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
The historic native state includes the native plant communities that occur on the site, including the historic climax plant community. This state includes other plant communities that naturally occupy the site following fire, drought, flooding, herbivores, and other natural disturbances. The historic climax plant community represents the natural climax community that eventually reoccupies the site with proper management.
Forest overstory. This site has a mixed plant community with an over-story of large trees like Arizona sycamore, cottonwood, walnut and ash, with an under-story of young trees, shrubs and perennial and annual grasses and forbs. Broadleaf tree species leaf out in March and shed their leaves with the first hard freeze of winter, usually in November. The aspect is deciduous riparian woodland.
Sycamore reproduces mainly by sprouting into clones. Walnut, cottonwood, willow and ash reproduce by seed that germinate in the spring. Tree canopy ranges from 40-70%.
Trees reach maximum size on this site. Numbers of large trees per acre range from 30-50.
Channel and stream bank erosion and sedimentation are natural features of the site. Numerous old channels lined with trees can be seen across these narrow floodplains. Tree species present are vigorous root and stem sprouters after cutting or injury from floods. Stream flow is intermittent, but surface flow occurs reliably during the winter-spring months. These warm temperature riparian forests require abundant water in March-April, when most of the tree species leaf out, set seed, and germinate. Summer rainfall usually does not result in sustained stream flow on this site, limiting the dominant riparian communities to vernally adapted species. Excessive ground water pumping, damming, or diversion can reduce cool season stream flow and eventually lead to thinning of the tree canopy on the site.
Forest understory. Under-story composition and reproduction varies greatly with percent canopy cover.
Management of the under-story should be aimed at leaving enough cover to protect soils from erosion and to trap sediment in the summer flooding season. Heavy winter runoff can cause much erosion due to prolonged flow and increased viscosity of the water (due to cooler temperatures) resulting in higher capacities to carry sediments.
Table 5. Annual production by plant type
| Plant type |
Low
(kg/hectare) |
Representative value
(kg/hectare) |
High
(kg/hectare) |
| Tree |
2242 |
4483 |
6725 |
| Grass/Grasslike |
1569 |
673 |
112 |
| Forb |
213 |
123 |
62 |
| Shrub/Vine |
420 |
224 |
56 |
| Total |
4444 |
5503 |
6955 |
Table 6. Soil surface cover
| Tree basal cover |
1-2%
|
| Shrub/vine/liana basal cover |
0-1%
|
| Grass/grasslike basal cover |
1-10%
|
| Forb basal cover |
1-5%
|
| Non-vascular plants |
0%
|
| Biological crusts |
0-5%
|
| Litter |
50-90%
|
| Surface fragments >0.25" and <=3" |
10-60%
|
| Surface fragments >3" |
1-15%
|
| Bedrock |
0%
|
| Water |
0-20%
|
| Bare ground |
0-25%
|
Table 7. Canopy structure (% cover)
| Height Above Ground (m) |
Tree |
Shrub/Vine |
Grass/
Grasslike |
Forb |
| <0.15 |
– |
– |
0-2% |
1-10% |
| >0.15 <= 0.3 |
– |
– |
1-10% |
1-10% |
| >0.3 <= 0.6 |
– |
0-1% |
5-10% |
1-5% |
| >0.6 <= 1.4 |
– |
0-5% |
1-10% |
0-5% |
| >1.4 <= 4 |
1-2% |
1-10% |
0-5% |
0-5% |
| >4 <= 12 |
5-20% |
– |
– |
– |
| >12 <= 24 |
35-65% |
– |
– |
– |
| >24 <= 37 |
– |
– |
– |
– |
| >37 |
– |
– |
– |
– |
| Jan |
Feb |
Mar |
Apr |
May |
Jun |
Jul |
Aug |
Sep |
Oct |
Nov |
Dec |
| J |
F |
M |
A |
M |
J |
J |
A |
S |
O |
N |
D |
State 2
Exotic grasses / forbs in under-story
Community 2.1
Exotic grasses / forbs in under-story
This state occurs where non-native grasses like Johnson grass, bermuda, tall fescue, dallis grass, weeping lovegrass and noxious weeds like whitetop, sweet clover and several species of thistle, rumex and centeaura have invaded the under-story plant community. Seed sources for these weeds can be many miles upstream. As these species become dominant in the under-story, native perennial grasses and forbs can decrease to minor amounts.
