Timothy J. Arkebauer

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.

Carbon dioxide exchange in a peatland ecosystem

Micrometeorological measurements of carbon dioxide exchange were made in an open peatland in north central Minnesota during two growing seasons (1991 and 1992). The vegetation at the site was dominated by Sphagnum papillosum, Scheuchzeria palustris, and Chamaedaphne calyculata. The objective of the study was to examine the diurnal and seasonal variations in canopy photosynthesis (P) and develop information on the net ecosystem CO2 exchange. The two seasons provided contrasting microclimatic conditions: as compared with 1991, the 1992 season was significantly wetter and cooler. Canopy photosynthesis was sensitive to changes in light, temperature, and moisture stress (as indicated by water table depth and atmospheric vapor pressure deficit). Under moderate conditions (temperature 18–28°C, vapor pressure deficit 0.7–1.5 kPa, and water table near the surface) during the peak growth period, midday (averaged between 1000–1400 hours) P values ranged from 0.15 to 0.24 mg m−2 s−1. Under high-temperature (30°–34°C) and moisture stress (water table 0.16–0.23 m below the surface and vapor pressure deficit 2.2–3.0 kPa) conditions, midday P was reduced to about 0.03–0.06 mg m−2 s−1. There was a high degree of consistency in the values of P under similar conditions in the two seasons. Seasonally integrated values of the daily net ecosystem CO2 exchange indicated that the study site was a source of atmospheric CO2, releasing about 71 g C m−2 over a 145-day period (May-October) in 1991. Over a similar period in 1992, however, this ecosystem was a sink for atmospheric CO2 with a net accumulation of about 32 g C m−2. These results are consistent with previous investigations on CO2 exchange in other northern wetland sites during wet and dry periods.

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.

Diel variation in lacunal CH4 and CO2 concentration and δ13C in Phragmites australis

We tested the hypothesis that the diurnal patterns of variationin lacunal gas concentrations and isotopic fractionationpreviously reported in a single plant genera (Typha)typified the patterns of all through-flow convective plantsby extending our observations to Phragmites australisCav. In daylight, Phragmites CH4 transport isdriven by internal pressurization which results in gas flowdown young green culms and its exit from one year old deadbrown culms. Flow rates of 10.4 ± 4.0 mL min−1 weremeasured in this study. At night, CH4 is transportedfrom the sediments to the atmosphere via the lacunal plantspaces by molecular diffusion. Within green culms, lacunalCH4 concentrations varied by a factor of 1000, from 3%(parts by volume) pre-dawn to lows of 25 ppmv during midday.Methane in brown culms varied by a factor of 10 diurnally,from 5% pre-dawn to 0.3% at midday. Lacunal CO2concentrations varied similarly.Concentrations of both gases varied inversely with lacunalpressure. In green culms, large isotopic fractionations wereobserved in CH4 and CO2 in the morning and eveningduring transitions in gas transport mode and were associatedwith slight downward flows counter to the upward diffusionof these gases. Methane δ13C as depletedas −100‰ was observed. In daylight, lacunal CH4 wassimilar to or 13C depleted relative to sedimentary andemitted CH4 isotopic values, but at night lacunalCH4 was 13C enriched relative to sedimentarymethane. Overall, the diurnal variations of CH4concentration and δ13C value inPhragmiteswere similar to those observed in Typha andindicate that these patterns should be consistent in otherconvective-flow plants. Furthermore, our results demonstratethat the large isotopic fractionations found in aquaticplants can result solely from isotopic fractionationassociated with gas transport.

Field measurements of internal pressurization in Phragmites australis (Poaceae) and implications for regulation of methane emissions in a midlatitude prairie wetland.

Emergent aquatic macrophytes in vegetated wetlands provide routes for methane (CH(4)) transport from sites of production in oxygen-poor sediments, where CH(4) concentrations are relatively high, to the atmosphere, which typically has much lower CH(4) concentrations. Transport can occur through aerenchymatous tissue via simple diffusion. Recently, the importance of convective throughflow (i.e., mass transport of gases through plants driven by pressure gradients) in enhancing gas transport has been demonstrated in several genera (e.g., Nuphar, Nymphaea, Nelumbo, Typha, and Phragmites). This study was conducted to elucidate the governing plant-mediated gas transport mechanisms in a midlatitude prairie wetland and to determine both their diel and seasonal variations and the importance of environmental controlling factors. Pressures inside culms of the two dominant emergent aquatic macrophytes (Scirpus acutus and Phragmites australis) were measured directly throughout the growing season and on selected days in midseason. Supporting measurements included solar radiation, air temperature, relative humidity, and windspeed. Results indicated pressures inside green healthy culms of Phragmites were above atmospheric pressure by up to 1650 Pa during the day. At night culm pressures were at or slightly above atmospheric. No pressurization was detected in Scirpus. Highest pressures in Phragmites occurred during midseason when biomass and foliage area index were at their maxima (920 g/m(2) and 2.8, respectively). High internal pressures also coincided with periods of high solar radiation (>500 W/m(2)), high temperature (>20°C), and low relative humidity (<60%). Periods of high internal pressures also coincided with periods of high CH(4) efflux from the wetland as measured in concomitant studies. Convective throughflow driven by internal pressure gradients in Phragmites thus explains much of the diel variation in methane efflux previously reported from this wetland.

