Ratio Of Gross Primary Production Research Articles

Sustained vegetation greening enhanced ecosystem water-use efficiency in the Loess Plateau of China in recent 20 years

Global vegetation greenness has already been confirmed to increase substantially since the early 1980s, and this phenomenon is extremely heterogeneous in both time and space. The Loess Plateau (LP) serves as an ecological barrier of northern China, where the ecological environment is characterized by exceptional fragility and sensitivity. However, the spatial pattern of vegetation dynamics in both greenness and productivity and their relationship with vegetation ecosystem water-use efficiency (WUE) are still poorly documented, especially in recent 20 years. In this study, the leaf area index (LAI) and gross primary productivity (GPP) were employed to characterize vegetation canopy leaf area and productivity, respectively. The time series data of ecosystem WUE in the LP were obtained by the ratio of GPP and ET during the period of 2000–2018, the dynamic trend of vegetation greenness and productivity in the LP was detected by using a linear fitting and nonlinear method, then, the extent to which the sustained greening in the LP revealed an increase of ecosystem WUE in recent 20 years was quantified. The results showed that sustained vegetation greening and increased GPP elucidated a great improvement in ecosystem WUE in the LP during 2000–2018, in which the sustained greening potentially reveals an increase of ecosystem WUE in >69.33% regions of the LP in recent 20 years. >93% regions of the LP have been experiencing a vegetation greening progress derived by both the annually averaged LAI (LAIAN) and growing season (April to October) maximum LAI (LAIGS). Furthermore, precipitation and soil moisture positively contributed to an interannual increase in ecosystem WUE; however, increasing temperature greatly resulted in a decrease in ecosystem WUE in the LP in recent 20 years. Importantly, this work also implied that ecological water is significantly consumed by the increasing vegetation photosynthesis derived from the significant growth of ET in the LP since the 2000s. These findings implied that the increasing ecological water consumption should be incorporated into indigenized ecosystem process models.

Increased carbon assimilation and efficient water usage may not compensate for carbon loss in European forests

Phenological responses of vegetation to global warming impact ecosystem gross primary production and evapotranspiration. However, high resolution and large spatial scale observational evidence of such responses in undisturbed core forest areas is lacking. Here, we analyse MODIS satellite data to assess monthly trends in gross primary productivity and evapotranspiration across undisturbed core forest areas in Europe between 2000 and 2020. Both parameters increased during the early spring and late autumn in nearly half of the total undisturbed core forest area (3601.5 km2). Enhanced productivity drove increased water-use-efficiency (the ratio of gross primary productivity to evapotranspiration). However, productivity increases during spring and autumn were not sufficient to compensate for summertime decreases in 25% of core forest areas. Overall, 20% of total gross primary productivity across all European forest core areas was offset by forest areas that exhibited a net decrease in productivity.

Ecological impact of polycyclic aromatic hydrocarbons on Baiyangdian Lake based on an ecosystem model

Conventional approaches to assessing the ecological impact of chemicals face uncertainty when extrapolating from single-species toxicity data to community-level effects on real ecosystems. Ecological models may be a useful tool to reduce that uncertainty because they enable a more mechanistic understanding of the effects of chemicals on species assemblages at the ecosystem level. Here, a case study was developed in AQUATOX 3.1 to simulate the effects of the polycyclic aromatic hydrocarbons (PAHs) on Baiyangdian (BYD) Lake in northern China. The model consisted of 12 groups of species: two phytoplankton, one macrophyte, three zooplankton, four benthic macroinvertebrates, one omnivorous fish, and one piscivorous fish. A simulation of a perturbed (or baseline) scenario over 1 year showed satisfactory calibration on annual average biomasses. Compared with a control scenario, the overall average biomass relative variation (V) decreases by 4.02%, the relative variation of eco-exergy (VEx) decreases by 5.64%, and the ratio of gross primary productivity to community respiration (P/R) increases by 5.58%. These results indicate that one-third of the species are remarkably affected, of which the biomass differences are larger than 20%. However, indirect effects due to ecological interactions simulated by the BYD model can either amplify or mitigate the effect of direct toxicity, leading to significant changes for some organisms’ responses to chemicals in real ecosystems. The case study demonstrates that the BYD model has the potential to be an effective and useful tool to evaluate the toxicity effects of organic pollutants at an ecosystem level. Some suggestions are put forward to improve the model's accuracy, including (1) more monitoring and testing data for biological and toxicological parameters, (2) a step-by-step strategy for model calibration, and (3) sensitivity analysis before calibrating the parameters.

