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1.
Net carbon flux partitioning was used to disentangle abiotic and biotic drivers of all important component fluxes influencing the overall sink strength of a Mediterranean ecosystem during a rapid spring to summer transition. Between May and June 2006 we analyzed how seasonal drought affected ecosystem assimilation and respiration fluxes in an evergreen oak woodland and attributed variations in the component fluxes (trees, understory, soil microorganisms and roots) to observations at the ecosystem scale. We observed a two thirds decrease in both ecosystem carbon assimilation and respiration (Reco) within only 15 days time. The impact of decreasing Reco on the ecosystem carbon balance was smaller than the impact of decreasing primary productivity. Flux partitioning of GPP and Reco into their component fluxes from trees, understory, soil microorganisms and roots showed that declining ecosystem sink strength was due to a large drought and temperature-induced decrease in understory carbon uptake (from 56% to 21%). Hence, the shallow-rooted annuals mainly composing the understory have a surprisingly large impact on the source/sink behavior of this open evergreen oak woodland during spring to summer transition and the timing of the onset of drought might have a large effect on the annual carbon budget. In response to seasonal drought Reco was increasingly dominated by respiration of heterotrophic soil microorganisms, while the root flux was found to be of minor importance. Soil respiration flux decreased with drought but its contribution to total daily CO2-exchange increased by 11.5%. This partitioning approach disentangled changes in respiratory and photosynthetic ecosystem fluxes that were not apparent from the eddy-covariance or the soil respiration data alone. By the novel combination of understory vs. overstory carbon flux partitioning with soil respiration data from trenched and control plots, we gained a detailed understanding of factors controlling net carbon exchange of Mediterranean ecosystems.  相似文献   

2.
Due to their high emission potential, the reporting of CO2 emissions from peatlands requires exact emission factors for different land use categories. Recently used emission factors are mainly based on CO2 flux measurements by chamber techniques or the micrometeorological eddy covariance (EC) method. However, evidence about the reliability and comparability of annual CO2 balances based on these methods is scarce. Therefore, manual chamber measurements of ecosystem respiration (RECO) and net ecosystem exchange (NEE) were conducted for two years (March 2012–April 2014) to model annual balances of two sites on fen soils with different land use intensity in northern Germany: an unutilized and rewetted grassland (UG) and an intensively utilized wet grassland (GW). Simultaneously, EC measurements of NEE were conducted on the sites. Two reasons for occasionally great deviations in NEE between the methods could be observed: (1) the accordance of both methods was most hampered during transition periods such as the beginning of the growing season and the onset of regrowth after a grassland defoliation due to different spatial scales of EC and chamber measurements and (2) RECO and gross primary production (GPP) partitioned from EC NEE measurements were systematically lower than those from the chamber‐based model, which could be a result of the EC energy balance gap. Differences were more pronounced for the managed site GW as a result of more frequent regrowth periods. It is concluded that the EC and chamber method can show comparable results for the CO2 exchange of grasslands on fen soils when the limitations of both methods are known and considered for the reporting of emission factors. These limitations are due to energy balance closure and potentially biased footprints for EC and a restricted representativeness especially during early stages of plant development for the chamber method.  相似文献   

