Flora and Fauna News

Patchy Green
Sonoran Desert

By Michael Plagens
Sonoran Desert Sciences

PHOENIX, Az. ----- The 2009-2010 winter rainy season started out well in December when a powerful Pacific storm swept through the Sonoran Desert. That storm produced significant precipitation in some places north of Phoenix but mostly modest amounts to the south. But then the next 30 days were bone dry and the year ended with a much lower annual rainfall total than the mean. Finally, this week significant to heavy rainfall is predicted throughout the region that should result in significant greening of the desert environment. Northern and eastern sections at slightly higher elevations have gotten more rain and should produce more spring wildflowers. This year had been described as ‘El Niño’ which generally means increased precipitation for the southwestern United States. If that prediction holds true and we get more rain then the coming year could be very good for spring wildflowers.

Wildflowers aren't the only thing adding 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 considerable 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 (or sometimes a cyanobacterium). 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 adaptation 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!