In red rock country, the bottom of a canyon can be a difficult place for a plant to live. Why?
Here you can see the depths of Boynton Canyon, where ponderosa pine (Pinus ponderosa) and blue spruce (Picea pungens) must grow tall and straight to survive. Growing deep in a canyon restricts the hours of direct sunlight to a scarce few – perhaps too few to support enough photosynthesis to offset respiration. On the other hand, canyons rarely endure major wildfires, preserving large populations of majestic trees that have grown up high enough to reach the sun.
And here, in Oak Creek Canyon, you can see the constant risk of drought. Canyons are often wet places, serving as the lowest points of large watersheds, but the vagaries of the canyon’s history of downcutting mean that many portions are exposed and dry. Here you can see a bigtooth maple (Acer grandidentatum) blindly seeking for water, with blind roots pushing through hidden fissures in the canyon walls.
Canyon life has other downsides, as well. Flash floods can erase centuries of growth as they scour canyon bottoms, and the narrow confines of the walls can prevent fertilization or dispersal of seeds and so reduce fitness. They are beautiful but harsh places, and I am glad to not be a plant as I walk through their hidden worlds.