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Bamboo Forest Water Use Efficiency in the Yangtze River Delta Region, China
... Their results showed that NPP in terrestrial ecosystems in the YRD region also E a r l y R e l e a s e showed a decreasing trend. This was mainly caused by global climate change, the frequency of summer droughts has increased in this region, and severe summer droughts have led to a decline in vegetation productivity (Zhao and Running 2010;Wang et al. 2016). Along with freezing rain and ice storms, GPP in the YRD region has gradually shown a decreasing trend (Xu et al. 2013). ...
Understanding the characteristics of water use efficiency (WUE) could enhance our understanding of the integrated response of terrestrial ecosystems to climate change. In this study, the vegetation photosynthesis model (VPM) and the revised remote sensing-Penman Monteith (RRS-PM) model were used to characterize the spatial-temporal dynamics of gross primary productivity (GPP) and evapotranspiration (ET) in the Yangtze River Delta (YRD) region. Our predicted GPP and ET results were in the range of those reported by other models. To be specific, the RRS-PM model could explain 85% of the variability in ET at the Anji site, China. The root mean square error between the simulation value and the measured ET was 8.543 mm month-1. For the evergreen broadleaf forest, deciduous needle leaf forest and deciduous broadleaf forest, GPP decreased and ET increased, thus the WUE = (GPP/ET) inevitably decreased. The increase in WUE in grassland was due to the decreased ET, for which the rate of decrease was greater than for GPP. For the spatial distribution of WUE, the maximum value occurred in the southern and northeastern parts of the YRD region, while the minimum value was distributed in the middle and northwest. This is likely due to the different land-cover types. In the south, there is more forest, while in the north the land-cover type is almost entirely cropland. In our study, GPP increased with increasing LAI across all of the plant function types. However, ET only increased with LAI when LAI was less than 2 m2 m-2 across all of the plant function types, once LAI was larger than 2 m2 m-2, ET did not continue to increase with LAI.
Knowledge of water use efficiency (WUE) at the ecosystem level is imperative as it is a critical ecophysiological index reflecting the coupling processes between carbon sequestration and water consumption. However, the temporal patterns of WUE and their underlying regulating factors during different seasons in tea plantations, which are under intense pruning practice, remain poorly understood. Based on carbon and water vapor flux data obtained by the eddy covariance technique in a subtropical hilly tea plantation in southeast China during 2014–2018, this study analyzed the seasonal variations in gross primary productivity (GPP), evapotranspiration (ET) and ecosystem WUE, and discussed the response of GPP, ET and WUE to environmental variables during different seasons. The results showed that the mean annual GPP, ET and WUE ranged from 1426.61 ~ 1896.05 g C m⁻², 607.05 ~ 805.83 mm and 2.22 ~ 2.68 g C kg⁻¹ H2O, respectively. At the daily time scale, GPP and ET responded significantly and positively to variations in air temperature (Ta), water vapor pressure deficit (VPD), and net radiation (Rn) in both the pruning season and late growing season, but daily WUE had a significantly negative response to variations in Ta, VPD and Rn. Rn was identified as the dominant factor in regulating GPP and ET during both the pruning season and late growing season, while the dominant factor controlling daily variations in WUE was VPD during the pruning season and Ta during the late growing season. Such results were mainly due to the divergent responses of carbon assimilation and water loss to different environmental variations and management strategies. These findings highlighted the importance of understanding the coupled processes between carbon assimilation and water loss in tea plantations.
This study reports eddy-covariance measurements of carbon and water fluxes in a boreal Scots pine (Pinus sylvestris) forest during the 1999-2008 growing seasons. We found that gross primary productivity (GPP) was limited by low rainfall and low summer temperatures, whereas evapotranspiration (ETP) was limited by temperature and precipitation. GPP was less sensitive to increased ETP in the years with little rainfall than in the years with abundant rainfall. During low-rainfall years with long-lasting drought periods and few rainy days (2000 and 2003), GPP was reduced, whereas during the low-rainfall year (2006) with evenly distributed seasonal precipitation it remained at the same level as that during the abundant-rainfall years. Ecosystem water-use efficiency (WUE) was found to be sensitive to increased ETP and vapour-pressure deficits. However, there was no difference between abundant- and low-rainfall years in the responses of WUE to climatic and edaphic variables, most likely due to the coupled feedback of the carbon and water cycles to drought. This study suggested that combined frequency (also seasonality) of precipitation and total amount of rainfall are needed to better evaluate the feedback of carbon and water budgets in boreal forests to drought, especially under expected climate change.
A better understanding of ecosystem water-use efficiency (WUE) will help us improve ecosystem management for mitigation as well as adaption to global hydrological change. Here, long-term flux tower observations of productivity and evapotranspiration allow us to detect a consistent latitudinal trend in WUE, rising from the subtropics to the northern high-latitudes. The trend peaks at approximately 51°N, and then declines toward higher latitudes. These ground-based observations are consistent with global-scale estimates of WUE. Global analysis of WUE reveals existence of strong regional variations that correspond to global climate patterns. The latitudinal trends of global WUE for Earth's major plant functional types reveal two peaks in the Northern Hemisphere not detected by ground-based measurements. One peak is located at 20° ~ 30°N and the other extends a little farther north than 51°N. Finally, long-term spatiotemporal trend analysis using satellite-based remote sensing data reveals that land-cover and land-use change in recent years has led to a decline in global WUE. Our study provides a new framework for global research on the interactions between carbon and water cycles as well as responses to natural and human impacts.
The water-use efficiency (WUE) of an eco-system—defined as the gross ecosystem production (GEP) divided by the evapotranspiration (ET)—is an important index for understanding the coupling of water and carbon and quantifying water–carbon trade-offs in forests. An open-path eddy covariance technique and a microclimate measurement system were deployed to investigate the WUE of a poplar plantation ecosystem in the Daxing District of Beijing, China, during the growing seasons in 2006, 2007, and 2008. We found that WUE values changed diurnally, peaking in early morning and showing a mini-mum between 2 pm and 3 pm. This pattern was regulated by photosynthetically active radiation, saturated vapor pressure deficit, and stomatal opening and closure. WUE had inter-daily variations but no substantial seasonal vari-ation. The WUE decreased with increasing soil water content due to the higher sensitivity of ET than GEP to increased soil moisture. Under moist soil conditions (i.e., relative extractable water content [0.4), GEP was stable and WUE was generally low. These results suggest that the poplar plantation does not effectively use the available soil water for carbon uptake, and that soil moisture is lost to the atmosphere through ET.
The carbon and water cycles for a southwestern Amazonian forest site were investigated using the longest time series of fluxes of CO2 and water vapor ever reported for this site. The period from 2004 to 2010 included two severe droughts (2005 and 2010) and a flooding year (2009). The effects of such climate extremes were detected in annual sums of fluxes as well as in other components of the carbon and water cycles, such as gross primary production and water use efficiency. Gap-filling and flux-partitioning were applied in order to fill gaps due to missing data, and errors analysis made it possible to infer the uncertainty on the carbon balance. Overall, the site was found to have a net carbon uptake of ≈5 t C ha(-1) year(-1), but the effects of the drought of 2005 were still noticed in 2006, when the climate disturbance caused the site to become a net source of carbon to the atmosphere. Different regions of the Amazon forest might respond differently to climate extremes due to differences in dry season length, annual precipitation, species compositions, albedo and soil type. Longer time series of fluxes measured over several locations are required to better characterize the effects of climate anomalies on the carbon and water balances for the whole Amazon region. Such valuable datasets can also be used to calibrate biogeochemical models and infer on future scenarios of the Amazon forest carbon balance under the influence of climate change.
We selected four sites of ChinaFLUX representing four major ecosystem types in China—Changbaishan temperate broad-leaved Korean pine mixed forest (CBS), Dinghushan subtropical evergreen broadleaved forest (DHS), Inner Mongolia temperate steppe (NM), and Haibei alpine shrub-meadow (HBGC)—to study the seasonal dynamics of ecosystem water use efficiency (WUE = GPP/ET, where GPP is gross primary productivity and ET is evapotranspiration) and factors affecting it. Our seasonal dynamics results indicated single-peak variation of WUE in CBS, NM, and HBGC, which were affected by air temperature (Ta) and leaf area index (LAI), through their effects on the partitioning of evapotranspiration (ET) into transpiration (T) (i.e., T/ET). In DHS, WUE was higher at the beginning and the end of the year, and minimum in summer. Ta and soil water content affected the seasonal dynamics of WUE through their effects on GPP/T. Our results indicate that seasonal dynamics of WUE were different because factors affecting the seasonal dynamics and their mechanism were different among the key ecosystems.
