Altitude, slope and aspect
Altitude (in m) was extracted from the SRTM (Shuttle Radar Topography Mission) 90 m Digital Elevation Data for the entire world provided by the CGIAR-CSI GeoPortal. We used the function gdalwrap from the GDAL library to mosaick the 5 degree by 5 degree tiles and resample the data at 1 km resolution in the projection WGS 84 / UTM 38S (epsg: 32738).
Slope and aspect were computed from altitude using the function r.slope.aspect from the GRASS GIS software. Slope is in degree. Aspect computed from r.slope.aspect is counterclockwise from East. We transformed the aspect data so that it is provided here clockwise from North in degree.
Solar radiation (in Wh.m-2.day-1) was computed from altitude, slope and aspect using the function r.sun from the GRASS GIS software. We incorporated the shadowing effect of terrain to compute the solar radiation. Solar radiation was computed for the Julian day 79 (20th of March for regular years=equinox).
We used the simplified geology map of Madagascar provided by Justin Moat and David Du Puy from the GIS unit of the Royal Botanical Gardens at Kew. The map was digitized from the geology map of Madagascar (Besairie, 1964). The 96 original categories were reclassified into 11 predominant rock types which seem to have an important effect on the vegetation they support. Caution: there are only 11 rock types but raster values range from 1 to 13 following original reclassification from Kew.
|Raster value||Rock type|
|1||Alluvial & Lake deposits|
|5||Tertiary Limestones + Marls & Chalks|
|7||Mesozoic Limestones + Marls (inc. "Tsingy")|
|9||Lavas (including Basalts & Gabbros)|
|10||Basement Rocks (Ign & Met)|
To be done
To be done
A recently proposed hypothesis (the “watershed hypothesis”) suggests that allopatric speciation driven by isolation in watersheds during Quaternary climate shifts provides a general explanation for patterns of local endemism across taxa in Madagascar (Wilmé et al. 2006). Watersheds with sources at high elevation (>2000 m) might have maintained mesic conditions during Quaternary climate shifts due to orographic precipitation. A map of these watersheds is provided by Pearson et al. (2009) who compared the watershed versus climatic gradient hypothesis for determining the local endemism for several species of lemurs, geckos and chameleons. The retreat-dispersion watersheds map is available on the Evolution scientific journal website.
Percentage of forest cover
Madagascar’s forest in 2010 was derived from the 30 m resolution 2000 forest map by Harper et al. (2007). On Harper’s map, ∼200,000 ha clouds are present over the ∼4.2 million ha moist forest ecoregion (4.8% of clouds). To remove these clouds, we used the 2000 cloud-free tree percent cover map provided by Hansen et al. (2013) (also at
30 m resolution) and we chose a threshold of 75% of tree cover to decide whether to replace cloud pixels with forest or non-forest pixels. We thus obtained a cloud-free year 2000 forest map for Madagascar. From this map, using deforestation data from 2000 to 2010 by Hansen et al. (2013), we obtained a cloud-free forest map in 2010 at 30 m resolution.
From the forest map in 2010 at 30 m resolution, we computed the percentage of forest cover in 1 km by 1 km grid cells.
Besairie H (1964) Carte Géologique de Madagascar au 1:1,000,000e, trois feuilles en couleur. Service Géologique, Antananarivo.
Hansen MC, Potapov PV, Moore R, et al. (2013) High-resolution global maps of 21st-
century forest cover change. Science, 342, 850-853.
Harper GJ, Steininger MK, Tucker CJ, Juhn D, Hawkins F (2007) Fifty years of deforestation and forest fragmentation in Madagascar. Environmental Conservation, 34, 325-333.
Pearson RG, Raxworthy CJ (2009) The evolution of local endemism in Madagascar: watershed versus climatic gradient hypotheses evaluated by null biogeographic models. Evolution, 63, 959-967.
Wilmé L, Goodman SM, Ganzhorn JU (2006) Biogeographic evolution of Madagascar's microendemic biota. Science, 312, 1063-1065