State 3
Shrub and tree invaded
Community 3.1
Shrub and tree invaded
Figure 7. Sandy Bottom, PLWR, POFR 16-20" pz, juniper
This state occurs where shrubby tree species like mesquite, alligator and one-seed juniper, and non-natives like tree of heaven, salt cedar, Chinese elm and Russian mulberry invade and increase to dominate openings in the native tree cover. This can occur in the presence of a seed source for these species and with continuous grazing of the understory vegetation. Once established these species could contribute, as ladder fuels, to the decline of native tree species in wildfires.
Community 4.1
Eroded channel
Figure 8. Sandy Bottom, PLWR, POFR 16-20" pz, eroded
This state occurs where the contributory watersheds are destabilized by the interactions of fire, drought and / or continuous grazing. Large amounts of runoff and sediment are delivered to the site causing gully and stream-bank erosion. Channel erosion can deepen floodways and reduce flooding of the stream terraces. Sediment can clog channels causing new channels to cut through floodplains, uprooting trees and removing cover and woody debris. Poorly designed stream crossings, bridges and culverts can cause short reaches of this state in areas immediately downstream.
Transition T1A
State 1 to 2
Continuous Heavy Grazing, introduction of a seed source of exotics like bermudagrass, tall fescue, Johnson grass, dallis grass, whitetop and thistle.
Transition T1B
State 1 to 3
Continuous Heavy Grazing, introduction of a seed source for mesquite, juniper, tree of heaven, Chinese elm, Russian mulberry and or salt cedar.
Transition T1C
State 1 to 4
Poor condition watershed areas produce excess amounts of runoff causing channels to erode. As channels deepen stream terraces no longer flood. Trees and large shrubs still dominate. Perennial herbs are replaced by annuals and half shrubs. Base level change in main stream causes down-cutting of bottoms. Poor condition watersheds can also deliver large amounts of sediment to bottoms, destabilizing them and causing new channels to cut through the floodplain. Poorly designed stream crossings, bridges or culverts can cause small areas of this state.
Restoration pathway R2A
State 2 to 1
Unknown. Possible herbicide treatment of exotics species and seeding of native grasses.
Transition T2A
State 2 to 4
Poor condition watershed areas produce excess amounts of runoff causing channels to erode. As channels deepen stream terraces no longer flood. Trees and large shrubs still dominate. Perennial herbs are replaced by annuals and half shrubs. Base level change in main stream causes down-cutting of bottoms. Poor condition watersheds can also deliver large amounts of sediment to bottoms, destabilizing them and causing new channels to cut through the floodplain. Poorly designed stream crossings, bridges or culverts can cause small areas of this state.
Restoration pathway R3A
State 3 to 1
Prescribed Grazing/No Grazing, mechanical and herbicide control of shrubby species.
Transition T3A
State 3 to 4
Poor condition watershed areas produce excess amounts of runoff causing channels to erode. As channels deepen stream terraces no longer flood. Trees and large shrubs still dominate. Perennial herbs are replaced by annuals and half shrubs. Base level change in main stream causes down-cutting of bottoms. Poor condition watersheds can also deliver large amounts of sediment to bottoms, destabilizing them and causing new channels to cut through the floodplain. Poorly designed stream crossings, bridges or culverts can cause small areas of this state.
Restoration pathway R4A
State 4 to 1
Prescribed Grazing/No Grazing of watershed areas with or without shrub control and planting of native grasses, maintenance treatments for shrubs, etc. Mechanical control of sheet, rill and gully erosion here possible. Reestablish flooding regime of bottoms.
Restoration pathway R4B
State 4 to 2
Prescribed Grazing/No Grazing of watershed areas with or without shrub control and planting of native grasses, maintenance treatments for shrubs, etc. Mechanical control of sheet, rill and gully erosion here possible. Reestablish flooding regime of bottoms.
Restoration pathway R4C
State 4 to 3
Prescribed Grazing/No Grazing of watershed areas with or without shrub control and planting of native grasses, maintenance treatments for shrubs, etc. Mechanical control of sheet, rill and gully erosion here possible. Reestablish flooding regime of bottoms.