Variation in carbon-isotope composition among and between wheat genotypes was correlated with variation in water-use efficiency in separate pot experiments conducted in spring-summer and in winter. In the main, winter experiment, the water-use efficiencies ranged from 2.0 to 3.7 mmolC/mol H2O (means of four replicates) while the corresponding isotope effects for leaf material ranged from 1.0225 to 1.0194. ¹³C was more abundant in grain than in leaves and stems. It is suggested that carbon-isotope analysis may be a useful tool in selection for improved water-use efficiency in breeding programmes for C3 species.
Through 2–3-year (2003–2005) continuous eddy covariance measurements of carbon dioxide and water vapor fluxes, we examined the seasonal, inter-annual, and inter-ecosystem variations in the ecosystem-level water use efficiency (WUE, defined as the ratio of gross primary production, GPP, to evapotranspiration, ET) at four Chinese grassland ecosystems in the Qinghai-Tibet Plateau and North China. Representing the most prevalent grassland types in China, the four ecosystems are an alpine swamp meadow ecosystem, an alpine shrub-meadow ecosystem, an alpine meadow-steppe ecosystem, and a temperate steppe ecosystem, which illustrate a water availability gradient and thus provide us an opportunity to quantify environmental and biological controls on ecosystem WUE at different spatiotemporal scales. Seasonally, WUE tracked closely with GPP at the four ecosystems, being low at the beginning and the end of the growing seasons and high during the active periods of plant growth. Such consistent correspondence between WUE and GPP suggested that photosynthetic processes were the dominant regulator of the seasonal variations in WUE. Further investigation indicated that the regulations were mainly due to the effect of leaf area index (LAI) on carbon assimilation and on the ratio of transpiration to ET (T/ET). Besides, except for the swamp meadow, LAI also controlled the year-to-year and site-to-site variations in WUE in the same way, resulting in the years or sites with high productivity being accompanied by high WUE. The general good correlation between LAI and ecosystem WUE indicates that it may be possible to predict grassland ecosystem WUE simply with LAI. Our results also imply that climate change-induced shifts in vegetation structure, and consequently LAI may have a significant impact on the relationship between ecosystem carbon and water cycles in grasslands.
This paper discusses the advantages and disadvantages of the different methods that separate net ecosystem exchange (NEE) into its major components, gross ecosystem carbon uptake (GEP) and ecosystem respiration ( R eco ). In particular, we analyse the effect of the extrapolation of night‐time values of ecosystem respiration into the daytime; this is usually done with a temperature response function that is derived from long‐term data sets. For this analysis, we used 16 one‐year‐long data sets of carbon dioxide exchange measurements from European and US‐American eddy covariance networks. These sites span from the boreal to Mediterranean climates, and include deciduous and evergreen forest, scrubland and crop ecosystems.
We show that the temperature sensitivity of R eco , derived from long‐term (annual) data sets, does not reflect the short‐term temperature sensitivity that is effective when extrapolating from night‐ to daytime. Specifically, in summer active ecosystems the long‐term temperature sensitivity exceeds the short‐term sensitivity. Thus, in those ecosystems, the application of a long‐term temperature sensitivity to the extrapolation of respiration from night to day leads to a systematic overestimation of ecosystem respiration from half‐hourly to annual time‐scales, which can reach >25% for an annual budget and which consequently affects estimates of GEP. Conversely, in summer passive (Mediterranean) ecosystems, the long‐term temperature sensitivity is lower than the short‐term temperature sensitivity resulting in underestimation of annual sums of respiration.
We introduce a new generic algorithm that derives a short‐term temperature sensitivity of R eco from eddy covariance data that applies this to the extrapolation from night‐ to daytime, and that further performs a filling of data gaps that exploits both, the covariance between fluxes and meteorological drivers and the temporal structure of the fluxes. While this algorithm should give less biased estimates of GEP and R eco , we discuss the remaining biases and recommend that eddy covariance measurements are still backed by ancillary flux measurements that can reduce the uncertainties inherent in the eddy covariance data.
Global climate change projections suggest an increasing frequency of droughts and extreme rain events in the steppes of the Eurasian region. Using the eddy covariance method, we measured carbon and water balances of a meadow steppe ecosystem in Northeast China during 2 years which had contrasting precipitation patterns in spring seasons in 2007 and 2008. The meadow steppe sequestrated only 64·2 gC m−2 year−1 in 2007 compared to 160·5 gC m−2 year−1 in 2008, due to a severe spring drought in 2007. The 2007 spring drought resulted in a dramatic reduction of leaf area index (LAI) and aboveground net primary productivity (ANPP). However, the meadow steppe still acted as a carbon sink in 2007. The strength of the sink was much greater than that in the typical steppes in Central Mongolia and Inner Mongolia. Spring drought also caused a reduction of plant transpiration (Tr) and total ecosystem evapotranspiration (ET). However, the suppression of ET in 2007 was relatively small in comparison to gross ecosystem productivity (GEP) reduction. Thus, ecosystem water use efficiency (WEU) (GEP/ET) in 2007 was reduced to 5·0 gCO2 kg−1 H2O or 75% of that of 2008. We concluded that spring drought detrimentally impacted meadow steppe ecosystem by reducing leaf areas, biomass, GEP, WUE and associated increases in soil evaporation (Es) that might aggravate soil salinization of the Songnen Plain. Copyright © 2011 John Wiley & Sons, Ltd.
Terrestrial gross primary production (GPP) is the largest global CO2 flux driving several ecosystem functions. We provide an observation-based estimate of this flux at 123 ± 8 petagrams of carbon
per year (Pg C year−1) using eddy covariance flux data and various diagnostic models. Tropical forests and savannahs account for 60%. GPP over
40% of the vegetated land is associated with precipitation. State-of-the-art process-oriented biosphere models used for climate
predictions exhibit a large between-model variation of GPP’s latitudinal patterns and show higher spatial correlations between
GPP and precipitation, suggesting the existence of missing processes or feedback mechanisms which attenuate the vegetation
response to climate. Our estimates of spatially distributed GPP and its covariation with climate can help improve coupled
climate–carbon cycle process models.
Five years of eddy-covariance and other measurements at a mature ponderosa pine forest and a nearby young plantation are used to contrast the carbon fluxes for long-term averages, seasonal patterns, diel patterns and interannual variability, and to examine the differing responses to water-stress. The mature forest with larger leaf area and wetter and cooler soils has a net uptake of carbon 3.3 times that of the young plantation. In the spring, photosynthesis is larger at the mature site as expected based on the difference in leaf area, however, another important factor is the reduction in springtime respiration at the mature site due to lower soil temperatures because of more shade from the canopy. Patterns of photosynthesis, inherent water-use efficiency (IWUE) and tree transpiration indicate that the young plantation responds to the seasonal drought sooner and to a more severe degree. Lower sensitivity to seasonal drought at the mature site is likely due to higher soil moisture reserves year round and a deeper root system that can access more water. Outside the seasonal drought period, the IWUE is the same at both sites, suggesting a species-specific value. Larger interannual variability at the plantation is associated with water-year drought and aggrading. Keywords: carbon fluxes; net ecosystem exchange of carbon; eddy-covariance; water-stress; net ecosystem production; water-use efficiency (Published: 13 February 2012) Citation: TellusB 2012, 64 , 17159, DOI: 10.3402/tellusb.v64i0.17159
Copyrighted by American Geophysical Union. We analyzed 7 years (2002–2008) of micrometeorological and concurrent biological observations of carbon and water fluxes at a mature ponderosa pine forest in central Oregon in a semiarid climate. We sought to evaluate the extent that gross primary productivity, net ecosystem exchange, ecosystem respiration, net primary productivity, net ecosystem productivity, tree transpiration, and evapotranspiration varied seasonally and interannually in this ecosystem subjected to varying periods and severity of droughts. To explain variation, we found it necessary to define seasons functionally within a hydroecological year rather than by fixed calendar dates. The interannual variability in growing season length was large (45 days), and the end date was more variable than the onset. Plant-available soil water was the main determinant of carbon fluxes. Spring evapotranspiration primarily used shallow water, whereas summer and fall evapotranspiration drew water from deeper in the soil profile. A multiyear drought (2001–2003) had a more severe and fundamentally different impact on carbon and water cycles than a single-year (2005) drought because of carryover effects in soil water and carbohydrate reserves in plant tissue. Calendar year–based analysis was inadequate to diagnose drought years in precipitation and ecosystem drought response. Extension of meteorological records back to 1982 showed that anomalies were coherent across the region and that the observations represented below-average precipitation and above-average temperatures coherent with a warm-phase Pacific Decadal Oscillation. The carbon sink of this seasonally water-limited ecosystem is anticipated to increase with increasing available soil water during the growing season.
