JARS v61n1 - Rhododendron occidentale and its Habitats

Rhododendron occidentale and its Habitats
Michael Oliver
Portland, Oregon

Rhododendron occidentale , the western azalea, grows from southwestern Oregon through California, and possibly into northwestern Mexico, though this has not been confirmed. Because of the attractiveness and wide variation of the flowers, several investigators have looked for new and interesting forms in the wild areas of its range, or at least beyond where it is specifically cultivated (3, 4, 6, 7, 8, 9). Three different examples of the variations of the flowers discovered are shown in Figs. 2-4.

SM30 flower, with much yellow on all 
five petals SM502 flower, a very nice reddish-purple 
Fig. 2. SM30 flower, with much yellow on all five petals.
Photo by Michael Oliver
Fig. 3. SM502 flower, a very nice reddish-purple picotee.
Photo by Michael Oliver
2406 flower, a good pink with yellow 
on all five petals
Fig. 4. 2406 flower, a good pink with yellow on all five petals.
Photo by Michael Oliver

Several maps (4, 10) show the range of R. occidentale over a large area in southwestern Oregon and through a broad swath of California, but the specific areas where it exists in undisturbed areas of the wild are relatively few.
The basic habitat for R. occidentale is on soils of specific rock types in situations where there is sufficient water. From the northern end of its range near Myrtle Creek, Oregon, to roughly the latitude of San Francisco on the south, this soil type is ultramafic, or as it is often referred to, serpentine. South of this, R. occidentale exists on other mafic rocks as well, but always near water.
There has been a moderate amount of study of vegetation on serpentine soils. Kruckeberg (5) has described the situation for California and has analyzed what he calls the "serpentine syndrome," i.e., that serpentine soils have a unique flora, with some species noticeably absent, and others present. Indeed, some of the species that occur in serpentine habitats occur only there. When serpentine areas are studied, one usually finds that plants normally found primarily or entirely on serpentine have the same neighboring species, or "ecological associates."
Serpentine rocks are rich in magnesium and sometimes iron, hence the term "mafic" with "ma" for magnesium and "fi" for iron. Serpentine is usually referred to as ultramafic, since the rock is usually very rich in magnesium and iron. Generally, serpentine rocks are also very poor in calcium. Only plant species that can tolerate the concentrations and mix of elements can survive in these soils. The interest in vegetation on serpentine soils is worldwide, and many interesting environments have been studied. An excellent book discussing this subject has been written by Robert R. Brooks. (1). From these studies, however, no clear-cut conclusions have been made on what precisely is the mechanism that excludes many species from existing on serpentine.
Interestingly, Kruckeberg gives little attention to R. occidentale . This may be for several reasons. As noted, adequate water is required for R. occidentale and other plants to exist, and, in California, many serpentine areas have little water, perhaps too little for R. occidentale , but enough for many other species. In both southern Oregon and northern California, R. occidentale exists in areas of high moisture, both near the coast and at higher elevations.
Another aspect of the serpentine effect is that there is a range of compositions of serpentine soils. Some serpentine soils are so inhospitable that they have very little vegetation of any kind growing on them. Occasionally such sites are given names such as that for "Devil's Half Acre," which is about fifteen miles north of Agness, Oregon. At the other extreme, very little difference is observed in the plant communities of some weak serpentine and neighboring non-serpentine soils. This behavior is commonly observed on and near the serpentine outcroppings along the east coast of the United States (2).
This range of composition of serpentine soils can lead to a subjective criterion for what is labeled serpentine on geologic maps. On some geologic maps, areas labeled serpentine have different plant communities than do adjacent areas but do not show the more distinct ecology associated with stronger serpentine areas. One such example is the small area labeled serpentine about 10 miles southeast of Agness, Oregon, along Shasta Costa Creek.
Another aspect result of this variation of substrates labeled serpentine is that some areas have plant communities associated with stronger serpentine rock, and often R. occidentale , while areas only a few miles away and part of the same contiguous serpentine region on the soil map have different plant communities and no R. occidentale . Such a situation exists in a serpentine area about 25 miles northeast of Clear Lake, California. There Stony Creek cuts through a serpentine area and has R. occidentale along its banks. About five miles south Little Stony Creek cuts through the serpentine area, and along that creek there is no R. occidentale . The associated vegetation is also substantially different from that of Stony Creek.
There are many serpentine rock areas in northern California and southern Oregon. In California all of them are west of the Sierra crest. A map, Fig. 1, taken from Kruckeberg (5) shows the largest concentration of serpentine in California is in the northwestern part of the state, with very little in the bottom half of the state. Where sufficient water is available, R. occidentale usually, but not always, exists. Away from the coast, water is available on lake edges, streams and bogs. In some cases, azaleas exist along draws that are dry for a portion of the summer, but apparently the soil underneath retains adequate moisture.

Serpentine areas of southern 
Oregon and California
Fig. 1. Serpentine areas of southern Oregon and California.
Reprinted by permission from California Serpentines: Flora,
Vegetation, Geology, Soils, and Management Problems
by Arthur Kruckeberg, University of California Press, 1984.

