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
Figure 5. Sandy Bottom, subirrigated POFR, 12-16" pz.
This site has a mixed plant community with an over-story of cottonwood and willow and an under-story of tree seedlings, perennial and annual grasses and forbs, and some shrubs. Both cottonwood and willow flower in spring and leaf out shortly after. Both lose their leaves in the fall with first frost. The aspect is deciduous riparian woodland.
Channel and stream-bank erosion and sedimentation are natural features of this site. Tree species present are vigorous root and stem sprouters after cutting or injury from floods. Establishment of the major species from seed can only occur with sedimentation. Both cottonwood and willow seedlings pioneer sandbars and beds of coarse textured alluvium left after large floods. Under-story production varies greatly with different percent of canopy cover. Canopy cover ranges from 60-80%. Trees reach maximum size on the site. Excessive ground water pumping can, over time, lower water tables beyond the loss of tree roots and cause excessive loss of trees. Natural fire may have been important in maintaining herbaceous under-stories. Salt cedar can invade and become dominant with destruction of the native tree cover. 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 Fremont cottonwood and Godding black willow. Over-story canopy cover varies along the stream from 50 to 80% in mature stands of trees. Sub-dominate over-story include species like Arizona ash, Arizona black walnut, coyote willow, Bonpland willow, Arizona sycamore, netleaf hackberry and mesquite. Note; the percent composition by species in the following table is percent by canopy cover (not frequency).
Forest understory. The production of under-story plants varies greatly with canopy cover. Open canopy areas will be dominated by grasses, sedges and rushes in the under-story. Closed canopy areas will be dominated by wetland forbs in the under-story.
Table 5. Annual production by plant type
| Plant type |
Low
(lb/acre) |
Representative value
(lb/acre) |
High
(lb/acre) |
| Tree |
4000 |
5500 |
7000 |
| Grass/Grasslike |
30 |
150 |
450 |
| Forb |
20 |
100 |
270 |
| Shrub/Vine |
5 |
50 |
260 |
| Total |
4055 |
5800 |
7980 |
Table 6. Soil surface cover
| Tree basal cover |
1%
|
| Shrub/vine/liana basal cover |
0-1%
|
| Grass/grasslike basal cover |
1-15%
|
| Forb basal cover |
1-10%
|
| Non-vascular plants |
0%
|
| Biological crusts |
0-5%
|
| Litter |
50-90%
|
| Surface fragments >0.25" and <=3" |
0-50%
|
| Surface fragments >3" |
0-20%
|
| Bedrock |
0%
|
| Water |
0-15%
|
| Bare ground |
0-20%
|
Table 7. Canopy structure (% cover)
| Height Above Ground (ft) |
Tree |
Shrub/Vine |
Grass/
Grasslike |
Forb |
| <0.5 |
– |
– |
0-5% |
1-10% |
| >0.5 <= 1 |
– |
– |
1-10% |
1-10% |
| >1 <= 2 |
– |
0-1% |
1-10% |
0-5% |
| >2 <= 4.5 |
– |
0-1% |
1-15% |
0-5% |
| >4.5 <= 13 |
5-10% |
1-10% |
– |
0-5% |
| >13 <= 40 |
5-20% |
– |
– |
– |
| >40 <= 80 |
40-60% |
– |
– |
– |
| >80 <= 120 |
0-5% |
– |
– |
– |
| >120 |
– |
– |
– |
– |
State 2
Cottonwood, seep willow strand
Community 2.1
Cottonwood, seep willow strand
Figure 7. Sandy Bottom, subirrigated POFR, strand vegetation
This site occurs where continuous grazing has greatly reduced the reproduction of cottonwood and willow. Seedlings of cottonwood and willow species are relished by livestock. As mature trees are lost to old age, fire or flooding, the plant community is reduced to a shrubby strand along the stream-banks with a variety of native and non-native grasses and forbs on the stream terraces. Seep willow, burrobrush and other shrubs line the banks along with occasional saplings of cottonwood and / or willow. Introduced grasses like bermuda grass, yellow nutsedge and Johnson grass are common on the floodplain. Non-native annuals grasses like rabbitfoot grass, annual bristlegrass, barnyard grass, stinkgrass and junglerice are common in this state. Introduced forbs include horehound, dandelion, horseweed, hoary cress, sow thistle, prickly lettuce and plantain.
State 3
Riverwash community
Community 3.1
Riverwash community
Figure 8. Sandy Bottom, subirrigated POFR, riverwash
This state occurs where groundwater pumping has dropped the water-table below the roots of the riparian tree species and stream-flow has been greatly reduced to flash floods following large storms. Stream-banks are largely unprotected. Channel areas widen, deepen and are colonized in places by shrubs like burrobrush, seep willow, mesquite and desert broom. Floodplain areas are lost to stream-bank erosion.
State 4
Salt Cedar invaded
Community 4.1
Salt Cedar invaded
Figure 9. Sandy Bottom, subirrigated POFR, salt cedar
This state occurs where salt cedar has invaded the plant community. As mature native trees are lost to old age, fire and / or flooding, salt cedar increases to dominate the plant community. As salt cedar canopies approach 70-80% the under-story is reduced to zero. Salt cedar concentrates salts in it's leaves, which return to the soil surface in litter-fall and increase surface salinity.
Transition T1A
State 1 to 2
Continuous Heavy Grazing. Mature cottonwood and willow die from age, fire or flood. Saplings of cottonwood and willow are heavily grazed and little recruitment occurs. Seepwillow and burrobrush dominate the streambank vegetation while bermudagrass and annuals dominate the under-story vegetation.
Transition T1B
State 1 to 3
Water-table depleted by groundwater pumping to greater than 20 feet. Mature trees die back to a reduced canopy that periodic flooding can support. Bank erosion widens channel area and wetland grasses and forbs are replaced by annuals. Seepwillow and burrobrush colonize areas of river-wash. No return if pumping is associated with urban areas or development.
Transition T1C
State 1 to 4
Introduction of a seed source for salt cedar. As fires, age and floods remove cottonwood and willow, salt cedar increases to assume dominance. As salt cedar canopy closes to 80%, the under-story vegetation is reduced to zero.
Restoration pathway R2A
State 2 to 1
Prescribed Grazing/No Grazing will result in return to cottonwood and willow dominance in less than 20 years. Trees will reach maturity in 50-70 years. Native perennial grasses and wetland forbs replace bermudagrass.
Transition T2A
State 2 to 4
Introduction of a seed source for salt cedar. As fires, age and floods remove POFR and SAGO, salt cedar increases to assume dominance. As salt cedar canopy closes to 80%, the under-story vegetation is reduced to zero.
Restoration pathway R3A
State 3 to 1
Cessation of groundwater pumping may allow water tables to rise over time to within 20 feet of the surface. Time depends on depth of depletion and amount of pumping reduced. This will not occur in urban or developing areas.
Transition T3A
State 3 to 4
Cessation of groundwater pumping and salt cedar introduction. 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 salt cedar and replanting of POFR and SAGO from dormant poles on limited acres.
Transition T4A
State 4 to 3
Water-table depleted by groundwater pumping to greater than 20 feet. Mature trees die back to a reduced canopy that periodic flooding can support. Bank erosion widens channel area and wetland grasses and forbs are replaced by annuals. Seepwillow and burrobrush colonize areas of river-wash. No return if pumping is associated with urban areas or development.