3.
We investigated the daily exchange of CO2 between undisturbed Larix gmelinii (Rupr.) Rupr. forest and the atmosphere at a remote Siberian site during July and August of 1993. Our goal was to measure and partition total CO2 exchanges into aboveground and belowground components by measuring forest and understory eddy and storage fluxes and then to determine the relationships between the environmental factors and these observations of ecosystem metabolism. Maximum net CO2 uptake of the forest ecosystem was extremely low compared to the forests elsewhere, reaching a peak of only ∼5 μmol m−2 s−1 late in the morning. Net ecosystem CO2 uptake increased with increasing photosynthetically active photon flux density (PPFD) and decreased as the atmospheric water vapor saturation deficit (D) increased. Daytime ecosystem CO2 uptake increased immediately after rain and declined sharply after about six days of drought. Ecosystem respiration at night averaged ∼2.4 μmol m−2 s−1 with about 40% of this coming from the forest floor (roots and heterotrophs). The relationship between the understory eddy flux and soil temperature at 5 cm followed an Arrhenius model, increasing exponentially with temperature (Q10∼2.3) so that on hot summer afternoons the ecosystem became a source of CO2. Tree canopy CO2 exchange was calculated as the difference between above and below canopy eddy flux. Canopy uptake saturated at ∼6 μmol CO2 m−2 s−1 for a PPFD above 500 μmol m−2 s−1 and decreased with increasing D. The optimal stomatal control model of Mäkelä et al. (1996) was used as a `big leaf' canopy model with parameter values determined by the non-linear least squares. The model accurately simulated the response of the forest to light, saturation deficit and drought. The precision of the model was such that the daily pattern of residuals between modeled and measured forest exchange reproduced the component storage flux. The model and independent leaf-level measurements suggest that the marginal water cost of plant C gain in Larix gmelinii is more similar to values from deciduous or desert species than other boreal forests. During the middle of the summer, the L. gmelinii forest ecosystem is generally a net sink for CO2, storing ∼0.75 g C m−2 d−1.  相似文献   

4.
The aim of this study was to measure the in situ soil CO2 flux from grassland, afforested land and reclaimed coalmine overburden dumps by using the automated soil CO2 flux system (LICOR‐8100® infrared gas analyzer, LICOR Inc., Lincoln, NE). The highest soil CO2 flux was observed in natural grassland (11·16 µmol CO2 m−2s−1), whereas the flux was reduced by 38 and 59 per cent in mowed site and at 15‐cm depth, respectively. The flux from afforested area was found 5·70 µmol CO2 m−2s−1, which is 50 per cent lower than natural grassland. In the reclaimed coalmine overburden dumps, the average flux under tree plantation was found to be lowest in winter and summer (0·89–1·12 µmol CO2 m−2s−1) and highest during late monsoon (3–3·5 µmol CO2 m−2s−1). During late monsoon, the moisture content was found to be higher (6–7·5 per cent), which leads to higher microbial activity and decomposition. In the same area under grass cover, soil CO2 flux was found to be higher (8·94 µmol CO2 m−2s−1) compared with tree plantation areas because of higher root respiration and microbial activity. The rate of CO2 flux was found to be determined predominantly by soil moisture and soil temperature. Our study indicates that the forest ecosystem plays a crucial role in combating global warming than grassland; however, to reduce CO2 flux from grassland, mowing is necessary. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

5.
This study presents a new method to estimate daytime respiration from the subcanopy of forests directly from conventional eddy covariance (EC) measurements. The method primarily considers the respiration signal from root, litter and microbial respiration, which are known to be the main components of ecosystem respiration, Re, as well as decomposition of coarse and fine woody detritus, and respiration from low understory vegetation (forbs, herbs and grasses). The conceptual framework is based on the premise that upward moving air parcels carry a specific and unambiguous signal in their CO2 and water vapour composition, which can be separated and distinguished into respiration and photosynthesis. The model employed a combination of conditional sampling methods, quadrant analysis and relaxed eddy accumulation with hyperbolic deadbands to identify respiration events and to quantify their flux contribution. Datasets from five sites, most of which had multiple sampling heights, were selected to test this technique among contrasting ecosystems and canopy structures.Respiration signals were successfully identified in daytime data of all sites. A hyperbolic deadband of size H = 0.25 applied to the plane constructed from perturbations of carbon dioxide c and water vapour q densities effectively separated the signals of respiration from photosynthesis. The time fraction of respiration events was ≤10% during daytime. The global correlation coefficient rc,q was found to be a universal predictor of this time fraction and was therefore used as a filter to identify periods of a meaningful and extractable respiration signal. Coherent structures, defined as ramp-shaped flow pattern observed in time series in and above forest canopies, are likely to be the underlying transport mechanism for these respiration events.Daytime subcanopy Re estimates derived from the new method agreed with those derived from: (i) the intercept of light-response curves and (ii) soil CO2 efflux chambers for three of the five sites. Limitations were posed by the dense, multi-layered deciduous canopy and the intense vertical turbulent mixing at one coniferous site. In addition, refixation of respired CO2 by the understorey (CO2 recycling) may cause an underestimation of daytime Re or pose a limitation to the method proposed here. An indicator relating the canopy shear length scale, Ls, to the adjustment length scale, Ld, was proposed to predict the skill of the new method, and found to be useful in four of the five sites. Analysis of vertical coupling in the plant canopy using exchange regimes could explain the failure of the new method for the remaining site.  相似文献   

