[Show abstract][Hide abstract] ABSTRACT: Interannual variability (IAV, represented by standard deviation) in net ecosystem exchange of CO2 (NEE) is mainly driven by climatic drivers and biotic variations (i.e., the changes in photosynthetic and respiratory responses to climate), the effects of which are referred to as climatic (CE) and biotic effects (BE), respectively. Evaluating the relative contributions of CE and BE to the IAV in carbon (C) fluxes and understanding their controlling mechanisms are critical in projecting ecosystem changes in the future climate. In this study, we applied statistical methods with flux data from 65 sites located in the Northern Hemisphere to address this issue. Our results showed that the relative contribution of BE (CnBE) and CE (CnCE) to the IAV in NEE was 57% ± 14% and 43% ± 14%, respectively. The discrepancy in the CnBE among sites could be largely explained by water balance index (WBI). Across water-stressed ecosystems, the CnBE decreased with increasing aridity (slope = 0.18% mm−1). In addition, the CnBE tended to increase and the uncertainty reduced as timespan of available data increased from 5 to 15 years. Inter-site variation of the IAV in NEE mainly resulted from the IAV in BE (72%) compared to that in CE (37%). Interestingly, positive correlations between BE and CE occurred in grasslands and dry ecosystems (r > 0.45, P < 0.05) but not in other ecosystems. These results highlighted the importance of BE in determining the IAV in NEE and the ability of ecosystems to regulate C fluxes under climate change might decline when the ecosystems experience more severe water stress in the future.
[Show abstract][Hide abstract] ABSTRACT: Gross photosynthesis is a key term and concept in carbon cycle science. It however turns out that this term has been and is used with different meanings by different communities - either with (historically referred to as apparent photosynthesis) or without (historically referred to as true photosynthesis) including photorespiration - which has been causing confusion. Here we review the history of these terms and the underlying theory to clarify the terminology and make recommendations about a consistent use of terms. We further show that eddy covariance CO2 flux partitioning, due to an overestimation of daytime mitochondrial respiration and an underestimation of photorespiration, yields estimates which are quantitatively closer to the definition of true photosynthesis (i.e. carboxylation only) despite aiming at estimating apparent photosynthesis (i.e. carboxylation minus photorespiration). The implications of these findings are discussed.
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Plant Cell and Environment 05/2015; DOI:10.1111/pce.12569 · 6.96 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Considerable amounts and varieties of biogenic volatile organic compounds (BVOCs) are exchanged between vegetation and the surrounding air. These BVOCs play key ecological and atmospheric roles that must be adequately represented for accurately modeling the coupled biosphere-atmosphere-climate earth system. One key uncertainty in existing models is the response of BVOC fluxes to an important global change process: drought. We describe the diurnal and seasonal variation in isoprene, monoterpene, and methanol fluxes from a temperate forest ecosystem before, during, and after an extreme 2012 drought event in the Ozark region of the central USA. BVOC fluxes were dominated by isoprene, which attained high emission rates of up to 35.4 mg m(-2) h(-1) at midday. Methanol fluxes were characterized by net deposition in the morning, changing to a net emission flux through the rest of the daylight hours. Net flux of CO2 reached its seasonal maximum approximately a month earlier than isoprenoid fluxes, which highlights the differential response of photosynthesis and isoprenoid emissions to progressing drought conditions. Nevertheless, both processes were strongly suppressed under extreme drought, although isoprene fluxes remained relatively high compared to reported fluxes from other ecosystems. Methanol exchange was less affected by drought throughout the season, confirming the complex processes driving biogenic methanol fluxes. The fraction of daytime (7-17 h) assimilated carbon released back to the atmosphere combining the three BVOCs measured was 2% of gross primary productivity (GPP) and 4.9% of net ecosystem exchange (NEE) on average for our whole measurement campaign, while exceeding 5% of GPP and 10% of NEE just before the strongest drought phase. The meganv2.1 model correctly predicted diurnal variations in fluxes driven mainly by light and temperature, although further research is needed to address model BVOC fluxes during drought events.
