Tree form has a genetic component, because some species are able to exist in an erect form where other species cannot. Examples of this are limber pine (Pinus flexilis) and bristlecone pines (P. aristata and P. longaeva), which are found in the Colorado Rocky Mountains in the United States. These species form erect trees where Engelmann spruce (Picea engelmanni) and Alpine fir (Abies lasiocarpa) can exist only as prostrate forms. One reason lies in the pines’ greater resistance to winter desiccation damage at high elevations, owing to the thick coating of wax and cuticle on the surface of their needles. These species differences can result in double timberlines, where one tree species or group of species forms a tree line at a different elevation from another species or group of species.

Low temperature is the main arbiter of timberlines. This is dramatically apparent in the higher timberlines that can be observed on the sunnier slopes of a mountain. Low temperature is also the reason for the increase in tree line in interior mountains with warmer summers, such as the Rocky Mountains (about 3,000–3,350 metres, or 10,000–11,000 feet), as opposed to coastal mountains, such as the White Mountains of New Hampshire, U.S. (approximately 1,400 metres, or 4,600 feet), where the summers are cooler and cloudier.