Climate‐driven uncertainties in modeling terrestrial gross primary production: a site level to global‐scale analysis

We used a land surface model to quantify the causes and extents of biases in terrestrial gross primary production (GPP) due to the use of meteorological reanalysis datasets. We first calibrated the model using meteorology and eddy covariance data from 25 flux tower sites ranging from the tropics to the northern high latitudes and subsequently repeated the site simulations using two reanalysis datasets: NCEP/NCAR and CRUNCEP. The results show that at most sites, the reanalysis‐driven GPP bias was significantly positive with respect to the observed meteorology‐driven simulations. Notably, the absolute GPP bias was highest at the tropical evergreen tree sites, averaging up to ca. 0.45 kg C m^?2 yr^?1 across sites (ca. 15% of site level GPP). At the northern mid‐/high‐latitude broadleaf deciduous and the needleleaf evergreen tree sites, the corresponding annual GPP biases were up to 20%. For the nontree sites, average annual biases of up to ca. 20–30% were simulated within savanna, grassland, and shrubland vegetation types. At the tree sites, the biases in short‐wave radiation and humidity strongly influenced the GPP biases, while the nontree sites were more affected by biases in factors controlling water stress (precipitation, humidity, and air temperature). In this study, we also discuss the influence of seasonal patterns of meteorological biases on GPP. Finally, using model simulations for the global land surface, we discuss the potential impacts of site‐level reanalysis‐driven biases on the global estimates of GPP. In a broader context, our results can have important consequences on other terrestrial ecosystem fluxes (e.g., net primary production, net ecosystem production, energy/water fluxes) and reservoirs (e.g., soil carbon stocks). In a complementary study (Barman et al., 2013 ), we extend the present analysis for latent and sensible heat fluxes, thus consistently integrating the analysis of climate‐driven uncertainties in carbon, energy, and water fluxes using a single modeling framework.     

Mechanism of cis-prenyltransferase reaction probed by substrate analogues

Undecaprenyl pyrophosphate synthase (UPPS) is a cis-type prenyltransferases which catalyzes condensation reactions of farnesyl diphosphate (FPP) with eight isopentenyl pyrophosphate (IPP) units to generate C₅₅ product. In this study, we used two analogues of FPP, 2-fluoro-FPP and [1,1-²H₂]FPP, to probe the reaction mechanism of Escherichia coli UPPS. The reaction rate of 2-fluoro-FPP with IPP under single-turnover condition is similar to that of FPP, consistent with the mechanism without forming a farnesyl carbocation intermediate. Moreover, the deuterium secondary KIE of 0.985 ± 0.022 measured for UPPS reaction using [1,1-²H₂]FPP supports the associative transition state. Unlike the sequential mechanism used by trans-prenyltransferases, our data demonstrate E. coli UPPS utilizes the concerted mechanism.     

Membrane-bound geranylgeranyl diphosphate phosphatases: purification and characterization from Croton stellatopilosus leaves

Geranylgeranyl diphosphate phosphatase is an enzyme catalyzing the dephosphorylation of geranylgeranyl diphosphate (GGPP) to form geranylgeraniol (GGOH). The enzyme activity of GGPP phosphatase was detected in leaves of Croton stellatopilosus, a Thai medicinal plant containing plaunotol, a commercial anti-peptic acyclic diterpenoid. Enzymological studies of GGPP phosphatase in C. stellatopilosis leaves revealed that the enzyme is a membrane-bound protein that could be removed from 20,000g pellet by 0.1% Triton X-100 without significant loss of enzyme activity. The solubilized enzyme preparation was separated into two activity peaks, PI and PII, by BioGel A gel filtration chromatography. PI and PII were both partially purified and characterized. PI appeared to be a tetrameric enzyme with its native molecular mass of 232kDa and subunit size of 58kDa, whereas PII was a monomeric enzyme with a molecular mass of 30-34kDa. Both phosphatases utilized GGPP as the preferred substrate over farnesyl and geranyl diphosphates. The apparent K(m) values for GGPP of PI and PII appeared to be 0.2 and 0.1mM, respectively. Both activities were Mg(2+) independent and exhibited slightly acidic pH optima, 6.0-6.5 for PI and 6.5-7.0 for PII. The catalytic activities of PII was strongly inhibited by 1.0mM of Zn(2+), Mn(2+) and Co(2+), whereas that of PI was not affected. Both enzyme preparations were very stable upon storage at -20 degrees C for 45 days without significant loss of phosphatase activity. The presence of GGPP phosphatase enzymes in C. stellatopilosus is consistent with its putative involvement in the biosynthetic pathway of plaunotol although whether PI or PII is the actual enzyme involved in the pathway remains to be clarified.     

