If you ever want to experience a different world, climb a mountain in the desert. Here in southern Arizona, you’ll see an immediate shift from low desert plants like saguaro, cholla, and creosote to intermediate elevation plants like yucca, juniper, and oak, all the way to high-elevation plants like Ponderosa pine and aspen. Adaptations to drought and heat become less important as those for cold tolerance and low resource availability. This is of course only part of the story – understanding the forces that structure different communities along environmental gradients is still a very active area of research!
Elevation is almost always a proxy for a more fundamental climate variable. Here, the most important factor that changes at high elevation is temperature. Higher parts of the mountain experience colder temperatures, and more extreme temperature swings. That can mean rain in the low desert, but heavy snow at the summit. In the picture above, I climbed Mica Mountain in the Rincons – only a few thousand feet above the desert floor, there were several inches of snow. It was treacherous walking, because the snow can hide prickly pears (Opuntia sp) and shindaggers (Agave schottii)!
Snow often reveals the movement of animals. Many use mountains as their homes, so each mountain becomes akin to a ‘sky island’ where different animal populations are separated. Here you can see the print of a mountain lion – the only other creature to disturb this snow for a week! Interestingly the ‘sky island’ picture is now being revised: analyses of genetic differences between populations of animals on different mountains are beginning to show high levels of gene flow, indicating that these apparently separate populations are actually interbreeding and remaining connected. The mystery, of course, is when and how the actual animals make the crossings between mountain ranges!