Community Composition and Structure Affect Ecosystem and Canopy Water Use Efficiency Across Three Typical Alpine Ecosystems.

Unique ecosystems distributed in alpine areas of the Qinghai–Tibetan Plateau play important roles in climate change mitigation, local food supply, and conservation of species diversity. To understand the water use efficiency (WUE) of this fragile and sensitive region, this study combined observed data from the eddy covariance system and the Shuttleworth–Wallace (S-W) model to measure the continuous mass exchange, including gross primary productivity (GPP), evapotranspiration (ET), and canopy transpiration (T) throughout 2 or 3 years (2016–2018) in three common alpine ecosystems (i.e., alpine steppe, alpine meadow, and alpine swamp). These ecosystems represent a water availability gradient and thus provide the opportunity to quantify environmental and biological controls on WUE at various spatiotemporal scales. We analyzed the ecosystem WUE (WUEe; defined as the ratio of GPP to ET) and canopy WUE (WUEc; defined as the ratio of GPP and canopy T). It was found that the yearly WUEe was 1.40, 1.63, and 2.16 g C kg–1 H2O, and the yearly WUEc was 8.93, 2.46, and 5.19 g C kg–1 H2O in the three typical ecosystems, respectively. The controlling factors of yearly WUE diverged between WUEe and WUEc. We found that plant functional group proportion (e.g., gramineous and Cyperaceae) highly explained the yearly WUEe variation across sites, and a good correlation was observed between community species diversity and WUEc. These findings suggest that community composition and trait change are critical in regulating WUEe and WUEc across different alpine ecosystems and that the regulation mechanisms may differ fundamentally between WUEe and WUEc.

Effects of soil water content on forest ecosystem water use efficiency through changes in transpiration/evapotranspiration ratio

Ecosystem water use efficiency (WUE), defined as the ratio of gross primary production (GPP) over evapotranspiration (ET), is a crucial variable that describes the tradeoff between ecosystem carbon uptake and water loss. However, the effects of soil moisture on WUE remain unclear. Based on hourly observations from 36 forest eddy-covariance sites globally and two percentile-based statistical models, we investigate the effects of soil water supply (i.e., volumetric soil water content, SWC) and vapor pressure deficit (VPD) on carbon-water coupling at the hourly timescale. We further decompose WUE into two components, i.e., the ratio between GPP and plant transpiration (T), WUEt (GPP/T), and the ratio of plant transpiration to total evapotranspiration, T/ET. Results show that the sensitivities of WUE to SWC significantly increased with the increase of VPD. The sensitivities of WUE to SWC are high at high VPD, due to a rapid decrease of T/ET with SWC at high VPD. At low VPD, WUE is largely independent of SWC. The contribution linked to WUEt is much weaker and no significant relationships are observed between the sensitivities of WUEt to SWC and VPD. Our study suggests that the change of ecosystem WUE with SWC is largely due to changes in T/ET, while WUEt remains rather constant.

Ecosystem Productivity and Evapotranspiration Are Tightly Coupled in Loblolly Pine (Pinus taeda L.) Plantations along the Coastal Plain of the Southeastern U.S.

Forest water use efficiency (WUE), the ratio of gross primary productivity (GPP) to evapotranspiration (ET), is an important variable to understand the coupling between water and carbon cycles, and to assess resource use, ecosystem resilience, and commodity production. Here, we determined WUE for managed loblolly pine plantations over the course of a rotation on the coastal plain of North Carolina in the eastern U.S. We found that the forest annual GPP, ET, and WUE increased until age ten, which stabilized thereafter. WUE varied annually (2–44%), being higher at young plantation (YP, 3.12 ± 1.20 g C kg−1 H2O d−1) compared to a mature plantation (MP, 2.92 ± 0.45 g C kg−1 H2O d−1), with no distinct seasonal patterns. Stand age was strongly correlated with ET (R2 = 0.71) and GPP (R2 = 0.64). ET and GPP were tightly coupled (R2 = 0.86). Radiation and air temperature significantly affected GPP and ET (R2 = 0.71 − R2 = 0.82) at a monthly scale, but not WUE. Drought affected WUE (R2 = 0.35) more than ET (R2 = 0.25) or GPP (R2 = 0.07). A drought enhanced GPP in MP (19%) and YP (11%), but reduced ET 7% and 19% in MP and YP, respectively, resulting in a higher WUE (27–32%). Minor seasonal and interannual variation in forest WUE of MP (age > 10) suggested that forest functioning became stable as stands matured. We conclude that carbon and water cycles in loblolly pine plantations are tightly coupled, with different characteristics in different ages and hydrologic regimes. A stable WUE suggests that the pine ecosystem productivity can be readily predicted from ET and vice versa. The tradeoffs between water and carbon cycling should be recognized in forest management to achieve multiple ecosystem services (i.e., water supply and carbon sequestration).