6.
CO2 exchange was measured on the forest floor of a coastal temperate Douglas-fir forest located near Campbell River, British Columbia, Canada. Continuous measurements were obtained at six locations using an automated chamber system between April and December, 2000. Fluxes were measured every half hour by circulating chamber headspace air through a sampling manifold assembly and a closed-path infrared gas analyzer. Maximum CO2 fluxes measured varied by a factor of almost 3 between the chamber locations, while the highest daily average fluxes observed at two chamber locations occasionally reached values near 15 μmol C m−2 s−1. Generally, fluxes ranged between 2 and 10 μmol C m−2 s−1 during the measurement period. CO2 flux from the forest floor was strongly related to soil temperature with the highest correlation found with 5 cm depth temperature. A simple temperature dependent exponential model fit to the nighttime fluxes revealed Q10 values in the normal range of 2–3 during the warmer parts of the year, but values of 4–5 during cooler periods. Moss photosynthesis was negligible in four of the six chambers, while at the other locations, it reduced daytime half-hourly net CO2 flux by about 25%. Soil moisture had very little effect on forest floor CO2 flux. Hysteresis in the annual relationship between chamber fluxes and soil temperatures was observed. Net exchange from the six chambers was estimated to be 1920±530 g C m−2 per year, the higher estimates exceeding measurement of ecosystem respiration using year-round eddy correlation above the canopy at this site. This discrepancy is attributed to the inadequate number of chambers to obtain a reliable estimate of the spatial average soil CO2 flux at the site and uncertainty in the eddy covariance respiration measurements.  相似文献   

7.
Boreal forest soils have the potential to sequester large amounts of carbon by accumulating charcoal from fire. Some suggest that sequestration rates could be large enough to account for some of the missing sink in the global CO2 budget, but further data on soil charcoal pools are necessary to adequately develop boreal carbon budgets under a changing climate and fire regime. The primary objective of this study was to determine the amount of charred wood in surface mineral soil horizons (Ah) of the Boreal Transition of Saskatchewan, a fire-prone grassland forest ecotone region of western Canada. A second objective was to use the charcoal data to infer vegetation dynamics and the development of these Ah horizons as a function of parent material type, i.e. glacio-fluvial, glacio-lacustrine and glacial till. The latter objective served to provide information in regards to future vegetation shifts and ecosystem C budgets of Boreal Plain ecosystems under climatic warming. The carbon fraction measured as charcoal is an important component of organic matter in Ah horizons of Chernozemic soils in Saskatchewan and differs from the classical model of humus fractions in Chernozems which suggests that it is a system created from microbial degradation of root litter only. The carbon sequestered as charcoal within the whole ecoregion was estimated at 36.1 Tg, which is the lower limit of the global annual rate of charcoal accumulation in terrestrial environments estimated from experimental fires. Charcoal pools were consistently lower in the fluvial soils relative to the lacustrine and till soils. We suggest a model where dry conditions and low water availability prevailing under the coarser fluvial soils during the Holocene favoured the dominance of low productivity herbaceous vegetation that led to a high ash to charcoal production ratio from fire and to the development of relatively thick Ah horizons through below ground additions of organic matter from root decay. We propose that the more available water in lacustrine and till soils favoured the growth of trembling aspen which, through less frequent/intense fires relative to grasslands and incomplete burning of the woody material, led to high charcoal accumulation rates in soil. The development of thick Ah horizons in lacustrine soils likely occurred during a warm and dry period of the early Holocene (i.e. the hypsithermal) when herbaceous vegetation invaded forested land or during dry spells in the mid to late Holocene (e.g. the Medieval Warm Period) when opening of forest canopies occurred, thus augmenting light transmission to the forest floor and favouring the growth of herbaceous vegetation in the understory. Such events did not create deep Ah horizons in the tills soils as a consistent rock impediment near the surface limited the penetration of understory roots at greater depth. These results suggest that fluvial sites my be the first shifting to herbaceous vegetation in the future due to climatic warming, followed by till sites and then lacustrine sites.  相似文献   

8.

