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AbstractLeaf area index (LAI) and total foliar nitrogen (TFN) are important canopy characteristics and crucial variables needed to simulate photosynthesis and ecosystem CO2fluxes. Although plant communities dominated by Cassiope tetragonaare widespread in the Arctic, LAI and TFN for this vegetation type have not been accurately quantified. We address this knowledge gap by (i) direct measurements of LAI and TFN for C. tetragona, and (ii) determining TFN-LAI and LAI–normalized difference vegetation index (NDVI) relationships for typical C. tetragonatundras in the subarctic (Sweden) and High Arctic (Greenland and Svalbard).Leaves of C. tetragonaare 2–6 mm long and closely appressed to their stems forming parallelepiped shoots. We determined the LAI of C. tetragonaby measuring the area of the leaves while still attached to the stem, then doubling the resulting one-sided area. TFN was determined from leaf N and biomass. The LAI-NDVI and TFN-LAI relationships showed high correlation and can be used to estimate indirectly LAI and TFN. The LAI-NDVI relationship for C. tetragonavegetation differed from a generic LAI-NDVI relationship for arctic tundra, whereas the TFN-LAI relationship did not. Overall, the LAI of C. tetragonatundra ranged from 0.4 to 1.1 m2m−2and TFN from 1.4 to 1.7 g N m−2.

Total soil organiccarbon SOC stocks for grassland soils in Flanders N Belgium were determined for the Kyoto Protocol reference year 1990 and 2000 in order to investigate whether these soils have been CO2sinks or sources during that period. The stocks were calculated by means of detailed SOC datasets, which were available at the community scale for the whole of Flanders. The total SOC stocks for Flemish grassland soils 1 m depth were estimated at 38 Mt SOC in 1990 and 34 Mt SOC in 2000. The loss of SOC resulted from a decrease in the SOC content of grassland soils 71 and could also partly 29 be explained by a decline in grassland area. Significant decreases in SOC for the 0–6 cm depth layer were found for the 1990s for the coarsertextured soils with SOC losses ranging between –0.3 and –0.5 over the 10 y period. Specific management practices that disturb the SOC balance such as conversion to temporary grassland and a reduction of animalmanure application are hypothesized to have contributed to the observed loss of SOC stocks. We furthermore conducted an analysis of uncertainty of the 1990 and 2000 grassland SOC–stocks calculation using Monte Carlo analysis. Probabilitydistribution functions were determined for each of the inputs of the SOCstock calculation, enabling us to assess the uncertainty on the 1990 and 2000 SOC stocks. The frequency distributions of these simulated stocks both closely approached lognormal distributions, and their 95confidence intervals ranged between 150 and 50 of the calculated mean SOC stock. The standard error on the measured decrease in SOC stocks in Flemish grassland soils during the 1990s was calculated to be 7–8 Tg SOC, which is equivalent to twice this decrease. This clearly shows that largescale changes in SOC stocks are uncertaintyridden, even when they are based on detailed datasets.

Two promising techniques to measure water uptake in a direct way were evaluated on substrate-grown truss tomato plants (Lycopersicon esculentum 'Clothilde'). The first technique, which is called the mass-balance technique, determines tomato water uptake using automated weight measurements of substrate mass and collected leachate. It was found that leachate could not be recorded appropriately in a 1 minute time interval by the tipping bucket gauges used in this study. Therefore, the tipping bucket readings had to be corrected and modified. The second technique computes the water uptake b) a single tomato plant based on the thermal energy balance of a stem segment and, therefore, it is called the heat-balance technique. The stem segment was carefully selected at the base of a tomato stem so that the whole-plant water uptake could be determined. However, it was found that an independent measurement of minimum water uptake during the night was needed to correctly calculate water uptake during the whole day. As a result, the major pitfalls of the two techniques were identified. With appropriate modifications, a good correlation between the measurements was found, even during a period of imposed drought stress. [ABSTRACT FROM AUTHOR]

