Whitebark Pine (Pinus albicaulis) tree at 2,300 meters elevation, Yellowstone National Park, Montana. Because they exist at the highest elevations at tree line these trees often occur as Krummholz, dwarfed, slow growing trees that remain low to the ground because of exposure to constant wind. Seeds of the species are an important food source for many alpine birds and mammals including grizzly bears. They are in serious decline because of an introduced fungus and also because of mountain pine beetle outbreaks which are increasingly reaching the higher elevations because of global warming.