Half-hourly measurements of the net exchanges of carbon dioxide and water vapor between terrestrial ecosystems and the atmosphere provide estimates of gross primary production (GPP) and evapotranspiration (ET) at the ecosystem level and on daily to annual timescales. The ratio of these quantities represents ecosystem water use efficiency. Its multiplication with mean daylight vapor pressure deficit (VPD) leads to a quantity which we call “inherent water use efficiency” (IWUE*). The dependence of IWUE* on environmental conditions indicates possible adaptive adjustment of ecosystem physiology in response to a changing environment. IWUE* is analyzed for 43 sites across a range of plant functional types and climatic conditions. IWUE* increases during short-term moderate drought conditions. Mean annual IWUE* varied by a factor of 3 among all sites. This is partly explained by soil moisture at field capacity, particularly in deciduous broad-leaved forests. Canopy light interception sets the upper limits to canopy photosynthesis, and explains half the variance in annual IWUE* among herbaceous ecosystems and evergreen needle-leaved forests. Knowledge of IWUE* offers valuable improvement to the representation of carbon and water coupling in ecosystem process models
Water-use efficiency (WUE) has been recognized as an important characteristic of vegetation productivity in various natural scientific disciplines for decades, but only recently at the ecosystem level, where different ways exist to characterize water-use efficiency. Hence, the objective of this research was (a) to systematically compare different ways of calculating ecosystem water-use efficiency (WUEe) from eddy-covariance measurements, (b) quantify the diurnal, seasonal and interannual variability of WUEe in relation to meteorological conditions, and (c) analyse between-site variability of WUEe as affected by vegetation type and climatic conditions, across sites in European forest ecosystems.
Day-to-day variability of gross primary productivity (GPP) and evapotranspiration (ET) were more strongly coupled than net ecosystem production (NEP) and ET, obviously because NEP also depends on the respiration that is not heavily coupled to water fluxes. However, the slope of daytime NEP versus ET (mNEP) from half-hourly measurements of a single day may also be used as a WUEe-estimate giving very similar results to those of the GPP-ET slope (mGPP), since the diurnal variation is dominated by GPP. Since ET is the sum of transpiration (linked to GPP) and evaporation from wet vegetation and soil surfaces (not linked to GPP) we expected that WUEe is increasing when days after rain are excluded from the analysis. However only very minor changes were found, justifying an analysis of WUEe related to vegetation type.
In most of the studied ecosystems the instantaneous WUEGPP was quite sensitive to diurnally varying meteorological conditions and tended to decline from the morning to the afternoon by more than 50% because of increasing vapour pressure deficits (VPD).
Seasonally, WUEGPP increased with a rising monthly precipitation sum and rising average monthly temperatures up to a threshold of 11, 14 and 18°C in boreal, temperate and Mediterranean ecosystems, respectively. Across all sites, the highest monthly WUEGPP-values were detected at times of positive anomalies of summer-precipitation. During drought periods with high temperatures, high VPD, little precipitation and low soil water content, the water-use efficiency of gross carbon uptake (WUEGPP) tended to decrease in all forest types because of a stronger decline of GPP compared to ET.
However the largest variation of growing season WUEGPP was found between-sites and significantly related to vegetation type: WUEGPP was highest in ecosystems dominated by deciduous trees ranging from 5.0 g CO2 kg H2O−1 for temperate broad-leaved deciduous forests (TD), to 4.5 for temperate mixed forests (TM), 3.5 for temperate evergreen conifers (TC), 3.4 for Mediterranean broad-leaved deciduous forests (MD), 3.3 for Mediterranean broad-leaved evergreen forests (Mbeg), 3.1 for Mediterranean evergreen conifers (MC), 2.9 for boreal evergreen conifers (BC) and only 1.2 g CO2 kg H2O−1 for a boreal wetland site (BT). Although vegetation type and meteorology co-vary, the WUEGPP variation was hardly related to meteorology, as we could show by comparing similar meteorological conditions only. Furthermore we compared across-site WUEGPP only under conditions when the 10% high GPP rates were exhibited. The between site differences remained, and at all sites ecosystem reached higher WUEGPP levels under this condition. This means when vegetation is most productive usually it also maximises the amount of carbon gained per water lost.
Overall our results show that water-use efficiency exhibits a strong time-scale dependency in the sense that at longer time-scale meteorological conditions play a smaller role compared to shorter time scale. Moreover, we highlight the role of vegetation in determining carbon-water relation at ecosystem level. Consequently, all predictions of changing carbon-water cycle under changing climate should take into this role and the differences between vegetation types. These results show the strong time-scale dependency of water-use efficiency
Short-term measurements of carbon dioxide, water, and energy fluxes were collected at four locations along a mean annual precipitation gradient in southern Africa during the wet (growing) season with the purpose of determining how the observed vegetation-atmosphere exchange properties are functionally related to the long-term climatic conditions. This research was conducted along the Kalahari Transect (KT), one in the global set of International Geosphere-Biosphere Program transects, which covers a north-south aridity gradient, all on a homogenous sand formation. Eddy covariance instruments were deployed on a permanent tower in Mongu, Zambia (879 mm of rainfall per year), as well as on a portable tower in Maun (460 mm yr(-1)), Okwa River Crossing (407 mm yr(-1)), and Tshane (365 mm yr(-1)), Botswana for several days at each site. The relationships between CO2 flux, F-c, and photosynthetically active radiation were described well by a hyperbolic fit to the data at all locations except for Mongu, the wettest site. Here, there appeared to be an air temperature effect on F-c. While daytime values of F-c routinely approached or exceeded -20 mumol m(-2) s(-1) at Mongu, the magnitude of F-c remained less than -10 mumol m(-2) s(-1) when the air temperature was above 27degreesC. Canopy resistances to water vapor transfer, r(c), displayed an overall decline from the wetter sites to the more arid sites, but the differences in r(c) could be almost exclusively accounted for by the decrease in leaf area index (LAI) from north to south along the KT. Ecosystem water use efficiency (WUE), defined as the ratio of net carbon flux to evapotranspiration, showed a general decrease with increasing vapor pressure deficit, D, for all of the sites. The magnitudes of WUE at a given D, however, were dissimilar for the individual sites and were found to be stratified according to the position of the sites along the long-term aridity gradient. For example, Mongu, which has the wettest climate, has a much lower WUE for like levels of D than Tshane, which historically has the most arid climate. Given the similar inferred stomatal resistances between the sites, the disparate carbon uptake behavior for the grass vs. woody vegetation is the likely cause for the observed differences in WUE along the aridity gradient. The short-term flux measurements provide a framework for evaluating the vegetation's functional adaptation to the long-term climate and provide information that may be useful for predicting the dynamic response of the vegetation to future climate change.
Moderate Resolution Imaging Spectroradiometer (MODIS) products and climate data collected from meteorological stations were used to characterize the spatial-temporal dynamics of gross primary productivity (GPP), evapotranspiration (ET), and water-use efficiency (WUE) in the Yangtze River Delta (YRD) region and the response of these three variables to meteorological factors. The seasonal patterns of GPP and WUE showed a bimodal distribution, with their peak values occurring in May and August, and April and October, respectively. By contrast, the seasonal variation of ET presented a unimodal pattern with its maximum in July or August. The spatial distribution of ET and GPP was similar to higher values occurring in the south. From 2001 to 2012, GPP in the eastern YRD decreased, while GPP in the western part increased. In comparison, over the 12 years, ET in the northern part of YRD decreased, while ET in the southern part increased. The spatial distribution and spatial variation of WUE were both similar to those of GPP. This implies that the changes in WUE are primarily controlled by the variations in GPP. The annual average WUE over vegetation types followed the order of: evergreen broadleaf forest (1.95 g C kg−1 H2O) > deciduous broadleaf forest (1.87 g C kg−1 H2O) > evergreen needle leaf forest (1.70 g C kg−1 H2O) > deciduous needle leaf forest (1.68 g C kg−1 H2O) > grassland (1.66 g C kg−1 H2O) > cropland (1.61 g C kg−1 H2O). Both GPP and ET increased with increasing annual mean temperature (Ta) and annual mean precipitation across all of the plant function types. WUE decreased as vapour pressure deficit (VPD) increased in all of the biomes. Interestingly, the relationship between WUE and VPD was the most significant in broadleaf forest. Whether this phenomenon is universal should be investigated in future studies.
Water use efficiency (WUE) reflects the coupling of carbon and water cycles. Analyzing the spatial variability of WUE can improve our understanding on the interaction between carbon and water cycles at a large scale, which also provides a basis for improving the regional carbon budget assessment. Based on China’s eddy covariance measurements, we examined the spatial variation of China’s WUE and its affecting factors. WUE showed a decreasing trend with the increasing altitude, which was the result of ecosystem type distribution resulting from the climatic gradient. After fully considering the vertical variation of WUE, we found that not only mean annual air temperature (MAT), mean annual precipitation (MAP), and mean leaf area index (MLAI) but also mean annual photosynthesis active radiation (MAR) affected the spatial variation of WUE. With the increasing MAT, MAP, and MLAI, WUE increased significantly but the increasing MAR decreased WUE. The spatial variation of WUE could be directly depicted by MLAI and altitude, the equation including which explained 65% of the spatial variation of WUE. The effects of MAT and MAP on the spatial variation of WUE may be achieved through altering MLAI, while the mechanism underlying the effect of MAR on the spatial variation of WUE was still unclear, which should be the subject of future investigations. This study reveals the vertical variation of WUE and provides a new approach to generate the spatial variation in WUE, which will benefit the regional carbon budget assessment.
