7th Workshop on Forest Fire Management - EARSeL, European ...

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IV - BURNED LAND MAPPING, FIRE SEVERITY DETERMINATION, AND VEGETATION RECOVERY ASSESSMENT
(Keeley, 1986), thus affecting the structure of plant communities and causing
a decline of long-term ecosystem productivity. The effective manifestation
of post-fire succession is determined by a combination of local physical
conditions, climatic factors and pre-fire vegetation (e.g. Lloret and Vilà,
2003; Whelan, 1995).
In the above-mentioned context, a considerable number of fire recovery
studies, based on remote sensing information, have been conducted in
regions characterized by Mediterranean climates. In particular, spectral
vegetation indices (Röder et al., 2008), namely NDVI, have proven to be
especiallty useful to monitor plant regeneration after fire (Díaz-Delgado et
al., 2003).
2 - Results
The response of vegetation was assessed based on fields of Normalised
Difference Vegetation Index (NDVI) as derived from images acquired by the
VEGETATION instrument on-board SPOT 4 and SPOT 5. Covering the period
1999-2006, data consist of the S10 products of the VITO database
(http://free.vgt.vito.be) at the 1km x 1km spatial scale.
Large burnt scars in Portugal during the 2003 and 2005 fire seasons were
identified using k-means clustering applied to 12 monthly NDVI anomalies,
from September of the year to August of the year after the fire season. As
shown in Figure 1, cluster analysis allows identifying two (four) clusters in
2003 (2005). In both years, the centroid associated to systematic negative
values of monthly anomalies throughout the year (black circles) is worth
being pointed out since the corresponding spatial distributions (Figure 2)
correspond to burnt scars (black pixels).
Figures 3 and 4 present observed NDVI values over areas corresponding to
four selected burnt scars, two of them respecting to the 2003 fire season
and the other two to 2005. The locations of the scars are indicated by the
red boxes in Figure 2. During the late spring preceding the occurrence of
the fire episodes, namely in May 2003 and 2005, all regions present high
levels of vegetation activity (left panels of Figures 3 and 4), with values of
NDVI around 0.7. Nine months after the fire episodes, in May 2004 and
2006 (middle panels), values of NDVI drop to around 0.35 on average. It
may be noted, however, that the spatial distributions of NDVI before the
fire season of 2003 present higher values than the ones before the fire season
of 2005, in particular over the burnt scar located in the south of
Portugal (Figure 3, bottom panel), a feature that relates with the severe
drought that hit the country during 2004/2005. A similar behaviour may be
observed with the NDVI values after the fire events, which provide an indication
of fire sevirity. Again, NDVI after the fire season of 2003 tends to
be larger than NDVI after the fire season of 2005. Finally, relative differences
between NDVI of May before and after the fire events (left panels of
Figures 3 and 4) present higher values in the case of the burnt scars locat-