Assessing MODIS GPP in Non-Forested Biomes in Water Limited Areas Using EC Tower Data

Flor Álvarez-Taboada,David Tammadge,Andrew Skidmore,Martin Schlerf

doi:10.3390/rs70303274

Abstract

Although shrublands, savannas and grasslands account for 37% of the world’s terrestrial area, not many studies have analysed the role of these ecosystems in the global carbon cycle at a regional scale. The MODIS Gross Primary Production (GPP) product is used here to help bridge this gap. In this study, the agreement between the MODIS GPP product (GPPm) and the GPP Eddy Covariance tower data (GPPec) was tested for six different sites in temperate and dry climatic regions (three grasslands, two shrublands and one evergreen forest). Results of this study show that for the non-forest sites in water-limited areas, GPPm is well correlated with GPPec at annual scales (r2 = 0.77, n = 12; SEE = 149.26 g C∙m−2∙year−1), although it tends to overestimate GPP and it is less accurate in the sites with permanent water restrictions. The use of biome-specific models based on precipitation measurements at a finer spatial resolution than the Data Assimilation Office (DAO) values can increase the accuracy of these estimations. The seasonal dynamics and the beginning and end of the growing season were well captured by GPPm for the sites where (i) the productivity was low throughout the year or (ii) the changes in the flux trend were abrupt, usually due to the restrictions in water availability. The agreement between GPPec and GPPm in non-forested sites was lower on a weekly basis than at an annual scale (0.44 ≤ r2 ≤ 0.49), but these results were improved by including meteorological data at a finer spatial scale, and soil water content and temperature measurements in the model developed to predict GPPec (0.52 ≤ r2 ≤ 0.65).

Highlights

Savannas and grasslands account for 37% of the world’s terrestrial area, many studies have been conducted to estimate how forests contribute to the global carbon cycle [1,2,3,4,5,6,7,8], but not many monitor the implications of grasslands/shrublands on the carbon cycle at a large scale [9,10]
The results show that GPPm is highly correlated with GPP Eddy Covariance tower data (GPPec) for grassland and shrubland areas on an annual basis (r2 = 0.94, standard error of the estimates (SEE) = 142.57), which agrees with the findings of [13] for three sites (r = 0.86) and [18] in an across-biome validation where 10 out of the 42 sites were in nonforested areas (r2 = 0.84, n = 42)
In non-forested sites in water-limited areas, the Moderate Resolution Imaging Spectroradiometer (MODIS) Gross Primary Production (GPP) product is well correlated with the GPP EC tower estimations when averaged at an annual level, it tends to overestimate

Summary

Introduction

Savannas and grasslands account for 37% of the world’s terrestrial area, many studies have been conducted to estimate how forests contribute to the global carbon cycle [1,2,3,4,5,6,7,8], but not many monitor the implications of grasslands/shrublands on the carbon cycle at a large scale [9,10] Monitoring these non-forested ecosystems in dry areas is important since they are very sensitive to changes in water availability [11] and because they have been suggested as potential candidate areas for major carbon storage efforts [12]. The GPP estimates from EC towers (GPPec) only represent fluxes at the tower footprint scale (between one hundred metres and several kilometres) [18], and the estimation of GPP at regional scale can only be made by using ecosystem models

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Conclusion