Global Change Biology 05/2015; 21(10). DOI:10.1111/gcb.12980 · 8.04 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The reliable simulation of gross primary productivity (GPP) at various spatial
and temporal scales is of significance to quantifying the net exchange of carbon between
terrestrial ecosystems and the atmosphere. This study aimed to verify the ability of a
nonlinear two-leaf model (TL-LUEn), a linear two-leaf model (TL-LUE), and a big-leaf
light use efficiency model (MOD17) to simulate GPP at half-hourly, daily and 8-day scales
using GPP derived from 58 eddy-covariance flux sites in Asia, Europe and North America
as benchmarks. Model evaluation showed that the overall performance of TL-LUEn was
slightly but not significantly better than TL-LUE at half-hourly and daily scale, while the
overall performance of both TL-LUEn and TL-LUE were significantly better (p < 0.0001)
than MOD17 at the two temporal scales. The improvement of TL-LUEn over TL-LUE was
relatively small in comparison with the improvement of TL-LUE over MOD17. However,
the differences between TL-LUEn and MOD17, and TL-LUE and MOD17 became less
distinct at the 8-day scale. As for different vegetation types, TL-LUEn and TL-LUE
performed better than MOD17 for all vegetation types except crops at the half-hourly scale.
At the daily and 8-day scales, both TL-LUEn and TL-LUE outperformed MOD17 for
forests. However, TL-LUEn had a mixed performance for the three non-forest types while
TL-LUE outperformed MOD17 slightly for all these non-forest types at daily and 8-day
scales. The better performance of TL-LUEn and TL-LUE for forests was mainly achieved
by the correction of the underestimation/overestimation of GPP simulated by MOD17
under low/high solar radiation and sky clearness conditions. TL-LUEn is more applicable
at individual sites at the half-hourly scale while TL-LUE could be regionally used at
half-hourly, daily and 8-day scales. MOD17 is also an applicable option regionally at the
[Show abstract][Hide abstract] ABSTRACT: Particle Investigations at a Northern Ozarks Tower: NOx, Oxidant, Isoprene Research (PINOT NOIR) were conducted in a Missouri forest dominated by isoprene emissions from May to October 2012. This study presents results of new particle formation (NPF) and the growth of new particles to cloud condensation nuclei (CCN)-active sizes (100 nm) observed during this field campaign. The measured sub-5 nm particles were up to 20,000 cm-3 during a typical NPF event. Nucleation rates (Js) were relatively high (11.0 +/-10.6 cm-3 s-1), and one order of magnitude higher than formation rates of 5 nm particles (Js). Sub-5 nm particle formation events were observed during 64% of measurement days, with a high preference in biogenic volatile organic compounds (BVOCs)- and SO2-poor northwesterly (90%) air masses than in BVOCs-rich southerly air masses (13%). About 80% of sub-5 nm particle events led to the further growth. While high temperatures and high aerosol loadings in the southerly air masses were not favorable for nucleation, high BVOCs in the southerly air masses facilitated the growth of new particles to CCN-active sizes. In overall, 0.4–9.4% of the sub-5 nm particles grew to CCN-active sizes within each single NPF event. During a regional NPF event period that took place consecutively over several days, concentrations of CCN size particles increased by a factor of 4.7 in average. This enhanced production of CCN particles from new particles was commonly observed during all 13 regional NPF events during the campaign period.
[Show abstract][Hide abstract] ABSTRACT: Understanding the climatic and biotic controls of interannual variability (IAV) in net ecosystem exchange (NEE) is important for projecting future uptake of CO2 in terrestrial ecosystems. In this study, a statistical modeling approach was used to partition climatic and biotic effects on the IAV in NEE, gross primary productivity (GPP) and ecosystem respiration (RE) at a subtropical evergreen plantation in China (QYZ), a deciduous forest (MOZ), and a grassland (DK1) in the USA. The climatic effects in the study are defined as the interannual anomalies in carbon (C) fluxes directly caused by climatic variations, whereas the biotic effects are those caused by the IAV in photosynthetic and respiratory traits. The results showed that the contribution of biotic effects to the IAV in NEE increased significantly as the temporal scale got longer from daily to annual scales. At the annual scale, the contribution of biotic effects to the IAV in NEE was 47, 69, and 77% at QYZ, MOZ, and DK1, respectively. However, the IAV in NEE was mainly controlled by GPP at QYZ, and by RE at DK1, whereas the contributions of GPP and RE to the IAV in NEE were similar at MOZ, indicating different mechanisms regulating the IAV in NEE among ecosystems. Interestingly, there was a strong negative correlation between the climatic and biotic effects at the annual scale from 2003 to 2009 at QYZ (r 2 = 0.80, P NEE and contributed to our understanding of their underlying mechanisms.
