Chaparral Shrublands

History

Water has historically affected populations occupying this region. Water related activities have been documented since about 200 B.C., when Hohokam Indians settled the Salt River Valley in central Arizona and constructed canals to irrigate their fields (Baker 1999b). European settlers in the Phoenix, AZ area in the late 1860s depended on irrigation water from the Salt River for agriculture. However, water supplies fluctuated greatly because the river often flooded in the winter and dried up during the summer. There were no impoundments to store water for the dry seasons. Therefore, the Salt River Water Users' Association, the largest irrigation district in Arizona, signed an agreement in 1904 with the United States government under the National Reclamation Act, to build a dam on the Salt River below the confluence with Tonto Creek.

The Roosevelt Dam, the first of 6 dams on the Salt and Verde Rivers, was completed in 1911. Watershed managers in the early 20th century became concerned that erosion on the adjacent and headwater watersheds of the Salt River would move sediment into the newly constructed Roosevelt Reservoir, which would decrease its capacity. Measurements indicated that 12,450 ha-m of coarse granitic sediments accumulated behind Roosevelt Dam between 1909 and 1925 (Baker 1999b). Because of the concern about these sediment accumulation, the Summit Plots were established in 1925 by the USDA Forest Service 24 km upstream from Roosevelt Dam to study the effects of vegetation recovery from livestock grazing (the dominate land use at the time), mechanical stabilization of disturbed soil, and reseeding on stormflow and sediment yields from the lower chaparral zone (Rich 1961).

The early research on the Summit Plots was expanded to consider the effects of watershed management practices on all the region's natural resource products and uses of the forests, woodlands, and shrublands. The influences of vegetation manipulation on the natural resources from chaparral shrublands were studied by the USDA Forest Service and its cooperators at Three-Bar, Whitespar, Mingus, and Battle Flat because they contained extensive areas of chaparral vegetation commonly found throughout the Southwest, thoroughly evaluating the effects of vegetative manipulations on the array of multiple uses from the ecosystems studied. Results from this research show that vegetation can often be managed to increase water yields, while still providing timber, forage, wildlife, and amenity values required by society in some optimal combination. This finding was not surprising, as many of the vegetation management practices studied to improve water yield were common in principle and application to other management programs often implemented to benefit other natural resources.

Description

The chaparral type is restricted almost entirely to the Lower Colorado River Basin, where it covers about 1.4 million ha, nearly all in Arizona (Hibbert 1979). Unlike the mountain brush in Colorado and Utah, chaparral species tend to be low-growing shrubs with thick, evergreen leaves well adapted to heat and drought. The type is common on rugged terrain from 900 to 2,000 m in elevation. Shrub live oak (Q. turbinella) is most abundant, followed by mountainmahogany. Other common shrubs are manzanita (Arctostaphylos spp.), Emory oak (Q. emoryi), silktassel (Garrya wrightii), desert ceanothus (Ceanothus greggii), and sugar sumac (Rhus ovata). Most species sprout prolifically from root crowns after burning or cutting and are difficult to eradicate.

Chaparral shrublands occur on rough, discontinuous, mountainous, terrain south of the Mogollon Rim in central Arizona. Average annual precipitation varies from about 380 mm at the lower limits to over 630 mm at the higher elevations (Hibbert 1979). Approximately 60% of the annual precipitation occurs as rain or snow between November and April. The summer rains fall in July and August, which are the wettest months of the year. Annual potential evaporation rates can approach 900 mm. Water yield varies greatly depending on precipitation, elevation, and soils. The overall average is 25 mm or more; the lower, drier sites produce little, while the wettest sites can yield 75 or 100 mm.

General Information

Plant Species—Chaparral shrublands occur on rough, discontinuous, mountainous, terrain south of the Mogollon Rim at elevations ranging from 3,000 to 7,000 ft. Chaparral stands consist of a heterogenous species mix in many locations, but often only 1 or 2 species dominate. Shrub live oak (Quercus turbinella) is the most prevalent species, while true (Cercocarpus montanus) and birchleaf mountain mahogany (C. Betuloides), Pringle (Arctostaphylos pringlei) and pointleaf (A. pungens) manzanita, yellowleaf (Garrya flavescens), hollyleaf buckthorn (Rhammus crocea), desert ceanothus (Ceanothus greggii), and other shrub species are included in the chaparral mixture of shrub species. Annual and perennial grasses, forbs, and half-shrubs are present, particularly where the overstory canopy is open or only moderately dense.

Precipitation—Average annual precipitation varies from about 14 inches at the lower limits of the chaparral shrubland type (3,000 ft) to 30 inches at the higher elevations (7,000 ft). Approximately 60 % of the annual precipitation occurs as rain or snow between November and April. Summer rains occur in July and August, which are the wettest months of the year. Annual potential evapotranspiration rates can approach 35 inches. Also see: water yield and potential increase.

Resources and Activities—Although the recreational value (hiking, camping, and hunting) of chaparral is lower than that of higher elevation vegetation types, its close proximity to major population centers provide it with an advantage. Research studies also have determined that chaparral areas are major sources of water if vegetation control is exercised. Chaparral rangelands are often grazed year-long by livestock, because evergreen plants common to the shrublands provide a continuous forage supply. A variety of wildlife species are found in chaparral shrublands, with comparatively high populations often concentrated in fringe areas.

Soils—Chaparral soils are typically coarse-textured, deep, and poorly developed. Granites occur on more than half of the shrublands. The topography is characterized by mountain ranges dissected by steep-walled gorges and canyons. Slopes of 60% to 70% are common.

Yavapai County Watersheds

Three sets of experimental watersheds (DeBano et al. 1999b), the Whitespar, Mingus, and Battle Flat watersheds, were established in Yavapai County to study chaparral management. The Whitespar watersheds played an important role in assessing the potential water yield increases that could be obtained by chaparral conversion practices in areas of moderately dense chaparral (40-60% cover density). This information extended the research results obtained from the Three-Bar watersheds, which were covered with a dense stand of chaparral (DeBano et al. 1999a). The Mingus watersheds contained a sparse cover of chaparral and were similar to the Natural Drainages on the Sierra Ancha Experimental Forest (Gottfried et al. 1999).

Images Available

There are currently over 150 images available in the image database illustrating various aspects and conditions found in chaparral ecosystems in southwestern United States. These can be accessed after getting into the database by using the key words chaparral. Additional key words are available for searching the images in a drop down list within the database.