Vegetation Zones of the Southwest - Design with the Desert: Conservation and Sustainable Development - page 145

vegetation as a function of both the availability of water and its seasonal distribution. These
two variables are used as a simple rubric for delineating four broad vegetation types within
the temperate zone, all of which occur within the southwestern region. This rubric gives
only a general guide to the correlation between climate and vegetation as numerous eco-
logically interesting exceptions occur. The broad pattern, however, is valid and useful for
understanding the patterns of vegetation across the landscape. As this pattern is consid-
ered, one must keep in mind that other environmental factors, particularly geology, soils,
and disturbance history, will add additional levels of complexity to the landscape mosaic.
We will focus on topographically defined patches that rise above the background matrix
of the hyperarid to semiarid desert. In these topographically defined patches, the general
pattern observed is that precipitation increases and temperature decreases as elevation
increases. The result is that these topographic patches have increasing P/ET ratios, and
the vegetation communities that develop are more diverse and have higher vegetative
abundance (measured as either primary productivity or standing biomass). Some of
these patches, which often abruptly rise out of the surrounding desert background, have
engendered considerable scientific interest and their study has been important in the
development of ecological theory. For instance, Merriam 23 chose to study changes in plant
and animal communities on Humphrey's Peak (12,635 ft) in northern Arizona “because of
its southern position, isolation, great altitude, and proximity to an arid desert.” His studies
led to the “life zone” concept, which is still a common approach to describing altitudinal
and latitudinal vegetation changes as a series of discrete biotic communities.
Whittaker and Niering 24 took a different approach on the Santa Catalina Mountains, a sky
island dominated by Mount Lemmon (9156 ft) in southern Arizona. Here they described
vegetation patterns as a continuum along which the distribution of individual plant
species independently varied in response to their specific adaptations to the elevation/
climatic gradient. Their work was fundamental in the development of the gradient concept
of vegetation distribution. When discussing sky islands, in the context of development, it
is also important to note that many of these mountain top ecosystems have been relatively
isolated at least since the late Pleistocene (Chapter 6) and contain many unique and
typically endangered species. The presence of these species presents special challenges to
landscape development apart from other ecological considerations.
Merriam 23 and Whittaker and Niering both describe the changes in vegetative
communities that occur over elevation gradients. The specific changes, however, also depend
on seasonal distribution of precipitation. In the western region of the North American
deserts, increases in elevation initially result in increases in winter precipitation relative to
summer precipitation due to the influence of an oceanic climate. Gleason and Cronquist 3
refer to the vegetation type developing under this climatic regime as the Sclerophyllous
Forest type (commonly referred to as chaparral communities). Dominant plants of this type
share the common theme of being able to withstand severe summer drought but also being
able to quickly begin photosynthesis as temperatures rise in the spring using moisture
accumulated during the wetter winter season. Common plant species are large shrubs
with thick, leathery, evergreen leaves that persist through dry periods but are available for
immediate photosynthesis when conditions are suitable. Because of the summer drought
characterizing these communities, fire is a common disturbance feature and species are
well adapted to regenerate following burns either through resprouting from the plant's
base or from fire-adapted seeds. While species in this type are well adapted to periodic
burning, these same fires become unacceptable as development occurs (see Chapter 11). As
in other fire-prone vegetation types, suppression of fire often results in accumulation of
fuel and the potential for higher intensity fires.