Timothy J. Griffis

Global and time-resolved monitoring of crop photosynthesis with chlorophyll fluorescence.

Significance Global food and biofuel production and their vulnerability in a changing climate are of paramount societal importance. However, current global model predictions of crop photosynthesis are highly uncertain. Here we demonstrate that new space-based observations of chlorophyll fluorescence, an emission intrinsically linked to plant biochemistry, enable an accurate, global, and time-resolved measurement of crop photosynthesis, which is not possible from any other remote vegetation measurement. Our results show that chlorophyll fluorescence data can be used as a unique benchmark to improve our global models, thus providing more reliable projections of agricultural productivity and climate impact on crop yields. The enormous increase of the observational capabilities for fluorescence in the very near future strengthens the relevance of this study. Photosynthesis is the process by which plants harvest sunlight to produce sugars from carbon dioxide and water. It is the primary source of energy for all life on Earth; hence it is important to understand how this process responds to climate change and human impact. However, model-based estimates of gross primary production (GPP, output from photosynthesis) are highly uncertain, in particular over heavily managed agricultural areas. Recent advances in spectroscopy enable the space-based monitoring of sun-induced chlorophyll fluorescence (SIF) from terrestrial plants. Here we demonstrate that spaceborne SIF retrievals provide a direct measure of the GPP of cropland and grassland ecosystems. Such a strong link with crop photosynthesis is not evident for traditional remotely sensed vegetation indices, nor for more complex carbon cycle models. We use SIF observations to provide a global perspective on agricultural productivity. Our SIF-based crop GPP estimates are 50–75% higher than results from state-of-the-art carbon cycle models over, for example, the US Corn Belt and the Indo-Gangetic Plain, implying that current models severely underestimate the role of management. Our results indicate that SIF data can help us improve our global models for more accurate projections of agricultural productivity and climate impact on crop yields. Extension of our approach to other ecosystems, along with increased observational capabilities for SIF in the near future, holds the prospect of reducing uncertainties in the modeling of the current and future carbon cycle.

Ecophysiological controls on the carbon balances of three southern boreal forests

Continuous measurements of carbon exchange using the eddy covariance (EC) technique were made at three boreal forest mature sites including Southern Old Aspen (SOA), Southern Old Black Spruce (SOBS) and Southern Old Jack Pine (SOJP) in 2000. Climatic conditions were slightly warmer than normal with precipitation exceeding evapotranspiration at each site. Annual ecosystem respiration (R) derived from daytime analyses of EC data was 1141, 815 and 52 1gCm −2 per year and was consistently lower than nighttime EC estimates of 1193, 897 and 57 8gCm −2 per year for SOA, SOBS and SOJP, respectively. The differences, however, were not statistically significant given the large uncertainty associated with each analytical technique. The uncertainty in annual net ecosystem productivity (NEP) was assessed by randomly simulating missing data and gap filling using simple biophysical algorithms. The uncertainty analysis supports the finding that each site was a net sink, and that differences in NEP were only significant between SOA and SOBS. The annual NEP and uncertainty for SOA, SOBS and SOJP was 122 (64–142), 35 (18–53) and 78 (61–91 )gCm −2 per year, respectively. These relatively old growth forests represent a weak to moderate carbon sink. Despite having the shortest growing period, carbon sequestration was greatest at SOA because of its relatively large photosynthetic capacity ( Amax). At the evergreen sites, Amax was marginally larger at SOBS; however, annual carbon sequestration was smaller as a result of greater R. The evergreen sites exhibited a pronounced mid-season reduction in NEP, which was attributed to a large increase in R while Amax had not reached its full capacity. Non-growing season R resulted in a carbon loss of 285, 120 and 6 4gCm −2 and accounted for 70, 80 and 46% of the summertime NEP at SOA, SOBS and SOJP, respectively. Six years of EC data at SOA indicate that carbon sequestration at boreal aspen sites may benefit from warmer climatic conditions because R is relatively conservative and photosynthesis increases in response to a longer growing period.