Bamboo forest ecosystem is the part of the forest ecosystem. The distribution area of bamboo forest is limited, but in somewhere, like south China, it has been cultivate for a long time with human management. As the climate change has been take great effect on forest carbon budget, many researchers pay attention to the carbon budget in bamboo forest. Moreover cultivative management had a significant impact on the bamboo forest carbon budget. In this study, we modified a terrestrial ecosystem model named Integrated Biosphere Simulator (IBIS) according the management of Lei bamboo forest. Some management, like fertilization, shoots harvesting and organic mulching in winter, had been incorporated into model. Then we had compared model results with the observation data from a Lei bamboo flux tower. The simulated and observed results had achieved good consistency. Our simulated Lei bamboo forest yearly net ecosystem productivity (NEP) was 0.41 kgC a-1 of carbon, which is very close to the observation data 0.45 kgC a-1 of carbon. And the monthly simulated results can take the change of carbon budget in each month, similar to the data we got from flux tower. It reflects that the modified IBIS model can characterize the growth of bamboo forest and perform the simulation well. And then two groups of simulations were set to evaluate effects of cultivative managements on Lei bamboo forests carbon budget. And results showed that both fertilization and organic mulching had taken positive effects on Lei bamboo forests carbon sequestration.
Water use efficiency (WUE) at the ecosystem level is an important ecophysiological index reflecting the coupling relationship between water and carbon cycles. CO2 and water vapor fluxes were measured by the eddy covariance method during the period 2006–2010 over a warm-temperate mixed plantation in the North China. The seasonal and interannual variations of gross primary productivity (GPP), evapotranspiration (ET) and ecosystem WUE were analyzed, and the impacts of climatic variables and soil moisture on GPP, ET and WUE were discussed. At the monthly scale, GPP and ET had similar relations with solar radiation, air temperature, vapor pressure deficit (VPD) and precipitation. It is suggested that photosynthesis and evapotranspiration were driven by climatic variables at the approximately equal strength. During the growing season, WUE decreased significantly with the increase of VPD and solar radiation. Cloudiness can improve photosynthesis and enhance WUE. GPP was 9–39% greater but ET was 8–26% lower under cloudy sky conditions than that under sunny sky conditions. Therefore, WUE was 29–72% higher under cloudy skies in comparison with that under sunny skies. Annual average WUE ranged from 1.76 to 2.41 g C kg−1 H2O. The major driver of interannual variability in WUE was soil water content in May.
Comparisons were made among Douglas-fir forest, aspen (broad leaf deciduous) forest and wheatgrass (C3) grassland for ecosystem-level water-use efficiency (WUE). WUE was defined as the ratio of photosynthetic CO2 assimilation rate and evapotranspiration (ET) rate. The ET data measured by eddy covariance were screened so that they overwhelmingly represented transpiration. The three sites used in this comparison spanned a range of vegetation (plant functional) types and environmental conditions within western Canada. When compared in the relative order Douglas-fir (located on Vancouver Island, BC), aspen (northern Saskatchewan), grassland (southern Alberta), the sites demonstrated a progressive decline in precipitation and a general increase in maximum air temperature and atmospheric saturation deficit (Dmax) during the mid-summer. The average (±SD) WUE at the grassland site was 2.6±0.7 mmol mol−1, which was much lower than the average values observed for the two other sites (aspen: 5.4±2.3, Douglas-fir: 8.1±2.4). The differences in WUE among sites were primarily because of variation in ET. The highest maximum ET rates were approximately 5, 3.2 and 2.7 mm day−1 for the grassland, aspen and Douglas-fir sites, respectively. There was a strong negative correlation between WUE and Dmax for all sites. We also made seasonal measurements of the carbon isotope ratio of ecosystem respired CO2 (δR) in order to test for the expected correlation between shifts in environmental conditions and changes to the ecosystem-integrated ratio of leaf intercellular to ambient CO2 concentration (ci/ca). There was a consistent increase in δR values in the grassland, aspen forest and Douglas-fir forest associated with a seasonal reduction in soil moisture. Comparisons were made between WUE measured using eddy covariance with that calculated based on D and δR measurements. There was excellent agreement between WUE values calculated using the two techniques. Our δR measurements indicated that ci/ca values were quite similar among the Douglas-fir, aspen and grassland sites, despite large variation in environmental conditions among sites. This implied that the shorter-lived grass species had relatively high ci/ca values for the D of their habitat. By contrast, the longer-lived Douglas-fir trees were more conservative in water-use with lower ci/ca values relative to their habitat D. This illustrates the interaction between biological and environmental characteristics influencing ecosystem-level WUE. The strong correlation we observed between the two independent measurements of WUE, indicates that the stable isotope composition of respired CO2 is a useful ecosystem-scale tool to help study constraints to photosynthesis and acclimation of ecosystems to environmental stress.
This study investigated evapotranspiration and CO2 exchange, observed by the eddy covariance method, over two larch-dominated forests, in the middle part of the Lena basin, eastern Siberia. Compared with the Spasskaya Pad (SP) forest, the Elgeeii (EG) forest had more plant biomass and a soil texture that allowed for larger water retention. Observations in three growing seasons revealed that the two sites had similar seasonal patterns of meteorological conditions and almost the same total evapotranspiration; however, the net and gross uptakes of CO2 at the EG forest were 1·61 and 1·31 times those at the SP forest, respectively. The difference in aboveground biomass was probably responsible for the higher productivity at EG. Less aboveground biomass would result in less transpiration by trees at SP, which would be compensated for in total evapotranspiration by soil evaporation and transpiration by the forest floor vegetation. It is likely that forest transpiration is commonly optimized to the same level of evapotranspiration under similar meteorological conditions through differences in the contributions of evapotranspiration from the forest floor vegetation and the soil water availability. Water use efficiency (WUE) displayed different patterns, reflecting the seasonal variations in each flux. Seasonal sequences in the WUE at EG indicated a midsummer depletion, which is commonly observed in forest ecosystems under a broad climate. Daily variation corresponding to the vapour pressure deficit and the cloud cover conditions (reduced solar radiation) was also observed. In contrast, WUE was relatively invariable at the seasonal and daily scales at SP. Copyright © 2013 John Wiley & Sons, Ltd.
Eddy covariance was used to measure above-canopy exchanges of CO2 and water vapor at an operational plantation of hybrid poplar (variety “Walker”) established on marginal agricultural land in east central Alberta, Canada. Winter ecosystem respiration (R
e) rates were inferred from seasonal changes in the normalized respiration rate at 10°C (R
10) for the growing season and observations of soil CO2 concentration measured with solid-state probes. Over five consecutive growing seasons following planting, gross ecosystem production (GEP) increased each year, ranging from 21 g C m−2 y−1 in year 1 to 469 g C m−2 y−1 in year 5. During this period, the annual carbon balance shifted from a net source of greater than 330 g C m−2 in year 1 to approximately C-neutral in year 5. Total carbon (C) release over 5 years likely exceeded 630 g C m−2. Intra- and interannual variations in temperature and soil water availability greatly affected annual C balance each year. GEP and R
e were particularly sensitive to temperature during spring and to soil water availability in summer: year 5 was notable because a cold spring and accumulating drought caused growth and carbon uptake to fall well below their potential. Annual evapotranspiration (ET) increased slightly with leaf area, from 281 mm in year 1 to 323 mm in year 4, but in year 5 it declined, while exceeding total precipitation (P). This trend of increasing annual ET/P suggests that annual GEP could become increasingly water-limited in years with below normal precipitation, as the plantation achieves maximum leaf area. Measured canopy albedos did not change appreciably over three winters, suggesting that estimates of increased radiative forcing resulting from afforestation in high latitudes could be exaggerated in regions where fast-growing deciduous plantations are managed on short (~20-year) rotations.
