| Vegetation biomass is an important component of the terrestrial ecosystem. In grasslandecosystems, the vegetation biomass represents the primary production and determines the herbivorecarrying capacity. Therefore, it is critical to estimate biomass carbon stock of grasslands and itsdynamics to study the regional carbon cycle and the sustainable use of grassland resources. In this study,we estimated biomass and its spatio-temporal pattern for Inner Mongolia’s grassland between2001and2012using field samples, MODIS-NDVI time series data and different statistical models for differentregions based on the relationship between NDVI and aboveground biomass (AGB), and furtherestimated belowground biomass (BGB) based a relationship between AGB and BGB. We also exploredpossible relationships between the spatio-temporal pattern of biomass and climatic factors (temperatureand precipitation). The main results were obtained:(1) The precision of NDVI-based models were generally higher than these of EVI. We also foundthat mean NDVI of peak season (July to August) were superior to the other models. The best modelsobtained for the east meadow grassland region (Regions I), the middle temperate grassland region(Regions II), and the west desert grassland and desert region (Regions III), were exponential (precisionis74%), power (precision is73%), and unitary linear (precision is67%), respectively. Our findingssuggested that establishing separate statistical models for different vegetation conditions may reduce theuncertainty of AGB estimation on a large spatial scale.(2) Using NDVI time series data combined with field-based sample data, we calculated thebiomass carbon storage in Inner Mongolia’s grassland between2001~2012. The total biomass carbonstorage was estimated to be131.77TgC over a total area of19.6×104km~2, with20.88TgC and110.90TgC occurring in AGB and BGB, respectively (average over the12years). Our result was lower thanprevious estimates. The differences among these estimates may be due to different data sources,estimating approaches, and studying periods.(3) The biomass in Inner Mongolia exhibited a large spatial heterogeneity, decreasing from thenortheast to the southwest. The mean biomass density was194.54gC/m~2over the whole study area overthe12–year study period, with30.82gC/m~2and163.72gC/m~2occurring in AGB and BGB, respectively.During2001~2012, the carbon storage in biomass of Inner Mongolia’s grassland experienced a generalfluctuation (coefficient of variation is10.51%), with no significant trend (R~2=0.03, P=0.57), implyingthat biomass in Inner Mongolia’s grassland was carbon neutral.(4) The AGB in Inner Mongolia’s grassland was showed a robust positive correlation withprecipitation, but different R~2were found in various periods (January~August, April~August,May~August, January~July, April~July, and May~July). The AGB was most closely related to maingrowing season precipitation (GSP, May~August), which explained40%of variation in AGB.(5) The AGB variation over the12-year period was closely coupled with the pattern ofprecipitation. Interannual variability in AGB decreased with precipitatin increased, and the AGB stability was gradually increasing along the precipitation gradient (R~2=0.43, P<0.0001). Our analysisalso showed a significant positive correlation between the interannual variability of AGB and that ofGSP (R~2=0.22，P<0.0001).(6) The GSP was also an important factor in the spatial pattern of AGB over the study area. Ourresult indicated that AGB increasd exponentially along the precipitation gradient, which explained68%of the spatial ANPP. The slope of the linear relationships between AGB and GSP appeared to be steeperin the Region I than that in the Region II and the Region III. A moisture index (K) that combined theeffects of precipitation and temperature explained more variation in AGB than did GSP alone (R~2=0.70,P<0.0001). |
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