchange ( NEE) above a desert steppe in Inner Mongolia was measured with eddy covariance technique in growing season of 2008. Environmental factors were also observed during the same period. Our results show that the minimum of growing season daily cumulative NEE appeared on August 26 (−1.6 g Cm −2), while the maximum appeared on June 11 (0.9 g Cm −2). Monthly cumulative NEE distributed in the growing season unevenly. The minimum of monthly cumulative NEE appeared in June (0.74 g Cm −2); the maximum appeared in September (−14.94 g Cm −2). The sum of the growing season NEE was −45.5 g Cm −2. The most important environmental factor affecting the half-hour average NEE was photosynthetically active radiation. The most important environmental factor affecting the daily cumulative NEE was 20–30 cm soil water content.

Carbon dioxide and water vapour exchange in a tropical dry forest as influenced by the North American Monsoon System (NAMS)

To better understand the effects and relationship between precipitation, net ecosystem carbon dioxide (NEE) and water vapor exchange (ET), we report a study conducted in the tropical dry forest (TDF) in the northwest of Mexico. Ecosystem gas exchange was measured using the eddy correlation technique during the presence of North American Monsoon System (NAMS) in 2006. Patterns in NEE and ET were different in wet and dry periods. Three markedly defined periods were found during the six-month study period. A pre-monsoon period, where gas exchange was close to zero. A monsoon period, divided in two stages: 1) early monsoon: a strong increase in the respiratory rate marked by a peak of positive values, with a maximum of 22 g CO 2 m −2 day −1, and, 2) late monsoon: an assimilation period occurred in the peak of the monsoon period, with sustained values around −20 g CO 2 m −2 day −1. The final was a post-monsoon period, where ecosystems returned to dormancy. NEE and ET trends in the TDF were similar to other seasonally dry ecosystems influenced by the NAMS. During the study period the TDF of Northwest Mexico acted as a sink capturing 374 g CO 2 m 2 with an ecosystem water use efficiency (-NEE/ET) comparable to other ecosystems in the region. Mechanistic information about biological and environmental variables controlling gas exchange dynamics is still necessary to predict how seasonally dry ecosystems would respond to climate change.