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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B8, 2012 
XXII ISPRS Congress, 25 August — 01 September 2012, Melbourne, Australia 
MONITORING SPATIAL PATTERNS OF VEGETATION PHENOLOGY IN 
AN AUSTRALIAN TROPICAL TRANSECT USING MODIS EVI 
Xuanlong Ma?*^, Alfredo Huete^; Qiang Yu*^, Kevin Davies?, and Natalia Restrepo Coupe? 
* Plant Functional Biology and Climate Change Cluster, University of Technology, Sydney, Australia 
P Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing, China 
Commission VIIV6 
KEY WORDS: NATT, tropical savannas, phenology, climate change, MODIS, EVI 
ABSTRACT: 
Phenology is receiving increasing interest in the area of climate change and vegetation adaptation to climate. The 
phenology of a landscape can be used as a key parameter in land surface models and dynamic global vegetation mod- 
els to more accurately simulate carbon, water and energy exchanges between land cover and atmosphere. However, 
the characterisation of phenology is lacking in tropical savannas which cover more than 30% of global land area, and 
are highly vulnerable to climate change. The objective of this study is to investigate the spatial pattern of vegetation 
phenology along the Northern Australia Tropical Transect (NATT) where the major biomes are wet and dry tropical 
savannas. For this analysis we used more than 11 years Moderate Resolution Imaging Spectroradiometer (MODIS) 
Enhanced Vegetation Index (EVI) product from 2000 to 2011. Eight phenological metrics were derived: Start of Sea- 
son (SOS), End of Season (EOS), Length of Season (LOS), Maximum EVI (MaxG), Minimum EVI (MinG), annual 
amplitude (AMP), large integral (LIG), and small integral (SIG) were generated for each year and each pixel. Our 
results showed there are significant spatial patterns and considerable interannual variations of vegetation phenology 
along the NATT study area. Generally speaking, vegetation growing season started and ended earlier in the north, and 
started and ended later in the south, resulting in a southward decrease of growing season length (LOS). Vegetation 
productivity, which was represented by annual integral EVI (LIG), showed a significant descending trend from the 
northern part of NATT to the southern part. Segmented regression analysis showed that there exists a distinguishable 
breakpoint along the latitudinal gradient, at least in terms of annual minimum EVI (EVI), which is located between 
18.84°S to 20.04° S. 
1 INTRODUCTION 
Phenology as a subject to study the life cycles of vegeta- 
tion and the interactions between vegetation and climate 
(White and Thornton, 1997) is receiving increasing in- 
terests in global change research. Vegetation phenology 
can be used as a key parameter in large scale ecosys- 
tem simulation models (Running and Hunt, 1993) and 
general circulation models (Sellers et al., 1996). At the 
same time, vegetation phenology is also an accurate in- 
dicator of influences by climate change on vegetation 
growth (Menzel et al., 2006). 
Phenological studies of vegetation traditionally utilised 
ground based techniques (Jeffree, 1960, Sparks and Jef- 
free, 2000), however, increasing number of studies utilise 
remote sensing to study vegetation phenology on a large 
scale (Schwartz, 1999, Zhang et al., 2003, Stóckli, 2004). 
Compared with field based cameras or visual inspection, 
space borne optical sensors such as MERIS (MEdium 
Resolution Imaging Spectrometer) and MODIS (Mod- 
erate Resolution Imaging Spectroradiometer) are able 
to provide daily measurements of variety biophysical 
and biochemical information of the earth's surface with 
moderate spatial resolution. 
* Corresponding address: Plant functional biology and cli- 
mate change cluster, University of Technology, Sydney, PO 
Box 123, Broadway, NSW, 2000, Australia, Tel: 461 2 9514 
4084, Email: alfredo.huete @uts.edu.au 
However, till now most remote sensing phenology stud- 
ies focused on temperature and light limited systems 
(Beurs and Henebry, 2010), with few conducted on wa- 
ter limited systems (Brown and de Beurs, 2008), and 
rarely on tropical savannas. Tropical savannas are gen- 
erally defined as a biome with discrete tree stratum and 
continuous grassy ground layer (Frost et al., 1986), which 
covers one-sixth of the global land surface, and con- 
tributes approximately 3096 of the gross primary pro- 
ductivity (GPP) of all terrestrial ecosystems (House and 
Hall, 2001). Tropical savannas are also considered par- 
ticularly vulnerable to climate change (Canadell et al., 
2003). Despite the importance of tropical savannas, stud- 
ies of its vegetation phenology are lacking regardless 
of the methods, thus restricting our capability to under- 
stand the impact of possible climate change scenarios on 
tropical savannas ecosystems. 
Previous studies showed that a biogeographical bound- 
ary existed in the NATT area, which may distributed 
around 16-20 °S. 18-20 °S was considered as the south 
limit of the influences from monsoonal rainfall (Bow- 
man, 1996, Burbidge, 1960), 15-16 °S was considered 
as the southern limit of wet season as well as the south- 
ern limit of monsoon tall-grass savannas (Cook and Heerde- 
gen, 2001). Meanwhile, in terms of vegetation family 
and species, the major changes occur around 16-17 °S 
(Egan and Williams, 1996). Based on these findings, 
we hypothesised that, if such a virtual biogeographical