Modeling gross primary productivity of alpine meadow in the northern Tibet Plateau by using MODIS images and climate data

The Vegetation Photosynthesis Model (VPM) was used to simulate the gross primary productivities (GPP) of the alpine meadow ecosystem in the northern Tibet Plateau at three different spatial resolutions of 0.5 km, 1.5 km and 2.5 km, respectively. The linear relationships between enhanced vegetation indices (EVI) and GPP, with higher correlative coefficients, were better than those between normalized difference vegetation indices (NDVI) and GPP at the three resolutions. VPM could well simulate the seasonal changes and inter-annual variations of GPP, with similar trends at the three resolutions. There were significant differences (P < 0.0001) among the three modeled GPP with the three resolutions. Therefore, the modeled GPP at high resolution could not be directly extrapolated to low resolution, and vice versa. The contribution levels of different model parameters, including photosynthetically active radiation (PAR), air temperature (Ta), NDVI, EVI and land surface water indices (LSWI), to modeled GPP could vary with spatial resolution based on multiple stepwise linear regression analysis. This indicated that it was important to choose parameters properly and consider their effects on modeled GPP.     

Is productivity of mesic savannas light limited or water limited? Results of a simulation study

A soil–plant–atmosphere model was used to estimate gross primary productivity (GPP) and evapotranspiration (ET) of a tropical savanna in Australia. This paper describes model modifications required to simulate the substantial C4 grass understory together with C3 trees. The model was further improved to include a seasonal distribution of leaf area and foliar nitrogen through 10 canopy layers. Model outputs were compared with a 5‐year eddy covariance dataset. Adding the C4 photosynthesis component improved the model efficiency and root‐mean‐squared error (RMSE) for total ecosystem GPP by better emulating annual peaks and troughs in GPP across wet and dry seasons. The C4 photosynthesis component had minimal impact on modelled values of ET. Outputs of GPP from the modified model agreed well with measured values, explaining between 79% and 90% of the variance and having a low RMSE (0.003–0.281 g C m^?2 day^?1). Approximately, 40% of total annual GPP was contributed by C4 grasses. Total (trees and grasses) wet season GPP was approximately 75–80% of total annual GPP. Light‐use efficiency (LUE) was largest for the wet season and smallest in the dry season and C4 LUE was larger than that of the trees. A sensitivity analysis of GPP revealed that daily GPP was most sensitive to changes in leaf area index (LAI) and foliar nitrogen (N_f) and relatively insensitive to changes in maximum carboxylation rate (V_cmax), maximum electron transport rate (J_max) and minimum leaf water potential (ψ_min). The modified model was also able to represent daily and seasonal patterns in ET, (explaining 68–81% of variance) with a low RMSE (0.038–0.19 mm day^?1). Current values of N_f, LAI and other parameters appear to be colimiting for maximizing GPP. By manipulating LAI and soil moisture content inputs, we show that modelled GPP is limited by light interception rather than water availability at this site.     

Subalpine grassland carbon dioxide fluxes indicate substantial carbon losses under increased nitrogen deposition,but not at elevated ozone concentration

Ozone (O₃) and nitrogen (N) deposition affect plant carbon (C) dynamics and may change ecosystem C‐sink/‐source properties. We studied effects of increased background [O₃] (up to [ambient] × 2) and increased N deposition (up to +50 kg ha^?1 a^?1) on mature, subalpine grassland during the third treatment year. During 10 days and 13 nights, distributed evenly over the growth period of 2006, we measured ecosystem‐level CO₂ exchange using a static cuvette. Light dependency of gross primary production (GPP) and temperature dependency of ecosystem respiration rates (R_eco) were established. Soil temperature, soil water content, and solar radiation were monitored. Using R_eco and GPP values, we calculated seasonal net ecosystem production (NEP), based on hourly averages of global radiation and soil temperature. Differences in NEP were compared with differences in soil organic C after 5 years of treatment. The high [O₃] had no effect on aboveground dry matter productivity (DM), but seasonal mean rates of both R_eco and GPP decreased ca. 8%. NEP indicated an unaltered growing season CO₂–C balance. High N treatment, with a +31% increase in DM, mean R_eco increased ca. 3%, but GPP decreased ca. 4%. Consequently, seasonal NEP yielded a 53.9 g C m^?2 (±22.05) C loss compared with control. Independent of treatment, we observed a negative NEP of 146.4 g C m^?2 (±15.3). Carbon loss was likely due to a transient management effect, equivalent to a shift from pasture to hay meadow and a drought effect, specific to the 2006 summer climate. We argue that this resulted from strongly intensified soil microbial respiration, following mitigation of nutrient limitation. There was no interaction between O₃ and N treatments. Thus, during the 2006 growing season, the subalpine grassland lost >2% of total topsoil organic C as respired CO₂, with increased N deposition responsible for one‐third of that loss.     

