Every mountain hiker knows that temperatures decrease as you go to higher elevations. The observant hiker will notice that different plants and animals occupy successively higher elevations and that the vertical range may be rather narrow for any particular species. Global warming moves these narrow microclimate ranges up the mountainside, but since plants cannot extend their ranges up the slope very quickly, the narrow ranges of habitability become even more narrow or may disappear altogether.
Krajick (2004) list numerous studies on several continents that have shown upslope movement of climate zones and accompanying loss of species at particular elevations. Low mountains are particularly vulnerable, since species near the top have no place to which to migrate.
Reference
Krajick, K., 2004: All downhill from here? Science, 303, 1600-1603.