Purpose

Carbon (C) dynamics in grassland ecosystem contributes to regional and global fluxes in carbon dioxide (CO2) concentrations. Grazing is one of the main structuring factors in grassland, but the impact of grazing on the C budget is still under debate. In this study, in situ net ecosystem CO2 exchange (NEE) observations by the eddy covariance technique were integrated with a modified process-oriented biogeochemistry model (denitrification–decomposition) to investigate the impacts of grazing on the long-term C budget of semiarid grasslands.

Materials and methods

NEE measurements were conducted in two adjacent grassland sites, non-grazing (NG) and moderate grazing (MG), during 2006–2007. We then used daily weather data for 1978–2007 in conjunction with soil properties and grazing scenarios as model inputs to simulate grassland productivity and C dynamics. The observed and simulated CO2 fluxes under moderate grazing intensity were compared with those without grazing.

Results and discussion

NEE data from 2-year observations showed that moderate grazing significantly decreased grassland ecosystem CO2 release and shifted the ecosystem from a negative CO2 balance (releasing 34.00 g C?m?2) at the NG site to a positive CO2 balance (absorbing ?43.02 g C?m?2) at the MG site. Supporting our experimental findings, the 30-year simulation also showed that moderate grazing significantly enhances the CO2 uptake potential of the targeted grassland, shifting the ecosystem from a negative CO2 balance (57.08?±?16.45 g C?m?2?year?1) without grazing to a positive CO2 balance (?28.58?±?14.60 g C?m?2?year?1) under moderate grazing. The positive effects of grazing on CO2 balance could primarily be attributed to an increase in productivity combined with a significant decrease of soil heterotrophic respiration and total ecosystem respiration.

Conclusions

We conclude that moderate grazing prevails over no-management practices in maintaining CO2 balance in semiarid grasslands, moderating and mitigating the negative effects of global climate change on the CO2 balance in grassland ecosystems.  相似文献   

9.
Northern wetlands are critically important to global change because of their role in modulating atmospheric concentrations of greenhouse gases, especially CO2 and CH4. At present, continuous observations for CO2 and CH4 fluxes from northern wetlands in Asia are still very limited. In this paper, two growing season measurements for CO2 flux by eddy covariance technique and CH4 flux by static chamber technique were conducted in 2004 and 2005, at a permanently inundated marsh in the Sanjiang Plain, northeastern China. The seasonal variations of CO2 exchange and CH4 flux and the environmental controls on them were investigated. During the growing seasons, large variations in net ecosystem CO2 exchange (NEE) and gross ecosystem productivity (GEP) were observed with the range of −4.0 to 2.2 (where negative exchange is a gain of carbon from the atmosphere) and 0-7.6 g C m−2 d−1, respectively. Ecosystem respiration (RE) displayed relatively smooth seasonal pattern with the range of 0.8-4.2 g C m−2 d−1. More than 70% of the total GEP was consumed by respiration, which resulted in a net CO2 uptake of 143 ± 9.8 and 100 ± 9.2 g C m−2 for the marsh over the growing seasons of 2004 and 2005, respectively. A significant portion of the accumulated NEE-C was lost by CH4 emission during the growing seasons, indicating the great potential of CH4 emission from the inundated marsh. Air temperature and leaf area index jointly affected the seasonal variation of GEP and the seasonal dynamic of RE was mainly controlled by soil temperature and leaf area index. Soil temperature also exerted the dominant influence over variation of CH4 flux while no significant relationship was found between CH4 emission and water table level. The close relationships between carbon fluxes and temperature can provide insights into the response of marsh carbon exchange to a changing climate. Future long term flux measurements over the freshwater marsh ecosystems are undoubtedly necessary.  相似文献   

