No. 12. Ceanothus-Manzanita Shrublands

Christopher B. Chappell and Jimmy Kagan

Geographic Distribution. This habitat ranges from southwestern Oregon south through much of California. Within Oregon, it is primarily located in the Rogue and Illinois valleys in Curry, Josephine, and Jackson counties; it is also found scattered in the Siskiyou Mountains of the same counties, in Douglas County, and in the southern Cascades of Jackson and western Klamath counties.

Physical Setting. Climate is mostly very warm and relatively dry (about 17-30 inches [43-76 cm] mean annual precipitation), but extends less commonly on serpentine or extremely dry sites into somewhat cooler and moister climates (up to 50 inches [127 cm] annual precipitation). Summers are very dry; winters are only slightly cool, much warmer than shrublands on the eastside. Primary elevation range is about 1,000-2,000 ft (305-610 m), but extends up to a maximum of 5,000 ft (1,524 m). Topography is typified by mainly lower valley slopes and foothills, but extends to nearly flat valley bottoms (where before European settlement this habitat was a major type) and sporadically onto mountain slopes. This habitat tends to occur on southern aspects when it does occur outside of low valleys. Soils are typically shallow to bedrock or are derived from coarse alluvial deposits. Ultramafic bedrock is a major parent material in the western Siskiyou Mountains, whereas the eastern Siskiyou Mountains and adjacent valleys are largely volcanic.

Landscape Setting. This habitat occurs adjacent to or in a mosaic with Southwest Oregon Mixed Conifer-Hardwood Forest, Westside Oak and Dry Douglas-fir Forest and Woodlands, Agriculture, and rarely, Westside Grassland. Urban is also adjacent in few areas. Westside Riparian-Wetlands habitat occurs as small inclusions within this habitat. This habitat covers large areas only in lower elevation valleys or on extensive areas of serpentine soils. At moderate to high elevations it is mainly small patches within a forest mosaic. Major land use of this habitat is grazing and low-density residential development.

Structure. This habitat consists mainly of shrubland dominated by sclerophyllous evergreen broadleaf shrubs, but can also include grasslands with scattered tall shrubs. Deciduous broadleaf shrubs are less important, but are in some cases dominant. The shrubs are mostly 3.3-13 ft (1-4 m) high. Shrub canopy ranges from very open to completely closed. Herbaceous cover varies inversely with shrub canopy cover. Perennial bunchgrasses are the dominant understory at sites in good condition, whereas annual grasses dominate at sites in poorer condition. If shrubs are not too dense, forbs are abundant. Historically, many of these shrublands were probably grasslands with scattered shrubs. Occasional conifers or broadleaf trees are sometimes scattered in the habitat.

Composition. Sclerophyllous and hemi-sclerophyllous shrubs that dominate are, most commonly, wedge-leaf ceanothus (Ceanothus cuneatus) and white-leaf manzanita (Arctostaphylos viscida), and less commonly, chaparral whitethorn (Ceanothus leucodermis), blueblossom (C. thyrsiflorus), deerbrush (C. integerrimus), and deer oak (Quercus sadleriana). Wedge-leaf ceanothus is the most abundant species at low elevations in the major valleys and is the shrub most tolerant of xeric conditions. Other common, but not typically dominant shrubs include hairy manzanita (Arctostaphylos columbiana), pinemat manzanita (A. nevadensis), birchleaf mountain-mahogany (Cercocarpus montanus var. glaber), Klamath plum (Prunus subcordata), bitter cherry (P. emarginata), chokecherry (P. virginiana), Brewer’s oak (Quercus garryana var. breweriana), huckleberry oak (Q. vacciniifolia), California yerba-santa (Eriodictyon californicum), and bearbrush (Garrya fremontii).

The native bunchgrasses are Idaho fescue (Festuca idahoensis), California fescue (F. californica), California oatgrass (Danthonia californica), Lemmon’s needlegrass (Stipa lemmonii), western needlegrass (S. occidentalis), and bluegrass (Poa secunda). Forb diversity is often high and common genera include Lilium, Calochortus, Fritillaria, Microseris, Monardella, and Erigeron. One of several species of oak (Quercus) or pine (Pinus), Douglas-fir (Pseudotsuga menziesii), or incense cedar (Calocedrus decurrens) are sometimes present as scattered individuals, especially on less xeric sites.

Other Classifications and Key References. Franklin and Dyrness ⁸⁸ referred to this habitat as sclerophyllous shrub communities in the interior valleys of Oregon. It is also called chaparral ^{125, 130, 141}. The Oregon Gap II Project ¹²⁶ and Oregon Vegetation Landscape-Level Cover Type ¹²⁷ that would represent this type is Siskiyou Mountain serpentine shrubland. Other references describe aspects of this habitat ^{69, 188}.

Natural Disturbance Regime. Fire is the major natural disturbance. Fire regimes have not been studied in Oregon, but in central California the fire-return interval has been estimated at 30-60 years ⁸⁴. Fire frequency may have been greater during historic based on chaparral fire regimes in southern California ¹⁵⁷. High-severity fires are typical, with most shrubs being top-killed.

Succession and Stand Dynamics. Wedge-leaf ceanothus and white-leaf manzanita are killed by fire. Some less common shrubs sprout after fire. The dominant shrub species regenerate abundantly after fire from a long-lived seedbank that is scarified by fire ¹⁸⁷. Many seedlings die in the first 3 years after fire ⁵⁰. Wedge-leaf ceanothus can maintain prominence for >100 years ¹²⁴. Shrub canopy cover generally increases up to 30 years or so after the last fire, and in the absence of another fire, the herbaceous understory can be reduced under dense late-successional shrub canopies. Wedge-leaf ceanothus is considered a climax, or late-successional, dominant species on low-elevation dry sites in the Rogue Valley and on some serpentine sites ^{14, 69}. On many other sites, this habitat seems to be maintained by occasional fires, and trees, especially oaks and ponderosa pine, will gradually increase in the absence of fire.

Effects of Management and Anthropogenic Impacts. Fire suppression has probably increased the predominance of dense, tall shrub stands versus a more open, patchy structure. It also seems to reduce the cover of bunchgrasses and forbs as stands become old. Grazing reduces native bunchgrasses in favor of exotics and/or the native rhizomatous California brome (Bromus carinatus). Exotic species that have successfully invaded the understory of this habitat are soft brome (Bromus mollis), medusahead (Taeniatherum caput-medusae), hedgehog dogtail (Cynosurus echinatus), and yellow star-thistle (Centaurea solstitialis).

Status and Trends. This habitat occupies a small area within this region; it has declined considerably because of conversion to agriculture, residential development, and fire suppression. Most of this habitat has been degraded by fire suppression, grazing, and exotic species invasions. This habitat is still declining in extent from development pressures. One out of 7 Oregon plant associations listed in the National Vegetation Classification is considered imperiled globally ¹⁰, but this type of vegetation has been poorly studied in Oregon and there may be other imperiled associations.