Tuesday, Jan.
31st, 2012
Vol. 15 No. 2
Patchy Green
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By Michael Plagens |
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PHOENIX, Az. ----- The 2011-2012 winter rainy season started out well in November-December when several Pacific storms swept through the Sonoran Desert dropping adequate precipitation. But then January 2012 produced only localized traces of rain. Near normal temperatures have kept evaporation low so many areas still show a lot of green and there are many small annuals hanging on. However, without significant additional rain this springs' wildflowers will not show much color. 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 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 (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 adaption to the extremely thin layer of suitable microhabitat. Current Weather Conditions in Phoenix, Arizona, USA, from the National Weather Service
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Flora
and Fauna News, now in its 14th year of publication, appears
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per month and provides current informaion about the birds, insects and plants
(natural history) living in the Arizona Sonoran Desert.
Copyright Michael J. Plagens, 2012
Send questions or comments to mjplagens@arizonensis.org
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