Tracking diurnal variation in photosynthetic down-regulation using low cost spectroscopic instrumentation.

Martin Van Leeuwen,Jan Van Aardt,Robert Kremens

doi:10.3390/s150510616

Abstract

Photosynthetic light-use efficiency (LUE) has gained wide interest as an input to modeling forest gross primary productivity (GPP). The photochemical reflectance index (PRI) has been identified as a principle means to inform LUE-based models, using airborne and satellite-based observations of canopy reflectance. More recently, low-cost electronics have become available with the potential to provide for dense in situ time-series measurements of PRI. A recent design makes use of interference filters to record light transmission within narrow wavebands. Uncertainty remains as to the dynamic range of these sensors and performance under low light conditions, the placement of the reference band, and methodology for reflectance calibration. This paper presents a low-cost sensor design and is tested in a laboratory set-up, as well in the field. The results demonstrate an excellent performance against a calibration standard (R2 = 0.9999) and at low light conditions. Radiance measurements over vegetation demonstrate a reversible reduction in green reflectance that was, however, seen in both the reference and signal wavebands. Time-series field measurements of PRI in a Douglas-fir canopy showed a weak correlation with eddy-covariance-derived LUE and a significant decline in PRI over the season. Effects of light quality, bidirectional scattering effects, and possible sensor artifacts on PRI are discussed.

Highlights

Terrestrial ecosystems store approximately 283 to 827 GtC [1,2,3] and play a major role in the exchange of carbon with the atmosphere [4], monitoring and understanding forest productivity are of vital importance to understanding global carbon cycles.A principle variable in the study of land-atmosphere interactions is the gross primary productivity (GPP), which describes the amount of CO2 sequestered by vegetation per unit area per unit time, before any losses due to growth and maintenance respiration are accounted for
A design was presented in this paper for a low-cost narrow-waveband sensor for the collection of photochemical reflectance index (PRI) data
Fine-spatial, in situ measurements of PRI may be an important source of information for the assessment of algorithms used to subdue atmospheric and surface BRDF effects present in airborne and tower-based data or to gain a better understanding of fine structure-function relationships

Summary

Introduction

Terrestrial ecosystems store approximately 283 to 827 GtC [1,2,3] and play a major role in the exchange of carbon with the atmosphere [4], monitoring and understanding forest productivity are of vital importance to understanding global carbon cycles.A principle variable in the study of land-atmosphere interactions is the gross primary productivity (GPP), which describes the amount of CO2 sequestered by vegetation per unit area per unit time, before any losses due to growth and maintenance respiration are accounted for. Terrestrial ecosystems store approximately 283 to 827 GtC [1,2,3] and play a major role in the exchange of carbon with the atmosphere [4], monitoring and understanding forest productivity are of vital importance to understanding global carbon cycles. A significant landmark in the estimation of forest productivity at the stand-level is the eddy-covariance technique that is used to measure fluxes of CO2 between ecosystem and atmosphere at approximately 10 ha footprints by measuring high-frequency wind-vector components and CO2-mixing ratios using sonic anemometers and gas analyzers [9], from which estimates of LUE can be derived. A network of over 500 permanent eddy-covariance installations covers a wide range of different biomes and is used to create dense time series of productivity estimates across naturally occurring environmental gradients

Methods

Results

Conclusion