Craig M. Trotter

Estimating photosynthetic light-use efficiency using the photochemical reflectance index: variations among species

Recent studies have shown that the photochemical reflectance index (PRI), derived from narrow waveband reflectance at 531 and 570 nm, can be used as a remote measure of photosynthetic light-use efficiency (LUE). However, uncertainty remains as to the consistency of the relationship between PRI and LUE across species. In this study we examined the relationship between the PRI and various photosynthetic parameters for a group of species with varying photosynthetic capacity. At constant irradiance, for the species group as a whole, the PRI was well correlated with LUE (r2=0.58) and with several other photosynthetic parameters, but best correlated with the ratio of carotenoids to chlorophylls contents (Caro / Chl). Despite the interspecific trends observed, determination of light response functions for the PRI in relation to photosynthetic parameters revealed that species-specific relationships were clearly stronger. For example, r2>0.90 for species-level PRI / LUE relationships. Also, the species-specific light-response data show that the magnitude of the PRI can be related to the magnitude of the saturated irradiance and the rate of CO2 uptake. As demonstrated here, a light response function provides a simple yet precise approach for characterising the relationship between the PRI and photosynthetic parameters, which should assist with improved evaluation of the usefulness of the PRI as a generalised measure of LUE.

Directional reflectance of vegetation measured by a calibrated digital camera

Obtaining directional reflectance information for vegetation canopies is often an expensive and time-consuming process. We present here a simple approach based on the use of an inexpensive digital camera equipped with a wide-angle lens. By the imaging of a large homogeneous area, a single image captures multiple views of a vegetation canopy. This gives a directional reflectance distribution fully sampled for view direction and free of variations in Sun elevation and azimuth. We determined the radiometric response of the camera sensor CCDs at the focal point and then extended this calibration to the full CCD array by using averaged images of clear blue sky. We evaluated the utility of the system by obtaining directional reflectance distributions of two vegetation targets, grass (Lolium spp) and pine forest (Pinus radiata), for red visible light. The precision of the derived biangular pattern of reflectance was +/-7%.

Estimating photosynthetic light- use efficiency using the photochemical reflectance index : the effects of short-term exposure to elevated CO2 and low temperature

There is growing interest in using the photochemical reflectance index (PRI) to estimate photosynthetic light‐use efficiency (LUE) in studies of terrestrial carbon exchange. However, evidence suggests that the PRI is correlated with LUE only while there is coordinated adjustment between PSII (photosystem II) photochemical efficiency (ΔF/Fm′) and the CO2 fixation rate. The range of environmental conditions under which this occurs has yet to be fully explored. In this study we examined for 13 species the response of ΔF/Fm′, LUE, and the PRI to short‐term exposure to elevated CO2 and low temperature. The PRI was linearly correlated with both ΔF/Fm′ and LUE under all conditions. Under elevated CO2, LUE was considerably enhanced without significantly affecting either ΔF/Fm′ or the PRI. This is consistent with current mechanistic understanding of the basis of the PRI/LUE relationship. In contrast, ΔF/Fm′, LUE, and the PRI were on average reduced proportionally by low temperature. This suggests simple multiplicative scaling may be sufficient to account across species for the effect of typical diurnal temperature variations in PRI/LUE relationships.

WEATHERING AND REGOLITH PROPERTIES AT AN EARTHFLOW SITE

Abstract The influence of weathering on the physical, chemical and geotechnical characteristics of soils has been examined for both unstable and adjacent stable ground at a site in North Island New Zealand where earthflows have developed in regolith overlying Tertiary calcareous mudrock of marine origin. The same sequence of weathering zones was visually identified during sample extrusion for both the unstable and stable ground, although the earthflow sites were characterized by extension of weathering to greater depths. Both the characteristics of the weathering zones and the relationships between calcite content and geotechnical index properties show significant similarities to those reported for the Fullers Earth in the UK. Variations in density and moisture content throughout the profiles are attributed to variations in the extent of stress relief fissuring, augmented by a calcite content which varies with depth and position on the slope. If the variation in dry density (and moisture content) due to calcite content alone is removed, the depth at which stress relief fissuring becomes prominent can be readily discerned and appears to be associated with the zone of basal shear in the earthflows. Physico-chemical swelling appears to be absent, even though the soil mineralogy is dominated by Casmectites. Post-failure weathering, clay mineral alteration and calcite removal are expected to provide a mechanism for promoting gradually increasing earthflow instability.

Optimizing the airborne thermal detection of possums

Brushtail possums (Trichosurus vulpecula