Montezuma Castle National Monument - Plants

In spite of the small sizes of the Castle and Well Units, together they support 379 species of plants and about 15 plant associations. This is an impressive amount for a semiarid upland area with less than 12 inches of precipitation per year. Yet the monument is highly disturbed because of prehistoric, historic and current human actions. Human use of the Montezuma Castle region has been documented since prehistoric times. Ancient Hohokam and Sinagua people practiced extensive agriculture at both units beginning as early as 1200 years ago. They installed irrigation systems, harvested crops, and built communities. With the decline of the Hohokam and the Sinagua, Native American use and disturbance of Montezuma Castle and its surroundings likewise diminished only to be intensified again with the arrival of European settlers in the mid-1800s. The resulting post-settlement conversion of the landscape to agriculture and urban use continues to the present day. The physical imprint of post-settlement agrarian and pastoral practices on the landscape is still highly visible. The vegetation of Montezuma Castle remains in the process of recovery and in some instances may never recover to a state which is more or less "natural", that is, one which would greatly resemble central Arizona vegetation types (i.e., plant associations) which had developed largely in the absence of human disturbance. Montezuma Castle National Monument"s bottomland vegetation is a prime example. The mesquite stands located on the Beaver Creek floodplain are relatively young and represent only about 60 years of recovery, at a maximum, from abandoned agricultural fields and pastures. Although mesquite dominates the vegetation"s tree layer, dense, self-sustaining and ecologically stable populations of Mediterranean annual grasses and weedy forbs dominate the understory vegetation. Recovery will likely proceed no further due to hydrological changes in Beaver Creek brought about by upstream water diversions and disturbance-induced water table lowering. The high volume of fine fuels also creates an unnatural fire hazard. Plants typical of upland comminuties and responsive to disturbance, such as snakeweed, are invading and changing the vegetation characteristics of Montezuma Castle"s bottomland plant associations. The unit"s bottomlands, in essence have become "desertified." At Montezuma Castle the highest species heterogeneity values of persistent perennial plant species are not in bottomland associations like the Arizona Sycamore - Velvet Ash Association, contrary to what one might expect and perhaps due to the extensive history of disturbance. In fact, the highest species heterogeneity values are observed in upland plant associations. At the Castle unit, these are the Crucifixion Thorn, Snakeweed - Catclaw Acacia - Velvet Mesquite, and Creosote Bush Mixed Scrub Associations. At the Well unit they are Needlegrass - Mixed Scrub and Four-wing Saltbush - Velvet Mesquite Associations. Species richness trends on the other hand to follow expectations. At the Castle unit, plant associations having the highest species richness values are the Crucifixion Thorn association, an upland, multistoried type and the Arizona Sycamore - Velvet Ash Association, a bottomland gallery forest or woodland type. At the Well unit estimated species richness of persistent plants is highest in Arizona Sycamore - Velvet Ash - Fremont Cottonwood Association. Lowest species heterogeneity and richness values were predictably observed in plant associations occupying the most highly disturbed sites. Of the major plant associations at Montezuma Castle, this is generally the Velvet Mesquite Association. Perhaps the only area of the monument relatively free of past disturbance, except for livestock grazing, is the hilly, rocky terrain in the southeastern part of the Castle unit. The current dominant plant species is crucfixion thorn but it is highly likely that perennial grasses were relatively more important in the past. Plant studies of the Montezuma Casle flora revealed its very high affinity to both Plains and Great Basin Grassland and Semidesert Grassland. Yet, this strong grassland affinity does not often reveal itself in the form of local plant associations dominated by native perennial grasses. An exception is a small area of the Well unit that supports a Needlegrass-Mixed Scrub association. Nevertheless, moderate and large-sized woody plants dominate the majority of the Well unit"s vegetation. Woody shrubs, in particular, crucfixion thorn and creosote bush, dominate upland plant associations of the Castle unit. Associations dominated by perennial grasses, namely purple three-awn and needlegrass, account for only a small portion of the upland vegetation and less than two percent of Montezuma Castle. Tree overstories are important in only two plant association, both very similar to one another and both occupy bottomland habitats. These are the Arizona Sycamore - Velvet Ash - Fremont Cottonwood Association (Well unit) and the Arizona Sycamore - Velvet Ash Association (Castle unit). Several native plant species are more or less ubiquitous at Montezuma Castle. Mesquite and snakeweed exist in a variety of both upland and bottomland plant associations. Neither can be considered major "differential" plant species. For example, upland vegetation is defined by the presence of either crucifixion thorn or creosote bush and bottomland vegetation is defined by the presence of Arizona Sycamore and Velvet Ash. Mesquite, though often cited and invoked as a bottomland indicator species, does not function as such at Montezuma Castle. Trees and shrubs are the dominant plant life forms in terms of cover contribution to the biotic community. However, according to numbers of representative species comprising each life-form category, the dominant life-form would be the hemicryptophytes, as is often cited for semiarid areas. Clearly, simple species list alone may not give an accurate picture of life-form diversity in an area"s vegetation.