Life zone
The life zone concept was developed by C. Hart Merriam in 1889 as a means of describing areas with similar plant and animal communities. Merriam observed that the changes in these communities with an increase in latitude at a constant elevation are similar to the changes seen with an increase in elevation at a constant latitude.[1]
The life zones Merriam identified are most applicable to western North America, being developed on the San Francisco Peaks, Arizona and Cascade Range of the northwestern USA. He tried to develop a system that is applicable across the North American continent, but that system is rarely referred to.
The life zones that Merriam identified, along with characteristic plants, are as follows:
- Lower Sonoran (low, hot desert): creosote bush, Joshua tree
- Upper Sonoran (desert steppe or Utah juniper
- Transition (open woodlands): ponderosa pine
- Canadian (fir forest): Rocky Mountain Douglas fir, quaking aspen
- Hudsonian (spruce forest): Rocky Mountains bristlecone pine
- Arctic-Alpine (alpine meadows or tundra): grass
The Canadian and Hudsonian life zones are commonly combined into a Boreal life zone.
This system has been criticized as being too imprecise. For example, the scrub oak chaparral in
Holdridge
In 1947, Leslie Holdridge published a life zone classification using indicators of:
- mean annual biotemperature(logarithmic)
- annual precipitation (logarithmic)
- ratio of annual potential evapotranspiration to mean total annual precipitation.
Biotemperature refers to all temperatures above freezing, with all temperatures below freezing adjusted to 0 °C, as plants are dormant at these temperatures. Holdridge's system uses biotemperature first, rather than the temperate latitude bias of Merriam's life zones, and does not primarily use elevation. The system is considered more appropriate to the complexities of tropical vegetation than Merriam's system.[4]
See also
References
- ISBN 9780816072293.
- ^ Ricketts, Taylor H.; Dinerstein, Eric; Olson, David M.; Loucks, Colby J.; et al. (1999). Terrestrial Ecoregions of North America: a Conservation Assessment. Washington DC: Island Press.
- ^ "Ecological Regions of North America: Toward a Common Perspective" (PDF). Commission for Environmental Cooperation. 1997. Archived from the original (PDF) on 2004-01-04.
- ^ "Holdridge's Life Zones". Geology class notes. Radford University. Archived from the original on 2008-01-10.