The measured net ecosystem exchange (NEE) of CO2 between the ecosystem and the atmosphere
reflects the balance between gross CO2 assimilation [gross primary production (GPP)]
and ecosystem respiration (Reco). For understanding the mechanistic responses of ecosystem
processes to environmental change it is important to separate these two flux components. Two
approaches are conventionally used: (1) respiration measurements made at night are extrapolated
to the daytime or (2) light–response curves are fit to daytime NEE measurements and
respiration is estimated from the intercept of the ordinate, which avoids the use of potentially
problematic nighttime data.We demonstrate that this approach is subject to biases if the effect
of vapor pressure deficit (VPD) modifying the light response is not included.We introduce an
algorithm for NEE partitioning that uses a hyperbolic light response curve fit to daytime NEE,
modified to account for the temperature sensitivity of respiration and the VPD limitation of
photosynthesis. Including the VPD dependency strongly improved the model’s ability to
reproduce the asymmetric diurnal cycle during periods with high VPD, and enhances the
reliability of Reco estimates given that the reduction of GPP by VPD may be otherwise
incorrectly attributed to higher Reco. Results from this improved algorithm are compared
against estimates based on the conventional nighttime approach. The comparison demonstrates that the uncertainty arising from systematic errors dominates the overall uncertainty of annual sums (median absolute deviation of GPP: 47 gCm�2 yr�1), while errors arising from the random error (median absolute deviation: � 2gCm�2 yr�1) are negligible. Despite sitespecific differences between the methods, overall patterns remain robust, adding confidence to statistical studies based on the FLUXNET database. In particular, we show that the strong correlation between GPP and Reco is not spurious but holds true when quasi-independent, i.e. daytime and nighttime based estimates are compared.
Bamboo stands are one of the most important forest types in China, covering an area of about 4.99 million hectares, and estimation of their carbon stocks is vital for China's national carbon accounting. Bamboo biomass and carbon fraction, as well as soil bulk density and soil organic matter content, data were collated from 40 publications describing conditions at 35 sites in 10 Chinese provinces where most bamboo stands are distributed. Carbon stocks and its changes in the living biomass and soil organic matter in bamboo stands in China in the past five decades were estimated based on these collated data together with the area of bamboo stands and number of bamboo culms derived from the National Forestry Inventory (NFI). Our estimates indicate that the carbon stocks in bamboo stands in China have been increasing since the 1950s with estimated values of 318.55Tg C (1950–1962), 427.37TgC (1977–1981), 463.80TgC (1984–1988), 493.00TgC (1989–1993), 548.79TgC (1994–1998) and 631.58TgC (1999–2003) accompanying the increase of bamboo stand area. Based on correlation between forest area and bamboo area, as well as the trends of forest area predicted in government strategy documents for forest development over the next five decades, the carbon stocks in bamboo stands for 2010, 2020, 2030, 2040 and 2050 are estimated to be 727.08TgC, 839.16TgC, 914.43TgC, 966.803TgC and 1017.64TgC, respectively.
Canopy water use efficiency (W), the ratio of crop productivity to evapotranspiration (ET), is critical in determining the production and water use for winter wheat (Triticum aestivum L.) in the North China Plain, where winter wheat is a major crop and rainfall is scarce and variable. With the eddy covariance (EC) technique, we estimated canopy W of winter wheat at gross primary productivity (WG) and net ecosystem productivity (WN) levels from revival to maturing in three seasons of 2002/2003, 2003/2004 and 2004/2005 at Yucheng Agro-ecosystem Station. Meanwhile we also measured the biomass-based water use efficiency (WB). Our results indicate that WG, WN and WB showed the similar seasonal variation. Before jointing (revival-jointing), WG, WN and WB were obviously lower with the values of 2.09–3.54gCkg−1, −0.71 to 0.06gCkg−1 and 1.37–4.03gkg−1, respectively. After jointing (jointing-heading), the winter wheat began to grow vigorously, and WG, WN and WB significantly increased to 5.26–6.78gCkg−1, 1.47–1.86gCkg−1 and 6.41–7.03gkg−1, respectively. The maximums of WG, WN and WB occurred around the stage of heading. Thereafter, WG, WN and WB began to decrease. During the observed periods, three levels of productivity: GPP, NEP and aboveground biomass (AGB) all had fairly linear relationships with ET. The slopes of GPP–ET, NEP–ET and AGB–ET were 4.67–6.12gCkg−1, 1.50–2.08gCkg−1 and 6.87–11.02gkg−1, respectively. Generally, photosynthetically active radiation (PAR) and daytime vapor pressure deficit (D) had negative effects on WG, WN and WB except for on some cloudy days with low PAR and D. In many cases, WG, WN and WB showed the similar patterns. While there were still some obvious differences between them besides in magnitude, such as their significantly different responses to PAR and D on cloudy and moist days.
Eddy covariance and sapflow data from three Mediterranean ecosystems were analysed via top-down approaches in conjunction with a mechanistic ecosystem gas-exchange model to test current assumptions about drought effects on ecosystem respiration and canopy CO2/H2O exchange. The three sites include two nearly monospecific Quercus ilex L. forests – one on karstic limestone (Puéchabon), the other on fluvial sand with access to ground water (Castelporziano) – and a typical mixed macchia on limestone (Arca di Noè). Estimates of ecosystem respiration were derived from light response curves of net ecosystem CO2 exchange. Subsequently, values of ecosystem gross carbon uptake were computed from eddy covariance CO2 fluxes and estimates of ecosystem respiration as a function of soil temperature and moisture. Bulk canopy conductance was calculated by inversion of the Penman-Monteith equation. In a top-down analysis, it was shown that all three sites exhibit similar behaviour in terms of their overall response to drought. In contrast to common assumptions, at all sites ecosystem respiration revealed a decreasing temperature sensitivity (Q10) in response to drought. Soil temperature and soil water content explained 70–80% of the seasonal variability of ecosystem respiration. During the drought, light-saturated ecosystem gross carbon uptake and day-time averaged canopy conductance declined by up to 90%. These changes were closely related to soil water content. Ecosystem water-use efficiency of gross carbon uptake decreased during the drought, regardless whether evapotranspiration from eddy covariance or transpiration from sapflow had been used for the calculation. We evidence that this clearly contrasts current models of canopy function which predict increasing ecosystem water-use efficiency (WUE) during the drought. Four potential explanations to those results were identified (patchy stomatal closure, changes in physiological capacities of photosynthesis, decreases in mesophyll conductance for CO2, and photoinhibition), which will be tested in a forthcoming paper. It is suggested to incorporate the new findings into current biogeochemical models after further testing as this will improve estimates of climate change effects on (semi)arid ecosystems' carbon balances.
To accurately predict ecosystem responses induced by climate warming at local-to-global scales, models are in need of more precise knowledge of response during periods of environmental stress such as drought. In this paper, we studied environmental control of canopy-level water use efficiency (WUE) during drought at an eddy flux site in an oak-hickory forest in central Missouri, USA. Two consecutive severe droughts in the summers of 2006 and 2007 afforded coverage of a broad range of environmental conditions. We stratified data to obtain subranges that minimized cross-correlations among putative WUE-controlling factors. Our results showed that WUE was subject to control by atmospheric saturation deficit (ASD), soil water potential (SWP) and the ratio of diffuse to total photosynthetically active radiation (If/It). Generally, WUE was found to scale with 1/(ASD)0.5, consistent with predictions from stomatal optimization theory. In contrast, SWP and If/It were related to WUE in a linear fashion. ASD was better correlated with WUE than either of the other two factors. It was also observed that the relationship between WUE and any single controlling factor was subject to influence of the other two. One such example was an opposite response of WUE to SWP between low and high ASD values, suggesting a breakdown of stomatal optimality under severe environmental stresses and a shift from optimal stomatal regulation to nonstomatal regulation at leaf scale. We have demonstrated that different data handling (stratified vs. nonstratified) or selection (hourly vs. daily) could lead to different conclusions on the relationship between WUE and its controls. For this reason, we recommend modelers to be cautious when applying WUE-response formulas at environmental conditions or at time scales different from those at which they are derived.
The eddy covariance technique and the cuvette method were used to investigate water use efficiency in an irrigated winter wheat (Triticum asetivum L.)/summer maize (Zea mays L.) rotation system in the North China Plain. The results show that ecosystem water use efficiency (WUEe) changed diurnally and seasonally. Daily maximal WUEe appeared in the morning. WUEe generally peaked in late April in wheat field and in late July/early August in maize field. From 2003 to 2006, seasonal mean WUEe was 6.7–7.4 mg CO2 g−1 H2O for wheat and 8.4–12.1 mg CO2 g−1 H2O for maize. WUEe was much lower than canopy water use efficiency (WUEc) under small leaf area index (LAI) but very close to WUEc under large LAI. With the increase in LAI, WUEe enlarged rapidly under low LAI but slowly when LAI was higher than one. WUEe was greater on the cloudy days than on the sunny days. Under the same solar radiation, WUEe was higher in the morning than in the afternoon. The ratio of internal to ambient CO2 partial pressure (Ci/Ca) decreased significantly with the increase in photosynthetically active radiation (PAR) when PAR was lower than the critical values (around 500 and 1000 μmol m−2 s−1 for wheat and maize, respectively). Beyond critical PAR, Ci/Ca was approximately constant at 0.69 for wheat and 0.42 for maize. Therefore, when LAI and solar radiation was large enough, WUEe has negative correlation with vapor pressure deficit in both of irrigated wheat and maize fields.
