Plants of Casa Grande Ruins National Monument - NPS Inventory ...

Vegetation Mapping
14
Larrea tridentata
shrubland
This community type is composed of a creosote monoculture averaging 1.5–
2.5 m tall and usually spaced 2–3 m apart on a flat, level landscape. In small
depressions where runoff is concentrated, usually adjacent to roads (especially
around the perimeter of the main unit) or archeological sites, shrubs are slightly
taller and more closely spaced. These depressions often contain inclusions of
one or more Velvet mesquite individuals, but these contribute less than 1% of
total cover in this alliance type. It is notable, however, that many large velvet
mesquite snags are scattered throughout areas in the main unit occupied by
this type. Their decline was documented by Judd (1971). Barrel cactus is the
only other common perennial found in this type, with individuals sparsely
and irregularly scattered throughout, often growing underneath cresosote
shrubs in apparent nurse relationships. Areas between shrubs are mostly
bare soil or gravel, but may contain a variety of annual plant species during
wet seasons. Litter is sparse and soil development poor, except directly under
creosote shrubs, which tend to grow on low mounds of soil retained by their
roots against strong aeolian erosive forces on the landscape. Within this
association are two small inclusions on an old, defunct asphalt road bordering
an old irrigation canal in the northeast corner of the main unit. Creosote still
dominates but individuals are larger and more widely spaced than in the rest of
the alliance. Desert broom and cattle saltbush are interspersed with creosote,
on opposite ends of the old road. These inclusions are notable because they
are the only place where either species is present in the eastern part of the
Monument. Moreover, herbaceous annuals are significantly less dense in
these inclusions. It is likely that these inclusions are the result of the altered soil
surface in this portion of the park.