[Show abstract][Hide abstract] ABSTRACT: In C3 plants, CO<sub>2</sub> concentrations drop considerably along mesophyll diffusion pathways from substomatal cavities to chloroplasts where CO<sub>2</sub> assimilation occurs. Global carbon cycle models have not explicitly represented this internal drawdown and therefore overestimate CO<sub>2</sub> available for carboxylation and underestimate photosynthetic responsiveness to atmospheric CO<sub>2</sub>. An explicit consideration of mesophyll diffusion increases the modeled cumulative CO<sub>2</sub> fertilization effect (CFE) for global gross primary production (GPP) from 915 to 1,057 PgC for the period of 1901–2010. This increase represents a 16% correction, which is large enough to explain the persistent overestimation of growth rates of historical atmospheric CO<sub>2</sub> by Earth system models. Without this correction, the CFE for global GPP is underestimated by 0.05 PgC/y/ppm. This finding implies that the contemporary terrestrial biosphere is more CO<sub>2</sub> limited than previously thought.
Proceedings of the National Academy of Sciences 10/2014; 111(44). DOI:10.1073/pnas.1418075111 · 9.67 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Peatlands harbor more than one-third of terrestrial carbon leading to the argument that the bryophytes, as major components of peatland ecosystems, store more organic carbon in soils than any other collective plant taxa. Plants of the genus Sphagnum is an important component of peatland ecosystems and are potentially vulnerable to changing climatic conditions. However, the response of Sphagnum to rising temperatures, elevated CO2 and shifts in local hydrology have yet to be fully characterized. In this review, we examine Sphagnum biology and ecology and explore the role of this keystone species and its associated microbiome in carbon and nitrogen cycling using literature review and model simulations. Several issues are highlighted including the consequences of a variable environment on plant-microbiome interactions, uncertainty associated with CO2 diffusion resistances and the relationship between fixed N and that partitioned to the photosynthetic apparatus. We note that the Sphagnum fallax genome is currently being sequenced and outline potential applications of population-level genomics and corresponding plant photosynthesis and microbial metabolic modeling techniques. We highlight Sphagnum as a model organism to explore ecosystem response to a changing climate, and to define the role that Sphagnum can play at the intersection of physiology, genetics and functional genomics.
[Show abstract][Hide abstract] ABSTRACT: Great uncertainty exists in the global exchange of carbon between the atmosphere and the terrestrial biosphere. An important source of this uncertainty lies in the dependency of photosynthesis on the maximum rate of carboxylation (Vcmax) and the maximum rate of electron transport (Jmax). Understanding and making accurate prediction of C fluxes thus requires accurate characterization of these rates and their relationship with plant nutrient status over large geographic scales. Plant nutrient status is indicated by the traits: leaf nitrogen (N), leaf phosphorus (P), and specific leaf area (SLA). Correlations between Vcmax and Jmax and leaf nitrogen (N) are typically derived from local to global scales, while correlations with leaf phosphorus (P) and specific leaf area (SLA) have typically been derived at a local scale. Thus, there is no global-scale relationship between Vcmax and Jmax and P or SLA limiting the ability of global-scale carbon flux models do not account for P or SLA. We gathered published data from 24 studies to reveal global relationships of Vcmax and Jmax with leaf N, P, and SLA. Vcmax was strongly related to leaf N, and increasing leaf P substantially increased the sensitivity of Vcmax to leaf N. Jmax was strongly related to Vcmax, and neither leaf N, P, or SLA had a substantial impact on the relationship. Although more data are needed to expand the applicability of the relationship, we show leaf P is a globally important determinant of photosynthetic rates. In a model of photosynthesis, we showed that at high leaf N (3 gm−2), increasing leaf P from 0.05 to 0.22 gm−2 nearly doubled assimilation rates. Finally, we show that plants may employ a conservative strategy of Jmax to Vcmax coordination that restricts photoinhibition when carboxylation is limiting at the expense of maximizing photosynthetic rates when light is limiting.
