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Plant Ecology Text

Chapter 2. Life Zones & Biomes

Life Zones:

Merriam's Life Zones
elevation SanFrancisco
Mts, AZ
Mts, NY
2800+ m snow n/a ?
2600-2800 m Alpine Alpine 1600+ m
2000-2600 m Hudsonian Hudsonian 1400-1600 m
1500-2000 m Canadian Canadian 600-1400 m
1100-1500 m Transition Transition 30-600 m
0-1100 m Sonoran n/a n/a
Merriam Life Zones modern Vegetation zones
Colorado Plateau area
Arctic-Alpine Alpine Tundra
Hudsonian Spruce-Fir or
Subalpine Conifer Forest
Canadian Mixed Conifer Forest
Transition Ponderosa Pine Forest
Upper Sonoran Piñon-Juniper Woodland,
Semi-arid Grasslands
Lower Sonoran Mojave Desert
Sonoran Desert
Chihuahuan Desert

In the 19th Century, Naturalists realized that the landscape (a broad overview, as if seen from some elevation above the terrain, of the vegetation across many communities) spanning broad areas of the Earth surface appears as “bands” of similar vegetation (the total plant cover of an area, consisting of one or more communities, quoted from Pierre Dansereau, 1957. Biogeography, an Ecological Perspective, The Ronald Press Co, New York, p. 336). The postulated belts of similar vegetation can be seen in natural color, visible light, cloud-free composite photographs of Earth taken by orbiting satellites, suggesting that these belts of similar vegetation actually exist (see, for example Google™ Earth image of the San Francisco Mountains below, as seen from ‘Eye altitude’ = 15.06 km [9.36 mi], with 2.4X ‘elevation exaggeration’).
San Francisco Mts, Google™ Earth image
The light colored roundish area at the Transition label and extending to the left into the Canadian is probably a recovering old burn area with a Gambel's oak (Quercus gambelii) - New Mexico locust (Robina strobiformis) community growing on it. The lighter patches further left along the Transition - Canadian border appear to be newer burn areas. The small, whitish spots below the newer burn areas along the Upper Sonoran - Transition boundary can be seen to be rock outcrops in a photograph on the “Land Use History of North America, Colorado Plateau” website (cited in the next paragraph). This photograph also shows one of the newer burn areas to be a burn area. The lines drawn to indicate the boundaries between the Life Zones are deliberately shown across only a small portion of the mountain so that you can see for yourself where they would be if extended. I expect that, if you attempt to extend the lines (at least in your imagination), you will discover how clearly the zones are bounded [this is easiest to see along the upper and lower limits of the Canadian Zone to the right of the drawn lines]. The mis-labelled “Lower Sonoran” in the illustration is Upper Sonoran Semi-Arid Grasslands (the tan spots below the Transition Zone, and since they are almost identical to the newer burn areas along the Transition Zone-Canadian Zone boundary, you can correctly interpret that the newer burn areas supported dry grasslands at the time the Google™ image was recorded).
    Merriam introduced the concept of Life Zones (Merriam, C. H. and Steineger, L. 1890. Results of a biological survey of the San Francisco mountain region and the desert of the Little Colorado, Arizona. North American Fauna Report 3. U.S. Department of Agriculture, Division of Ornithology and Mammalia, Washington, D.C., 136 pp.) to describe the belts of vegetation (and animal communities of the vegetation). Merriam later (1893) completed a map delineating the major life zones of the entire North American continent. Inherent in the Life Zones concept is the expectation that the Life Zones separate on the basis of altitude (in mountain ranges), but also on the basis of latitude. “Merriam believed that climatic gradients, especially temperature, largely determined what type of vegetative community one may find in a given location (Land Use History of North America, Colorado Plateau, downloaded 15 Feb 2011).” Expanding the concept to the entire Earth, it was suggested that moving north by 160km (100 mi) was equivalent to going up 300m (1,000 ft):
N . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Equator
Tundra, Boreal Forest, Temperate deciduous forest, Subtropical forest, Tropical forest


Biomes of the World
precip, cm  
250..450 tropical
130..250 trop.seasonal
taiga n/a
100..130 thorn forest woodland
70..100 savanna grassland steppe tundra
40..70 thorn scrub shrubland
0..40 desert semi-desert desert grassland
temp ° 30 .. 18 18 .. 3 3 .. -5 -5 .. -14

