Plant Community Dynamics
Primary plant succession occurs as the historical development of the ecological site takes place. Plant succession is the progressive replacement of plant communities on an ecological site that leads to a climax or characteristic plant community. Succession occurs over time and is a result of environmental factors, including natural disturbances. Retrogression is the degradation of, or shift away from, the historic plant community and is a reflection of changes in the site condition. Commonly, the site condition changes irreversibly and a different vegetative state develops. This vegetative state may be relatively steady and resistant to change.
Range similarity index is a rating used to evaluate an ecological site. It is based on a comparison of the present plant community to either the historic climax plant community or another vegetative state community. The similarity index is a percentage, in annual weight, of a specific vegetative state plant community that is presently on a site. The index serves to describe the seral stage of the plant community when compared to the historic climax plant community and indicates its position in the state and transition model when compared to other plant communities in the site description. It provides an indication of the extent of change needed to establish the desired plant community state or the historic climax plant community.
Rangeland health assessment is an inventory method used to determine the overall health of a site. Seventeen attributes are evaluated against a matrix of conditions and are placed in one of five categories—extreme, moderate to extreme, moderate, slight to none, and none. The rating of a rangeland site is compared to the potential for that site. The attributes impact the biotic integrity, soil site stability, and hydrologic function of a rangeland site. The assessment identifies the attributes that are not functioning properly and the overall functions that might be at risk. It gives managers the ability to focus limited resources on repairing or redirecting only the indicated attributes. Generally, if an attribute is rated extreme, moderate to extreme, or in some cases, moderate, it can be considered to be at risk and requires some management changes to improve the overall health of the site.