Ecology and Evolution 08/2014; 4(16). DOI:10.1002/ece3.1173 · 2.32 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The exchange of carbon dioxide is a key measure of ecosystem metabolism and a critical intersection between the terrestrial biosphere and the Earth's climate. Despite the general agreement that the terrestrial ecosystems in North America provide a sizeable carbon sink, the size and distribution of the sink remain uncertain. We use a data-driven approach to upscale eddy covariance flux observations from towers to the continental scale by integrating flux observations, meteorology, stand age, aboveground biomass, and a proxy for canopy nitrogen concentrations from AmeriFlux and Fluxnet-Canada Research Network as well as a variety of satellite data streams from the MODIS sensors. We then use the resulting gridded flux estimates from March 2000 to December 2012 to assess the magnitude, distribution, and interannual variability of carbon fluxes for the U.S. and Canada. The mean annual gross primary productivity (GPP), ecosystem respiration (ER), and net ecosystem productivity (NEP) of the U.S. over the period 2001–2012 were 6.84, 5.31, and 1.10 Pg C yr−1, respectively; the mean annual GPP, ER, and NEP of Canada over the same 12-year period were 3.91, 3.26, and 0.60 Pg C yr−1, respectively. The mean nationwide annual NEP of natural ecosystems over the period 2001–2012 was 0.53 Pg C yr−1 for the U.S. and 0.49 Pg C yr−1 for the conterminous U.S. Our estimate of the carbon sink for the conterminous U.S. was almost identical with the estimate of the First State of the Carbon Cycle Report (SOCCR). The carbon fluxes exhibited relatively large interannual variability over the study period. The main sources of the interannual variability in carbon fluxes included drought and disturbance. The annual GPP and NEP were strongly related to annual evapotranspiration (ET) for both the U.S. and Canada, showing that the carbon and water cycles were closely coupled. Our gridded flux estimates provided an independent, alternative perspective on ecosystem carbon exchange over North America.
[Show abstract][Hide abstract] ABSTRACT: Climate feedbacks from soils can result from environmental change followed by response of plant and microbial communities, and/or associated changes in nutrient cycling. Explicit consideration of microbial life-history traits and functions may be necessary to predict climate feedbacks owing to changes in the physiology and community composition of microbes and their associated effect on carbon cycling. Here we developed the microbial enzyme-mediated decomposition (MEND) model by incorporating microbial dormancy and the ability to track multiple isotopes of carbon. We tested two versions of MEND, that is, MEND with dormancy (MEND) and MEND without dormancy (MEND_wod), against long-term (270 days) carbon decomposition data from laboratory incubations of four soils with isotopically labeled substrates. MEND_wod adequately fitted multiple observations (total C-CO2 and (14)C-CO2 respiration, and dissolved organic carbon), but at the cost of significantly underestimating the total microbial biomass. MEND improved estimates of microbial biomass by 20-71% over MEND_wod. We also quantified uncertainties in parameters and model simulations using the Critical Objective Function Index method, which is based on a global stochastic optimization algorithm, as well as model complexity and observational data availability. Together our model extrapolations of the incubation study show that long-term soil incubations with experimental data for multiple carbon pools are conducive to estimate both decomposition and microbial parameters. These efforts should provide essential support to future field- and global-scale simulations, and enable more confident predictions of feedbacks between environmental change and carbon cycling.The ISME Journal advance online publication, 11 July 2014; doi:10.1038/ismej.2014.120.