As suggested in the definition of vegetation above, vegetation consists of communities (the populations of all species [usually of a single high level taxon, such as Order] occupying the same site). When all of the communities (Mammal, Bird, Reptile, Amphibian, Insect, Spider, worm, Plant, Fungi & microbiological… ) are considered along with the physical-chemical environment of the area, we have an Ecosystem. When we look across the late 20th to early 21st Century landscape, we see a mosaic of patches mostly caused by Human fragmentation of the natural landscape to carve out our agricultural and urban spaces. However, if we could look across the pre-Columbian North American landscape, we would probably still see a mosaic of patches made up of a few to several Ecosystems (with the number of Ecosystems depending on our ‘eye altitude’) [but no corn fields nor suburban housing tracts], just as the landscape in the remote areas I have seen in my professional life time tend to represent a mosaic of patches. The only exceptions I have seen are a vast piñon (Pinus edulis) - juniper (Juniperus sp) shrublands of the Colorado Plateau in south-east Utah, and the vast Douglas fir (Pseudotsuga menziesii) - Redwood (Sequoia sempervirens) forests of Mendocino County, California, between Fort Bragg and Willets. In the pre-Columbian North America landscape, the mosaic patches were caused by topographic features of the environment (slope [how steep], exposure [surface angle to solar radiation], and microclimatology [climate near the ground]), depth to the perched water table, recent fire history etc.

    Early in the 20th Century, it was proposed that the community was effectively a “super organism,” and potentially [my interpretation of the paper by Clements (Frederic Clements, 1936. “Nature and structure of the climax,” Journal of Ecology 24(1): 253-284.) in which he presented the hypothesis] an additional ‘fundamental unit’ in Ecology. There followed classification schemes (notably, Braun-Blanquet, 1932. Plant Sociology (trnaslated by Conrad & Fuller), McGraw-Hill Book Co, New York, 439 pp.) to place the plant Communities into Formation Classes and, with the animal communities included, into Biomes. If one were to take a “road-trip” across North America from west (Monterey, CA) to east (Norfolk VA) [ignoring Mountains], one would see the following Biomes:
            { scrubland, semi-desert, steppe, prairie, woodland, temperate forest }.
    Perhaps the most important contribution to ecological theory from this effort was the concept that “Plant formations, as has been emphasized previously, must be described in terms of vegetation and vegetation alone (not climate, drainage, soil, etc.); and for the same reason, habitat must be approached in terms of the environment itself (not of structure, or composition, or dynamics of vegetation), since a correlation is being sought between the two” (Dansereau, 1957. Biogeography, an Ecological Perspective, the Ronald Press Company, New York. p. 128). This admonition was needed because the most familiar classification of climate regimes (Köppen) was based in part on the vegetation, while the Formations were based in part on the climate. The classic example of this is the concept of a desert: climatologically a desert has (among other characteristics) large spaces between the plants which include cacti (and other plants adapted for water conservation), while biologically a desert (among other characteristics) receives less than 150 mm (6 inches) of rainfall per year. Because of the circular nature of these definitions, we cannot logically conclude that desert plants are adapted to desert climates! If we defined a desert climatologically to receive less than 150 mm of rainfall per year, and biologically as widely spaced plants with adaptations to reduce water loss, we could conclude that desert plant communities are restricted to desert climates [except that it's not true; some cacti were collected from Indiana with annual average rainfall of 860 - 890 mm/yr (34 - 35 inches/yr) when the first major immigration of Humans of European descent was occurring. I have seen a Prickly-pear cactus (Opuntia sp [requires well drained soils]) growing about 100 mm (4 inches) from a Bog orchid (unidentified [requires saturated soils]) in the habitat [Lake Michigan sand dunes] from which the oldest known specimen of the cactus was collected in Indiana. The cactus was growing on the top of a 200 mm (8 inch) tall sand “dune;” the orchid, in standing water between the miniature dunes].
    As plant Ecologists began to define the vegetation carefully in terms of biological properties, and environmental factors in terms of physical/chemical properties, it became clear that the Community can not be considered to be a ‘super-organism,’ although it can still be considered to be a fundamental unit in Biological systems. As a result, the ‘super-organism community’ has been replaced by the “individualistic community” concept (which is explained at length in chapter 8).

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revised: 16 Feb 2011