Measuring forest floor CO2 fluxes in a Douglas-fir forest

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

Response of Net Ecosystem Productivity of Three Boreal Forest Stands to Drought

In 2001–03, continuous eddy covariance measurements of carbon dioxide (CO2) flux were made above mature boreal aspen, black spruce, and jack pine forests in Saskatchewan, Canada, prior to and during a 3−year drought. During the 1st drought year, ecosystem respiration (R) was reduced at the aspen site due to the drying of surface soil layers. Gross ecosystem photosynthesis (GEP) increased as a result of a warm spring and a slow decrease of deep soil moisture. These conditions resulted in the highest annual net ecosystem productivity (NEP) in the 9 years of flux measurements at this site. During 2002 and 2003, a reduction of 6% and 34% in NEP, respectively, compared to 2000 was observed as the result of reductions in both R and GEP, indicating a conservative response to the drought. Although the drought affected most of western Canada, there was considerable spatial variability in summer rainfall over the 100−km extent of the study area; summer rainfalls in 2001 and 2002 at the two conifer sites minimized the impact of the drought. In 2003, however, precipitation was similarly low at all three sites. Due to low topographic position and consequent poor drainage at the black spruce site and the coarse soil with low water-holding capacity at the jack pine site almost no reduction in R, GEP, and NEP was observed at these two sites. This study shows that the impact of drought on carbon sequestration by boreal forest ecosystems strongly depends on rainfall distribution, soil characteristics, topography, and the presence of vegetation that is well adapted to these conditions.

Interannual variability of net ecosystem CO2 exchange at a subarctic fen

Landscape-scale net ecosystem CO 2 exchange (NEE) and the energy balance of a subarctic fen were studied during five growing seasons near Churchill, Manitoba. Interannual variability in NEE was large and ranged from a net sink of -235 g CO 2 m -2 in 1996 to a net source of +76 g CO 2 m -2 in 1994. Annual estimates of CO 2 exchange indicate that during the present period the fen is losing carbon nearly 3 times faster than its long-term historical gain of about 11 g CO 2 m -2 yr -1 . Our estimates suggest that gross ecosystem photosynthesis may be more variable than ecosystem respiration on diurnal, seasonal, and interannual timescales. Our data strongly indicate that an early snowmelt combined with wet and warm conditions during the spring period lead to large carbon acquisition even when drier conditions were experienced over the majority of the growing season. The phenological stage of the vegetation relative to the climatic conditions experienced is an important cause of the interannual variability in NEE. An accurate representation of phenology in climate models is, therefore, critical to the success of forecasting the carbon budgets of northern wetlands.

δ18O of water vapour, evapotranspiration and the sites of leaf water evaporation in a soybean canopy

Stable isotopes in water have the potential to diagnose changes in the earths hydrological budget in response to climate change and land use change. However, there have been few measurements in the vapour phase. Here, we present high-frequency measurements of oxygen isotopic compositions of water vapour (delta(v)) and evapotranspiration (delta(ET)) above a soybean canopy using the tunable diode laser (TDL) technique for the entire 2006 growing season in Minnesota, USA. We observed a large variability in surface delta(v) from the daily to the seasonal timescales, largely explained by Rayleigh processes, but also influenced by vertical atmospheric mixing, local evapotranspiration (ET) and dew formation. We used delta(ET) measurements to calculate the isotopic composition at the sites of evaporative enrichment in leaves (delta(L,e)) and compared that with the commonly used steady-state prediction (delta(L,s)). There was generally a good agreement averaged over the season, but larger differences on individual days. We also found that vertical variability in relative humidity and temperature associated with canopy structure must be addressed in canopy-scale leaf water models. Finally, we explored this data set for direct evidence of the Péclet effect.

Northern Canadian wetlands : Net ecosystem CO2 exchange and climatic change

Northern Canadian peatlands represent a long term sink for atmospheric carbon dioxide (CO2), however there is concern they may become a net source of CO2 due to climatic change. Climatic change is expected to result in significant changes in regional hydrology in boreal and subarctic regions of Canada. A hydrologic model predicted a summer water table drop of 0.14 m in northern Canadian fens given an increase in summer temperature and rainfall of 3°C and 1 mm d-1, respectively. Moreover, surface peat temperature increased by 2.3°C. Net ecosystem exchange of CO2 was modelled using these modelled hydrologic and thermal changes with respiration:peat temperature and water table:net ecosystem production relationships developed from measurements at wetlands in northern Sweden and near Churchill, Manitoba. Model results indicate that the net atmospheric CO2 sink function of fens may be enhanced under future 2 × CO2 scenarios, while bogs may become a net source of atmospheric CO2. If the net ecosystem productivity response to the new hydrologic conditions was ignored then the model predicts a decrease in summer carbon storage for all peatland types.

Identification and correction of spectral contamination in 2H/1H and 18O/16O measured in leaf, stem, and soil water.