The ISME Journal 07/2014; 9(1). DOI:10.1038/ismej.2014.120 · 9.30 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The realistic representation of key biogeophysical and biogeochemical functions is the fundamental of process-based ecosystem models. A functional test platform is designed to create direct linkages between site measurements and the process-based ecosystem model within the Community Earth System Models (CESM). The platform consists of three major parts: 1) interactive user interfaces, 2) functional test models and 3) observational datasets. It provides much needed integration interfaces for both field experimentalists and ecosystem modelers to improve the model's representation of ecosystem processes within the CESM framework without large software overhead.
[Show abstract][Hide abstract] ABSTRACT: The widespread invasion of exotic cool-season grasses, such as Kentucky bluegrass and smooth brome, in the mixed-grass rangelands is diminishing the hope of bringing back the natural native plant-dominated communities. A best management strategy for these rangelands, according to several plant community studies, is perhaps to live with these cool-season exotics to some extent while reducing their cover in favor of a diverse plant community. However, there are uncertainties for the future of these invaded rangelands as the ecophysiological mechanisms generally are lacking to explain the relative competitiveness between the native and exotic species. Our research provides a broad comparison of leaf-level photosynthesis among 26 plant species co-occurring in the mixed-grass prairie near Streeter, ND USA. The highlight is on the correlations between photosynthetic potential and plant success, with special reference to Kentucky bluegrass and smooth brome. Photosynthetic potential is defined as a composite variable summarizing the intrinsic limiting factors for photosynthesis as obtained from the net assimilation (A) vs. internal CO2 (Ci) response curves from plants grown under well-watered greenhouse conditions. Plant success was defined as the average frequency measured over 25 years (1988-2012) in the over-flow ecological sites across five levels (no-grazing, moderate, light, heavy and extremely heavy) of grazing intensity. The correlation between photosynthetic potential and plant frequency was negative (n=26 species, p<0.05), suggesting that the two cool-season grasses, Kentucky bluegrass and smooth brome, do not rely on their superior leaf-level photosynthesis for competitive success. Instead, some other traits, such as early and prolonged growth, may be more important for them to gain dominance in the mixed-grass prairie. This is compatible with the management suggestions provided by plant community-based studies for controlling exotic grass dominance and encouraging plant diversity through proper use of early-season grazing and fire.
Cool-Season Invasive Grasses of the Northern Great Plains Workshop, Fargo, North Dakota; 03/2014
[Show abstract][Hide abstract] ABSTRACT: In recent years, temperate bamboo species have been introduced in Europe for multiple uses such as renewable bio-based materials (wood, composites, fibres, biochemicals…) and numerous ecological functions (soil and water conservation, erosion control, phytoremediation…). Despite their interesting potential, little is known on the ecophysiology of these plants in their new habitat. Therefore, we studied gas exchange parameters on a full soil bamboo plantation of Phyllostachys humilis on a test field in Ireland (Europe). We evaluated the seasonal, diurnal and vertical variation of the parameters of two commonly used photosynthetic models, i.e. the light response curve (LRC) model and the model of Farquhar, von Caemmerer and Berry (FvCB). Furthermore, we tested if there were environmental effects on the photosynthetic parameters of these models and if a correlation between photosynthetic parameters and fluorescence parameters was present, fluorescence parameters can be easily and fast determined. Our results show that the gas exchange parameters do not vary diurnally or vertically. Only seasonal variations were found and should, therefore, be taken into account when using the LRC or FvCB model when modelling canopy growth. Therefore, a big-leaf model or a sunlit-shade model can be used for modelling bamboo growth in Western Europe. There is no straightforward relation between environmental variables and the photosynthetic parameters. Although fluorescence parameters showed a correlation with the photosynthetic parameters, application of such correlation may be limited.
Photosynthesis Research 03/2014; 120(3). DOI:10.1007/s11120-014-9992-9 · 3.50 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Dormancy is an essential strategy for microorganisms to cope with environmental stress. However, global ecosystem models typically ignore microbial dormancy, resulting in notable model uncertainties. To facilitate the consideration of dormancy in these large-scale models, we propose a new microbial physiology component that works for a wide range of substrate availabilities. This new model is based on microbial physiological states and the major parameters are the maximum specific growth and maintenance rates of active microbes and the ratio of dormant to active maintenance rates. A major improvement of our model over extant models is that it can explain the low active microbial fractions commonly observed in undisturbed soils. Our new model shows that the exponentially-increasing respiration from substrate-induced respiration experiments can only be used to determine the maximum specific growth rate and initial active microbial biomass, while the respiration data representing both exponentially-increasing and non-exponentially-increasing phases can robustly determine a range of key parameters including the initial total live biomass, initial active fraction, the maximum specific growth and maintenance rates, and the half-saturation constant. Our new model can be incorporated into existing ecosystem models to account for dormancy in microbially-driven processes and to provide improved estimates of microbial activities.
