Disentangling the Relationships between Net Primary Production and Precipitation in Southern Africa Savannas Using Satellite Observations from 1982 to 2010

Likai Zhu,Jane Southworth

doi:10.3390/rs5083803

Abstract

To obtain a better understanding of the variability in net primary production (NPP) in savannas is important for the study of the global carbon cycle and the management of this particular ecosystem. Using satellite and precipitation data sets, we investigated the variations in NPP in southern African savannas from 1982 to 2010, and disentangled the relationships between NPP and precipitation by land cover classes and mean annual precipitation (MAP) gradients. Specifically, we evaluate the utility of the third generation Global Inventory Monitoring and Modeling System (GIMMS3g) normalized difference vegetation index (NDVI) dataset, in comparison with Moderate-resolution Imaging Spectroradiometer (MODIS) derived NPP products, and find strong relationships between the overlapping data periods (2000–2010), such that we can apply our model to derive NPP estimates to the full 29-year NDVI time-series. Generally, the northern portion of the study area is characterized by high NPP and low variability, whereas the southern portion is characteristic of low NPP and high variability. During the period 1982 through 2010, NPP has reduced at a rate of −2.13 g∙C∙m−2∙yr−1 (p < 0.1), corresponding to a decrease of 6.7% over 29 years, and about half of bush and grassland savanna has experienced a decrease in NPP. There is a significant positive relationship between mean annual NPP and MAP in bush and grassland savannas, but no significant relationship is observed in tree savannas. The relationship between mean annual NPP and MAP varies with increases in MAP, characterized as a linear relationship that breaks down when MAP exceeding around 850–900 mm.

Highlights

Vegetation over terrestrial ecosystems is a major component of the global carbon cycle since it regulates climate through the exchange of energy, water vapor, and momentum between the land surface and the atmosphere, controls CO2 and currently absorbs about one-third of anthropogenic fossil fuel emissions to the atmosphere [1,2,3]
We address three main research questions: (1) Can we use the GIMMS3g products to estimate net primary production (NPP) from 1982 to 2010 based only on its relationship to the Moderate-resolution Imaging Spectroradiometer (MODIS) MOD17 NPP product? (2) How has NPP changed from 1982 to 2010 across the study area? and (3) What are the relationships between mean annual precipitation (MAP)
Our results show that MODIS NPP has a significant linear relationship with GIMMS3g normalized difference vegetation index (NDVI) across the entire study area

Summary

Introduction

Vegetation over terrestrial ecosystems is a major component of the global carbon cycle since it regulates climate through the exchange of energy, water vapor, and momentum between the land surface and the atmosphere, controls CO2 and currently absorbs about one-third of anthropogenic fossil fuel emissions to the atmosphere [1,2,3]. Several studies have developed a global and regional understanding of NPP variations though methods, time periods used and results have varied. Nemani et al [4] has reported a global increase in NPP by 3.4 petagrams of carbon between 1982 and 1999, based on estimates modeled using. AVHRR data inputs and climate variables, with 80% of the increase being attributable to ecosystems in tropical regions (mainly rainforests) and those in the high latitudes of the North Hemisphere (NH). Studies have demonstrated that terrestrial NPP has increased in the middle and high latitudes in the NH [3,4,5,6,7,8,9] during the 1980–1999 time period. Zhao and Running [10] concluded that

Results

Discussion

Conclusion