The European CARBOEUROPE/FLUXNET monitoring sites, spatial remote sensing observations via the EOS-MODIS sensor and ecosystem modelling provide independent and complementary views on the effect of the 2003 heatwave on the European biosphere's productivity and carbon balance. In our analysis, these data streams consistently demonstrate a strong negative anomaly of the primary productivity during the summer of 2003. FLUXNET eddy-covariance data indicate that the drop in productivity was not primarily caused by high temperatures (‘heat stress’) but rather by limitation of water (drought stress) and that, contrary to the classical expectation about a heat wave, not only gross primary productivity but also ecosystem respiration declined by up to more than to 80 gC m−2 month−1. Anomalies of carbon and water fluxes were strongly correlated. While there are large between-site differences in water-use efficiency (WUE, 1–6 kg C kg−1 H2O) here defined as gross carbon uptake divided by evapotranspiration (WUE=GPP/ET), the year-to-year changes in WUE were small (<1 g kg−1) and quite similar for most sites (i.e. WUE decreased during the year of the heatwave). Remote sensing data from MODIS and AVHRR both indicate a strong negative anomaly of the fraction of absorbed photosynthetically active radiation in summer 2003, at more than five standard deviations of the previous years. The spatial differentiation of this anomaly follows climatic and land-use patterns: Largest anomalies occur in the centre of the meteorological anomaly (central Western Europe) and in areas dominated by crops or grassland. A preliminary model intercomparison along a gradient from data-oriented models to process-oriented models indicates that all approaches are similarly describing the spatial pattern of ecosystem sensitivity to the climatic 2003 event with major exceptions in the Alps and parts of Eastern Europe, but differed with respect to their interannual variability.
Short-term measurements of carbon dioxide, water, and energy fluxes were collected at four locations along a mean annual precipitation gradient in southern Africa during the wet (growing) season with the purpose of determining how the observed vegetation–atmosphere exchange properties are functionally related to the long-term climatic conditions. This research was conducted along the Kalahari Transect (KT), one in the global set of International Geosphere-Biosphere Program transects, which covers a north–south aridity gradient, all on a homogenous sand formation. Eddy covariance instruments were deployed on a permanent tower in Mongu, Zambia (879 mm of rainfall per year), as well as on a portable tower in Maun (460 mm yr−1), Okwa River Crossing (407 mm yr−1), and Tshane (365 mm yr−1), Botswana for several days at each site. The relationships between CO2 flux, Fc, and photosynthetically active radiation were described well by a hyperbolic fit to the data at all locations except for Mongu, the wettest site. Here, there appeared to be an air temperature effect on Fc. While daytime values of Fc routinely approached or exceeded −20 μmol m−2 s−1 at Mongu, the magnitude of Fc remained less than −10 μmol m−2 s−1 when the air temperature was above 27°C. Canopy resistances to water vapor transfer, rc, displayed an overall decline from the wetter sites to the more arid sites, but the differences in rc could be almost exclusively accounted for by the decrease in leaf area index (LAI) from north to south along the KT. Ecosystem water use efficiency (WUE), defined as the ratio of net carbon flux to evapotranspiration, showed a general decrease with increasing vapor pressure deficit, D, for all of the sites. The magnitudes of WUE at a given D, however, were dissimilar for the individual sites and were found to be stratified according to the position of the sites along the long-term aridity gradient. For example, Mongu, which has the wettest climate, has a much lower WUE for like levels of D than Tshane, which historically has the most arid climate. Given the similar inferred stomatal resistances between the sites, the disparate carbon uptake behavior for the grass vs. woody vegetation is the likely cause for the observed differences in WUE along the aridity gradient. The short-term flux measurements provide a framework for evaluating the vegetation's functional adaptation to the long-term climate and provide information that may be useful for predicting the dynamic response of the vegetation to future climate change.
A system is described which is intended to calculate vertical fluxes of heat, moisture, momentum, and certain atmospheric pollutants at sites that are less than ideal. Fluxes, along with other turbulence statistics, are computed in real-time and printed at the end of each averaging period. The main elements of the program are (1) detrending (by use of running mean removal), (2) calculation of the entire stress tensor (which allows a three-dimensional coordinate rotation to be performed on the covariances), (3) software-adjustable timing delays for each instrument channel, and (4) real-time graphic presentation of the raw data as stripchart images. The first two of these program elements tend to relax the normal site and sensor-leveling requirements. Sample results are presented, and the sensitivities of the calculated quantities to coordinate rotation and to mean removal time are examined for both ideal and non-ideal sites.
To understand the seasonal variations of water use efficiency (WUE) of coniferous plantation in the subtropical monsoon area,
the experiment was conducted in 2003 and 2004 which presented two distinguished climatic conditions (severe summer drought
in 2003 and normal climatic condition in 2004). The water stress influenced WUE greatly, which caused a special seasonal WUE
pattern. WUE reached the minimum in summer drought and the maximum in winter, which was contrary to the variation of gross
primary production (GPP) and canopy evaporation (Fw). In winter, GPP and Fw increased along with the increasing of air temperature
and vapor pressure deficit (VPD), with the similar increasing rate. However, in drought summer, there was an adverse trend
among GPP/Fw and air temperature and VPD, and the decreasing rate of GPP was far larger than that of Fw. In summer, the conservation
of WUE was changed because of the environmental factors, resulting in the decreasing WUE. The photosynthesis and transpiration
of vegetation were mainly controlled by the environmental factors in winter, and the impact of stomatal regulation was relatively
weak. In summer, Fw was mainly controlled by the stomatal closure and GPP by both environmental factors and stomatal closure.
Keywordswater use efficiency-gross primary production-canopy evapotranspiration-canopy conductance-eddy covariance
This paper reports on results from eddy covariance measurements of carbon uptake and evapotranspiration in the eucalypt site of Espirra in Southern Portugal (38 degrees 38'N, 8 degrees 36'W). This site was included in the "Carboeurope" European network and is part of a 300 ha eucalypt forest, with about 1100 trees ha(-1), intensively managed as a coppice for pulp production and characterized by a 12-month annual growing period. The climate is of Mediterranean type with a long term (1961-1990) annual average precipitation of 709 mm and an annual average air temperature of 15.90 degrees C. During the measurement period (2002-2009) two main events took place, which changed the annual sink pattern of the forest: a drought period of two years (2004-2005) and a tree felling (October and November 2006). We analyzed the daily, seasonal and inter-annual variation of carbon uptake and evapotranspiration, and their relationships with the events and the variability of the main meteorological variables. Before the felling, annual net ecosystem exchange (NEE) increased from -865.56 g C m(-2) in 2002 to -356.64 g C m(-2) in 2005 together with a deep decrease in rainfall from 748 mm in 2002 to 378.58 mm and 396.64 mm in 2004 and 2005, respectively. For the same period, seasonal patterns of carbon uptake showed maximum values in April and decreased in July-August. The eucalypt stand recovered its carbon sink ability since June 2007 and had a NEE of -209.01 g C m(-2) in 2009. After the felling, the carbon uptake occurred from mid-February to mid-October, following an almost opposite pattern than that of the trees in the term of their productive cycle. A quantitative approach using generalized estimating equations (GEEs) was made for the period before the felling to relate monthly NEE and GPP with accumulated photosynthetic active radiation, water vapour pressure and precipitation. In conclusion, our study showed the relevant effects of water stress and anthropogenic interventions in the daily, seasonal and annual patterns of carbon uptake, under a context of good environmental conditions for carbon sequestration.
On local scale, the eddy covariance technique is suited to estimate gross primary production (GPP). Scaling up such observations to the regional and continental level, however, remains a challenge. Here, we show that there is a surprisingly robust stoichiometric relationship between vegetation CO2 and H2O fluxes, mediated by vapor pressure deficit (VPD), across many different forest vegetation types. This relationship is used to provide a data-driven estimate of Europe's GPP from its water balance. Namely, watershed-wide evapotranspiration (ET), as derived from precipitation (P) and river runoff (R), is multiplied by the ratio of GPP to ET as derived from eddy covariance measurements (water-use efficiency, WUE). In doing so, GPP of Europe is estimated to range between 3.9 and 5.8 PgC/a (median 5 PgC/a). Such GPP estimate is an important independent benchmark for large-scale ecosystem models and may be extended to global scale when relevant data becomes available.
