The mountains dataset has been overlaid with a global data set on percent tree cover taken from MODIS 1-km resolution percent tree cover data, courtesy of University of Maryland Global Land Cover Facility. Species richness, density and forest height tend to reduce with increasing altitude; the boundary between forest vegetation and more open ground cover at higher elevation 'the treeline' is an ecological marker signifying the transition to more extreme climatic conditions. 

To provide a global context for a discussion of mountain forests, it is first necessary to define the locations and types of mountain forests, and this in turn requires a definition of mountains or mountain areas. Altitude and slope and the environmental gradients they generate are key components of such a definition, but their combination is problematic. Simple altitude thresholds both exclude older and lower mountain systems and include areas of relatively high elevation that have little topographic relief and few environmental gradients. Using slope as a criterion on its own or in combination with altitude can resolve the latter problem, but not the former. The mountains dataset shows the location of mountain land estimated from a digital elevation model using criteria based on elevation alone (the upper three classes: > 2 500 metres) and at lower elevation, on a combination of elevation, slope and local elevation range. This is an update of the Mountain's of the World 2000 and was produced for the UNEP-WCMC publication Mountain Watch, 2002.