Andrew E. Suyker

Relationship between gross primary production and chlorophyll content in crops: Implications for the synoptic monitoring of vegetation productivity

CO2/m 2 s in maize (GPP ranged from 0 to 3.1 mg CO2/m 2 s) and less than 0.2 mg CO2/m 2 s in soybean (GPP ranged from 0 to 1.8 mg CO2/m 2 s). Validation using an independent data set for irrigated and rainfed maize showed robustness of the technique; RMSE of GPP prediction was less than 0.27 mg CO2/m 2 s.

Relationship Between Ecosystem Productivity and Photosynthetically Active Radiation for Northern Peatlands

We analyzed the relationship between net ecosystem exchange of carbon dioxide (NEE) and irradiance (as photosynthetic photon flux density or PPFD), using published and unpublished data that have been collected during midgrowing season for carbon balance studies at seven peatlands in North America and Europe. NEE measurements included both eddy-correlation tower and clear, static chamber methods, which gave very similar results. Data were analyzed by site, as aggregated data sets by peatland type (bog, poor fen, rich fen, and all fens) and as a single aggregated data set for all peatlands. In all cases, a fit with a rectangular hyperbola (NEE = α PPFD Pmax/(α PPFD + Pmax) + R) better described the NEE-PPFD relationship than did a linear fit (NEE = β PPFD + R). Poor and rich fens generally had similar NEE-PPFD relationships, while bogs had lower respiration rates (R = −2.0μmol m−2s−1 for bogs and −2.7 μmol m−2s−1 for fens) and lower NEE at moderate and high light levels (Pmax = 5.2 μmol m−2s−1 for bogs and 10.8 μmol m−2s−1 for fens). As a single class, northern peatlands had much smaller ecosystem respiration (R = −2.4 μmol m−2s−1) and NEE rates (α = 0.020 and Pmax = 9.2μmol m−2s−1) than the upland ecosystems (closed canopy forest, grassland, and cropland) summarized by Ruimy et al. [1995]. Despite this low productivity, northern peatland soil carbon pools are generally 5–50 times larger than upland ecosystems because of slow rates of decomposition caused by litter quality and anaerobic, cold soils.

Phase and amplitude of ecosystem carbon release and uptake potentials as derived from FLUXNET measurements

As length and timing of the growing season are major factors explaining differences in carbon exchange of ecosystems, we analyzed seasonal patterns of net ecosystem carbon exchange (FNEE) using eddy covariance data of the FLUXNET data base (http://www-eosdis.ornl.gov/FLUXNET). The study included boreal and temperate, deciduous and coniferous forests, Mediterranean evergreen systems, rainforest, native and managed temperate grasslands, tundra, and C3 and C4 crops. Generalization of seasonal patterns are useful for identifying functional vegetation types for global dynamic vegetation models, as well as for global inversion studies, and can help improve phenological modules in SVAT or biogeochemical models. The results of this study have important validation potential for global carbon cycle modeling. The phasing of respiratory and assimilatory capacity differed within forest types: for temperate coniferous forests seasonal uptake and release capacities are in phase, for temperate deciduous and boreal coniferous forests, release was delayed compared to uptake. According to seasonal pattern of maximum nighttime release (evaluated over 15-day periods, Fmax) the study sites can be grouped in four classes: (1) boreal and high altitude conifers and grasslands; (2) temperate deciduous and temperate conifers; (3) tundra and crops; (4) evergreen Mediterranean and tropical forests. Similar results are found for maximum daytime uptake (Fmin) and the integral net carbon flux, but temperate deciduous forests fall into class 1. For forests, seasonal amplitudes of Fmax and Fmin increased in the order tropical C3-crops>temperate deciduous forests>temperate conifers>boreal conifers>tundra ecosystems. Due to data restrictions, our analysis centered mainly on Northern Hemisphere temperate and boreal forest ecosystems. Grasslands, crops, Mediterranean ecosystems, and rainforests are under-represented, as are savanna systems, wooded grassland, shrubland, or year-round measurements in tundra systems. For regional or global estimates of carbon sequestration potentials, future investigations of eddy covariance should expand in these systems.

Land management and land-cover change have impacts of similar magnitude on surface temperature

The direct effects of land-cover change on surface climate are increasingly well understood, but fewer studies have investigated the consequences of the trend towards more intensive land management practices. Now, research investigating the biophysical effects of temperate land-management changes reveals a net warming effect of similar magnitude to that driven by changing land cover.

