Monday, September 26, 2011

Soil, Roots, and Desert Plants

In no other ecosystem does soil structure influence where plants grow more strongly than in the desert.  Soils in all ecosystems have strong influences on where plants grow, but the effect is particularly strong in deserts. Being most desert plant adaptations center around accumulating and conserving water, and desert soils have strong influences on where water accumulates, this connection between soil and plants makes sense.  Furthermore, desert soils are relatively diverse when compared to soils in other regions.  The presence or absence of argillic and or caliche horizons show how extremely different one soil can be from another in the desert.  Other regions have too much rain to develop nearly impenetrable argillic and caliche horizons.  Or regions with more rain or freezing temperatures create conditions that are too unstable to develop soils as long and as strong as some desert soils.

There are three common soil-plant communities in the desert.  1. Shallow rooted plant communities growing in soils with caliche and or argillic horizons, 2. Deep rooted plant communities growing on soils without caliche or argillic horizons, and 3. deep rooted plants growing in soils with massive amounts of caliche.  Lastly there are also mountain soils which we will not talk about here due to their non-uniform and unstable nature.  Mountain soils often are not considered soils at all in most situations due to their unstable nature.
The different ages of alluvial fans along bajadas determine how these soils and corresponding plant communities are distributed.  Check out this previous blog entry on alluvial fans and desert soils.  An understanding of how alluvial fans are deposited and form bajadas in the desert is important to understanding soil and plant distributions.
The shallow roots of this Foothills Palo Verde are growing just above the whitish layer of caliche.  Erosion has removed the soil above the caliche in this picture.  After erosion, harder caliche is left in place and shallow roots growing just above the impenetrable soil horizon are exposed.
1. Shallow rooted plant communities growing on soils with argillic and caliche horizons
On the upper bajada, the alluvial fan closest to the mountain, shallow rooted plants compose the community. These are older soils on the bajada and therefore have highly developed argillic and caliche horizons.  These horizons are thick and heavy, resisting water infiltration and root penetration.  Though this may seem like extremely poor conditions, most desert plants are highly adapted and even thrive under these circumstances.  Shallow impenetrable soils horizons allow only shallow rooted plants to root themselves.  Shallow roots avoid impenetrable horizons by accumulating their roots just above.  Water pools in the soil above the caliche allowing an abundance for water thirsty plants.  When only caliche is present, slightly deeper shallow rooted plant occupy the area, such as Saguaros and Foothills Palo Verde.  Areas with an arigillic horizon and caliche have the most shallow rooted plants such as Cholla cacti and Triangle-Leaf Bursage.

How to identify these areas:  Reddish colored soils closer to the mountain, shallow rooted plants such as Saguaro, Triangle-Leaf Bursage, Cholla cacti, Foothills Palo Verde.  These areas generally are sloping gently.  In areas of erosion such as along washes whitish colored soils indicating caliche may be present.
A Creosote Brush community growing on light colored soil without caliche or argillic horizons.
2. Deeper rooted plant communities growing on soils without caliche or argillics
The younger alluvial fans far away from the mountain have very little soil development.  This means no argillic or caliche soil horizons allowing for deeper water and root penetration.  Primarily Creosote Brush occupy these areas with their often six foot deep roots.  Shallow rooted plants generally cannot survive in these areas being water penetrates deeper than their roots reach.

How to identify these areas: Lighter colored soils further away from the mountain that are nearly level.  Plants are generally Creosote Brush and sometimes White Ratany, which is parasitic to Creosote.   White Bursage is also often common in these areas.
Desert pavement.  Usually indicates massive caliche development under the surface.  
3. Deep rooted plants growing on massive amounts of cliche
These are extremely old alluvial fans with very thick caliche horizons.  The massive amount of caliche prevents water from penetrating and the only place for plants to grow are through cracks.  Plants are few and far between and typically only Creosote Brush are present.  The soil surface will typically be nearly level, covered with black pebbles called desert varnish, and be far away from the mountain.  Generally these are most common in the western portions of the Sonoran Desert around the Colorado River.

How to identify these areas: Nearly level areas of desert varnish generally far away from the mountain.  Creosote Brush will be about the only plant and they will be few and far between.  Sometimes patches of Teddy-Bear Cholla will be present.

Transitional areas: exceptions to the above rules
Of course, these are not hard and fast rules, they are generalizations.  For example, transitional areas between community and soils 1 and 2 above will often have a mix of Creosote Brush and Triangle-Leaf Bursage, the first being deep rooted and the second shallow rooted.  This typically indicates a poorly developed caliche that allows for the shallow rooted Bursage to grow.  The poorly developed caliche also allows for deep Creosote roots to penetrate.

Also, for plant community-soil 3 above, areas adjacent to desert pavement without the pavement have had erosional disturbance.  This disturbance allows more water to penetrate the soil as well more roots to penetrate.  Typically there is still caliche so these communities are dominated by shallow rooted plants.

So a little knowledge of how to identify soils and about the rooting patterns of specific plants goes a long way in helping you understand the local Sonoran Desert environment.  When exploring the Sonoran Desert, by looking for these indicators you can begin to piece together what is happening along a bajada.

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