Towards operational remote sensing of forest carbon balance across Northern Europe

P Olofsson,J Lindström,L Klemedtsson,A Lindroth,L Eklundh,W Kutsch,F Lagergren

doi:10.5194/bg-5-817-2008

Abstract

Abstract. Monthly averages of ecosystem respiration (ER), gross primary production (GPP) and net ecosystem exchange (NEE) over Scandinavian forest sites were estimated using regression models driven by air temperature (AT), absorbed photosynthetically active radiation (APAR) and vegetation indices. The models were constructed and evaluated using satellite data from Terra/MODIS and measured data collected at seven flux tower sites in northern Europe. Data used for model construction was excluded from the evaluation. Relationships between ground measured variables and the independent variables were investigated. It was found that the enhanced vegetation index (EVI) at 250 m resolution was highly noisy for the coniferous sites, and hence, 1 km EVI was used for the analysis. Linear relationships between EVI and the biophysical variables were found: correlation coefficients between EVI and GPP, NEE, and AT ranged from 0.90 to 0.79 for the deciduous data, and from 0.85 to 0.67 for the coniferous data. Due to saturation, there were no linear relationships between normalized difference vegetation index (NDVI) and the ground measured parameters found at any site. APAR correlated better with the parameters in question than the vegetation indices. Modeled GPP and ER were in good agreement with measured values, with more than 90% of the variation in measured GPP and ER being explained by the coniferous models. The site-specific respiration rate at 10°C (R10) was needed for describing the ER variation between sites. Even though monthly NEE was modeled with less accuracy than GPP, 61% and 75% (dec. and con., respectively) of the variation in the measured time series was explained by the model. These results are important for moving towards operational remote sensing of forest carbon balance across Northern Europe.

Highlights

The global carbon balance is the result of fluxes of carbon into and out of ocean and land
The aim of this paper is to investigate the possibilities of implementing a model for obtaining net ecosystem exchange (NEE), gross primary production (GPP) and respiration over large areas
The models were estimated using measured values on air temperature (AT) and PAR, with absorbed photosynthetically active radiation (APAR) being the product of measured PAR and fractional absorption of PAR (FAPAR) estimated from satellite

Summary

Introduction

The global carbon balance is the result of fluxes of carbon into and out of ocean and land. There is scientific evidence of a missing carbon sink in the global carbon cycle, probably located in the Northern hemisphere (Tans et al, 1990; Keeling et al, 1996). With boreal forests covering large parts of the Northern hemisphere, knowledge of the carbon sink/strength of these regions is if great importance. It is possible to measure the net exchange of CO2 between atmosphere and biosphere for long periods of time using eddy covariance methods. These measurements have proven to be of great importance for studies concerning carbon budgets and their seasonal patterns

Objectives

Results

Conclusion