Abstract
Abstract. Drylands, covering nearly 30% of the global land surface, are characterized by high climate variability and sensitivity to land management. Here, two satellite-observed vegetation products were used to study the long-term (1988–2008) vegetation changes of global drylands: the widely used reflective-based Normalized Difference Vegetation Index (NDVI) and the recently developed passive-microwave-based Vegetation Optical Depth (VOD). The NDVI is sensitive to the chlorophyll concentrations in the canopy and the canopy cover fraction, while the VOD is sensitive to vegetation water content of both leafy and woody components. Therefore it can be expected that using both products helps to better characterize vegetation dynamics, particularly over regions with mixed herbaceous and woody vegetation. Linear regression analysis was performed between antecedent precipitation and observed NDVI and VOD independently to distinguish the contribution of climatic and non-climatic drivers in vegetation variations. Where possible, the contributions of fire, grazing, agriculture and CO2 level to vegetation trends were assessed. The results suggest that NDVI is more sensitive to fluctuations in herbaceous vegetation, which primarily uses shallow soil water, whereas VOD is more sensitive to woody vegetation, which additionally can exploit deeper water stores. Globally, evidence is found for woody encroachment over drylands. In the arid drylands, woody encroachment appears to be at the expense of herbaceous vegetation and a global driver is interpreted. Trends in semi-arid drylands vary widely between regions, suggesting that local rather than global drivers caused most of the vegetation response. In savannas, besides precipitation, fire regime plays an important role in shaping trends. Our results demonstrate that NDVI and VOD provide complementary information and allow new insights into dryland vegetation dynamics.
Highlights
Drylands cover nearly 30 % of the global land surface, they are characterized by high climate variability and are sensitive to land use practice (Tietjen et al, 2010)
For the arid drylands, mean Vegetation Optical Depth (VOD) values were generally lower than Normalized Difference Vegetation Index (NDVI), with increasing humidity and biomass VOD increased faster than NDVI (Fig. 2a and b)
Following Liu et al (2011a), we found that VOD showed stronger increase than NDVI, moving from low to high biomass regions (Figs. 2a, b and 3)
Summary
Drylands cover nearly 30 % of the global land surface, they are characterized by high climate variability and are sensitive to land use practice (Tietjen et al, 2010). The primary drivers of vegetation dynamics in drylands include: (i) climate (Herrmann et al, 2005; Bai et al, 2008); (ii) fire regime (Bond and Keeley, 2005; Archibald et al, 2010); (iii) grazing (Asner et al, 2004; Liu et al, 2013b); (iv) agriculture (Piao et al, 2003; Jeyaseelan et al, 2007); and (v) atmospheric CO2 concentrations (Bond and Midgley, 2012; Donohue et al, 2013). Where ρ is the wavelength reflectance and f the fraction canopy cover, with subscripts denoting overstory (o), understory (u) and soil surface (s) This is a somewhat simplified approach, as light reflecting from one component can affect reflectance of another component by multiple scattering and transmittance (Roberts et al , 1993). All else being equal, an increase in NDVI can be explained by an increase in total canopy cover or a relative increase in recurrent vegetation canopy cover, or both
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
