A new approach to evaluate the MODIS annual NPP product (MOD17A3) using forest field data from Turkey


Gulbeyaz O. , Bond-Lamberty B., Akyurek Z. , West T. O.

INTERNATIONAL JOURNAL OF REMOTE SENSING, cilt.39, ss.2560-2578, 2018 (SCI İndekslerine Giren Dergi) identifier identifier

  • Cilt numarası: 39 Konu: 8
  • Basım Tarihi: 2018
  • Doi Numarası: 10.1080/01431161.2018.1430913
  • Dergi Adı: INTERNATIONAL JOURNAL OF REMOTE SENSING
  • Sayfa Sayıları: ss.2560-2578

Özet

In this study, we present the first evaluation of the MODIS (Moderate Resolution Imaging Spectroradiometer) annual net primary production (NPP) for Turkey's forest ecosystems using field measurements. Due to lack of country scale field measurements (i.e. flux tower for forest ecosystems), tree DBH (diameter at breast height) data set provided by Ministry of Forest and Water Affairs (MFWA) of Turkey is used to calculate NPP of Turkey's forest ecosystems. The lack of a reliable NPP data set leads the researchers to use global NPP models such as MODIS annual NPP product. The MODIS MOD17A3 product of vegetation NPP is one of the most highly used data sources for studies of global carbon cycle. However, it is still necessary to test its predictions in multiple biomes, especially for heterogeneous areas in terms of its accuracy and potential bias. Here, we studied a new approach to evaluate coarse scale NPP estimates from the MODIS NPP-MOD17A3 data product, using 2008-2013 field measurements of tree growth throughout Turkey. Three different methods were used to calculate field NPP, including standardized growth coefficients (ministry coefficients [MC]), growth coefficients from North America (Jenkins coefficients [JC]), and annual expected increment (AEI). The average NPP values for all the country is calculated as 2.06kgC m(-1)/5years (0.412kgC m(-2)year(-1)) (SD=1.15kgC m(-1)/5years) from MOD17A3, 0.90kgC m(-1)/5years (0.18kgC m(-2)year(-1)) (SD=0.57kgC m(-1)/5years) with MC, 0.63kgC m(-1)/5years (0.126kgC m(-2)year(-1)) (SD=0.37kgC m(-1)/5years) with JC and 0.58kgC m(-2)year(-1) (SD=0.29kgC m(-1)/5years) with AEI for the studied plots. We found that the MODIS NPP product has a clear relation with both the NPP estimates obtained by using MC (R-2=0.34, root mean square error (RMSE)=1.51kgC m(-1)/5years) and JC (R-2=0.32, RMSE=1.73kgC m(-1)/5years). In addition to that, the relation between MOD17A3 product and AEI-derived NPP is relatively strong (R-2=0.48, RMSE=0.26kgC m(-2)year(-1)). We discuss possible reasons for these trade-offs among different methods. This study lays out a new approach to validate coarse scale MODIS product using field data directly, including for highly heterogeneous areas.