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Short wavelength (C-band) radar backscatter intensity from forests is insufficient for estimating age related tree parameters because the microwave radiation does not penetrate far into the forest canopy. However, extra information that is not available in the SAR image tone may be provided by the image texture, the spatial variation in image intensity. This paper demonstrates that a simple measure of texture in high spatial resolution C-band airborne SAR imagery of a tropical forest region can assist in the discrimination of forest age classes that have very similar average backscatter responses. As this effect is not seen in similar studies of a temperate coniferous forest, it is suggested that the relationship between texture and age is associated with the process of forest development particular to tropical forests. >

This study describes the development of a semiempirical model for the retrieval of above-ground biomass density of regenerating tropical forest using JERS-1 Synthetic Aperture Radar (SAR). The magnitude and variability of the response of the L-band SAR to above-ground biomass density was quantified using field data collected at Tapajos in central Amazonia and imagery from a series of dates. A simple backscatter model was fitted to this response and validated using image and field data acquired independently at Manaus, 500 km to the west of Tapajos. The sources of variability in biomass density and SAR backscatter were investigated so as to determine confidence limits for the subsequent retrieval of biomass density using the model. This analysis suggested that only three broad classes of regenerating forest biomass density may be positively distinguished. While the backscatter appears to saturate at around 60 tonnes per hectare, the biomass limit for retrieval purposes which is tolerant to both speckle and image texture is only 31 tonnes per hectare. The spatial distribution of biomass density in central Amazonia was estimated by applying the model to a mosaic of 90 JERS-1 images. A favorable comparison of this distribution to a map of regeneration derived from NOAA AVHRR imagery suggested that L-band SAR will provide a useful method of monitoring tropical forests on a regional scale.

Tilman et al . (Reports, 8 December 2006, p. 1598) argued that low-input high-diversity grasslands can provide a substantial proportion of global energy needs. We contend that their conclusions are not substantiated by their experimental protocol. The authors understated the management inputs required to establish prairies, extrapolated globally from site-specific results, and presented potentially misleading energy accounting.

Evaluating the impact of land use practices on ground water quality has been difficult because few techniques are capable of monitoring the quality and quantity of soil water flow below the root zone without disturbing the soil profile and affecting natural flow processes. A recently introduced method, known as equilibrium tension lysimetry, was a major improvement but it was not a true equilibrium since it still required manual intervention to maintain proper lysimeter suction. We addressed this issue by developing an automated equilibrium tension lysimeter (AETL) system that continuously matches lysimeter tension to soil-water matric potential of the surrounding soil. The soil-water matric potential of the bulk soil is measured with a heat-dissipation sensor, and a small DC pump is used to apply suction to a lysimeter. The improved automated approach reported here was tested in the field for a 12-mo period. Powered by a small 12-V rechargeable battery, the AETLs were able to continuously match lysimeter suction to soil-water matric potential for 2-wk periods with minimal human attention, along with the added benefit of collecting continuous soil-water matric potential data. We also demonstrated, in the laboratory, methods for continuous measurement of water depth in the AETL, a capability that quantifies drainage on a 10-min interval, making it a true water-flux meter. Equilibrium tension lysimeters have already been demonstrated to be a reliable method of measuring drainage flux, and the further improvements have created a more effective device for studying water drainage and chemical leaching through the soil matrix.

The use of net radiation as a critical variable in models of surface-atmosphere exchange is routine; however, its measurement is not. The discrete measurement of incoming and outgoing solar radiation with pyranometers and long-wave radiation with pyrgeometers is widely thought to be the most accurate method for estimating net radiation, but deployment at multiple sites is cost prohibitive. A recently developed single-sensor net radiometer differs from other commonly used net radiometers in that it has no domes covering the thermopile sensor. Long-term field data from this net radiometer were compared to reference net radiation calculated as the sum of the four independently measured components. The domeless net radiometer agreed well with the reference measurements, out-performing a similarly priced, widely used, domed net radiometer under most conditions. However, our analyses suggest that the domeless net radiometer has differing sensitivities to short and long-wave radiation, which is not accounted for in the calibration. Additionally, precipitation affected the domeless net radiometer much more severely than the domed model, which could be a significant limitation in areas with frequent rainfall. Despite these shortfalls, the domeless net radiometer evaluated here could be useful in a variety of field settings.

Reicosky, D.C., Winkelman, L.J., Baker, J.M. and Baker, D.G., 1989. Accuracy of hourly air temperatures calculated from daily minima and maxima. Agric. For. Meteorol., 46: 193-209. Temperature is one of the critical variables that drives biological systems and is of fundamental importance in crop growth models. The objective of this work was to determine the accuracy of several methods for calculating hourly air temperatures from daily maxima and minima. Methods that have as inputs daily minimum and maximum temperature were selected from the literature based on their use in existing growth models and simplicity. Four years of hourly air temperature data collected during the growing season at 2 m over well-watered grass were used to test the various methods. Six days from each growing season were randomly selected for detailed analysis, and an additional 9 days were selected to cover a range of daily maximum temperatures and solar radiation. The absolute mean error within a 24-h period ranged from 0.5 to 9.3 ° C for the 6 randomly selected days for all 4 years of the data. All methods worked reasonably well on clear days but with limited success on overcast days. Daily maximum temperature did not appear to affect the accuracy of any of the methods. If accurate timing of temperature input to models is critical, the results indicate direct measurement of hourly temperature may be necessary.

With any method for measuring soil water content it is useful to know the spatial sensitivity of the measurement, i.e., what volume of soil is measured and what the distribution of sensitivity is within that volume. We determined this information for the method of time-domain reflectometry (TDR), us

(...) When the potential gradient between center and outer compartments reached 1.0 MPa, the gamma readings showed overnight movement of water from wet to dry soil that could only have taken place through the root systems of the outer row plants. The overnight transfer of water, while significant, was not sufficient to fully supply the day-time transpiration demand of the unirrigated center row plants (...)

Heat balance methods of stem flow measurement offer the opportunity to measure directly the mass flow rate of water in plants. We have tested one such approach; the constant power heat balance method of Sakuratani (1981). The results supported his statement of an approximate accuracy of 10% when measuring the transpiration rate of herbaceous plants. The response to sudden changes in stem flow rate is not instantaneous, but investigation of the time constant shows that it decreases as stem flow increases, to the extent that, at flow rates typical of daytime conditions the system is capable of accurately tracking changes in stem flow within 5 min or less. We describe a new gauge design that is relatively rugged, simple to use with an appropriate digital datalogger and suitable for field use over prolonged periods of time. It does not injure or penetrate the stem, is amenable to continuous and direct recording of the mass flow rate of water in the stem and requires no calibration. A further refinement, which should improve both the accuracy and the dynamic response of the system, is proposed.