Ecological dynamics
The plant communities found on an ecological site are naturally variable. Composition and production will vary with yearly conditions, location, aspect, and the natural variability of the soils. The Historical Climax Plant Community represents the natural potential plant communities found on relict or relatively undisturbed sites. Other plant communities described here represent plant communities that are known to occur when the site is disturbed by factors such as fire, grazing, or drought.
Production data provided in this site description is standardized to air dry weight at the end of the summer growing season. The plant communities described in this site description are based on near normal rainfall years.
NRCS uses a Similarity Index to compare existing plant communities to the plant communities described here. Similarity index is determined by comparing the production and composition of a plant community to the production and composition of a plant community described in this site description. To determine Similarity index, compare the production (air dry weight) of each species to that shown in the plant community description. For each species, count no more than the maximum amount shown for the species, and for each group, count no more than the maximum amount shown for each group. Divide the resulting total by the total normal year production shown in the plant community description. If the rainfall has been significantly above or below normal, use the total production shown for above or below normal years. If field data is not collected at the end of the summer growing season, then the field data must be corrected to the end of the year production before comparing it to the site description. The growth curve can be used as a guide for estimating production at the end of the summer growing season.
State 1
Historic Climax Plant Community
Community 1.1
Historic Climax Plant Community
This site has a mixed plant community with an over-story of mesquite and an under-story of perennial grasses, shrubs and annuals. Other tree species usually occur in clumps or along the banks of stream channels. Mesquite leafs out in mid-to-late spring after the last frost, flowers in early summer, and has mature fruits ready to drop by early July. The trees lose their leaves with the first hard freeze in the fall. The aspect is deciduous woodland.
The site is very susceptible to channel and stream bank erosion. Drainage-ways are deeply entrenched and, if streamside vegetation is disturbed, bank cutting becomes a serious problem. If the over-story of mesquite is reduced, plants such as greythorn and wait-a-bit bush can quickly increase to dominate the plant community. Herbaceous under-story varies greatly with different percent canopy cover. Excessive groundwater pumping can, over time, lower water tables beyond the reach of tree roots and cause extensive loss of trees. Tree species present on the site are vigorous sprouters after fire or woodcutting. Natural fire may have been important in maintaining herbaceous under-stories. Salt cedar can invade and become a minor component of the plant community, usually growing on or near the banks of the stream channel. Bermuda and Johnson grasses are introduced species commonly found on this site.
Forest overstory. The over-story part of the plant community is dominated by velvet mesquite and / or western honey mesquite. Overstory canopy cover varies from 25 to 65% in mature stands of trees. Trees reach maximum size on this site (40-50 feet). Note; The percent composition in the following table is percent by canopy cover (not frequency).
Forest understory. The production of under-story plants can vary greatly with canopy cover. Open canopy areas will be dominated by shrubs and perennial grasses. Closed canopy areas will be dominated by annual forbs and grasses. Cool season annual forbs and grasses grow very well as mesquite is defoliated in the winter and spring. They can make tremendous production in El Nino years.
