Monitoring seasonal and diurnal changes in photosynthetic pigments with automated PRI and NDVI sensors

J A Gamon,S R Garrity,O Kovalchuck,A Harris,C Y S Wong

doi:10.5194/bg-12-4149-2015

Abstract

Abstract. The vegetation indices normalized difference vegetation index (NDVI) and photochemical reflectance index (PRI) provide indicators of pigmentation and photosynthetic activity that can be used to model photosynthesis from remote sensing with the light-use-efficiency model. To help develop and validate this approach, reliable proximal NDVI and PRI sensors have been needed. We tested new NDVI and PRI sensors, "spectral reflectance sensors" (SRS sensors; recently developed by Decagon Devices, during spring activation of photosynthetic activity in evergreen and deciduous stands. We also evaluated two methods of sensor cross-calibration – one that considered sky conditions (cloud cover) at midday only, and another that also considered diurnal sun angle effects. Cross-calibration clearly affected sensor agreement with independent measurements, with the best method dependent upon the study aim and time frame (seasonal vs. diurnal). The seasonal patterns of NDVI and PRI differed for evergreen and deciduous species, demonstrating the complementary nature of these two indices. Over the spring season, PRI was most strongly influenced by changing chlorophyll : carotenoid pool sizes, while over the diurnal timescale, PRI was most affected by the xanthophyll cycle epoxidation state. This finding demonstrates that the SRS PRI sensors can resolve different processes affecting PRI over different timescales. The advent of small, inexpensive, automated PRI and NDVI sensors offers new ways to explore environmental and physiological constraints on photosynthesis, and may be particularly well suited for use at flux tower sites. Wider application of automated sensors could lead to improved integration of flux and remote sensing approaches for studying photosynthetic carbon uptake, and could help define the concept of contrasting vegetation optical types.

Highlights

The photochemical reflectance index (PRI) was originally derived as a measure of xanthophyll cycle activity determined using proximal remote sensing of leaves and canopies on a diurnal timescale (Gamon et al, 1992, 1997)
The goals of this study were to (1) develop field measurement protocols for inexpensive, automated sensors; (2) compare the normalized difference vegetation index (NDVI) and PRI measured by these sensors to independent spectrometer measurements; (3) explore the complementary behavior of PRI and NDVI in deciduous and evergreen canopies; and (4) evaluate whether the PRI signals obtained in this way can distinguish the facultative and constitutive pigment responses
We derived an empirical equation for each sensor band, enabling automatic correction of the midday PRI and NDVI values

Summary

Introduction

The photochemical reflectance index (PRI) was originally derived as a measure of xanthophyll cycle activity determined using proximal remote sensing of leaves and canopies on a diurnal timescale (Gamon et al, 1992, 1997) In this context, the xanthophyll cycle is a facultative response that changes readily as a means of dissipating extra light energy non-destructively (Demmig-Adams and Adams, 1992). During seasonal transitions from a dormant to an active growth phase, evergreen plants adjust their chlorophyll : carotenoid ratios over many weeks in response to changing temperatures (Adams et al, 2002), and this adjustment can be readily detected by PRI (Stylinski et al, 2002; Filella et al, 2009; Porcar-Castell et al, 2012; Wong and Gamon, 2015a) Both the facultative and constitutive PRI responses are strongly correlated with photosynthetic activity but over different timescales and using different mechanisms, both of which involve photoprotective carotenoid pigments

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