Coblentz, D. and P. L. Keating. 2008.Topographic controls on the distribution of tree islands in
the high Andes of south-western Ecuador. Journal of Biogeography (J. Biogeogr.) (2008) 35,
2026–2038.
Coblentz is with Los Alamos National Laboratory, Los Alamos, NM, USA; Keating is with the
Department of Geography, Indiana University, Bloomington, IN, USA
2) the research problem or question (1 or 2 sentences)
What is the role of topography, especially as manifest as fire regime, in maintaining the spatial
distribution of Polylepis spp. forest patches in the Ecuadoran Andes?
3) the general background knowledge leading up to this study (2 sentences)
Polylepis, the highest-living tree genus, occurs in and often dominates forest patches from 2700
to 4300 m in Andean Ecuador, where stands are imbedded in a landscape mosaic of vegetation
types ranging from natural vegetation to areas with intense human activities that could have
lasted for as long as 7000 years. Although pre-human distribution of the forests was probably
controlled by climate variation, fires, grazing, and woodcutting have reduce the previously
widespread forest to the current patchy distribution. Tree islands in Cajas National Park are
found in areas that have lower fire frequencies because of topographic factors such as very steep
slopes. The authors hypothesize that fire frequency, which is controlled by human activities,
which in turn is controlled by topographic factors, determine the distribution of Polylepis forests,
and test the hypothesis by analyzing the topography of the park.
4) the methods used by the investigators (1 or 2 sentences)
The Normalized Difference Vegetation Index (NDVI) calculated from Landsat data acquired in
1987 distinguished forest from non-forested areas, and aerial photography and field work was
used to verify the classification. An “eigenvalue ratio method” (not described in the paper itself,
but described in the online supplemental material) “to quantify topographic roughness,
organization and gradient across the landscape” was applied to a 30-m digital elevation model
(DEM). Forest patches were mapped on surfaces that describe the topographic variables.
5) the results of the study (2 sentences and one figure or table)
When compared to randomly selected sites of similar areas, tree islands are located in areas with
the “geomorphometric parameters” of high topographic roughness, high gradient (slope), and
low “organization” (a variable that is not well described in the paper). The tree islands are
restricted to sites where human activites that lead to fire are minimal.
6) what are the weaknesses of the study
The digital elevation model has a 30-m spatial resolution, which smoothes important microtopography. The relationship between topography and the forest patches is a correlation, and no
cause-effect relationship can be demonstrated. Other factors are also important for determining
forest distribution, but they were not studied.
7) what we learned new from the study (2 sentences).
Most deforestation in land easier for humans to access this area happened in the distant past, and
rough topography has excluded wood gathering and grazing, and consequent fires so Polylepis
forests remain in rough areas. Many factors may control the distribution of these forests, but
rough topography is among the most important factors.
MWB Notes: I’ve inferred from the paper’s text that fires are a consequence of human activities,
but that isn’t the only reason that inaccessible spots are fire-resistant. They are often moister,
have less organic matter fuel, and have other characteristics that have nothing to do with humans.
Nonetheless, these factors coupled with the inaccessibility seem to me to be important.