10.
A Lagrangian experiment was conducted over Iowa during the daytime (9:00–17:30 LT) on June 19, 2007 as part of the North American Carbon Program's Mid-Continent Intensive using a light-weight and operationally flexible aircraft to measure a net drawdown of CO2 concentration within the boundary layer. The drawdown can be related to net ecosystem exchange when anthropogenic emissions are estimated using a combination of the Vulcan fossil fuel emissions inventory coupled with a source contribution analysis using HYSPLIT. Results show a temporally and spatially averaged net CO2 flux of −9.0 ± 2.4 μmol m−2 s−1 measured from the aircraft data. The average flux from anthropogenic emissions over the measurement area was 0.3 ± 0.1 μmol CO2 m−2 s−1. Large-scale subsidence occurred during the experiment, entraining 1.0 ± 0.2 μmol CO2 m−2 s−1 into the boundary layer. Thus, the CO2 flux attributable to the vegetation and soils is −10.3 ± 2.4 μmol m−2 s−1. The magnitude of the calculated daytime biospheric flux is consistent with tower-based eddy covariance fluxes over corn and soybeans given existing land-use estimates for this agricultural region. Flux values are relatively insensitive to the choice of integration height above the boundary layer and emission footprint area. Flux uncertainties are relatively small compared to the biospheric fluxes, though the measurements were conducted at the height of the growing season.  相似文献   

11.
Pasture and afforestation are land-use types of major importance in the tropics, yet, most flux tower studies have been conducted in mature tropical forests. As deforestation in the tropics is expected to continue, it is critical to improve our understanding of alternative land-use types, and the impact of interactions between land use and climate on ecosystem carbon dynamics. Thus, we measured net ecosystem CO2 fluxes of a pasture and an adjacent tropical afforestation (native tree species plantation) in Sardinilla, Panama from 2007 to 2009. The objectives of our paired site study were: (1) to assess seasonal and inter-annual variations in net ecosystem CO2 exchange (NEE) of pasture and afforestation, (2) to identify the environmental controls of net ecosystem CO2 fluxes, and (3) to constrain eddy covariance derived total ecosystem respiration (TER) with chamber-based soil respiration (RSoil) measurements. We observed distinct seasonal variations in NEE that were more pronounced in the pasture compared to the afforestation, reflecting changes in plant and microbial activities. The land conversion from pasture to afforestation increased the potential for carbon uptake by trees vs. grasses throughout most of the year. RSoil contributed about 50% to TER, with only small differences between ecosystems or seasons. Radiation and soil moisture were the main environmental controls of CO2 fluxes while temperature had no effect on NEE. The pasture ecosystem was more strongly affected by soil water limitations during the dry season, probably due to the shallower root system of grasses compared to trees. Thus, it seems likely that predicted increases in precipitation variability will impact seasonal variations of CO2 fluxes in Central Panama, in particular of pasture ecosystems.  相似文献   

12.
Carbon dioxide budget of maize   总被引：1，自引：0，他引：1  
Measurements made during 1982, 1983 and 1984 were used to study the CO2 budget of maize (Zea mays L.). Above-canopy CO2 flux density, which represents most of the CO2 absorption by crops, was monitored throughout each growing season using the eddy correlation technique. Intercepted solar radiation was calculated on an hourly basis using measurements of incident solar radiation, leaf area index and solar elevation. The observed relationships between above-canopy CO2 flux densities and intercepted solar radiation, for each growing season, were then used to estimate hourly above-canopy CO2 flux densities. Assimilation of soil-respired CO2 and nighttime losses of plant respiratory CO2 were also estimated, based on experimental data, and combined with above-canopy CO2 flux densities to determine net photosynthesis. Although clear short term relationships between above-canopy CO2 flux density and intercepted solar radiation have been observed for maize, a great variability in CO2 flux density as a function of estimated intercepted solar radiation is observed over the whole growing season. Comparison of estimated CO2 budget based on gaseous exchange estimates and destructive plant sampling are presented. For 1982 and 1983, both estimates agreed within ±1 standard error while for 1984 the estimates based on gaseous exchanges were consistently lower. The relative magnitudes of gross photosynthesis, soil and plant respiration are presented and techniques for improving our ability for closing the CO2 budget using gaseous exchanges estimates are discussed.  相似文献   

