Ecological dynamics
The historic climax plant community (HCPC) for a site in North America is the plant community that existed at the time of European immigration and settlement. It is the plant community that was best adapted to the unique combination of environmental factors associated with the site. The historic climax plant community was in dynamic equilibrium with its environment. It is the plant community that was able to avoid displacement by the suite of disturbances and disturbance patterns (magnitude and frequency) that naturally occurred within the area occupied by the site. Natural disturbances, such as drought, fire, grazing of native fauna, and insects, were inherent in the development and maintenance of these plant communities. The effects of these disturbances are part of the range of characteristics of the site that contribute to that dynamic equilibrium. Fluctuations in plant community structure and function caused by the effects of these natural disturbances establish the boundaries of dynamic equilibrium. They are accounted for as part of the range of characteristics for an ecological site. Some sites may have a small range of variation, while others have a large range.
The historic climax plant community of an ecological site is not a precise assemblage of species for which the proportions are the same from place to place or from year to year. In all plant communities, variability is apparent in productivity and occurrence of individual species. Spatial boundaries of the communities; however, can be recognized by characteristic patterns of species composition, association, and community structure. The HCPC for this ecological site has been estimated by sampling relict or relatively undisturbed sites and/or reviewing historic records.
Plant communities that are subjected to abnormal disturbances and physical site deterioration or that are protected from natural influences, such as fire and grazing, for long periods seldom typify the historic climax plant community. The physical site deterioration caused by the abnormal disturbance results in the crossing of a threshold or irreversible boundary to another state, or equilibrium, for the ecological site. There may be multiple thresholds and states possible for an ecological site, determined by the type and or severity of abnormal disturbance. The known states and transition pathways for this ecological site are described in the accompanying state and transition model.
The Plant Community Plant Species Composition table provides a list of species and each specie’s or group of species’ annual production in pounds per acre (air-dry weight) expected in a normal rainfall year. Low and high production yields represent the modal range of variability for that species or group of species across the extent of the ecological site.
The Annual Production by Plant Type table provides the median air-dry production and the fluctuations to be expected during favorable, normal, and unfavorable years.
The present plant community on an ecological site can be compared to the various common vegetation states that can exist on the site. The degree of similarity is expressed through a similarity index. To determine the similarity index, compare the production 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 shown for the group. Divide the resulting total by the total representative value shown in the Annual Production by Plant Type table for the reference plant community. Variations in production due to above or below normal rainfall, incomplete growing season or utilization must be corrected before comparing it to the site description. The Worksheet for Determining Similarity Index is useful in making these corrections. The accompanying growth curve can be used as a guide for estimating percent of growth completed.
The State and Transition model shows the most common occurring plant communities likely to be encountered on this ecological site. This model may not show every possible plant community, but only those that are most prevalent and observed through field inventory. As more data is collected and research is available, these plant communities may be revised, removed, and even added to reflect the ecological dynamics of this site.
State 1
Reference State
This site exists on summits and side slopes of plateaus, mesas, and pediments. The site is characterized by a mix of shrubs with scattered perennial grasses and forbs. Primary shrubs are blackbrush and Torrey's jointfir with James' galleta and Indian ricegrass as the understory. Other shrubs present include cliffrose, snakeweed and Greenes' rabbitbrush.
Community 1.1
Shrubland with Native Grasses
The Shrubland with Native Grasses plant commmunity is characterized by a dominance of shrubs with scattered perennial grasses and forbs. Primary shrubs are blackbrush and Torrey's jointfir with James' galleta and Indian ricegrass as the understory. Other shrubs present include cliffrose, snakeweed and Greenes' rabbitbrush.
Table 5. Annual production by plant type
| Plant type |
Low
(lb/acre) |
Representative value
(lb/acre) |
High
(lb/acre) |
| Shrub/Vine |
140 |
160 |
180 |
| Grass/Grasslike |
20 |
35 |
50 |
| Forb |
10 |
15 |
20 |
| Total |
170 |
210 |
250 |
Table 6. Soil surface cover
| Tree basal cover |
0%
|
| Shrub/vine/liana basal cover |
0-5%
|
| Grass/grasslike basal cover |
0-2%
|
| Forb basal cover |
0-2%
|
| Non-vascular plants |
0%
|
| Biological crusts |
0-15%
|
| Litter |
5-10%
|
| Surface fragments >0.25" and <=3" |
30-60%
|
| Surface fragments >3" |
0-20%
|
| Bedrock |
0-10%
|
| Water |
0%
|
| Bare ground |
10-40%
|
Table 7. Canopy structure (% cover)
| Height Above Ground (ft) |
Tree |
Shrub/Vine |
Grass/
Grasslike |
Forb |
| <0.5 |
– |
5-10% |
0-2% |
0-2% |
| >0.5 <= 1 |
– |
1-5% |
0-2% |
0-1% |
| >1 <= 2 |
– |
0-1% |
– |
0-1% |
| >2 <= 4.5 |
– |
0-1% |
– |
– |
| >4.5 <= 13 |
– |
– |
– |
– |
| >13 <= 40 |
– |
– |
– |
– |
| >40 <= 80 |
– |
– |
– |
– |
| >80 <= 120 |
– |
– |
– |
– |
| >120 |
– |
– |
– |
– |
| Jan |
Feb |
Mar |
Apr |
May |
Jun |
Jul |
Aug |
Sep |
Oct |
Nov |
Dec |
| J |
F |
M |
A |
M |
J |
J |
A |
S |
O |
N |
D |
| Jan |
Feb |
Mar |
Apr |
May |
Jun |
Jul |
Aug |
Sep |
Oct |
Nov |
Dec |
| J |
F |
M |
A |
M |
J |
J |
A |
S |
O |
N |
D |
Community 1.2
Blackbrush Shrubland
Blackbrush Shrubland - The dominant aspect of this site is of blackbrush. Other shrubs are present but in small amounts. Perennial forbs and grasses are present, but in minor amounts. Grasses commonly present are James galletaa and Indian ricegrass. Shrubs commonly present include blaackbrush, Torrey jointfir along with other native shrubs in small amounts.
Pathway 1.1a
Community 1.1 to 1.2
Drought, Domestic/Wildlife Grazing
Pathway 1.2a
Community 1.2 to 1.1
Favorable precipitation, Time without disturbance, Prescribed Grazing
State 2
Blackbrush Shrubland with Introduced Annuals
Community 2.1
Blackbrush Shrubland with Introduced Annuals
Blackbrush Shrubland - The dominant aspect of this site is of blackbrush. Other shrubs are present but in small amounts. Perennial forbs and grasses are present, but in minor amounts. Grasses commonly present are James galletaa and Indian ricegrass. Shrubs commonly present include blaackbrush, Torrey jointfir along with other native shrubs in small amounts. Introduced annuals occur, but are a very minor part of the plant community, usually only found in trace amounts.
Transition T1A
State 1 to 2
Introduction of Non-native Annuals create a irreversible change in plant community
