Species composition refers to the contribution of each plant species to the vegetation. Botanical composition is another term used to describe species composition. Species composition is generally expressed as a percent, so that all species components add up to 100%.
Species composition can be expressed on either an individual species basis, or by species groups that are defined according to the objectives of the inventory or monitoring program (eg., Aristida spp., perennial forage grasses, etc.).
Species composition is a commonly determined attribute in rangeland inventory and monitoring. It is regarded as an important indicator of ecological and management processes at a site.
Ecological indicators - species composition provides the essential description of the character of the vegetation at a site. Certain images are readily understood when major species are mentioned, eg., pinon (Pinus sp.) - juniper (Juniperus sp.) woodland, and other common species are also presumed to be present as one becomes familiar with the vegetation. These distinctions form the basis of rangeland mapping and the delineation of range site boundaries.
The relative contribution of a species also signifies its dominance in the vegetation and its ability to capture resources. Slightly different inferences of competitive ability are suggested if species composition is expressed on the basis of cover, density, or biomass measurements.
Management indicators - most objectives in rangeland management are directly concerned with the assessment or manipulation of species composition. For example, carrying capacity is influenced by the relative abundance of desirable forage species at a site. Wildlife habitat is also influenced by the relative contribution of various species that provide sources of shelter and food. Species composition is used to determine range condition and range trend, which are valuable tools to judge the impact of previous management and guide future decisions.
Special Considerations for Species Composition Sampling
Here is a critical issue to be considered when designing sampling protocols to determine species composition:
Methods to Determine Species Composition
Species composition is generally determined by sampling methods based on assessing the contribution of individual species or species groups in sample units. This section describes methods commonly used to determine species composition.
References and Further Reading
Barbour, M.G., Burk, J.H., and W.D. Pitts. 1987. Terrestrial plant ecology. Benjamin Cummins Publishing Co, Menlo Park, CA. 2nd ed. pp. 191-193.
Bureau of Land Management. 1996. Sampling Vegetation Attributes. Interagency Technical Reference, BLM/RS/ST-96/002+1730. pp. 28-29.
Society for Range Management. 1989. A Glossary of Terms Used in Range Management. Society for Range Management, Denver, CO. 3rd ed. p.13.