We measured eddy covariance fluxes of CO2 and H2O over a flat irrigated olive orchard during growth, in different periods from Leaf Area Index (LAI) of 0.3–1.9; measurements of soil respiration were also collected. The daily net ecosystem exchange flux (FNEE) was practically zero at LAI around 0.4 or when the orchard intercepted 11% of the incoming daily radiation; at the end of the experiment, with LAI of 1.9 (and the fraction of intercepted daily radiation close to 0.5), FNEE was around 10 g CO2 m−2 day−1. The night-time ecosystem respiration (Reco), calculated from eddy fluxes in well-mixed night conditions, show a clear but non-linear dependence with LAI; it ranged from 0.05 to 0.15 mg CO2 m−2 s−1 (in average), being the lower limit ideally close to the heterotrophic soil respiration at the site. The gross primary production flux (FGPP) was linearly related to LAI within the LAI range of this experiment (with 11 g CO2 m−2 day−1 increments per unit of LAI) and to the fraction of intercepted radiation. The maximum rates of FGPP (0.75 mg CO2 m−2 s−1) were obtained in the summer mornings of 2002, at LAI close to 1.9. FGPP was strongly modulated by vapour pressure deficit (VPD) through the canopy conductance, even in absence of water stress. Hence, especially in the summer, the maximum rates of carbon assimilation are reached always before noon. The daily course of FGPP shows a two-phase pattern, first related to irradiance and then to canopy conductance. The water use efficiency (WUE) was, in average, 3.8, 6.3 and 7 g CO2 L−1 in 1999, 2001 and 2002, respectively, with maxima always in the early morning. Hourly WUE was strongly related to VPD (WUE = −10.25 + 22.52 × VPD−0.34). Our results suggest that drip irrigated orchards in general, and olive in particular, deserve specific carbon exchange and carbon budget studies and cannot be easily included in other biomes.
In this study, we used the remotely-sensed data from the Moderate Resolution Imaging Spectrometer (MODIS), meteorological and eddy flux data and an artificial neural networks (ANNs) technique to develop a daily evapotranspiration (ET) product for the period of 2004–2005 for the conterminous U.S. We then estimated and analyzed the regional water-use efficiency (WUE) based on the developed ET and MODIS gross primary production (GPP) for the region. We first trained the ANNs to predict evapotranspiration fraction (EF) based on the data at 28 AmeriFlux sites between 2003 and 2005. Five remotely-sensed variables including land surface temperature (LST), normalized difference vegetation index (NDVI), normalized difference water index (NDWI), leaf area index (LAI) and photosynthetically active radiation (PAR) and ground-measured air temperature and wind velocity were used. The daily ET was calculated by multiplying net radiation flux derived from remote sensing products with EF. We then evaluated the model performance by comparing modeled ET with the data at 24 AmeriFlux sites in 2006. We found that the ANNs predicted daily ET well (R2 = 0.52–0.86). The ANNs were applied to predict the spatial and temporal distributions of daily ET for the conterminous U.S. in 2004 and 2005. The ecosystem WUE for the conterminous U.S. from 2004 to 2005 was calculated using MODIS GPP products (MOD17) and the estimated ET. We found that all ecosystems' WUE-drought relationships showed a two-stage pattern. Specifically, WUE increased when the intensity of drought was moderate; WUE tended to decrease under severe drought. These findings are consistent with the observations that WUE does not monotonously increase in response to water stress. Our study suggests a new water-use efficiency mechanism should be considered in ecosystem modeling. In addition, this study provides a high spatial and temporal resolution ET dataset, an important product for climate change and hydrological cycling studies for the MODIS era.
Eddy covariance measurements of carbon dioxide (CO2) and water vapour fluxes were made from June 2002 to May 2003 over a 65-year-old temperate conifer plantation forest in southern Ontario, Canada. This site is part of a newly initiated long-term tower flux measurement program over a chronosequence of white pine plantation forests, known as the Turkey Point Flux Station. Net ecosystem productivity (NEP) showed a strong dependence on environmental variables such as temperature, light regime, and vapour pressure deficit. In the summer, saturation water deficit exerted a strong control on photosynthetic uptake through decreased bulk surface conductance. There was a linear relationship between monthly carbon uptake and water loss. For each kilogram of water evaporated from the stand, approximately 2.15 g of carbon was sequestered. Annual NEP was 196 g C m−2 from June 2002 to May 2003. Annual gross ecosystem productivity (GEP) was 1442 g C m−2 and annual ecosystem respiration (R) was 1247 g C m−2. A cross-site comparison of this site with 19 other, planted and natural, temperate conifer forests showed that NEP at the Turkey Point plantation was relatively low compared to other plantation forests of similar age group in Europe and North America. This analysis also showed that GEP and R of plantation forests was significantly higher than that of natural forests. In plantation forests, GEP and NEP linearly decreased with forest age, while R decreased little. Natural stands showed an increase in GEP and R with stand age, except for an old growth stand in western USA. This showed that photosynthesis, rather than respiration, plays a dominant role in net carbon uptake of both plantation and natural forests. In both, plantation and natural, stands there was a weak or no relationship between NEP and annual temperature and precipitation, while annual GEP and R was positively correlated to these environmental variables. The differences in carbon uptake among plantations and between plantation and natural temperate conifer forests were due more to physical and physiological differences among stands (e.g. stand age, tree density, leaf area index, site history, and adopted management practices in case of plantation forests), rather than differences in environmental variables.
Heightened awareness of global change issues within both science and political communities has increased interest in using the global network of eddy covariance flux towers to more fully understand the impacts of natural and anthropogenic phenomena on the global carbon balance. Comparisons of net ecosystem exchange (FNEE) responses are being made among biome types, phenology patterns, and stress conditions. The comparisons are usually performed on annual sums of FNEE; however, the average data coverage during a year is only 65%. Therefore, robust and consistent gap filling methods are required.We review several methods of gap filling and apply them to data sets available from the EUROFLUX and AmeriFlux databases. The methods are based on mean diurnal variation (MDV), look-up tables (LookUp), and nonlinear regressions (Regr.), and the impact of different gap filling methods on the annual sum of FNEE is investigated. The difference between annual FNEE filled by MDV compared to FNEE filled by Regr. ranged from −45 to +200 g C m−2 per year (MDV−Regr.). Comparing LookUp and Regr. methods resulted in a difference (LookUp−Regr.) ranging from −30 to +150 g C m−2 per year.We also investigated the impact of replacing measurements at night, when turbulent mixing is insufficient. The nighttime correction for low friction velocities (u∗) shifted annual FNEE on average by +77 g C m−2 per year, but in certain cases as much as +185 g C m−2 per year.Our results emphasize the need to standardize gap filling-methods for improving the comparability of flux data products from regional and global flux networks.
When the atmospheric turbulent flux of a minor constituent such as CO2 (or of water vapour as a special case) is measured by either the eddy covariance or the mean gradient technique, account may need to be taken of variations of the constituent's density due to the presence of a flux of heat and/or water vapour. In this paper the basic relationships are discussed in the context of vertical transfer in the lower atmosphere, and the required corrections to the measured flux are derived.
If the measurement involves sensing of the fluctuations or mean gradient of the constituent's mixing ratio relative to the dry air component, then no correction is required; while with sensing of the constituent's specific mass content relative to the total moist air, a correction arising from the water vapour flux only is required. Correspondingly, if in mean gradient measurements the constituent's density is measured in air from different heights which has been pre‐dried and brought to a common temperature, then again no correction is required; while if the original (moist) air itself is brought to a common temperature, then only a correction arising from the water vapour flux is required.
If the constituent's density fluctuations or mean gradients are measured directly in the air in situ , then corrections arising from both heat and water vapour fluxes are required.
These corrections will often be very important. That due to the heat flux is about five times as great as that due to an equal latent heat (water vapour) flux. In CO2 flux measurements the magnitude of the correction will commonly exceed that of the flux itself. The correction to measurements of water vapour flux will often be only a few per cent but will sometimes exceed 10 per cent.
We used the eddy-covariance technique to measure evapotranspiration (E) and gross primary production (GPP) in a chronosequence of three coastal Douglas-fir (Pseudotsuga menziesii) stands (7, 19 and 58 years old in 2007, hereafter referred to as HDF00, HDF88 and DF49, respectively) since 1998. Here, we focus on the controls on canopy conductance (gc), E, GPP and water use efficiency (WUE) and the effect of interannual climate variability at the intermediate-aged stand (DF49) and then analyze the effects of stand age following clearcut harvesting on these characteristics. Daytime dry-foliage Priestley-Taylor α and gc at DF49 were 0.4-0.8 and 2-6 mm s-1, respectively, and were linearly correlated (R2 = 0.65). Low values of α and gc at DF49 as well at the other two stands suggested stomatal limitation to transpiration. Monthly E, however, showed strong positive linear correlations to monthly net radiation (R2 = 0.94), air temperature (R2 = 0.77), and daytime vapour pressure deficit (R2 = 0.76). During July-September, monthly E (mm) was linearly correlated to monthly mean soil water content (θ, m3 m-3) in the 0-60 cm layer (E = 453θ - 21, R2 = 0.69), and GPP was similarly affected. Annual E and GPP of DF49 for the period 1998-2007 varied from 370 to 430 mm and from 1950 to 2390 g C m-2, respectively. After clearcut harvesting, E dropped to about 70% of that for DF49 while ecosystem evapotranspiration was fully recovered when stand age was ∼12 years. This contrasted to GPP, which varied hyperbolically with stand age. Monthly GPP showed a strong positive linear relationship with E irrespective of the stand age. While annual WUE of HDF00 and HDF88 varied with age from 0.5 to 4.1 g C m-2 kg-1 and from 2.8 to 4.4 g C m-2 kg-1, respectively, it was quite conservative at ∼5.3 g C m-2 kg-1 for DF49. N-fertilization had little first-year response on E and WUE. This study not only provides important results for a more detailed validation of process-based models but also helps in predicting the influences of climate change and forest management on water vapour and CO2 fluxes in Douglas-fir forests.