Plant water extracts typically contain organic materials that may cause spectral interference when using isotope ratio infrared spectroscopy (IRIS), resulting in errors in the measured isotope ratios. Manufacturers of IRIS instruments have developed post-processing software to identify the degree of contamination in water samples, and potentially correct the isotope ratios of water with known contaminants. Here, the correction method proposed by an IRIS manufacturer, Los Gatos Research, Inc., was employed and the results were compared with those obtained from isotope ratio mass spectrometry (IRMS). Deionized water was spiked with methanol and ethanol to create correction curves for δ(18)O and δ(2)H. The contamination effects of different sample types (leaf, stem, soil) and different species from agricultural fields, grasslands, and forests were compared. The average corrections in leaf samples ranged from 0.35 to 15.73‰ for δ(2)H and 0.28 to 9.27‰ for δ(18)O. The average corrections in stem samples ranged from 1.17 to 13.70‰ for δ(2)H and 0.47 to 7.97‰ for δ(18)O. There was no contamination observed in soil water. Cleaning plant samples with activated charcoal had minimal effects on the degree of spectral contamination, reducing the corrections, by on average, 0.44‰ for δ(2)H and 0.25‰ for δ(18)O. The correction method eliminated the discrepancies between IRMS and IRIS for δ(18)O, and greatly reduced the discrepancies for δ(2)H. The mean differences in isotope ratios between IRMS and the corrected IRIS method were 0.18‰ for δ(18)O, and -3.39‰ for δ(2)H. The inability to create an ethanol correction curve for δ(2)H probably caused the larger discrepancies. We conclude that ethanol and methanol are the primary compounds causing interference in IRIS analyzers, and that each individual analyzer will probably require customized correction curves.

Canopy-scale kinetic fractionation of atmospheric carbon dioxide and water vapor isotopes

Received 18 August 2008; revised 16 October 2008; accepted 21 October 2008; published 4 February 2009. [1] The carbon and oxygen isotopes of CO2 and the oxygen isotopes of H2 Oa re powerful tracers for constraining the dynamics of carbon uptake and water flux on land. The role of land biota in the atmospheric budgets of these isotopes has been extensively explored through the lens of leaf-scale observations. At the ecosystem scale, kinetic fractionation is associated with molecular and turbulent diffusion. Intuitively, air turbulence, being nondiscriminative in diffusing materials, should act to erase the kinetic effect. Using the first canopy-scale isotopic flux measurements, we show just the opposite: that in the terrestrial environment, air turbulence enhances the effect, rather than suppressing it. The sensitivity of kinetic fractionation to turbulence is striking in situations where the canopy resistance is comparable to or lower than the aerodynamic resistance. Accounting for turbulent diffusion greatly improves land surface model predictions of the isoforcing of 18 O-CO2 and transpiration enrichment of leaf water in 18 O-H2O in field conditions. Our results suggest that variations in surface roughness across the landscape can contribute to spatial variations in the composition of atmospheric 18 O-CO2 and that temporal trends in wind circulation on land can play a role in the interannual variability of atmospheric 18 O-CO2. In comparison, air turbulence has a limited effect on the isoforcing of 13 C-CO2.

Carbon dioxide fluxes in a northern fen during a hot, dry summer

Atmospheric gradient techniques were used to measure the net ecosystem exchange of CO2 for a subarctic sedge fen near Churchill, Manitoba, during the summer of 1994. This was the second driest and wannest summer since 1943. The mean daily temperature was 2°C above average, and the rainfall was 55% below normal. More than half of the rain fell after the main growth period. The fen landscape comprises hummocks and hollows. Equilibrium retention storage occurs at an average standing water depth of 80 mm above the hollow bottoms (water table reference is O). During the summer of 1994 the average water table position at −117 mm resided well below the zero equilibrium retention depth. Periodically this decreased to −265 mm, well below a 30-year average depth of −70 mm. During the full summer period, measurements indicate that the fen was a source of CO2. Only during a relatively short period of most active photosynthesis in midseason was there a small net CO2 uptake. A deep and warm soil aerobic layer promoted a large respiration flux, and this exceeded the photosynthetic CO2 uptake of the stressed sedge community. Diurnally, changes in surface temperature and incident solar radiation can explain most of the changes in the net CO2 exchange. It is hypothesized that in 1994 photosynthesis was significantly decreased and the respiration loss enhanced by the hot, dry conditions. If this hypothesis is correct, by analogy, climate warming would need to be accompanied by a substantial rainfall increase to maintain a condition of net CO2 gain to this peatland.