Parallel Seasonal Patterns of Photosynthesis, Fluorescence, and Reflectance Indices in Boreal Trees

Kyle Springer,John Gamon,Ran Wang

doi:10.3390/rs9070691

Kyle Springer, John Gamon + Show 1 more

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https://doi.org/10.3390/rs9070691

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Abstract

Tree species in the boreal forest cycle between periods of active growth and dormancy alter their photosynthetic processes in response to changing environmental conditions. For deciduous species, these changes are readily visible, while evergreen species have subtler foliar changes during seasonal transitions. In this study, we used remotely sensed optical indices to observe seasonal changes in photosynthetic activity, or photosynthetic phenology, of six boreal tree species. We evaluated the normalized difference vegetation index (NDVI), the photochemical reflectance index (PRI), the chlorophyll/carotenoid index (CCI), and steady-state chlorophyll fluorescence (FS) as a measure of solar-induced fluorescence (SIF), and compared these optical metrics to gas exchange to determine their efficacy in detecting seasonal changes in plant photosynthetic activity. The NDVI and PRI exhibited complementary responses. The NDVI paralleled photosynthetic phenology in deciduous species, but not in evergreens. The PRI closely paralleled photosynthetic activity in evergreens, but less so in deciduous species. The CCI and FS tracked photosynthetic phenology in both deciduous and evergreen species. The seasonal patterns of optical metrics and photosynthetic activity revealed subtle differences across and within functional groups. With the CCI and fluorescence becoming available from satellite sensors, they offer new opportunities for assessing photosynthetic phenology, particularly for evergreen species, which have been difficult to assess with previous methods.

Highlights

Forests cover approximately 4 billion hectares of the earth’s land surface [1]; the circumpolar boreal forest accounts for approximately one-quarter (1132 million hectares) of that total [2].The boreal region, known to be a substantial store of carbon [3], is experiencing a significant change in climate
When evaluating the efficacy of canopy reflectance indices in tracking photosynthetic phenology, we found that the normalized difference vegetation index (NDVI) was capable of tracking midday photosynthetic activity of deciduous species associated with the development and loss of green foliage across seasons, as previously reported [17]
Our results show the varying efficacy of FS and reflectance indices (NDVI, photochemical reflectance index (PRI), and chlorophyll/carotenoid index (CCI)) in tracking photosynthetic phenology for evergreen and deciduous species

Summary

Introduction

Forests cover approximately 4 billion hectares of the earth’s land surface [1]; the circumpolar boreal forest accounts for approximately one-quarter (1132 million hectares) of that total [2].The boreal region, known to be a substantial store of carbon [3], is experiencing a significant change in climate. The combined effects of increasing atmospheric carbon dioxide (CO2 ) levels and increasing temperatures associated with climate change will impact a number of plant processes [4], and these effects are amplified at higher latitudes. These changes in climate are expected to impact growing-season length, and directly influence phenology by altering the timing of seasonal transitions that are defined by changing temperatures and photoperiod [5]. In boreal forests, tree species cycle between periods of active growth and dormancy in response to a changing temperature and day-length [5]. Evergreens, have subtler changes in foliage during seasonal transitions

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