The effects of climatic factors and vegetation type on evapotranspiration (E) and water use efficiency (WUE) were analyzed using tower-based eddy-covariance (EC) data for nine mature forest sites, two peatland sites and one grassland site across an east-west continental-scale transect in Canada during the period 2003-2006. The seasonal pattern of E was closely linked to growing-season length and rainfall distribution. Although annual precipitation (P) during the observation period was highly variable among sites (250-1450 mm), minimum annual E was not less than 200 mm and was limited to 400-500 ram where annual P exceeded 700 mm. Site-specific interannual variability in E could be explained by either changes in total P or variations in solar irradiance. A highly positive linear correlation was found between monthly mean values of E and net radiation (R-n) at the grassland site (AB-GRL), the two peatland sites (AB-WPL and ON-EPL), and only one of the forest sites (coastal Douglas-fir, BC-DF49) whereas a hysteretic relationship at the other forest sites indicated that E lagged behind the typical seasonal progression of R-n. Results of a cross-correlation analysis between daily (24-h) E and R-n revealed that site-specific lag times were between 10 and 40 days depending on the lag of vapour pressure deficit (D) behind R-n and the decoupiing coefficient, Omega. There was significant seasonal variation in daytime mean dry-foliage Priestley-Taylor alpha with maxima occurring in the growing season at all sites except BC-DF49 where it was relatively constant (similar to 0.55) throughout all years. Annual means of daytime dry-foliage alpha mostly ranging between 0.5 and 0.7 implied stomatal limitation to transpiration. Increasing D significantly decreased canopy conductance (g(c)) at the forest sites but had little effect at the peatland and grassland sites, while variation in soil water content caused only minor changes in g(c). At all sites, a strong linear correlation between monthly mean values of gross primary production (GPP) and E resulted in water use efficiency being relatively constant. While at most sites, WUE was in the range of 2.6-3.6 g C kg(-1) H2O, the BC-DF49 site had the highest WUE of the twelve sites with values near 6.0 g C kg(-1) H2O. Of the two peatland sites. AB-WPL, a western treed fen, had a significantly higher WUE (similar to 3.0 g C kg(-1) H2O) than ON-EPL, an eastern ombrotrophic bog (similar to 1.8 g C kg(-1) H2O), which was related to peatland productivity and plant functional type.
The objective of this research was to compare seasonal and annual estimates of CO2 and water vapor exchange across sites in forests, grasslands, crops, and tundra that are part of an international network called FLUXNET, and to investigating the responses of vegetation to environmental variables. FLUXNETs goals are to understand the mechanisms controlling the exchanges of CO2, water vapor and energy across a spectrum of time and space scales, and to provide information for modeling of carbon and water cycling across regions and the globe. At a subset of sites, net carbon uptake (net ecosystem exchange, the net of photosynthesis and respiration) was greater under diffuse than under direct radiation conditions, perhaps because of a more efficient distribution of non-saturating light conditions for photosynthesis, lower vapor pressure deficit limitation to photosynthesis, and lower respiration associated with reduced temperature. The slope of the relation between monthly gross ecosystem production and evapotranspiration was similar between biomes, except for tundra vegetation, showing a strong linkage between carbon gain and water loss integrated over the year (slopes=3.4 g CO2/kg H2O for grasslands, 3.2 for deciduous broadleaf forests, 3.1 for crops, 2.4 for evergreen conifers, and 1.5 for tundra vegetation). The ratio of annual ecosystem respiration to gross photosynthesis averaged 0.83, with lower values for grasslands, presumably because of less investment in respiring plant tissue compared with forests. Ecosystem respiration was weakly correlated with mean annual temperature across biomes, in spite of within site sensitivity over shorter temporal scales. Mean annual temperature and site water balance explained much of the variation in gross photosynthesis. Water availability limits leaf area index over the long-term, and inter-annual climate variability can limit carbon uptake below the potential of the leaf area present.
Heightened awareness of global change issues within both science and political communities has increased interest in using the global network of eddy covariance flux towers to more fully understand the impacts of natural and anthropogenic phenomena on the global carbon balance. Comparisons of net ecosystem exchange (F NEE) responses are being made among biome types, phenology patterns, and stress conditions. The comparisons are usually performed on annual sums of F NEE ; however, the average data coverage during a year is only 65%. Therefore, robust and consistent gap filling methods are required. We review several methods of gap filling and apply them to data sets available from the EUROFLUX and AmeriFlux databases. The methods are based on mean diurnal variation (MDV), look-up tables (LookUp), and nonlinear regressions (Regr.), and the impact of different gap filling methods on the annual sum of F NEE is investigated. The difference between annual F NEE filled by MDV compared to F NEE filled by Regr. ranged from −45 to +200 g C m −2 per year (MDV−Regr.). Comparing LookUp and Regr. methods resulted in a difference (LookUp−Regr.) ranging from −30 to +150 g C m −2 per year. We also investigated the impact of replacing measurements at night, when turbulent mixing is insufficient. The nighttime correction for low friction velocities (u *) shifted annual F NEE on average by +77 g C m −2 per year, but in certain cases as much as +185 g C m −2 per year. Our results emphasize the need to standardize gap filling-methods for improving the comparability of flux data products from regional and global flux networks.
Understanding the variability of plant WUE and its control mechanism can promote the comprehension to the coupling relationship of water and carbon cycle in terrestrial ecosystem, which is the foundation for developing water-carbon coupling cycle model. In this paper, we made clear the differences of net assimilation rate, transpiration rate, and WUE between the two species by comparing the experiment data of soybean (Glycine max Merr.) and maize (Zea mays L.) plants under water and soil nutrient stresses. WUE of maize was about two and a half times more than that of soybean in the same weather conditions. Enhancement of water stresses led to the marked decrease of Am and Em of two species, but water stresses of some degree could improve WUE, and this effect was more obvious for soybean. WUE of the two species changed with psiL in a second-order curve relation, and the WUE at high fertilization was higher than that at low fertilization, this effect was especially obvious for maize. Moreover, according to the synthetic model of photosynthesis-transpiration based on stomatal behavior (SMPTSB) presented by Yu et al. (2001), the WUE model and its applicability were discussed with the data measured in this experiment. The WUE estimated by means of the model accorded well with the measured values. However, this model underestimated the WUE for maize slightly, thus further improvement on the original model was made in this study. Finally, by discussing some physiological factors controlling Am and WUE, we made clear the physiological explanation for differences of the relative contributions of stomata- and mesophyll processes to control of Am and WUE, and the applicability of WUE model between the two species. Because the requirement to stomatal conductance by unit change of net assimilation rate is different, the responses of opening-closing activity of stomata to environmental stresses are different between the two species. To obtain the same level of net assimilation rate, soybean has to open its stomata more widely to keep small stomatal resistance, as compared with maize.
Carbon (C) and water cycles of terrestrial ecosystems are two coupled ecological processes controlled partly by stomatal behavior. Water-use efficiency (WUE) reflects the coupling relationship to some extent. At stand and ecosystem levels, the variability of WUE results from the trade-off between water loss and C gain in the process of plant photosynthetic C assimilation. Continuous observations of C, water, and energy fluxes were made at three selected forest sites of ChinaFLUX with eddy covariance systems from 2003 to 2005. WUE at different temporal scales were defined and calculated with different C and water flux components. Variations in WUE were found among three sites. Average annual WUE was 9.43 mg CO(2) g(-1) H(2)O at Changbaishan temperate broad-leaved Korean pine mixed forest, 9.27 mg CO(2) g(-1) H(2)O at Qianyanzhou subtropical coniferous plantation, and 6.90 mg CO(2) g(-1) H(2)O at Dinghushan subtropical evergreen broad-leaved forest. It was also found that temperate and subtropical forest ecosystems had different relationships between gross primary productivity (GPP) and evapotranspiration (ET). Variations in WUE indicated the difference in the coupling between C and water cycles. The asynchronous response of GPP and ET to climatic variables determined the coupling and decoupling between C and water cycles for the two regional forest ecosystems.