Analysis on water use efficiency of Populus euphratica forest ecosystem in arid area

Water use efficiency (WUE, the ratio of gross primary productivity (GPP) to evapotranspiration (ET)) reflects the coupled relationship between water loss and carbon gain in the process of plant photosynthetic carbon assimilation. As a dominant tree species in arid area, Populus euphratica plays an important ecological role in slowing desertification. Here, continuous observations of carbon, water, and energy fluxes were carried out in Populus euphratica forest with eddy covariance (EC) technique in 2018. We systematically explained the variation characteristics of energy fluxes and WUE at different time scales, and explored the main controlling factors of WUE in drought-stressed environment based on the synchronous meteorological data. Results showed that the carbon exchange of the Populus euphratica forest ecosystem occurred mainly during the growing seasons (April–October). During this period, the entire ecosystem appeared as a carbon sink with the potential to sequester atmospheric carbon dioxide. The average daily WUE was 2.2 g C/kg H2O, which was lower than other temperate forests (2.57–6.07 g C/kg H2O) but higher than grassland, wetland, and cropland. We also concluded that an increase in carbon dioxide concentration (CCO2) and air relative humidity (RH) could promote the increase of WUE. Nevertheless, WUE was negatively correlated with air temperature (Ta), photosynthetically active radiation (PAR), and normalized difference vegetation index (NDVI). Additionally, WUE increased under moderate soil water content (SWC), but decreased due to the continuously rising SWC. WUE was more strongly affected by factors affecting water consumption than carbon uptake. Under the conditions of high temperature, strong radiation and low humidity in the summer, the growth rate of ET was much larger than that of GPP. This study not only contributes to our understanding of the carbon, water, and energy dynamics of the Populus euphratica forest ecosystem but also provides an important reference for ecological conservation and ecological restoration in arid regions.

Net community metabolism of a Posidonia oceanica meadow

AbstractWe report a 12‐yr data set (August 2006–October 2018) of nearly continuous estimates (n = 3275) of gross primary production (GPP), community respiration (CR), and net community production (NCP) in a Posidonia oceanica seagrass meadow, computed from O2 measurements on a mooring at 10 m bottom depth in the Bay of Revellata (Corsica). Both NCP and CR were correlated to GPP and followed the leaf biomass seasonal cycle. The meadow was net autotrophic (NCP of 23 ± 8 mol O2 m−2 yr−1, GPP [83 ± 16 mol O2 m−2 yr−1] > −CR [−60 ± 9 mol O2 m−2 yr−1]), in agreement with oxygen oversaturation (104% at annual scale, 101% in winter, and 109% in summer). Calcification (CAL) and CaCO3 dissolution (DIS) rates were evaluated from dissolved inorganic carbon measurements in benthic chamber incubations (August 2006–2009). The meadow was found to be a net sink of CaCO3 (DIS > CAL) at an annual rate of 7 mol CaCO3 m−2 yr−1 that matched estimates of CaCO3 deposition on the meadow by sedimentation from the water column. CAL from epiphyte coralline algae was correlated to GPP, but CAL : GPP ratio (0.1) was lower than reported for coralline algae in cultures (0.6) due to the additional contribution of Posidonia to GPP. Both NCP and net DIS contributed to an annual CO2 sink of −30 mol CO2 m−2 yr−1 distinctly stronger than the estimated net air‐sea CO2 flux (−1 mol CO2 m−2 yr−1). This suggests that CO2 input by vertical mixing and/or transport by horizontal advection also strongly contribute to the net atmospheric CO2 exchange.

