Fig 1.
Location of the study area, with (a) land cover distribution derived from MCD12Q1 in 2010, (b) temperature zone, and (c) drought zone. The dots in (a) indicate the field observations for land cover validation. The black and red triangles in (b) indicate the field measured net primary production (NPP) and observations from 4 flux tower sites for MODIS NPP validation. The stars in (c) indicate weather stations where data are obtained to calculate Standardized Precipitation Index (SPI). Abbreviations in (a): Evergreen Needleleaf Forest (ENF), Evergreen Broadleaf Forest (EBF), Deciduous Needleleaf Forest (DNF), Deciduous Broadleaf Forest (DBF), Mixed Forest (MF). Closed Shrublands and Open Shrublands have been combined as ‘Shrub’; Woody Savannas and Savannas as ‘Savannas’; Croplands and Cropland/Natural Vegetation Mosaics as ‘Cropland’; and Permanent Wetlands, Urban and Built-up areas, Snow and Ice, and Barren or Sparsely Vegetated areas have been combined as ‘Others’.
Table 1.
International Geosphere-Biosphere Program (IGBP) land cover classification scheme (a) and illogical transition matrix (b).
Illogical transitions in the matrix are labeled “X”. Adapted from ([60], the transitions from cropland (IGBP classes 12 and 14) and barren or sparsely vegetated land (IGBP 16) to forest (IGBP 1 and 3–5) have been taken as logical because of the known effects of several important Ecological Restoration Programs, including afforestation and the return of grain plots to forestry. A transition from IGBP 12, 14 and 16 to evergreen broadleaf forest (IGBP 2) is considered illogical because it is difficult for evergreen broadleaf forest to survive in the TNSP zone, which is characterized as an arid or semi-arid region.
Fig 2.
Flow chart for the decomposition analysis.
The four components shown were used to find the relative contributions of drought, land cover conversion (LCC), and other natural or anthropogenic (ONA) factors to inter-annual changes in net primary production (NPP).
Fig 3.
(a) Comparison of MODIS net primary production (NPP) with reference flux tower observations and field measured NPP, and (b) the comparison between the Standardized Precipitation Evapotranspiration Index (SPEI) product and Standardized Precipitation Index (SPI) calculated from precipitation data over 50 weather stations.
Fig 4.
Spatial patterns of the 10-year (2001–2010) averaged MODIS net primary production (NPP).
The bar graph shows the NPP distribution for different land cover types shown in the map.
Fig 5.
Spatial patterns of the temporal trend in terrestrial MODIS net primary production (NPP) for 2001–2010.
The general annual values of NPP for 2001–2010 in the Three-North Shelterbelt Program (TNSP) zone are shown in the graph.
Fig 6.
The temporal trend of Standardized Precipitation Evapotranspiration Index (SPEI) for 2001–2010 (a), and the spatial patterns of the correlation between annual net primary production and SPEI for 2001–2010. The general annual variation of SPEI for 2001–2010 in the Three-North Shelterbelt Program (TNSP) zone is showed by the line graph in part (a). SPCL in part (b) stands for ‘significant positive correlation level’.
Fig 7.
The relationship between total net primary production (NPP) values and land cover areas for 2001–2010.
* and ** denote significant correlation at the 0.05 and 0.01 p-values, respectively.
Fig 8.
The area percentage (AP) (a), change in NPP value (b), and contribution rate (CR) (c) for each component of the NPP variation for consecutive years. The changes in NPP were mainly attributable to land cover conversion (LCCM), drought but still with some influences of LCC (SPEI(LCC)M), drought (SPEIM), or other natural or anthropogenic factors excluding LCC (ONAM). The subscripts I and D indicate increases and decreases, respectively. The values showing in the grey rectangular area are the net changes in NPP (Tg C) resulting from changes in NPP.
Fig 9.
The relationships between total grasslandnet primary production (NPP) values and livestock numbers (LN) for 2001–2010, in Inner Mongolia, Ningxia, Gansu, and Xinjiang.