Flora and Fauna News

Patchy Green

Sonoran Desert

By Michael Plagens
Sonoran Desert Sciences

PHOENIX, Az. ----- The 2003 - 2007-2008 winter rainy season has brought areas of light rain showers to some areas of the Sonoran Desert. On the other hand the northern and eastern sections at slightly higher elevations have gotten significant rain so far with more predicted for the coming weekend (Jan 26-27, 2008). Spring wildflowers should be good to very good this year depending on the area's recent precipitation.

Wildflowers aren't the only thing that can add splashes of color to our often drab desert mountains. Bright green, orange, and yellow lichens can be found on the surfaces of exposed boulders and cliff faces seemingly without access to any water what so ever. Lichens are synonymous with survival and as such they have some important tricks. The surfaces upon which they grow tend to face the north and so get just a few hours or less of direct sunshine each day, thus they remain quite cold through the day. Come nightfall these rocks become much colder again. In fact their temperature can fall significantly lower than the air temperature because of radiative cooling into clear skies. These super cooled surfaces yield a microclimate with significantly higher relative humidity. Some mornings dew or frost will form on these rocks yielding precious moisture that the lichens quickly take up. Within this higher-humidity microclimate the lichens may also take moisture directly from the air aided as well by hygroscopic salts or peptides. In addition many of the rocks contain condiderable liquid water for many weeks following a rainstorm. This is particularly true of sandstones, siltstones, and volcanic tuff.

Lichens are actually a symbiotic relationship between two organisms, a fungus and an alga. Different "species" of lichen are expressions of various paired combinations of alga and fungal species. Some of these algae and fungi have not been found growing solo anywhere else, and so are obligate symbionts. The lichens now flourishing in the Phoenix Mountains are bright yellow-green and of the crustose form, that is they are very tightly appressed to the rock surface, an obvious adaption to the extremely thin layer of suitable microhabitat.

Photo by Stephen Sharnoff. Please visit his site NORTH AMERICAN LICHEN PROJECT and take the full tour through the fabulous Picture Gallery!