Potential of Sun‐Induced Chlorophyll Fluorescence for Indicating Mangrove Canopy Photosynthesis

AbstractAccurate characterization of gross primary productivity (GPP) is critically important in assessing mangrove carbon budgets, but the current knowledge of the temporal variations of GPP in evergreen mangroves is very limited. Remote sensing of sun‐induced chlorophyll fluorescence (SIF) has emerged as a promising approach to approximating GPP across ecosystems, but its capability for tracking GPP in evergreen mangroves has not been assessed. The SIF‐GPP link at a subtropical mangrove and its environmental controls are explored using 1‐year time‐series measurements from tower‐based hyperspectral and eddy covariance systems. Both the relationship between SIF and GPP as well as that between SIFy (SIF yield: the ratio of SIF over absorbed photosynthetically active radiation [APAR]) and LUE (light use efficiency: the ratio of GPP over APAR) at diurnal and seasonal time scales are analyzed. The temporal variations of SIF and GPP shared overall similar changing patterns, but their functional relationship tended to be time scale‐dependent. Midday depressions in SIF were observed when environmental stresses occurred around noon (including excess light and high VPD), and the strength of the SIF‐GPP link was affected by changing environmental conditions. The SIFy‐LUE relationship was temporally more dynamic, tending to match during midday hours but diverge from each other during morning and afternoon hours. These findings confirm SIF can serve as a potential remotely sensed indicator of mangrove canopy photosynthesis. This paper provides the first, high temporal‐resolution, continuous SIF measurements in mangroves, and highlights the importance of the impacts of environmental conditions on the SIF‐GPP relationship.

Water use efficiency in terrestrial ecosystem over East Asia: Effects of climate regimes and land cover types

Water use efficiency (WUE) is an environmental factor to account for the metabolism of terrestrial ecosystems using various climate systems and vegetation types. It is estimated by the ratio of gross primary productivity (GPP) to evapotranspiration (ET), the largest carbon and water fluxes with respect to plant respiration. In this study, the WUE was calculated using GPP and ET from the community land model version 4.0 (CLM4.0), inclusive of the prognostic carbon-nitrogen model in the community earth system model (CESM). The estimated WUE in East Asia was analyzed for climate zones, land cover types, and water- and energy-limited zones, with aridity index (AI). Spatial variations from 2001 to 2015 in annual WUE gradually increased as latitude decreased, though small year-to-year differences appeared between monthly GPP and ET. Monthly WUE was lower in summer than fall because the water loss rate in summer was higher than the carbon assimilation increase. The WUE under arid conditions (AI<0.5) was lower than under humid conditions. The GPP, ET, and WUE were higher in the forest, savannas, cropland, and permanent wetland with dense vegetation or abundant water resources than in other land cover types. The WUE was lower in water-limited zones than in energy-limited zones due to the low amount of water to use for the physical processes of GPP and ET. Based on this study, we identified general spatial and temporal variations of carbon fluxes in East Asia with various climate zones and land cover types.

Seasonality of phytoplankton cell size and the relation between photosynthesis and respiration in the Ría de Vigo (NW Spain)

The Ría de Vigo is a dynamic and productive upwelling ecosystem. We measured 14C incorporation (TO14CP) and gross primary production (GPP), community respiration (DCR), net production (NCP) and size-fractioned chlorophyll a (chl a) fortnightly from May 2012 to May 2013 in the euphotic layer of the Ría. Our aim was to improve the depiction of plankton metabolism in the Ría and to test the general hypothesis that community structure determines the degree of heterotrophy in planktonic ecosystems. Higher primary production was measured after upwelling episodes and during the spring bloom, when the community was dominated by microphytoplankton (&gt;70% chl a&gt;20μm). Lower primary production was observed during summer stratification periods (~65% chl a&gt;20μm), and during the pico- and nanophytoplankton-dominated winter (~25% chl a&gt;20μm). Coupling between phytoplankton photosynthesis and biomass varied seasonally, mainly driven by environmental conditions. DCR was 3 times lower and 8 times less variable than GPP, and its variability was mainly driven by the changes in chl a. The integrated metabolic balance was autotrophic most of the year, despite the negative NCP rates at depth. There was an inverse relationship between the DCR:GPP ratio and the percentage of microphytoplankton (% chl a&gt;20μm) only in the summer. However, DCR:GPP and DCR:chl a ratios were similar in winter and spring, despite the seasonal differences in primary production and size structure. The similar TO14CP:NCP and chl a:DCR relations in spring (&gt;70% chl a&gt;20μm) and winter (~25% chl a&gt;20μm), and the differences in summer (&gt;70% chl a&gt;20μm) confirm cell size independence in trophic functioning. We conclude that respiration variability is relevant for the metabolic balance in the Ría, and that the degree of heterotrophy is not systematically related to phytoplankton size over the scales of our study.

