Montane forests occur between the submontane zone and the subalpine zone. The elevation at which one habitat changes to another varies across the globe, particularly by latitude. The upper limit of montane forests, theforest line or timberline, is often marked by a change to hardier species that occur in less dense stands. For example, in the Sierra Nevada of California, the montane forest has dense stands of lodgepole pine and red fir, while the Sierra Nevada subalpine zone contains sparse stands of whitebark pine.The lower bound of the montane zone may be a "lower timberline" that separates the montane forest from drier steppe or desert region.Montane forests differ from lowland forests in the same area. The climate of montane forests is colder than lowland climate at the same latitude, so the montane forests often have species typical of higher-latitude lowland forests. Humans can disturb montane forests through forestry and agriculture. On isolated mountains, montane forests surrounded by treeless dry regions are typical "sky island" ecosystems.