Measurement of Diurnal Variation in Needle PRI and Shoot Photosynthesis in a Boreal Forest

Matti Mõttus,Caroline Nichol,Vincent Markiet,Viljami Perheentupa,Juho Aalto,Rocío Hernández-Clemente,Jaana Bäck

doi:10.3390/rs10071019

Matti Mõttus, Caroline Nichol + Show 5 more

Open Access

https://doi.org/10.3390/rs10071019

Copy DOI

Abstract

The photochemical reflectance index (PRI) is calculated from vegetation narrowband reflectance in two bands in the visible part of the spectrum. Variations in PRI are associated with changes in the xanthophyll cycle pigments which regulate the light use efficiency of vegetation. Correlations have been found between remotely-sensed PRI and various photosynthetic productivity parameters at the scales from leaves to landscapes. Environmental satellites can provide only an instantaneous value of this index at the time of overpass. The diurnal course of needle (leaf) PRI needs to be known in order to link the instantaneous values robustly with photosynthetic parameters at time scales exceeding one day. This information is not currently available in the scientific literature. Here we present the daily cycle of Scots pine needle and canopy PRI in a southern boreal forest zone in the presence of direct solar radiation during the peak growing season of two consecutive years. We found the PRI of the needles which are exposed to direct radiation to have a distinct diurnal cycle with constant or slightly increasing values before noon and a daily minimum in the afternoon. The cycle in needle PRI was not correlated with that in the incident photosynthetic photon flux density (PPFD). However, when PPFD was above 1000 μmol m−2 s−1, approximately between 8 a.m. and 5 p.m., needle PRI was correlated with the light use efficiency (LUE), measured with shoot chambers. The timing of the minimum needle PRI coincided with the minimum canopy value, as measured by an independent sensor above the canopy, but the correlation between the two variables was not significant. Our field results corroborate the applicability of needle PRI in monitoring the daily variation in LUE. However, to apply this to remote sensing of seasonal photosynthetic productivity, the daily cycle of leaf PRI needs to be known for the specific vegetation type.

Highlights

Successful quantification of the terrestrial carbon cycle requires determination of the efficiency with which the absorbed photosynthetically active radiation (PAR) is utilized in the fixation of carbon by plants [1,2]
We provide the first account of the diurnal cycle of needle photochemical reflectance index (PRI) in a boreal Scots pine stand, under natural illumination, and its relationship with primary productivity
In 2014, shaded needle PRI decreased simultaneously with sunlit needle PRI within the 90% confidence margin (Figure 3a); in 2015, shaded needle PRI remained constant, or increased slightly, while the values for sunlit needles decreased to recover in the evening (Figure 3b)

Summary

Introduction

Successful quantification of the terrestrial carbon cycle requires determination of the efficiency with which the absorbed photosynthetically active radiation (PAR) is utilized in the fixation of carbon by plants [1,2]. This efficiency, known as the light use efficiency (LUE), depends on the amount of vegetation cover and is constrained by various environmental factors, such as temperature and the availability of water. Many unresolved issues exist, a potential tool for achieving this is the photochemical reflectance index (PRI) [3,4], a narrowband spectral index which can be calculated from remotely sensed data It is defined as follows [5]: PRI

Objectives

Methods

Results

Conclusion