Effects of Drought on the Relationship Between Photosynthesis and Chlorophyll Fluorescence for Maize

Structural–physiological factors affect the accurate estimation of vegetation gross primary production (GPP) under various types of environmental stress. Solar-induced chlorophyll fluorescence (SIF), which is directly linked to photosynthesis, has been effectively used to estimate and monitor GPP. However, understanding of the physiological mechanism linking SIF to GPP under stress remains limited. In this article, the link between SIF and GPP at diurnal and seasonal timescales was explored for a maize field and its response to drought stress (as defined by the crop water stress index, CWSI) using three-years of continuous tower-based measurements was investigated. The results show that the ratio of GPP to SIF decreased with increasing drought stress levels, and the canopy stomata conductance (Gs) declined synchronously. Compared to two canopy structural factors (NDVI and NIRv), both the Pearson and partial correlation coefficients for the relationship between Gs and the ratio of GPP to the total SIF was higher (0.38, p ${{\bf \Phi }_F}$ tracked the changes in LUE well under drought conditions (CWSI > 0.6), which demonstrated that SIF can be a powerful parameter for estimating GPP under drought stress. However, there was a smaller drop in ${{\bf \Phi }_F}$ under drought stress (slope = –0.002) compared to the slope for the relationship between LUE and CWSI (–0.08). The response of light reactions to drought stress may be muted compared to the stomatal response. These findings confirm that the Gs is sensitive to drought and is important for the SIF-based GPP estimation model. It also provides reliable evidence that SIF data include a large amount of physiological information and can serve as a potential indicator for detecting drought and estimating GPP.

Shifts in ecosystem water use efficiency on china's loess plateau caused by the interaction of climatic and biotic factors over 1985–2015

Ecosystem water use efficiency (WUE), defined as the ratio of gross primary productivity (GPP) to total ecosystem evapotranspiration (ET), is an important indicator of the coupling between the terrestrial carbon and water cycles. China's Loess Plateau (CLP) has been experiencing climatic warming in recent decades and large-scale revegetation since the early 2000s. Understanding the combined effects of revegetation and climate change on ecosystem WUE patterns is important for a better prediction of future changes of the water and carbon cycles in the region. In this study, we decompose ecosystem WUE into a two-component process, i.e., the ratio of carbon uptake to plant transpiration, GPP/T, and the ratio of plant transpiration to total evapotranspiration, T/ET. Based on this, we investigate the temporal variations of ecosystem WUE and examine the underlying controls of WUE dynamics on CLP during 1985–2015. We find that ecosystem WUE on CLP remains more or less unchanged during 1985–1999 as a result of an increased T/ET that is largely offset by a decreased GPP/T induced by increases in atmospheric vapor pressure deficit. In comparison, ecosystem WUE significantly increases during 2000–2015, owing to the increased leaf area index, which promotes the fraction of plant transpiration over total ET (i.e., T/ET). In facing the upcoming peak of greenness and continuing climate warming, our results suggest that CLP's ecosystem WUE may experience a downward trend in the future due to diminishing positive effects of leaf area index but increasing negative effects of atmospheric vapor pressure deficit. Our study highlights the importance of decomposing ecosystem WUE into component processes to better understand the mechanisms that underlying the changes of ecosystem WUE and to predict future ecosystem WUE dynamics.

Integrating SIF and Clearness Index to Improve Maize GPP Estimation Using Continuous Tower-Based Observations.

