北海道大学 大学院農学院 修士論文発表会，2019 年 2 月 7 日

Estimation of ecosystem photosynthesis parameters using normalized difference
spectral indexes from hyper-spectral images
環境資源学専攻 地域環境学講座 生態環境物理学 李 静

1. Introduction
Nowadays, the increased global CO2 concentration leads to various kinds of environmental impacts. Thus it
is important to estimate the carbon dynamics of terrestrial ecosystems through photosynthesis (Gross PrimaryProduction: GPP). To do it for a wide area and know a spatial variation, satellite remote sensing with opticalsensors could be a powerful tool. For example, the parameters of the light curve of GPP were determined using
the reflected radiance of two broad wavebands in shortwave radiation from the ecosystem surface. To improve
the accuracy of the GPP estimation, it is necessary to measure the spectral reflectance for a narrower waveband
and to explore the best combination of wavelengths, which are highly correlated to the parameters. Therefore,
field experiments were conducted at three different ecosystems in Hokkaido using a spectral camera.

2. Materials and Methods
The field study was conducted in a snow-free period at three sites: Teshio experimental forest of Hokkaido
University in 2016 to 2018, Bibai Mire in 2016 and Tomakomai national forest in 2015. At each site, thespectral images of the plant canopy were taken every 15 min in the daytime from 460 to 780 nm at intervals of
10 nm with a liquid crystal tunable filter (LCTF) camera installed on a tower. The reflected radiance at eachpixel was converted to reflectance (Ri: i denotes a wavelength) using spectral solar irradiance measured with aspectro-radiometer. The reflectance at each wavelength was averaged within the region of interest (ROI) set inthe image. For every combination of two wavelengths, normalized difference spectral indices (NDSI[i,j] = (Ri– Rj) / (Ri + Rj)) were calculated. Simultaneously, net ecosystem CO2 exchange (NEE) was measured
continuously by the eddy covariance technique. After quality control, NEE was partitioned into GPP andecosystem respiration (RE) using an empirical method. Using a 5-day-long moving window, a non-rectangular
hyperbola was fitted to the relationship of GPP with photosynthetic photon flux density (PPFD), and the twoparameters of initial slope (φ) and maximum GPP (GPPmax) were determined every day. Then, to explore thebest combination of two wavelengths, correlation between NDSIs and the parameters were analyzed.

3. Results and Discussion
The best combination of wavelengths with the highest coefficient of determination (R2) to determine theparameters was 780 nm in near infrared and 710 nm in red edge at every site and in every year. Using NDSIsof this combination, it is possible to estimate the initial slope (φ) and GPP from solar radiation. After thecomparison of observed GPP and estimated GPP, the R2 of Teshio in each year was 0.89, 0.90 and 0.90, andthose of Bibai and Tomakomai were 0.85 and 0.86, respectively. This result showed that there is possibility toestimate GPP with high accuracy from NDSI determined hyperspectral images taken with the LCTF camera.
Also, interannual variations and inter-site differences in phenology between plant species could be confirmed
through NDSI[780,710] of different periods of plant respectively.

4. Conclusions
In this study, the best combination of two wavelengths that highly related to the light curve of GPP is thesame despite different ecosystems in different years. The result may provide useful insights for the assessmentof radiation use efficiency and photosynthetic capacity in ecosystems using spectral data.