Eddy correlation measurement of CO2 flux using a closed-path sensor: Theory and field tests against an open-path sensor

We have examined the potential of using a closed-path sensor to accurately measure eddy fluxes of CO2. Five inlet tubeflow configurations were employed in the experimental setup. The fluxes of CO2 were compared against those measured with an open-path sensor. Sampling air through an intake tube causes a loss of flux, due to the attenuation of CO2 density fluctuations. Adjustments need to be made to correct for this loss and to account for density effects due to the simultaneous transfer of heat and water vapor. Theory quantifying these effects is discussed.The “raw” CO2 flux measured with the closed-path sensor was smaller than that measured with the open-path sensor by about 15% (on average) for the turbulent tubeflow configurations with a short (≈3 m) intake tube, by 31% for turbulent tubeflow with a longer (≈6 m) intake tube and by 24% for laminar tubeflow. The difference was, in part, caused by tube attenuation of the CO2 density fluctuations and inadequate sensor time response. The elimination of the flux adjustment for the simultaneous transfer of sensible heat (i.e., the attenuation of ambient temperature fluctuations in the intake tube) generally accounted for the rest of this difference.The raw flux measured with the closed-path sensor was corrected for frequency response and density effects. Except in the case of laminar tubeflow, the corrected closed-path flux agreed consistently with the corrected open-path flux within a few percent (<5%). These results suggest that closed-path sensors, with appropriate corrections, can be used to measure CO2 flux accurately. Recommendations are included on selecting an “optimum” flow configuration to minimize the effect of sampling air through a tube.

Season‐long measurement of carbon dioxide exchange in a boreal fen

Atmospheric CO 2nexchange was measured in a boreal minerotrophic patterned fen in central Saskatchewan, Canada, using the eddy correlation technique. The study was conducted from mid-May to early October 1994, as part of the Boreal Ecosystem-Atmosphere Study (BOREAS). Herbaceous vegetation was dominated by buckbean (Menyanthes trifoliata) and various species of sedges (Carex and Eriphorumnspp). Bog birch (Betula pumila) and willow species (Salvenspp.) were dominant shrubs. Brown mosses were the predominant nonvascular vegetation. Canopy photosynthesis approached light saturation for PAR (photosynthetically active radiation) above 1000n1200 mmol m m2ns m1. High temperature (g20dC) and vapor pressure deficit (g1.5 kPa) decreased photosynthesis significantly. On cool days with low vapor pressure deficit, canopy photosynthesis tended to follow incident PAR. The diurnal pattern of canopy photosynthesis exhibited a midmorning maximum on days with high temperature and vapor pressure deficit. Canopy photosynthesis reached a peak of 0.59 mg CO 2nm m2ns m1n(midday) in early July, corresponding to the period of maximum leaf area index. Another increase in photosynthesis occurred in late August as the canopy recovered from a brief rise in water table that inundated some of the leaf area. The daily net CO 2nexchange showed significant day-to-day variability resulting from changes in environmental conditions. The integrated value of the net ecosystem-CO 2nexchange during the measurement period (mid-May to early October) was about 88 g C m m2. Consistent with the high productivity and high water table, this fen exhibited magnitudes of CO 2nexchange larger than other northern wetlands reported in the literature.

Synoptic Monitoring of Gross Primary Productivity of Maize Using Landsat Data

There is a growing interest in monitoring the gross primary productivity (GPP) of crops due mostly to their carbon sequestration potential. Both within- and between-field variability are important components of crop GPP monitoring, particularly for the estimation of carbon budgets. In this letter, we present a new technique for daytime GPP estimation in maize based on the close and consistent relationship between GPP and crop chlorophyll content, and entirely on remotely sensed data. A recently proposed chlorophyll index (CI), which involves green and near-infrared spectral bands, was used to retrieve daytime GPP from Landsat Enhanced Thematic Mapper Plus (ETM+) data. Because of its high spatial resolution (i.e., 30 30 m/pixel), this satellite system is particularly appropriate for detecting not only between- but also within-field GPP variability during the growing season. The CI obtained using atmospherically corrected Landsat ETM+ data was found to be linearly related with daytime maize GPP: root mean squared error of less than 1.58 in a GPP range of 1.88 to 23.1 ; therefore, it constitutes an accurate surrogate measure for GPP estimation. For comparison purposes, other vegetation indices were also tested. These results open new possibilities for analyzing the spatiotemporal variation of the GPP of crops using the extensive archive of Landsat imagery acquired since the early 1980s.