PLoS ONE 02/2014; 9(2):e89252. DOI:10.1371/journal.pone.0089252 · 3.23 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Ecosystem fluxes of isoprene emissions were measured during the majority of the 2011 growing season at the University of Missouri's Baskett Wildlife Research and Education Area in central Missouri, USA (38.7° N, 92.2° W). This broadleaf deciduous forest is typical of forests common in the Ozarks region of the central United States. The goal of the isoprene flux measurements was to test our understanding of the controls on isoprene emission from the hourly to the seasonal timescale using a state-of-the-art emission model, MEGAN (Model of Emissions of Gases and Aerosols from Nature). Isoprene emission rates from the forest were very high with a maximum of 53.3 mg m−2 h−1 (217 nmol m−2 s−1), which to our knowledge exceeds all other reports of canopy-scale isoprene emission. The fluxes showed a clear dependence on the previous temperature and light regimes, which was successfully captured by the existing algorithms in MEGAN. During a period of drought, MEGAN was unable to reproduce the time-dependent response of isoprene emission to water stress. Overall, the performance of MEGAN was robust and could explain 90% of the observed variance in the measured fluxes, but the response of isoprene emission to drought stress is a major source of uncertainty.
[Show abstract][Hide abstract] ABSTRACT: Studies with the variable J method have reported that mesophyll conductance (gm ) rapidly decreases with increasing intercellular CO2 partial pressures (Ci ) or decreasing irradiance. Similar responses have been suggested with the online isotope discrimination method, although with less consistency. Here we show that even when the true gm is constant, the variable J method can produce an artifactual dependence of gm on Ci or irradiance similar to those reported in previous studies for any of the following factors: day respiration and chloroplastic CO2 photocompensation point are estimated with Laisk method; Ci or electron transport rate is positively biased; net photosynthetic rate is negatively biased; insufficient NADPH is assumed while insufficient ATP limits RuBP regeneration. The isotopic method produces similar artifacts if fractionation of carboxylation or Ci are positively biased or Δ(13) negatively biased. A nonzero chloroplastic resistance to CO2 movement results in a qualitatively different dependence of gm on Ci or irradiance and this dependence is only sensitive at low Ci . We thus cannot rule out the possibility that previously reported dependence of gm on Ci or irradiance is a methodological artifact. Recommendations are made to take advantage of sensitivities of the variable J and isotopic methods for estimating gm .
[Show abstract][Hide abstract] ABSTRACT: Worldwide measurements of nearly 130 C3 species covering all major plant functional types are analyzed in conjunction with model simulations to determine the effects of mesophyll conductance (gm ) on photosynthetic parameters and their relationships estimated from A/Ci curves. We find that an assumption of infinite gm results in up to 75% underestimation for maximum carboxylation rate Vcmax , 60% for maximum electron transport rate Jmax , and 40% for triose phosphate utilization rate Tu . Vcmax is most sensitive, Jmax is less sensitive, and Tu has the least sensitivity to the variation of gm . Due to this asymmetrical effect of gm , the ratios of Jmax to Vcmax , Tu to Vcmax , and Tu to Jmax are all overestimated. An infinite gm assumption also limits the freedom of variation of estimated parameters and artificially constrains parameter relationships to stronger shapes. These findings suggest the importance of quantifying gm for understanding in-situ photosynthetic machinery functioning. We show that a nonzero resistance to CO2 movement in chloroplasts has small effects on estimated parameters. A nonlinear function with gm as input is developed to convert the parameters estimated under an assumption of infinite gm to proper values. This function will facilitate gm representation in global carbon cycle models.