The extended sensitivity of the EVI has facilitated phenology studies in dense
tropical rainforests, where MODIS and SPOT-VGT EVI were shown to discriminate phenology cycles in Amazon rainforests (Huete et al. 2006; Xiao et al. 2005,
2006) (Fig. 1.13a). Using 250 m and 0.05°EVI data from MODIS, Huete et al.
(2006) found Amazon rainforests to green-up by 25 % in the dry season in
response to the increased availability of sunlight. Disturbed forest areas, on the
other hand, showed dry-season declines in EVI, presumably because the more
shallow-rooted vegetation had reduced access to deep soil water. This was verified
by a strong linear and consistent relationship between seasonal EVI and towercalibrated GPP measurements of carbon fluxes in both intact rainforest and forest
conversion to pasture/agriculture sites in the Amazon (Fig. 1.13b).
Saleska et al. (2007) later analyzed Amazon rainforest EVI response to drought
and found a positive greening response to a short drought event in 2005, rather than
the expected negative response. Samanta et al. (2010) reported a much smaller
spatial extent in greening after more rigorous screening of contaminated pixels,
however, the proportion of acceptable quality pixels that showed greening remained
the same, and there was little drought-induced browning observed. Other studies
have confirmed the positive greening response to the 2005 drought, while reporting a
definitive negative response to a more severe drought in 2010 that clearly suppressed
photosynthetic activity (Xu et al. 2011; Anderson 2012; Brando et al. 2010).
The MODIS vegetation phenology product (MOD12Q2) uses maximum
inflections in seasonal NBAR-EVI profiles to produce a global set of phenology
metrics based on key transition dates related to vegetation growth activity (Zhang
et al. 2003; Ganguly et al. 2010). The NBAR-EVI has been successfully used to
map the phenology of single, double, and triple rice cropping patterns in the
Mekong delta where previously this was only accomplished with SAR data
(Sakamoto et al. 2006). NBAR-EVI was also used to show the effect of urban
climates on vegetation phenology transition dates in North American cities (Zhang
et al. 2004). Strong heat island effects were found in urban areas with increases in
the growing season of *15 days and delays in the onset of dormancy, relative to
adjacent non-disturbed ecosystems, a pattern that decays exponentially with distance from urban areas.
1.5.2 Carbon and Water Science
Vegetation indices have demonstrated their utility in studies of ecosystem functions
which affect net ecosystem exchange of CO 2 and water between the land and the
atmosphere. Most carbon exchange models use a light-use efficiency (LUE) relationship in which gross primary productivity (GPP) is related to the amount of PAR
absorbed by green vegetation multiplied by the efficiency with which the absorbed
light is used in carbon fixation, or photosynthesis (Monteith and Unsworth 1990),
GPP ¼ LUE Â APAR ¼ LUE Â f APAR Â PAR:
ð1:13Þ
1 Indices of Vegetation Activity
23
tropical rainforests, where MODIS and SPOT-VGT EVI were shown to discriminate phenology cycles in Amazon rainforests (Huete et al. 2006; Xiao et al. 2005,
2006) (Fig. 1.13a). Using 250 m and 0.05°EVI data from MODIS, Huete et al.
(2006) found Amazon rainforests to green-up by 25 % in the dry season in
response to the increased availability of sunlight. Disturbed forest areas, on the
other hand, showed dry-season declines in EVI, presumably because the more
shallow-rooted vegetation had reduced access to deep soil water. This was verified
by a strong linear and consistent relationship between seasonal EVI and towercalibrated GPP measurements of carbon fluxes in both intact rainforest and forest
conversion to pasture/agriculture sites in the Amazon (Fig. 1.13b).
Saleska et al. (2007) later analyzed Amazon rainforest EVI response to drought
and found a positive greening response to a short drought event in 2005, rather than
the expected negative response. Samanta et al. (2010) reported a much smaller
spatial extent in greening after more rigorous screening of contaminated pixels,
however, the proportion of acceptable quality pixels that showed greening remained
the same, and there was little drought-induced browning observed. Other studies
have confirmed the positive greening response to the 2005 drought, while reporting a
definitive negative response to a more severe drought in 2010 that clearly suppressed
photosynthetic activity (Xu et al. 2011; Anderson 2012; Brando et al. 2010).
The MODIS vegetation phenology product (MOD12Q2) uses maximum
inflections in seasonal NBAR-EVI profiles to produce a global set of phenology
metrics based on key transition dates related to vegetation growth activity (Zhang
et al. 2003; Ganguly et al. 2010). The NBAR-EVI has been successfully used to
map the phenology of single, double, and triple rice cropping patterns in the
Mekong delta where previously this was only accomplished with SAR data
(Sakamoto et al. 2006). NBAR-EVI was also used to show the effect of urban
climates on vegetation phenology transition dates in North American cities (Zhang
et al. 2004). Strong heat island effects were found in urban areas with increases in
the growing season of *15 days and delays in the onset of dormancy, relative to
adjacent non-disturbed ecosystems, a pattern that decays exponentially with distance from urban areas.
1.5.2 Carbon and Water Science
Vegetation indices have demonstrated their utility in studies of ecosystem functions
which affect net ecosystem exchange of CO 2 and water between the land and the
atmosphere. Most carbon exchange models use a light-use efficiency (LUE) relationship in which gross primary productivity (GPP) is related to the amount of PAR
absorbed by green vegetation multiplied by the efficiency with which the absorbed
light is used in carbon fixation, or photosynthesis (Monteith and Unsworth 1990),
GPP ¼ LUE Â APAR ¼ LUE Â f APAR Â PAR:
ð1:13Þ
1 Indices of Vegetation Activity
23