13.
Fluctuations in atmospheric CO2 density can arise from static pressure fluctuations but their effect on the long-term eddy covariance (EC) CO2 flux measurement is poorly known. In this paper, we report the results of a 1-year direct measurement of the static pressure fluctuations and the velocity-pressure covariance over a mixed forest in Northeast China. The results show that the pressure-vertical velocity covariance was primarily controlled by friction velocity and air stability. Without the pressure correction, the open-path EC measurement of the nighttime ecosystem respiration was biased low and that of the daytime photosynthetic CO2 uptake was biased high. Over the 1-year measurement period, the cumulative pressure correction was 40 gCm−2, which was about 20% of the annual net ecosystem production of this forest. Using the friction velocity data found in the literature, we estimated the magnitudes of the pressure correction for the major ecosystem types in the long-term global EC network (FluxNet).  相似文献   

14.
Warland and Thurtell (2000) proposed an analytical dispersion Lagrangian analysis (hereafter WT analysis) to relate the mean scalar concentration field to source profiles inside the canopy. The first objective of this study was to evaluate the performance of the WT analysis with existing turbulence statistics parameterizations in a corn canopy, by comparing its inferred net ecosystem CO2 exchange (NEE) and latent heat flux (λE) with eddy covariance measurements. The second objective was to assess the performance of the WT analysis to infer the soil CO2 flux. Four parameterizations of turbulence statistics were used to estimate Lagrangian time scale (TL) and standard deviation of vertical wind velocity (σw) profiles. The estimated TL and σw profiles were then corrected for atmospheric stability conditions. The field experiment was carried out in a corn field from August to October 2007 and 2008. Profiles of water vapour and CO2 mixing ratios were measured using a multiport sampling system connected to an infrared gas analyzer. Wind velocity within and above the canopy and eddy covariance measurements over the canopy were taken. The soil respiration, estimated using the WT analysis, was compared to estimates obtained by an empirical model. WT analysis fluxes showed good correlation (R2 = 0.77-0.88) with NEE and λE obtained by the eddy covariance technique, but overestimated net fluxes, especially when corrections for atmospheric stability were applied. The optimization of TL and σw profiles using in-canopy turbulence measurements improved the agreement between measured and modeled NEE and λE. Inferred soil CO2 fluxes were underestimated and were poorly correlated (R2 = 0.02-0.01) with estimates obtained using an empirical model based on soil temperature. This poor performance in estimating the soil respiration is likely caused by the decoupling between inside and above canopy flows.  相似文献   

15.
The exchange of CO2 between the atmosphere and a beech forest near Sorø, Denmark, was measured continuously over 14 years (1996-2009). The simultaneous measurement of many parameters that influence CO2 uptake makes it possible to relate the CO2 exchange to recent changes in e.g. temperature and atmospheric CO2 concentration. The net CO2 exchange (NEE) was measured by the eddy covariance method. Ecosystem respiration (RE) was estimated from nighttime values and gross ecosystem exchange (GEE) was calculated as the sum of RE and NEE. Over the years the beech forest acted as a sink of on average of 157 g C m−2 yr−1. In one of the years only, the forest acted as a small source. During 1996-2009 a significant increase in annual NEE was observed. A significant increase in GEE and a smaller and not significant increase in RE was also found. Thus the increased NEE was mainly attributed to an increase in GEE. The overall trend in NEE was significant with an average increase in uptake of 23 g C m−2 yr−2. The carbon uptake period (i.e. the period with daily net CO2 gain) increased by 1.9 days per year, whereas there was a non significant tendency of increase of the leafed period. This means that the leaves stayed active longer. The analysis of CO2 uptake by the forest by use of light response curves, revealed that the maximum rate of photosynthetic assimilation increased by 15% during the 14-year period. We conclude that the increase in the overall CO2 uptake of the forest is due to a combination of increased growing season length and increased uptake capacity. We also conclude that long time series of flux measurements are necessary to reveal trends in the data because of the substantial inter-annual variation in the flux.  相似文献   