Genotypic variation of gas exchange parameters and carbon isotope discrimination in winter wheat

Summary Carbon isotope discrimination (Δ) has been suggested as an indirect selection tool for plant water use efficiency and yield potential in wheat ( Triticum aestivum L.). Plant available soil water content (PASW) and vapor pressure deficit (VPD) are among some important factors affecting gas exchange and Δ. A two-year field experiment was conducted to (1) investigate the differences in gas exchange parameters: net CO 2 assimilation rate (An), stomatal conductance (Gs), intercellular CO 2 concentration (Ci), An/Ci, transpiration rate (E), water use efficiency (WUE), and Δ between older and newer cultivars in winter wheat; and (2) determine the relationships between Δ and gas exchange parameters, WUE and grain yield as influenced by PASW and VPD. Differences in An and An/Ci between the two years was influenced more by PASW conditions than by cultivar. In general, WUE decreased in all cultivars when PASW

Stability of leaf anatomy and light response curves of field grown maize as a function of age and nitrogen status

Summary Little information exists linking leaf anatomical characteristics, gas exchange rates and the effects of irrigation and nitrogen applications in field grown maize. The objectives of this study were: (i) to characterize the light response curves of leaves 13 and 17; (ii) to characterize leaf anatomical features (leaf thickness; bundle sheath diameter; separation between bundle sheaths; number of mesophyll cells between bundle sheaths; number of mesophyll cells separating the bundle sheaths from the epidermis; bundle sheath area ratio) over the season as a function of leaf position (leaves 13 and 17, counting from stem base), and (iii) to relate these anatomical features to the net assimilation rate at a PPFD of 2000 µmol m –2 s –1 (Amax) and to the convexity parameter of a light response curve of field grown maize in response to extremes in irrigation and nitrogen. Experiments were conducted in 1995 and 1996 with the maize hybrid Pioneer 3394. A randomized complete block with split-plot treatment design with two replications was used. Irrigation/no irrigation was the main plot and nitrogen (120 kg nitrogen ha –1 or no nitrogen) was the split plot treatment. Leaves from tagged plants were measured and then fixed for anatomical measurements. Environmental conditions during each year were markedly different providing a wide range of responses. Leaf thickness as well as other anatomical features were influenced by water stress and nitrogen in a coordinated fashion such that environmental conditions had little or no effect on the ratio of mesophyll and bundle sheath tissue. Amax increased with leaf thickness but the correlation was weak. The convexity parameter and Amax decreased with leaf age, with no correlation to the anatomical features described here.

Environmental Triggers of Winter Annual Weed Emergence in the Midwestern United States

