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clumping patterns with plants  RSS feed

 
Tom Connolly
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I have taken a number of hikes in hot arid lands and noticed a common phenomenon. plants exist in clumps, with patches of sandy hardpan in between. Does that happen because those parts of soil had a weakness that allowed the seeds into the soil?
 
Andrew Parker
pollinator
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Location: Salt Lake Valley, Utah, hardiness zone 6b/7a
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Competition for soil moisture by competing root systems establishes the spacing. The interrelation between species can be quite complex. Invasive species, especially cheat grass (downy brome) can be extremely disruptive of this ecosystem.

The desert hardpan, or desert crust, is composed of microbes (mostly green algae) that bind the soil particles together, protecting it from erosion by wind and sudden downpours. (This may be related to gleying a pond without clay.) Desert crust, once disturbed, does not easily recover.
 
Jon La Foy
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Location: Hopkinsville, KY (Western KY) Zone 7
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Tom, I have noticed the same thing, particularly with grasses. What I think happens is that after the plant seeds it tends to die. The seeds establish new bunches of grass and there is a void of vegetation between the old and new. When it rains or the wind blows, the exposed dirt/dust is forced up against the vegetation.
Not sure if all that makes sense, but I did my best. Plants, in the desert, tend to be bunched up around rocks or other objects for various reasons: one, that's where the seed ends up. Whether by wind or water it just can't migrate any further. Two, the object will provide shade to the plant. And Three, it'll protect against that dry wind.
 
Tom Connolly
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Thanks. So if I want to help improve the quality of desert soil, I need to find a way to fill in the gaps without destroying the crust which protects the soil and plant roots. Filling in the gaps with some nitrogen fixing plants would improve the soil chemistry and help the soil retain more moisture. Balancing the two seems like a complicated task.
 
Brett Andrzejewski
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Location: Buffalo, NY
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In the desert you are always going to be moisture limited as the evaporation rate greatly exceeds the precipitation rate. I totally agree with Andrew and his statement. The plants are performing a delicate optimization of moisture. Too many plants and they consume all the moisture, too few plants and they don't have the water holding potential. The clumping pattern is solution to this optimization problem.

Tom, do you have a copy of Harvesting Rainwater for Drylands and Beyond? If you are considering land in the desert this book is a must. Many viable solutions to filling in the gap pattern are given in the book.

I've read that the algae that live and die in the sand eventually create the precursors for soil in a process that can take decades. This was from a sign that I read in White Sands National Park explaining how plants were able to live in such harsh environments.
 
Andrew Parker
pollinator
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Location: Salt Lake Valley, Utah, hardiness zone 6b/7a
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http://www.soilcrust.org/crust.pdf

Go to page 35, section 3.6 Water Relations. Page 38, starting on the second paragraph, is particularly pertinent to this discussion.
 
Tom Connolly
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Thank you. That information is very helpful - gives me much to think about.
 
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