Solar-induced chlorophyll fluorescence (SIF) has been proven to be well correlated with vegetation photosynthesis. Although multiple studies have found that SIF demonstrates a strong correlation with gross primary production (GPP), SIF-based GPP estimation at different temporal scales has not been well explored. In this study, we aimed to investigate the quality of GPP estimates produced using the far-red SIF retrieved at 760 nm (SIF760) based on continuous tower-based observations of a maize field made during 2017 and 2018, and to explore the responses of GPP and SIF to different meteorological conditions, such as the amount of photosynthetically active radiation (PAR), the clearness index (CI, representing the weather condition), the air temperature (AT), and the vapor pressure deficit (VPD). Firstly, our results showed that the SIF760 tracked GPP well at both diurnal and seasonal scales, and that SIF760 was more linearly correlated to PAR than GPP was. Therefore, the SIF760–GPP relationship was clearly a hyperbolic relationship. For instantaneous observations made within a period of half an hour, the R2 value was 0.66 in 2017 and 2018. Based on daily mean observations, the R2 value was 0.82 and 0.76 in 2017 and 2018, respectively. Secondly, it was found that the SIF760–GPP relationship varied with the environmental conditions, with the CI being the dominant factor. At both diurnal and seasonal scales, the ratio of GPP to SIF760 decreased noticeably as the CI increased. Finally, the SIF760-based GPP models with and without the inclusion of CI were trained using 70% of daily observations from 2017 and 2018 and the models were validated using the remaining 30% of the dataset. For both linear and non-linear models, the inclusion of the CI greatly improved the SIF760-based GPP estimates based on daily mean observations: the value of R2 increased from 0.71 to 0.82 for the linear model and from 0.82 to 0.87 for the non-linear model. The validation results confirmed that the SIF760-based GPP estimation was improved greatly by including the CI, giving a higher R2 and a lower RMSE. These values improved from R2 = 0.66 and RMSE = 7.02 mw/m2/nm/sr to R2 = 0.76 and RMSE = 6.36 mw/m2/nm/sr for the linear model, and from R2 = 0.71 and RMSE = 4.76 mw/m2/nm/sr to R2 = 0.78 and RMSE = 3.50 mw/m2/nm/sr for the non-linear model. Therefore, our results demonstrated that SIF760 is a reliable proxy for GPP and that SIF760-based GPP estimation can be greatly improved by integrating the CI with SIF760. These findings will be useful in the remote sensing of vegetation GPP using satellite, airborne, and tower-based SIF data because the CI is usually an easily accessible meteorological variable.

Drought can offset potential water use efficiency of forest ecosystems from rising atmospheric CO2

Increasing atmospheric CO2 is both leading to climate change and providing a potential fertilisation effect on plant growth. However, southern Australia has also experienced a significant decline in rainfall over the last 30 years, resulting in increased vegetative water stress. To better understand the dynamics and responses of Australian forest ecosystems to drought and elevated CO2, the magnitude and trend in water use efficiency (WUE) of forests, and their response to drought and elevated CO2 from 1982 to 2014 were analysed, using the best available model estimates constrained by observed fluxes from simulations with fixed and time-varying CO2. The ratio of gross primary productivity (GPP) to evapotranspiration (ET) (WUEe) was used to identify the ecosystem scale WUE, while the ratio of GPP to transpiration (Tr) (WUEc) was used as a measure of canopy scale WUE. WUE increased significantly in northern Australia (p < 0.001) for woody savannas (WSA), whereas there was a slight decline in the WUE of evergreen broadleaf forests (EBF) in the southeast and southwest of Australia. The lag of WUEc to drought was consistent and relatively short and stable between biomes (≤3 months), but notably varied for WUEe, with a long time-lag (mean of 10 months). The dissimilar responses of WUEe and WUEc to climate change for different geographical areas result from the different proportion of Tr in ET. CO2 fertilization and a wetter climate enhanced WUE in northern Australia, whereas drought offset the CO2 fertilization effect in southern Australia.

Divergent spatial responses of plant and ecosystem water-use efficiency to climate and vegetation gradients in the Chinese Loess Plateau