Abstract Winter annual weeds are becoming prolific in agricultural fields in the midwestern United States. The objectives of this research were to understand the roles of soil temperature (daily average and fluctuation) and moisture on the emergence of nine winter annual weed species and dandelion and to develop predictive models for weed emergence based on the accumulation of modified thermal/hydrothermal time (mHTT). Experiments were established at Lincoln, NE; Mead, NE; and at two sites (irrigated and rainfed) near Clay Center, NE, in 2010 and 2011. In July of each year, 1,000 seeds of each species were planted in 15 by 20 by 6-cm mesh baskets installed between soybean rows. Soil temperature and water content were recorded at the 2-cm depth. Emerged seedlings were counted and removed from the baskets on a weekly basis until no additional emergence was observed in the fall, resumed in late winter, and continued until emergence ceased in late spring. Weather data were used to accumulate mHTT beginning on August 1. A Weibull function was selected to fit cumulative emergence (%) on cumulative mHTT (seven base temperature [Tbase] by six base water potential [&PSgr;base] by three base temperature fluctuation [Fbase] candidate threshold values  =  126 models); it was also fit to days after August 1 (DAA1), for a total of 127 candidate models per species. The search for optimal base thresholds was based on the theoretic-model comparison approach (Akaike information criterion [AIC]). All three components (Tbase, &PSgr;base, and Fbase) were only important for Virginia pepperweed. For downy brome and purslane speedwell, including Tbase and &PSgr;base resulted in the best fit, whereas for dandelion including Tbase and Fbase resulted in the best fit. A model including only Tbase resulted in the best fit for most species included in this study (Carolina foxtail, shepherds-purse, pinnate tansymustard, henbit, and field pansy). For field pennycress, the model based on DAA1 resulted in the best fit. Threshold values were species specific. Soil temperature was the major environmental factor influencing winter annual weed emergence. Even though soil moisture and often temperature fluctuation are essential for seed germination, &PSgr;base and Fbase were not as critical in the predictive models as initially expected. Most seedlings (> 90%) of downy brome, pinnate tansymustard, Carolina foxtail, henbit, and field pansy emerged during the fall. Virginia pepperweed, purslane speedwell, dandelion, shepherds-purse, and field pennycress seedlings emerged during both fall and spring. The results of this research provide robust information on the prediction of the time of winter annual weed emergence, which can help growers make better management decisions. Nomenclature: Carolina foxtail, Alopecurus carolinianus Walt. ALOCA; dandelion, Taraxacum officinale G.H. Weber ex Wiggers TAROF; downy brome, Bromus tectorum L. BROTE; field pansy, Viola bicolor Pursh VIORA; field pennycress, Thlaspi arvense L. THLAR; henbit, Lamium amplexicaule L. LAMAM; pinnate tansymustard, Descurainia pinnata (Walt.) Britt. DESPI; purslane speedwell, Veronica peregrina L. VERPG; shepherds-purse, Capsella bursa-pastoris (L.) Medik. CAPBP; Virginia pepperweed, Lepidium virginicum L. LEPVI; Glycine max (L.) Merr.

Assessment of canopy chlorophyll content retrieval in maize and soybean: Implications of hysteresis on the development of generic algorithms

Canopy chlorophyll content (Chl) closely relates to plant photosynthetic capacity, nitrogen status and productivity. The goal of this study is to develop remote sensing techniques for accurate estimation of canopy Chl during the entire growing season without re-parameterization of algorithms for two contrasting crop species, maize and soybean. These two crops represent different biochemical mechanisms of photosynthesis, leaf structure and canopy architecture. The relationships between canopy Chl and reflectance, collected at close range and resampled to bands of the Multi Spectral Instrument (MSI) aboard Sentinel-2, were analyzed in samples taken across the entirety of the growing seasons in three irrigated and rainfed sites located in eastern Nebraska between 2001 and 2005. Crop phenology was a factor strongly influencing the reflectance of both maize and soybean. Substantial hysteresis of the reflectance vs. canopy Chl relationship existed between the vegetative and reproductive stages. The effect of the hysteresis on vegetation indices (VI), applied for canopy Chl estimation, depended on the bands used and their formulation. The hysteresis greatly affected the accuracy of canopy Chl estimation by widely-used VIs with near infrared (NIR) and red reflectance (e.g., normalized difference vegetation index (NDVI), enhanced vegetation index (EVI) and simple ratio (SR)). VIs that use red edge and NIR bands (e.g., red edge chlorophyll index (CIred edge), red edge NDVI and the MERIS terrestrial chlorophyll index (MTCI)) were minimally affected by crop phenology (i.e., they exhibited little hysteresis) and were able to accurately estimate canopy Chl in two crops without algorithm reparameterization and, thus, were found to be the best candidates for generic algorithms to estimate crop Chl using the surface reflectance products of MSI Sentinel-2.

Robust estimates of soil moisture and latent heat flux coupling strength obtained from triple collocation

Land surface models (LSMs) are often applied to predict the one-way coupling strength between surface soil moisture (SM) and latent heat (LH) flux. However, the ability of LSMs to accurately represent such coupling has not been adequately established. Likewise, the estimation of SM/LH coupling strength using ground-based observational data is potentially compromised by the impact of independent SM and LH measurements errors. Here we apply a new statistical technique to acquire estimates of one-way SM/LH coupling strength which are nonbiased in the presence of random error using a triple collocation approach based on leveraging the simultaneous availability of independent SM and LH estimates acquired from (1) LSMs, (2) satellite remote sensing, and (3) ground-based observations. Results suggest that LSMs do not generally overestimate the strength of one-way surface SM/LH coupling.