Ecological dynamics
The pinyon-juniper forest is generally a climax vegetation type throughout its range, reaching climax about 300 years after disturbance, with an ongoing trend toward increased tree density and canopy cover and a decline in understory species over time. Singleleaf pinyon seedling establishment is episodic. Population age structure is affected by drought, which differentially reduces seedling and sapling recruitment more than other age classes. The ecotones between singleleaf pinyon forests and adjacent shrublands and grasslands provide favorable microhabitats for singleleaf pinyon seedling establishment since they are active zones for seed dispersal, nurse plants are available, and singleleaf pinyon seedlings are only affected by competition from grass and other herbaceous vegetation for a couple of years.
Several natural and anthropogenic processes can lead to changes in the spatial distribution of pinyon-juniper forests over time. These include 1) tree seedling establishment during favorable climatic periods, 2) tree mortality (especially seedlings and saplings) during periods of drought, 3) expansion of trees into adjacent grassland in response to overgrazing and/or fire suppression, and 4) removal of trees by humans, fire, or other disturbance episodes. Specific successional pathways after disturbance in singleleaf pinyon stands are dependent on a number of variables such as plant species present at the time of disturbance and their individual responses to disturbance, past management, type and size of disturbance, available seed sources in the soil or adjacent areas, and site and climatic conditions throughout the successional process.
Fire Ecology:
In the Great Basin, there is evidence of both frequent, low-severity fires carried by once-abundant perennial grasses, and less frequent, localized stand-replacement fires during extreme conditions. Fires burned in irregular patterns, producing a mosaic of burned and unburned landscape. On high-productivity sites where sufficient fine fuels existed, fires burned every 15 to 20 years, and on less productive sites with patchy fuels, fire intervals may have been in the range of 50 to 100 years or longer. Fire frequency in singleleaf pinyon communities varies with fuel loads and ignition source that, in turn, vary with habitat type, aspect, topography, stand history, and climatic conditions.
Major Successional Stages of Forest Development:
Herbaceous: Vegetation is dominated by grasses and forbs under full sunlight. This stage is experienced after a major disturbance such as crown fire. Skeleton forest (dead trees), remaining after fire or residual trees left following harvest, have little or no affect on the composition and production of the herbaceous vegetation.
Shrub Herbaceous: Herbaceous vegetation and woody shrubs dominate the site. Various amounts of tree seedlings (less than 20 inches in height) may be present up to the point where they are obviously a major component of the vegetal structure.
Sapling: In the absence of disturbance, tree seedling develops into saplings (20 inches to 4.5 feet in height) with a range in canopy cover of about 5 to 10 percent. Vegetation consists of grasses, forbs, and shrubs in association with tree samplings.
Immature Forest: The visual aspect and vegetal structure are dominated by Utah juniper trees and singleleaf pinyon trees greater than 4.5 feet in height. The upper crown of dominant and co-dominant trees are cone or pyramidal shaped. Seedling and saplings of pinyon and Utah juniper are present in the understory. Dominants are the tallest trees on the site; co-dominants are 65 to 85 percent of the highest of dominant trees. Understory vegetation is moderately influenced by a tree overstory canopy of about 10 to 25 percent.
Mature Forest (Reference Plant Community): The visual aspect and vegetal structure are dominated by singleleaf pinyon and Utah juniper that have reached or are near maximal heights for the site. Dominant trees average greater than five inches in diameter at one-foot stump. Upper crowns of singleleaf pinyon and Utah juniper are typically either irregularly or smoothly flat-topped or rounded. Tree canopy cover ranges from 35 to about 45 percent. Understory vegetation is strongly influenced by tree competition, overstory shading, duff accumulation, etc. Infrequent, yet periodic wildfire is presumed to be a natural disturbance influencing the understory of mature pinyon-juniper forests. This stage of community development is assumed to be representative of this woodland site in the pristine environment.
Over-Mature Forest: In the absence of wildfire or other naturally occurring disturbances, the tree canopy on this site can become very dense. This stage is dominated by singleleaf pinyon and Utah juniper that have reached maximal heights for the site. Dominant and co-dominant trees average greater than five inches in diameter at one-foot stump height. Upper crowns are typically irregularly flat-topped or rounded. Understory vegetation is sparse or absent due to tree competition, overstory shading, duff accumulation, etc. Tree canopy cover is commonly greater than 40 percent.
State 1
Reference Plant Community
Community 1.1
Reference Plant Community
The reference plant community is dominated by singleleaf pinyon and Utah juniper. An overstory canopy cover of about 40 percent is assumed to be representative of tree dominance on this site in the pristine environment. Overstory tree canopy composition is about 40 to 60 percent singleleaf pinyon and 60 to 40 percent Utah juniper. The understory includes curlleaf mountainmahogany, Gambel oak, muttongrass and blue grama.
Table 5. Ground cover
| Tree foliar cover |
35-45%
|
| Shrub/vine/liana foliar cover |
5-10%
|
| Grass/grasslike foliar cover |
1-5%
|
| Forb foliar cover |
1-5%
|
| Non-vascular plants |
0%
|
| Biological crusts |
0%
|
| Litter |
60-80%
|
| Surface fragments >0.25" and <=3" |
10-30%
|
| Surface fragments >3" |
0-5%
|
| Bedrock |
0%
|
| Water |
0%
|
| Bare ground |
10-20%
|
Table 6. Canopy structure (% cover)
| Height Above Ground (ft) |
Tree |
Shrub/Vine |
Grass/
Grasslike |
Forb |
| <0.5 |
1-5% |
1-5% |
1-5% |
1-5% |
| >0.5 <= 1 |
1-5% |
1-5% |
1-5% |
1-5% |
| >1 <= 2 |
1-5% |
1-5% |
– |
1-2% |
| >2 <= 4.5 |
1-10% |
5-10% |
– |
– |
| >4.5 <= 13 |
10-40% |
1-2% |
– |
– |
| >13 <= 40 |
10-20% |
– |
– |
– |
| >40 <= 80 |
– |
– |
– |
– |
| >80 <= 120 |
– |
– |
– |
– |
| >120 |
– |
– |
– |
– |