16.
Efforts to increase our understanding of the terrestrial carbon balance have resulted in a dense global network of eddy covariance towers, which are able to measure the net ecosystem exchange of CO2, H2O and energy between ecosystems and the atmosphere. However, the typical set-up on an eddy covariance tower does not monitor lateral CO2- and carbon fluxes such as dissolved organic carbon (DOC). By ignoring DOC fluxes eddy covariance-based CO2 balances overestimate the carbon sink of ecosystems as part of the DOC drains into the inland waters and get respired outside the footprint of the eddy covariance tower. In this study we quantify 7 years (2000-2006) of DOC fluxes from a temperate Scots pine forest in Belgium and analyse its inter-annual variability. On average, 10 gC m−2 year−1 is leached from the pine forest as DOC. If the DOC fluxes are considered relative to the gross ecosystem carbon fluxes we see that DOC fluxes are small: 0.8 ± 0.2% relative to gross primary productivity, 1.0 ± 0.3% relative to ecosystem respiration, and (2.4 ± 0.4%) relative to soil respiration. However, when compared to net fluxes such as net ecosystem productivity and net biome productivity the DOC flux is no longer negligible (11 ± 7% and 17%, respectively), especially because the DOC losses constitute a systematic bias and not a random error. The inter-annual variability of the DOC fluxes followed that of annual water drainage. Hence, drainage drives DOC leaching at both short and long time scales. Finally, it is noted that part of the carbon that is leached from the ecosystem as DOC is respired or sequestered elsewhere, so the physical boundaries of accounting should always be reported together with the carbon budget.  相似文献   

17.
Large-scale weather events such as the El Niño Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and droughts are known to cause substantial interannual variation in the net ecosystem productivity (NEP) of tropical, temperate and boreal forests. Hypotheses for the impacts on NEP of changes in air temperature (Ta) and precipitation associated with these events were tested at diurnal, seasonal and annual time scales using the terrestrial ecosystem model ecosys with measurements of CO2 and energy exchange from 1998 to 2006 at eddy covariance (EC) flux towers along a transcontinental transect of forest stands in the Fluxnet-Canada Research Network (FCRN).1 These tests were supported at seasonal time scales by remotely-sensed vegetation indices, and at decadal time scales by wood growth increments from tree-ring and inventory studies. Collectively, results from this testing indicate that large-scale weather events during the study period caused spatially coherent changes in NEP, although these changes may vary with climate zone, species and topography. High Ta episodes, such as occurred with greater frequency during ENSO/PDO events, adversely affected diurnal CO2 exchange of temperate and boreal conifers, but had little effect on that of a boreal deciduous forest. These contrasting responses of CO2 exchange to Ta were attributed in the model to greater xylem resistance to water uptake in coniferous vs. deciduous trees. Sustained warming such as occurred during ENSO/PDO events extended the period of net C uptake and thus raised annual NEP at boreal coniferous and deciduous sites, but did not do so at a temperate coniferous site where annual NEP was reduced. However the rise in NEP of boreal conifers with warming was partially offset by the adverse effects of high Ta on diurnal CO2 exchange, so that the rise in NEP with warming remained smaller than that at a boreal deciduous site. A 3-year drought during the study period adversely affected annual NEP of well-drained boreal deciduous forests but did not affect that of poorly-drained boreal conifers. This lack of effect was attributed in the model to low coniferous evapotranspiration rates and to subsurface water recharge. Drought effects on NEP were therefore largely determined by topography. These contrasting responses of different forest stands to warming and drought indicate divergent changes in forest growth with interannual changes in weather. Such divergent changes are consistent with the complex changes in forest NDVI and net C uptake observed over time in several large-scale remote-sensing studies.  相似文献   

