From Desert to "Sky Island" Vegetation
Main sources: Bowers, Janice E., 1988, A Sense of Place: the Life and Work of Forrest Shreve, Tucson: University of Arizona Press; Whittaker, R.H., & W.A. Niering, 1965, "Vegetation of the Santa Catalina Mountains, Arizona: a Gradient Analysis of the South Sope, Ecology Early Summer, Vol. 6 No. 4:429-452; Warshall, Peter, 1995, "The Madrean Sky Island Archipelago", in Debano, Leonard, ed., Biodiversity and the Management of the Madrean Archipelago, Diane Publishing, with thanks to the Sky Island Alliance.
To quote Peter Warshall (cited just above), "Sky islands are a type of continental or inland terrain made up of a sequence of valleys and mountains. All sky islands have a stack of biotic communities that allow vertical (as well as aspect) migration annually or during one of the planet's long-term climatic events. The valleys act as barriers or bridges to the colonization by new species that attempt to cross the intervening valley. The valleys become barriers when they contain an ecology alien to the migrating species."
What follows below elaborates on development of ideas related to this notion, which really got under way for our area with the work of Forrest Shreve (today culminating with the grasp of scientists like Warshall):
1) Forrest Shreve's research 1908-1915:
When Forrest Shreve first ascended the Santa Catalina Mountains in July 1908, he and his companions had to ride on horseback from the mountain's desert base at around 3,000 feet elevation to a Mount Lemmon summit at just over 9,100 feet. During that ascent, he observed "a continually shifting panorama of vegetation", and recognized that gradual changes in vegetation he had observed while crossing the entire United States were here compressed into a few thousand feet of elevation on one desert mountain. Bowers, p. 47)
Shreve's work followed the earlier "life-zone" system devised by biologist C. Hart Merriam in 1894, which identified zones of vegetation primarily in relation to cirumpolar belts of temperature, and the recognition of Merriam and others like Edgar Transeau in 1905 that vegetation patterns involved not one but four major climatic factors: temperature, relative humidity, wind velocity, and rainfall. (ibid., p. 71) Working now with steep gradients of topography, Shreve sought to identify how topography determined these local climate factors and through them, vegetation (ibid., p. 50).
2) Shreve's Basic Designation of "Vegetation Regions":
The vegetation of these mountains, he found, were layered in three broad "regions":
- Desert, where "the vegetation is open, low, and diversified in the assemblage of growth-forms, with a predominance of microphyllous trees and shrubs and an abundance of cacti",
- Encinal (the spanish word for Evergreen Oak is encino), dominated by Evergreen Oak trees, the lower division showing open and orchardlike stands, the upper division showing dense, closed stands; and
- Forest, with Pine Forest in the lower division and Fir Forest further up. (ibid., p. 51)
Carefully examining the variables of rainfall, soil moisture, evaporation, humidity, air temperature, and soil temperature, he found that, while others had assumed the gradient was determined by altitude, moisture and temperature were primary. These of course do vary with altitude, but the relation between evaporation rates and soil moisture are crucial: "Inadequate soil moisture, he concluded, prevented the downward movement of forest plants into encinal and of encinal plants into desert," whiletemperature factors, particularly cold (duration of frost, consecutive times of freezing, absolute minima) limited upward distribution of many desert plants and some encinal plants. (ibid., 52-3)
While others had observed the effects of slope on vegetation in the West -- that north-facing and south-facing slopes differ in solar radiation, soil temperture, and soil evaporation -- he noted that in the Santa Catalina Mountains these differences increase at higher elevations. In canyons, surface flow and underflow of water "bring components of upland vegetation of each altitude down along the streamways of the altitudes just below", with mountain plants withstanding the heat of lower levels where moisture supply approximates that of the mountains. On the other hand, cold-air drainage lowers temperatures in canyon bottoms, hence limits upward movement of desert plants. "'Each of the leading types of vegetation in the Santa Catalinas reaches the uppermost limit of its occurrence on ridges and high south-facing slopes,' causing 'an interdigitation of the vegetistic regions.'" (ibid., p. 51) Conversely, plants typical of higher regions reach lower on north-facing slopes, for example on Mica Mountain in the Rincons, where -- though the peak itself does not have the higher types of Fir Forest flora, high north-facing slopes below the top do.
3) Long-term Relationships:
Shreve then turned to consider the broader geographical and historical. affinities of the vegetation of the region, and found that of the Desert Region species, some 90% came from either the "Arizona-Sonora Desert" or the "Texas-Chihuahua Desert", while only a few derived from the Mojave or Great Basin Deserts. In other words, their geohistorical affiliations aligned distinctly toward the south. Of the Encinal Region species, the dominant ones ranged far southward along both sides of the Sierra Madre, but few were found farther north than the Mogollon Rim in Central Arizona. On the other hand, the Forest Region species had divided affinities, "both with the Mexican codillera and with the Rocky Mountains of Colorado and their southern extension in New Mexico." (ibid., p. 63-64) These observations were largely corroborated by later research.
4) R.H. Whittaker and W.A. Niering Elaboration in 1965:
Source: Whittaker, R.H., & W.A. Niering, 1965, "Vegetation of the Santa Catalina Mountains, Arizona: a Gradient Analysis of the South Sope, Ecology Early Summer, Vol. 6 No. 4:429-452.
The landmark work by these two scholars elaborated greatly on Shreve's vegetation work, developing methods of gradient analysis and sampling by transect grids to construct a picture of great precision, summarized below in their Figure 2 on page 436 of the cited paper: (Click on the image to enlarge it.)
These studies show how canyon-and-ridge corridors work in both directions, and of course animals do the same if and when they can, moving from one vegetational "refuge" to another by ascending or descending the mountain.
For more details on specific Sky-Islands of the Middle San Pedro River valley, see:
5) "Archipelago" Models:
For a much broader (indeed, global) contemporary view of Sky Island systems, see this link: Peter Warshall on "The Madrean Sky Island Archipelago". Warshall presents here a remarkable synthesis of the ecological implications of these kinds of systems for our planet and its future.