Joint control of terrestrial gross primary productivity by plant phenology and physiology

Significance Terrestrial gross primary productivity (GPP), the total photosynthetic CO2 fixation at ecosystem level, fuels all life on land. However, its spatiotemporal variability is poorly understood, because GPP is determined by many processes related to plant phenology and physiological activities. In this study, we find that plant phenological and physiological properties can be integrated in a robust index—the product of the length of CO2 uptake period and the seasonal maximal photosynthesis—to explain the GPP variability over space and time in response to climate extremes and during recovery after disturbance. Terrestrial gross primary productivity (GPP) varies greatly over time and space. A better understanding of this variability is necessary for more accurate predictions of the future climate–carbon cycle feedback. Recent studies have suggested that variability in GPP is driven by a broad range of biotic and abiotic factors operating mainly through changes in vegetation phenology and physiological processes. However, it is still unclear how plant phenology and physiology can be integrated to explain the spatiotemporal variability of terrestrial GPP. Based on analyses of eddy–covariance and satellite-derived data, we decomposed annual terrestrial GPP into the length of the CO2 uptake period (CUP) and the seasonal maximal capacity of CO2 uptake (GPPmax). The product of CUP and GPPmax explained >90% of the temporal GPP variability in most areas of North America during 2000–2010 and the spatial GPP variation among globally distributed eddy flux tower sites. It also explained GPP response to the European heatwave in 2003 (r2 = 0.90) and GPP recovery after a fire disturbance in South Dakota (r2 = 0.88). Additional analysis of the eddy–covariance flux data shows that the interbiome variation in annual GPP is better explained by that in GPPmax than CUP. These findings indicate that terrestrial GPP is jointly controlled by ecosystem-level plant phenology and photosynthetic capacity, and greater understanding of GPPmax and CUP responses to environmental and biological variations will, thus, improve predictions of GPP over time and space.

Methane flux in a boreal fen: Season‐long measurement by eddy correlation

Eddy correlation measurements of methane flux were made at a fen in central Saskatchewan, as part of the Boreal Ecosystem Atmosphere Study (BOREAS) in 1994. Data were collected from mid-May to early October. The water table was above the average peat surface throughout the measurement period. Detailed (near continuous) measurements allowed examination of temporal variability at hourly and daily time-scales. As compared with the average nighttime flux, the average daytime methane flux was 25–45% higher in July and in August and 5–15% higher earlier and later in the season. Distribution of midday (1130–1430 LT) methane emission showed varying trends in different parts of the season. From mid-May to early July, methane flux gradually increased from near zero to 4.1 mg m−2 h−1. The water table height (above an average hollow surface) varied from 0.09 to 0.18 m, but the trend in methane flux followed peat temperature (at 0.1-m depth) more closely. The peat warmed from 3.4° to 16.3°C during this time period. Methane flux was negligible until peat temperature (at 0.1-m depth) was above 12°C. From early July to early August there was a fivefold increase in methane flux from 4.1 to its seasonal peak of 19.5 mg m−2 h−1 on August 1. The water table ranged from 0.23 m to a brief seasonal plateau of 0.30 m on July 20–23. Sharp increases in the water table were followed by increasing trends in methane flux by approximately 12 days. Peat temperature reached its seasonal maximum (19.3°C) the same time when the methane flux peaked. After early August the methane flux declined steadily and reached a value of 2.4 mg m−2 h−1 in early October. The water table and peat temperature showed comparable trends and decreased steadily to 0.06 m and 5.7°C, respectively. The seasonally integrated methane emission (mid-May to early October) was estimated at 16.3 g C m−2. Nonlinear regression analysis of methane flux against water table (lagged by 12 days) and peat temperature was performed for different periods of the season. Except for a brief period of very high water table (when many hummocks were inundated) the regression using water table and peat temperature explained between 68 and 94% of the variability in methane flux. The sensitivity of methane flux to water table (or the slope of the log CH4 flux/water table relationship) obtained from our daily flux values ranged from 3.4 × 10−4 to 5.0 × 10−4 m−1.

A Production Efficiency Model-Based Method for Satellite Estimates of Corn and Soybean Yields in the Midwestern US

Remote sensing techniques that provide synoptic and repetitive observations over large geographic areas have become increasingly important in studying the role of agriculture in global carbon cycles. However, it is still challenging to model crop yields based on remotely sensed data due to the variation in radiation use efficiency (RUE) across crop types and the effects of spatial heterogeneity. In this paper, we propose a production efficiency model-based method to estimate corn and soybean yields with MODerate Resolution Imaging Spectroradiometer (MODIS) data by explicitly handling the following two issues: (1) field-measured RUE values for corn and soybean are applied to relatively pure pixels instead of the biome-wide RUE value prescribed in the MODIS vegetation productivity product (MOD17); and (2) contributions to productivity from vegetation other than crops in mixed pixels are deducted at the level of MODIS resolution. Our estimated yields statistically correlate with the national survey data for rainfed counties in the Midwestern US with low errors for both corn (R2 = 0.77; RMSE = 0.89 MT/ha) and soybeans (R2 = 0.66; RMSE = 0.38 MT/ha). Because the proposed algorithm does not require any retrospective analysis that constructs empirical relationships between the reported yields and remotely sensed data, it could monitor crop yields over large areas.