When winter above 5,000 feet in Northern Arizona became uncomfortable, we took a road trip south to a favorite sanctuary down on the Mexican border: Organ Pipe Cactus National Monument.
The Monument has been designated a "Biosphere Reserve" by the United Nations ... but even within its desert boundaries, life and death struggles play out ... often slowly.
Mistletoe has bled nutrients from this tree with obvious and inevitable results.
This Ironwood tree doesn't have a chance. As a "nurse tree" for the Saguaro cactus it will eventually succumb to the aggressive root system of the dominant cactus. The mistletoe will will aid in its demise but, like many accomplices, it will share the fate of its host.
Good thought, Slow struggles! In wet environments the struggles are faster as a fight for space and light takes place. Glad to see your post from the desert.
ReplyDeleteThanks, Anna.
DeleteWell, that's what it looks like. But: Ironwood roots are tapping into water resources that are far below the surface and usually are fed by rainfall further up slope. We have washes on our property that haven't had surface water in years but they are lined by healthy Ironwoods. Mesquites and Ironwoods both reach much deeper than Palo Verdes and for that matter Saguaros. The Saguaro soaks up rain water very much from the surface with its widespread, shallow root system. After the water seeps down it's mostly out of reach for the cactus that then lives of the stored water. So the competition with the Ironwood is far less direct than it seems. We are here (Picture Rocks) at the margin of Ironwood habitat because it is very dry (rain shadow of the Tucson Mountains and it gets so cold that some pre-damaged trees have a hard time recovering from freeze damage. But we have also several Ironwoods with 60 to 80 year old saguaros right inside the tree and others with skeletons of old saguaros leaning against the branches.
ReplyDeleteSame complex situation with the mistletoe: This plant does NOT use the trees nutrients (It does its own photosynthesis). Its tap root enters the water carrying Xylem, but not the sugar carrying phloem of the host. So as long as there is enough water (and with that I mean the limited desert precipitation that these plant communities have adapted to over several thousands of years) the host and the half parasite are both fine. The problem arises now, as the climate is changing and less and less rain at higher temps are stressing the system. A clear indicator is the fact that in Organ Pipe there are several species of smaller cacti (that always thrived there) now at the brink of extinction because they just do not have the storage capacity of a saguaro (or the long roots of the leguminous trees)
I thought about this for some time. There's a high level of "truth value" in what you say but it's my thought that three organisms cannot successfully occupy 1 or 2 sq. ft. of ground in the desert. My bet for survival is the Saguaro :)
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