18.
The ecosystem fluxes of mass and energy were quantified for a riparian cottonwood (Populus fremontii S. Watson) stand, and the daily and seasonal courses of evapotranspiration, CO2 flux, and canopy conductance were described, using eddy covariance. The ecosystem-level evapotranspiration results are consistent with those of other riparian studies; high vapor pressure deficit and increased groundwater depth resulted in reduced canopy conductance, and the annual cumulative evapotranspiration of 1095 mm was more than double the magnitude of precipitation. In addition, the cottonwood forest was a strong sink of CO2, absorbing 310 g C m−2 from the atmosphere in the first 365 days of the study. On weekly to annual time scales, hydrology was strongly linked with the net atmosphere-ecosystem exchange of CO2, with ecosystem productivity greatest when groundwater depth was ∼2 m below the ground surface. Increases in groundwater depth beyond the depth of 2 m corresponded with decreased CO2 uptake and evapotranspiration. Saturated soils caused by flooding and shallow groundwater depths also resulted in reduced ecosystem fluxes of CO2 and water.  相似文献   

19.
Open-path gas analyzers are popular in eddy covariance flux measurements of trace gasses (i.e. CO2). The quality of the data, however, may be influenced by several factors. Exposure in an outdoor environment invariably causes the instrument to become colder or warmer than the air temperature. Instruments with internal temperature regulation and/or from heat generated by active electrical components can also influence the sensor temperature. In addition, sensors can have condensation problems on their optical windows thus affecting the quality of the measurement. Unreasonable measurements have been widely discussed, especially in moist, high-latitude regions. As this is a very important research problem facing flux studies, we examined how wetness (dew and raindrops) on the surface of the focus lens of the popular LI-COR LI-7500 infrared gas analyzer may affect flux measurements from the open-path eddy-covariance system. Field experiments showed that additional sensor heating may inhibit dew formation yet greatly improve the quality of flux measurements. A detailed energy balance approach was used to model the gas analyzer window temperature under environmental conditions and dew effect through a pair of LI-COR LI-7500, with and without heat treatment, in a grassland ecosystem in the Netherlands. With the proposed model, existing datasets can be filtered for dew events. Data from three different flux measurement sites were then used to assess the magnitude of dew effects on longer time-scales; 2 years from the Netherlands and 3 weeks of data from an arid coastal desert. About 30% of the measurements were affected by dew in the grassland area versus 4% in the arid region during the dry season. Sensor heating suppresses dew formation but might lead to errors in trace gas fluxes evaluated over long periods, thus we analyzed how sensor heating or cooling affects trace gas flux measurements. Additions to a recent (2006) correction and application to a horizontally and vertically oriented LI-COR LI-7500 are presented as they deal with sensor heating problems in eddy-covariance systems. The sensor energy balance model, together with the proposed modified sensor heating corrections, were used to estimate sensor temperature effects on long-term scale CO2 flux measurements and showed that additional heating does affect the turbulent trace gas CO2 fluxes but is very minor, especially for a horizontally mounted LI-COR LI-7500 gas analyzer. Further efforts are urgently needed to improve the data quality and quality of flux measurements.  相似文献   

20.
对东莞大岭山撂荒的荔枝林(品种为“妃子笑”)林下植被进行样方调查, 分析了荔枝林林下植物群落的结构、多样性及生态位特征, 并提出撂荒荔枝林未来发展的对策。共记录到林下植被102 种, 隶属于50 科85 属。其中, 蕨类植物13 种, 隶属于9 科9 属; 双子叶植物65 种, 隶属于31 科52 属; 单子叶植物24 种, 隶属于10 科24 属。林下植被以灌木和乔木(幼苗)为优势生活型。灌木层与草本层物种多度分布差异不显著, 其物种丰富度、均匀度、Shannon-Wiener 指数、Simpson 指数差异亦均不显著。林下层优势物种中有春花(Raphiolepis indica)等8 种灌木, 芒萁(Dicranopteris dichotoma)等7 种草本, 玉叶金花(Mussaenda pubescens)等5 种藤本。灌木层主要种群的生态位宽度与生态位重叠值变化范围分别为0.909~1.450、0.567~0.955, 草本层分别为0.945~1.566、0.270~0.984。所调查的荔枝林林下自然演替的植被物种丰富, 区系成分为亚热带地带性种类, 结构亦较为复杂。因此, 在荔枝产业面临转型的阶段, 对于立地条件或荔枝品质较差不再经营的林分,可维持植被现状, 或向生态公益林方向改造, 以利于东莞荔枝产业的可持续发展。  相似文献   

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