Vegetation Zones of the Southwest - Design with the Desert: Conservation and Sustainable Development - page 146

Chaparral communities in the coastal regions (California) are very diverse and include
numerous habitats and common plant species include chamise ( Adenostoma fasciculatum ),
several species of manzanita ( Arctostaphylos spp.), and several oaks ( Quercus spp.). 3 Species
of pine including Coulter pine ( Pinus coulteri ) and digger pine ( Pinus sabiniana ) often occur
at the upper altitudinal limits of the California chaparral type.
Chaparral communities resembling those of California also occur in parts of Arizona
and New Mexico. The physiognomy of these communities is very similar; however, the
general affinities of the flora are with the Sonoran Province as opposed to the Californian
Province. 3 Dominant species include turbinella oak ( Quercus turbinella ), pointleaf manzanita
( Arctostaphylos pungens ), and desert ceanothus ( Ceanothus greggii ).
The seasonal distribution of precipitation on elevated patches is reversed in the eastern
regions of the North American deserts. Here, increases in elevation result in increases in
summer precipitation relative to winter precipitation due to the presence of a continental
climate. This creates conditions under which the Grassland type develops. During the
warmer months, water is available for growth and development of the dominant grasses,
and during the drier, cooler months, the aboveground portion of most grass plants die.
Regrowth occurs the next season from perennating buds that are located near or below the
ground surface and that are well protected against water loss by the dead litter of previous
years' leaves. Raunkiaer 25 referred to plants of this life form as hemicryptophytes (literally
half-hidden plants) since the perennating buds are well protected from the elements
leading to either stress (water loss) or disturbance (fire and grazing in this case). Fire has
historically been a common disturbance feature in many grasslands and its suppression
along with the associated overgrazing by livestock has contributed to invasion of shrubby
species (see Chapters 11 and 12). 26 While the extent of grasslands has diminished over
the last century, spectacular grasslands, nevertheless, remain around many sky islands
in southern Arizona and northern Mexico. Common species in these grasslands include
several gramas (e.g., Bouteloua gracilis , B. hirsuta , and B. eriopoda ), three-awns ( Aristida spp.),
curly mesquite ( Hilaria belangeri ), and tobosa.
Across the entire region, at elevations above sclerophyllous forest or grassland types,
water becomes much less limiting during either the summer or winter season. This allows
for the development of a variety of Forest types. Trees, which dominate the forest type,
have their perennating buds elevated above the ground and exposed to the elements year
round. Raunkiaer 25 referred to this life form as phanerophytes (literally exposed plants).
While the upper limit for the distribution of trees (elevation or latitude) is typically set by
the length and warmth of the summer season, the lower limit is set by the ability of the
trees to avoid desiccation during any season. The piñon-juniper woodland occurs at lower
elevations within the forest type and is a xeric forest in which several species of juniper
( Juniperus spp.) and piñon pine ( Pinus edulis ) occur with a highly variable understory.
Above the piñon-juniper woodlands, are large areas of forest dominated by ponderosa
pine ( Pinus ponderosa ). Associated species are highly viable and include junipers and oaks
in drier, warmer, lower-elevation areas and aspen ( Populus tremuloides ) and Douglas fir
( Pseudotsuga menzeisii ) in wetter, cooler, higher-elevation areas. The understory again is
highly variable but often includes grasses such as blue grama or Arizona fescue ( Festuca
arizonica ). Both the piñon-juniper woodland and ponderosa pine forests are well adapted
to frequent fires (see Chapter 11).
At increasing elevations with higher P/ET ratios, the forest types correspond to those
also found at higher latitudes. A spectacular example of this is the spruce-fir forest atop
Mount Graham, a sky island located in southern Arizona (10,718 ft). Merriam 23 classified
this forest type as a Hudsonian boreal forest equivalent to ecosystems occurring in