This spring I went out for a walk on one of the bald hills on the outskirts of Missoula, just east of Hellgate Canyon. I walked the crest of the hill and saw how the strong wind on these exposed ridges blows the soil away, leaving a gravelly surface. The plants growing on this stony pavement are different from the typical grassland species on the slopes.
There is a strong tendency for these plants to be low and compact, like little cushions. In addition, a number of species have white leaves. Why do all these different kinds of plants have the same growth form, and what’s the advantage to having white leaves?
High ridge crests receive the full force of the wind. In the summer, plants growing on these sites can be damaged by blowing particles of soil. During the winter, they are exposed to windblown ice crystals because these ridges rarely have snow cover to protect the plants. By staying low to the ground, cushion plants avoid the abrasive forces of the wind.
The wind and the lack of snow make these ridge sites drier than the surrounding slopes. Plants must green up and bloom early before spring moisture runs out. But early spring temperatures are often less than warm, and warmth is needed for efficient photosynthesis. The warmest place on an open windy ridge is flat against the ground, another advantage to the cushion growth form.
So cushion plants are favored in cold, windy sites because this growth habit keeps them warmer and helps prevent abrasive damage to their leaves. But why do so many of these ridge-inhabiting species also have white leaves? Although light is needed for photosynthesis, too much solar radiation can be harmful. During the summer, when moisture is limited, intense irradiation can damage leaves by overheating them. A dense coast of light-colored hairs can protect the leaf by reflecting some of the light and reducing the temperature at the surface by as much as 20 degrees F. White, hairy leaves are also common in deserts where the same problems of drought and intense sunlight are encountered. The dense light-colored spines of many cacti serve the same purpose as the white covering of hair observed on our ridge-top plants.
These adaptations — cushion-growth and white leaves — can be seen in many windy, dry environments, from valley bottom hills and outcrops to alpine summits. Next time you’re up on a sunny, windswept ridge and you get cold, lie down on the ground. It’s warmer, and you can get a better look at the cushion plants.