Monday, May 14, 2012

Desert Ironwood: Olneya tesota

Desert Ironwood flowers.
The Desert Ironwood is a subtle but one of the more interesting desert plants.  Saguaros and other cacti get a lot of attention as hard core desert plants, but the iron like endurance of Ironwoods makes them legendary.  Well at least I think it should, so let me explain.

A few weeks ago the Ironwoods lost most of their leaves in the process of getting ready to bloom.  Nearly every year this happens I fear the lack of rain in past months is killing the tree.  Not so.  Now in mid-May however, Ironwoods in the Sonoran Desert are shows off their stuff with purple blooms that cover the tree.  These flowers are quite beautiful and create quite an array of busyness for several weeks.  When hiking up to a blooming Ironwood this time of year the entire tree appears to be buzzing.  Thousands upon thousands of bees, gnats and other insects swarm the tree, feeding upon the nectar and helping the tree out by pollinating it.  Birds, such as Gnat Catchers and Fly Catchers, also actively feed on this buzzing swarm of insects.  With the minimal amount of rain the desert has received this past winter it is quite amazing that the trees can produce such a spectacular amount of flowers. 
A blooming Ironwood.
How does the Ironwood produce such great displays year after year, even when there is scant rainfall?  As I mentioned earlier, the Ironwood is an iron-like champion of desert conditions.  First off their roots penetrate far deeper than any other Sonoran Desert plant, some say up to 100 or more feet deep.  This allows them access to deep moisture, out of reach of other desert plants and protected from evaporating into the atmosphere.  Access to this highly stable water source is essential to its desert survival.  The problem with this however is that only a very tiny amount of moisture ever penetrates deep into desert soils, nearly all of it only soaks in a foot or two at the most before the heat evaporates it away.  This would make deep roots nearly useless except for one neat little strategy Ironwoods and other deep rooted plants often use.  When rain penetrates only the most shallow layers of soil, Ironwoods use their shallow roots to absorb moisture.  Some of this moisture is of course transported to the leaves to carry out photosynthesis but some of it also is transported downward through the deep roots and deposited into the soil deep below the surface.  Then, when everything drys out above ground the Ironwood reabsorbs and utilizes this moisture stored deep underground, allowing the plant access to water during long periods without rain.

About the only place where water penetrates deep into the desert soil is in the numerous dry washes that thread through the landscape.  This deep moisture penetration in dry washes, and the deep roots of the Ironwood, make dry washes a perfect habitat for the Ironwood.  The deep roots do well in the loose sandy soils of washes making Ironwoods one of the more common dry wash plants.  In-fact, they have been labeled as one of the "Big-Four" dry wash plants of the Sonoran Desert.  Even in areas of the desert that receive only four inches of rain, where it is two dry for much of anything to occupy the uplands, Ironwoods will occupy the washes.  These Ironwoods can survive quite well off of the moisture that seeps deep into wash sediments due to infrequent flash floods.

Ironwood actually derives its name from the very nature of its wood, another important desert adaptation.  Like iron, the wood is extremely hard, and is so dense it actually sinks in water.  The hard nature of this wood helps siphon water up from the roots to the leaves even under extreme drought conditions.  Softer wood would cause a breakage of the water column being siphoned up the trunk and to the leaves.

These adaptations make the Ironwood the largest desert tree of the dry uplands.  I have found Ironwoods with trunks three foot in diameter and thirty foot tall.  No other desert tree attains these sizes unless they are along a perennial river.  In the large limbs and trunks I have often found large bee hives or pack-rat middens.  I have also found Kit Foxes hiding high up in the trees.  Other desert trees typically never attain sizes near enough to support these types of things.  Ironwoods also survive cutting being they readily re-sprout from their trunks.  In some areas I have found nearly every Ironwood has been cut sometime in the past by the presence of old stumps.  These stumps however re-sprouted and now support very healthy trees.  It is very rare to find a dead Ironwood, especially in comparison to other desert trees.  I also suspect Ironwoods to be one of the oldest desert plants.  I am sure they can live for several hundred years without problem. 


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