Results from measurement of sap flow by heat balance sensors on the stem of a young oak tree (Quercus robur L.) revealed that thermal disequilibrium (i.e. heat storage) within the heated stem segment can introduce considerable errors in the measured sap-flow rates. The magnitude, sign and significance of these errors depend on the sap-flow rate and on the relationship between stem temperature and air temperature. Sap-flow rates were found to be more prone to errors caused by heat storage effects under low flow conditions than at higher rates of sap flow. Furthermore, daytime fluctuations of air temperature and stem temperature inside the heat balance sensor were either in phase when a low, or in opposite phase when a high sap-flow rate was passing through the stem of the young tree. To investigate this relationship, we developed an experimental set-up with cut stem segments through which tap water could be pressed. This set-up allowed the effects of air temperature and sap-flow rates on stem temperatures within heat balance sensors to be clearly separated. Good mathematical relationships were obtained and were successfully used to assess the relative importance of air temperature and sap-flow rate with respect to the fluctuations in stem temperature of the young oak tree. Based on the established relationships, a novel approach was put forward to correct for errors introduced into sap-flow calculations caused by heat storage effects if no measured data on stem temperature are available.

This report is one of a series of papers describing reflectant‐induced modification of the canopy radiation balance. Water‐saving effects are expected since net radiation, which is the driving force for evapotranspiration, should be reduced by the reflectant treatment. The experiments were planned to establish reflection patterns over soybeans (Glycine max.[L.] Merr. ‘Amsoy’) at Mead, Nebraska. This study was undertaken to determine reflection coefficients at various solar elevations and at two values of leaf area index (LAI = 2.5 and 5.5) of a normal, unreflectorized soybean canopy so as to develop a base line against which to evaluate the effectiveness of various factorial treatments of reflectant materials, times and amounts of application. Therefore, net radiation, incoming and reflected radiation (300 to 3,000 nm) and the corresponding energy spectra within the 380 to 1,550 nm wavelength band were measured using a net radiometer, an upright and inverted pyranometer and a spectroradiometer, respectively. The shortwave reflection coefficient, αs, as well as the coefficients for photosynthetically active radiation α(PAR) and near‐infrared radiation α(NIR) remained nearly constant throughout most of the day, but increased significantly when solar altitude was less than 35° The dependence of albedo on LAI was a function of wavelength. At medium to high solar elevations α(PAR) decreased with increasing LAI while α(NIR) increased sharply. At lower solar altitudes both α(PAR) and α(NIR) increased with increasing LAI. The radiant energy reflected from the soybean crop showed a strong spectral shift toward NIR wavelengths with the increase in leaf area.

Shootlets of Rosa multiflora L. cv. Montse were cultured in vitro with four different levels of sucrose (0, 1, 3 and 5%). Chloroplasts of shootlets grown in a medium without sucrose contained numerous, large plastoglobuli and were lacking in starch granules. The size and number of starch granules increased with the level of sucrose in the culture medium. Starch content in leaves of shootlets grown with 5% sucrose was higher (ca 1, 3%) than those grown with 3% (ca 0, 45%) and 1% sucrose (ca 0, 27%). Starch might be used by the in vitro shootlets during the acclimation period.

Soybeans (Glycine maxL. ‘Amsoy’) were coated with Celite and kaolinite reflectants to increase reflection and reduce net radiation. Such treatment is intended to decrease evapotranspiration and improve water use efficiency. The study was undertaken to establish the specific reflection patterns at various wavelengths of reflectorized soybeans. The effectiveness of the materials and the influence of additional applications applied factorially during the growing season was also evaluated. Both reflectant materials are most effective in the waveband of photosynthetically active radiation (PAR: 400–700 nm). Reflection coefficients in this waveband were double that from a natural canopy. The effect on near infrared radiation (NIR: 750–1,550 nm) was small. The average increase in shortwave reflection (300–3,000 nm) ranged from 10 to 20%. Reduction of net radiation measured over the treated canopies ranged from 5 to 10%. Within the treated canopies net radiation was greater than in the untreated crop. Application of reflectants at the beginning of the growing season was more effective in increasing reflection and reducing net radiation than were applications at later stages of crop maturity. The effects of additional treatments as the crop developed were not cumulative. Therefore additional applications should be made only to maintain the quality of the coating. Celite is the more effective material, producing the same degree of reflection with half as much material as is required when kaolinite is used.

A method has been developed to incorporate actual canopy architecture in radiation models. The mathematical procedure involved, introduces a leaf angle and azimuthal distribution function, and yields an integrated extinction coefficient which can be substituted into basic light extinction formulae.