Abstract Water-use efficiency (WUE) is an important indicator of carbon–water interactions and is defined as the ratio of vegetation productivity to water loss. However, how WUE varies along climate and vegetation gradients at the regional scale remains elusive. In this study, we investigated the spatial patterns of plant-canopy WUE (PWUE, i.e., ratio of gross primary productivity to plant transpiration) and ecosystem WUE (EWUE, i.e., ratio of gross primary productivity to evapotranspiration) in the Chinese Loess Plateau (CLP), which has seen large changes in the biosphere-atmosphere carbon and water cycles due to large-scale revegetation with the CLP. Spatial responses of PWUE and EWUE variations to the mean annual precipitation (MAP), mean annual air temperature (MAT), and normalized difference vegetation index (NDVI) gradients were examined based on remote-sensing and geostatistical model-based datasets. Results showed that mean EWUE estimated from two approaches was 1.26 ± 0.28 and 1.37 ± 0.68 g C kg−1 H2O, respectively, lower than the mean PWUE (3.16 ± 0.71 g C kg−1 H2O) across the CLP. EWUE and PWUE estimates showed similar spatial distributions, generally with higher values in the areas with more water available. Precipitation sensitivities of EWUE and PWUE appeared to be positive except the very cold regions, and gradually decreased with increasing MAT in the forest-steppe and forest zones. Spatial variation in EWUE is intrinsically affected by both of PWUE and ecosystem water allocation (i.e., ratio of transpiration to evapotranspiration), and NDVI sensitivity of EWUE is dominant by ecosystem water allocation, leading to postive sensitivity of EWUE to NDVI for most MAP range in the CLP. PWUE variation depends on the geographic patterns of vegetation communities determined by precipitation pattern, leading to relatively lower and stable NDVI sensitivity than EWUE along the MAP gradient in the CLP. Our study revealed the divergent spatial responses of WUE to climate and vegetation gradients at the plant-canopy and ecosystem levels, which could enhance our understanding on the regional-scale carbon-water relationships across multiple organismic levels, and provide essential information for the WUE upscaling and modeling efforts.

Revegetation has increased ecosystem water-use efficiency during 2000–2014 in the Chinese Loess Plateau: Evidence from satellite data

Vegetation coverage has been effectively increased across the Chinese Loess Plateau (CLP) mainly because of the implementation of large-scale revegetation programs. Understanding the entire CLP’s ecosystem carbon-water relationship and its response to the vegetation coverage increase and climate change is essential for the water resources management in this semi-humid to arid region. Water-use efficiency (WUE), defined as the ratio of gross primary productivity (GPP) over evapotranspiration (ET), is an important indicator linking carbon and water cycles at the ecosystem scale. In this study, the spatial and temporal variations in the ecosystem WUE were analyzed to investigate the responses of WUE variation to regional climate and vegetation coverage changes in the CLP using MODerate Resolution Imaging Spectroradiometer (MODIS) datasets of GPP and ET from 2000 to 2014. Results indicate that the spatial pattern of annual WUE was different from that of WUE over active growing season (AGS), generally with higher annual WUE but lower AGS-WUE for the semi-humid southeastern CLP areas. Significant increases were observed in annual and autumn WUE during the study period at both the per-pixel and spatial-average levels, while AGS-WUE, spring WUE, and summer WUE remained quite stable. Correlations between WUE trends and interannual variability of vegetation coverage (expressed as Normalized Difference Vegetation Index or NDVI), precipitation (P), and air temperature (Ta) showed that WUE trends were positively correlated with NDVI variation due to faster increases in GPP as compared to ET caused by enhanced NDVI. Precipitation variation was generally negatively correlated to WUE variation by contributing to faster increases in ET than GPP especially at different seasons. The positive correlations between interannual variability of WUE and Ta were subject to negative feedbacks between ET and Ta. Similar results were also found using net primary productivity (NPP) to calculate ecosystem annual WUE (i.e., NPP/ET). Our findings suggest that revegetation-induced vegetation coverage change rather than P and Ta changes is the primary cause for the increasing ecosystem annual WUE in the CLP. This study contributes to our enhanced understanding of the coupled carbon-water dynamics over regional terrestrial ecosystem under climate and vegetation coverage changes.

Testing of Automated Photochemical Reflectance Index Sensors as Proxy Measurements of Light Use Efficiency in an Aspen Forest.

Commercially available autonomous photochemical reflectance index (PRI) sensors are a new development in the remote sensing field that offer novel opportunities for a deeper exploration of vegetation physiology dynamics. In this study, we evaluated the reliability of autonomous PRI sensors (SRS-PRI) developed by METER Group Inc. as proxies of light use efficiency (LUE) in an aspen (Populus tremuloides) forest stand. Before comparisons between PRI and LUE measurements were made, the optical SRS-PRI sensor pairs required calibrations to resolve diurnal and seasonal patterns properly. An offline diurnal calibration procedure was shown to account for variable sky conditions and diurnal illumination changes affecting sensor response. Eddy covariance measurements provided seasonal gross primary productivity (GPP) measures as well as apparent canopy quantum yield dynamics (α). LUE was derived from the ratio of GPP to absorbed photosynthetically active radiation (APAR). Corrected PRI values were derived after diurnal and midday cross-calibration of the sensor’s 532 nm and 570 nm fore-optics, and closely related to both LUE (R2 = 0.62, p < 0.05) and α (R2 = 0.72, p < 0.05). A LUE model derived from corrected PRI values showed good correlation to measured GPP (R2 = 0.77, p < 0.05), with an accuracy comparable to results obtained from an α driven LUE model (R2 = 0.79, p < 0.05). The automated PRI sensors proved to be suitable proxies of light use efficiency. The onset of continuous PRI sensors signifies new opportunities for explicitly examining the cause of changing PRI, LUE, and productivity over time and space. As such, this technology represents great value for the flux, remote sensing and modeling community.