Table 5. Annual production by plant type
| Plant type |
Low
(kg/hectare) |
Representative value
(kg/hectare) |
High
(kg/hectare) |
| Tree |
3363 |
4483 |
6165 |
| Forb |
504 |
191 |
67 |
| Grass/Grasslike |
560 |
179 |
56 |
| Shrub/Vine |
387 |
163 |
39 |
| Total |
4814 |
5016 |
6327 |
Table 6. Soil surface cover
| Tree basal cover |
1%
|
| Shrub/vine/liana basal cover |
0-1%
|
| Grass/grasslike basal cover |
0-5%
|
| Forb basal cover |
0-5%
|
| Non-vascular plants |
0%
|
| Biological crusts |
0-10%
|
| Litter |
60-80%
|
| Surface fragments >0.25" and <=3" |
0-5%
|
| Surface fragments >3" |
0-1%
|
| Bedrock |
0%
|
| Water |
0%
|
| Bare ground |
0-40%
|
Table 7. Canopy structure (% cover)
| Height Above Ground (m) |
Tree |
Shrub/Vine |
Grass/
Grasslike |
Forb |
| <0.15 |
– |
– |
0-2% |
0-5% |
| >0.15 <= 0.3 |
– |
– |
1-10% |
1-10% |
| >0.3 <= 0.6 |
– |
0-5% |
1-10% |
1-20% |
| >0.6 <= 1.4 |
– |
0-5% |
1-20% |
0-10% |
| >1.4 <= 4 |
0-10% |
1-2% |
0-5% |
0-5% |
| >4 <= 12 |
25-55% |
– |
– |
– |
| >12 <= 24 |
– |
– |
– |
– |
| >24 <= 37 |
– |
– |
– |
– |
| >37 |
– |
– |
– |
– |
State 2
Mesquite thorn-scrub
Community 2.1
Mesquite thorn-scrub
This state occurs where mesquite has been cleared for cultivation and subsequently abandoned; or where mesquite has been cut for fuel-wood. The water table is intact. Mesquite re-growth and / or sprouts form a thorn-scrub with a 25-50% canopy 15 years after wood harvest or clearing that leaves root systems alive. Mesquite will take much longer to re-colonize areas that have been cleared and cultivated. Mesquite seeds need to be introduced to these areas and new plants will take 15 to 20 years for roots to reach the water table. It will take from 50 to 100 years with no further mesquite harvest for trees to reach maturity on the site.
State 3
Mesquite scrub-land
Community 3.1
Mesquite scrub-land
This state occurs where groundwater pumping has lowered the water table beyond the reach of mesquite roots (60 ft.). Mature mesquites die back to a sparse canopy (10-20%) of shrubby trees (10-15 ft. tall) which rainfall can support. Other shrubs like greythorn, catclaw acacia, wolfberry and whitethorn can make up considerable part of the perennial plant community. Annual grasses and forbs dominate the herbaceous layer. If the pumping is associated with urban areas or other commercial development, this state is not reversible.
State 4
Mesquite bosque, exotic annuals
Community 4.1
Mesquite bosque, exotic annuals
This state occurs where the under-story has been invaded and dominated by non-native annual forbs and grasses. The mature mesquite bosque is unaffected by this change in the under-story vegetation. These species are mostly winter annuals like London rocket, foxtail barley, red brome, rescue brome that can produce tremendous amounts of herbage in wet winters. Other non-native annuals include tumbleweed, stinkgrass, barnyard grass, annual bristlegrass, junglerice and horseweed. These annuals species can, over time, diminish the seed-bank of native annual species and may persist as the new under-story.
Transition T1A
State 1 to 2
Mesquite clearing for either wood harvest or cultivation, Irrigation abandoned, greythorn, other shrubs with shrubby mesquite form a scrubland. Water table intact.
Transition T1B
State 1 to 3
Water-table depleted by groundwater pumping to greater than 60 feet. Mature mesquites die back to a shrubby growth that rainfall can support. No return, if pumping is associated with urban areas or development.
Transition T1C
State 1 to 4
Introduction of a seed source for non native annuals like London rocket, foxtail barley, red brome and rescue brome
Restoration pathway R2A
State 2 to 1
50 to 100 years with no further mesquite harvest or control to go back to mature mesquite trees rooted 50 foot deep.
Restoration pathway R3A
State 3 to 1
Cessation of groundwater pumping may allow water tables to rise over time to within 50 feet of the surface. Time depends on depth of depletion and amount of pumping reduced. This will not occur in urban or developing areas.n, possible herbicide control of exotic annuals.
Restoration pathway R3A
State 3 to 4
Cessation of groundwater pumping may allow water tables to rise over time to within 50 feet of the surface. Time depends on depth of depletion and amount of pumping reduced. This will not occur in urban or developing areas.
Restoration pathway R4A
State 4 to 1
Unknown, possible herbicide control of exotic annuals.
Transition T4A
State 4 to 3
Water-table depleted by groundwater pumping to greater than 60 feet. Mature mesquites die back to a shrubby growth that rainfall can support. No return, if pumping is associated with urban areas or development.