三江源地区人工草地的生态系统CO2净交换、总初级生产力及其影响因子

为了揭示三江源区垂穗披碱草(Elymus nutans)人工草地生态系统(100°26′-100°41′ E, 34°17′-34°25′ N, 海拔3 980 m)的净生态系统CO₂交换(NEE), 该研究利用2006年涡度相关系统观测的数据分析了该人工草地的NEE, 总初级生产力(GPP)、生态系统呼吸(R_eco)以及R_eco/GPP的变化特征及其影响因子。CO₂日最大吸收值为6.56 g CO₂·m^-2·d^-1, 最大排放值为4.87 g CO₂·m^-2·d^-1。GPP年总量为1 761 g CO₂·m^-2, 其中约90%以上被生态系统呼吸所消耗, CO₂的年吸收量为111 g CO₂·m^-2。5月的R_eco/GPP略高于生长季的其他月份, 为90%; 6月R_eco/GPP比值最低, 为79%。生态系统的呼吸商(Q₁₀)为4.81, 显著高于其他生态系统。该研究表明: 生长季的NEE主要受光量子通量密度(PPFD)、温度和饱和水汽压差(VPD)的影响, 生态系统呼吸则主要受土壤温度的控制。     

Spatial-temporal variability of terrestrial vegetation productivity in the Yangtze River Basin during 2000-2009

Aims Terrestrial net primary production (NPP), the balance of gross primary production (GPP) and autotrophic respiration (AR), is a critical measure of carbon sequestration capacity for the Earth's land surface. The aim of this study was to understand the spatio-temporal variability of NPP associated with GPP and AR in the Yangtze River Basin (YRB), China, from 2000 to 2009 during which the basin warmed significantly.Methods We first derived AR and carbon-use efficiency (CUE) from the improved Moderate Resolution Imaging Spectroradiometer GPP/NPP products (MOD17) and then conducted spatial analysis to quantify how NPP relates to GPP, AR and their relationship with key observed climate variables (temperature, precipitation and sunshine percentage) in the YRB during 2000–2009.Important findings The spatial pattern of NPP in the YRB was predominantly determined by GPP and further modified by AR. Higher GPP and relatively low AR made the southern Jinshajiang sub-basin the most productive area in NPP in the YRB. A large portion of the YRB experienced a warmer and drier climate trend in the growing season during 2000–2009. In the upper reaches of the basin, possessing a relatively low temperature base, increases in temperature led to greater increases in GPP than those in AR, resulting in greater increased NPP. However, in the middle and lower reaches of the basin where the base temperature is relatively high, increases in temperature led to greater increases in AR than those in GPP, leading to decreases in NPP. Overall, 86.7% of the vegetated area showed a consistent GPP and NPP trend through time with 71.3% of the vegetated area having a positive trend both in GPP and NPP, and the remaining 13.3% of vegetated areas showed an opposite trend in GPP and NPP, with positive GPP and negative NPP trajectories dominating (10.1% of vegetated area) the trend. Although climate warming generally had positive effects on vegetation growth in most areas of the basin, areas with increased NPP (74.5%) were less extensive than those with increased GPP (81.4%) due to the wider increase in AR (82.2%). During the study period, increases in AR offset 62% of the total increased GPP, leading to a substantial decline of CUE, particularly in the warmer lower altitude regions in the southeast. Our work reveals the diverse responses of NPP associated with GPP and AR as the climate warms and generally suggests that NPP in the middle and lower sub-basins in the YRB is more sensitive to future climate warming. These findings enhance our understanding of terrestrial ecosystem carbon dynamics in response to global warming and provide a scientific basis for managing ecosystem productivity in the YRB, China.     

Paired comparisons of carbon exchange between undisturbed and regenerating stands in four managed forests in Europe

The effects of harvest on European forest net ecosystem exchange (NEE) of carbon and its photosynthetic and respiratory components (GPP (gross primary production) and TER (total ecosystem respiration)) were examined by comparing four pairs of mature/harvested sites in Europe via a combination of eddy covariance measurements and empirical modeling. Three of the comparisons represented high coniferous forestry (spruce in Britain, and pines in Finland and France), while a coppice‐with‐standard oak plantation was examined in Italy. While every comparison revealed that harvesting converted a mature forest carbon sink into a carbon source of similar magnitude, the mechanisms by which this occurred were very different according to species or management practice. In Britain, Finland, and France the annual sink (source) strength for mature (clear‐cut) stands was estimated at 496 (112), 138 (239), and 222 (225) g C m^?2, respectively, with 381 (427) g C m^?2 for the mature (coppiced) stand in Italy. In all three cases of high forestry in Britain, Finland, and France, clear‐cutting crippled the photosynthetic capacity of the ecosystem – with mature (clear‐cut) GPP of 1970 (988), 1010 (363), and 1600 (602) g C m^?2– and also reduced ecosystem respiration to a lesser degree – TER of 1385 (1100), 839 (603), and 1415 (878) g C m^?2, respectively. By contrast, harvesting of the coppice oak system provoked a burst in respiration – with mature (clear‐cut) TER estimated at 1160 (2220) gC m^?2– which endured for the 3 years sampled postharvest. The harvest disturbance also reduced GPP in the coppice system – with mature (clear‐cut) GPP of 1600 (1420) g C m^?2– but to a lesser extent than in the coniferous forests, and with near‐complete recovery within a few years. Understanding the effects of harvest on the carbon balance of European forest systems is a necessary step towards characterizing carbon exchange for timberlands on large scales.