GOLUM-CNP v1.0: a data-driven modeling of carbon, nitrogen and phosphorus cycles in major terrestrial biomes

Abstract. Global terrestrial nitrogen (N) and phosphorus (P) cycles are coupled to the global carbon (C) cycle for net primary production (NPP), plant C allocation, and decomposition of soil organic matter, but N and P have distinct pathways of inputs and losses. Current C-nutrient models exhibit large uncertainties in their estimates of pool sizes, fluxes, and turnover rates of nutrients, due to a lack of consistent global data for evaluating the models. In this study, we present a new model–data fusion framework called the Global Observation-based Land-ecosystems Utilization Model of Carbon, Nitrogen and Phosphorus (GOLUM-CNP) that combines the CARbon DAta MOdel fraMework (CARDAMOM) data-constrained C-cycle analysis with spatially explicit data-driven estimates of N and P inputs and losses and with observed stoichiometric ratios. We calculated the steady-state N- and P-pool sizes and fluxes globally for large biomes. Our study showed that new N inputs from biological fixation and deposition supplied &gt;20 % of total plant uptake in most forest ecosystems but accounted for smaller fractions in boreal forests and grasslands. New P inputs from atmospheric deposition and rock weathering supplied a much smaller fraction of total plant uptake than new N inputs, indicating the importance of internal P recycling within ecosystems to support plant growth. Nutrient-use efficiency, defined as the ratio of gross primary production (GPP) to plant nutrient uptake, were diagnosed from our model results and compared between biomes. Tropical forests had the lowest N-use efficiency and the highest P-use efficiency of the forest biomes. An analysis of sensitivity and uncertainty indicated that the NPP-allocation fractions to leaves, roots, and wood contributed the most to the uncertainties in the estimates of nutrient-use efficiencies. Correcting for biases in NPP-allocation fractions produced more plausible gradients of N- and P-use efficiencies from tropical to boreal ecosystems and highlighted the critical role of accurate measurements of C allocation for understanding the N and P cycles.

Disentangling Climate and LAI Effects on Seasonal Variability in Water Use Efficiency Across Terrestrial Ecosystems in China

AbstractWater use efficiency (WUE), the ratio of gross primary productivity (GPP) over evapotranspiration (ET), is a critical ecosystem function. However, it is difficult to distinguish the individual effects of climatic variables and leaf area index (LAI) on WUE, mainly due to the high collinearity among these factors. Here we proposed a partial least squares regression‐based sensitivity algorithm to confront the issue, which was first verified at seven ChinaFlux sites and then applied across China. The results showed that across all biomes in China, monthly GPP (0.42–0.65), ET (0.33–0.56), and WUE (0.01–0.31) showed positive sensitivities to air temperature, particularly in croplands in northeast China and forests in southwest China. Radiation exerted stronger effects on ET (0.55–0.78) than GPP (0.19–0.65), resulting in negative responses (−0.44 to 0.04) of WUE to increased radiation among most biomes. Increasing precipitation stimulated both GPP (0.06–0.17) and ET (0.05–0.12) at the biome level, but spatially negative effects of excessive precipitation were also found in some grasslands. Both monthly GPP (−0.01 to 0.29) and ET (0.02–0.12) showed weak or moderate responses to vapor pressure deficit among biomes, resulting in weak response of monthly WUE to vapor pressure deficit (−0.04 to 0.08). LAI showed positive effects on GPP (0.18–0.60), ET (0–0.23), and WUE (0.13–0.42) across biomes, particularly on WUE in grasslands (0.42 ± 0.30). Our results highlighted the importance of LAI in influencing WUE against climatic variables. Furthermore, the sensitivity algorithm can be used to inform the design of manipulative experiments and compare with factorial simulations for discerning effects of various variables on ecosystem functions.