In general, the annual rainfall increases in going from south to north throughout the range of R. occidentale . Also, higher elevations receive more rain than nearby lower areas. Between the increased rain and the higher moisture retention of the some of the soils throughout the year in the northern part of its range, R. occidentale and other serpentine tolerant plants often thrive in mountainous areas away from streams or other sources of water.
Near the ocean, the mist and light but frequent rains from the ocean can also keep vegetation soaked even with little organic or other retentive matter in the soil. One area that has no streams, but much moisture from the ocean, is the Red Mountain about ten miles east of the mouth of the Klamath River into the Pacific Ocean in northern California. (Note: there are several mountains called "Red Mountain" on serpentine rock in California and Oregon). There, R. occidentale is abundant.
One other group of habitats where R. occidentale exists on undisturbed sites is in bogs or other wet and often sandy coastal areas in southwest Oregon or northern California. One boggy area along US #101 just north of Port Orford, Oregon, is filled with R. occidentale . Another example is near Sea Ranch, east of Point Arena on the California coast, where a sandy bog area supports R. occidentale as well.

Southern California
In southern California, there are several sites where R. occidentale is known to exist and prosper. They all are near water sources, such as streams or springs. The soil substrate is not serpentine, but on the California geological map, it is a mafic, igneous rock labeled gabbro. Note, from Fig. 1, there is very little serpentine in the southern half of California, and only small fraction of the area has adequate water for vegetation throughout the year. The gabbro region that contains most, if not all, of the R. occidentale sites reported by Michael McCullough (7) and others (3) is the region from the Mt. Palomar area south through areas such as Idyllwild to Cuyamaca State Park. This area is in eastern San Diego County. The Yosemite area, which is near the middle of the state, is another area which is well known for R. occidentale , and its habitat there appears to be on or near gabbro.
At least some of these gabbro (mafic) areas of southern California provide good habitat for R. occidentale as does serpentine (ultramafic) rock elsewhere in its range. The characteristics of the southern California (San Diego County) R. occidentale plants differ in several ways from their northern counterparts. The leaves are never glossy and have a fine hairy cover. This may indicate a needed adaptation for this area and it is not unreasonable to assume that other adaptations may have occurred as well. When the genetic study of R. occidentale has advanced sufficiently, comparisons of these different populations may be a useful area of investigation.

Disturbed Areas
Much, if not most, of the study of R. occidentale over the past half century has been on the areas greatly altered by civilization. Investigators such as Frisbie (4) and Smith and Mossman (8, 9) saw so many plants near towns and farms that the few on serpentine did not stand out. Also, most serpentine lands are poor areas for farming, though some very wet areas support some pasture. In the forests, the government has classified serpentine areas as having no long-term commercial timber value since the few trees that exist grow so slowly. The absence of agriculture potential often then leads to low human activity in serpentine areas, and hence few access roads.
Rhododendron occidentale has several valuable characteristics that enable it to expand its range and prosper, though perhaps only for a few years. When competing vegetation is removed from an area, through human or natural events such as fire, R. occidentale can seed down and grow quickly, often several miles from any apparent natural source. Areas where this has occurred are areas converted to farmland, logged areas and forest fire burn areas. It is necessary, of course, for the habitat to have adequate water. These colonized areas will support R. occidentale until competing plants crowd it out. Such areas that were originally converted to farmland include the well-known Stage Coach Hill area (9), now an Azalea Reserve, south of Orick, California. This particular site has been regularly visited by R. occidentale enthusiasts for over 35 years, because of the large variety of attractive flower types among the plants that have grown and still grow there.
The history of R. occidentale on Stage Coach Hill has been described well elsewhere (9). Britt Smith and Frank Mossman explored the entire area thoroughly in the late 1960s and 1970s. It is on blue schist soil in view of the Pacific Ocean, which is about a mile away. The hill is in the same fog belt as the Redwood National Park, several miles to the north. There, the near-continual mist bath from the ocean allows the world's tallest trees to grow. The same bath allows the vegetation on Stage Coach Hill, including R. occidentale , to grow luxuriantly. When the area was actively farmed, over fifty years ago, this rapid growth of plants required that the farmers burn the fields every several years to maintain pasture. Now that farming has ceased, volunteer groups are removing some of the competing vegetation on a few acres of this hill, and this has allowed many R. occidentale plants to continue to thrive.
When Smith and Mossman explored the area, they found many attractive flowers. They propagated them by cuttings and wrote several interesting articles for enthusiasts. Several of the plants that Smith and Mossman originally found were outside of the areas where competing vegetation has been removed and have been shade killed. One especially attractive picotee, which they labeled SM (Smith-Mossman) 502 in 1970, was finally totally enclosed by a Sitka spruce forest, and it died in about 2000. Fortunately, most of the unusual, labeled forms have been propagated by cuttings and are gracing several gardens in the west.
Another example of the response of R. occidentale to disturbance is the response to the 2002 Biscuit fire in southern Oregon. The fire consumed over 100,000 acres and much of the area consumed was on serpentine rock. There was a sparse forest on the serpentine areas, with a moderate amount of understory vegetation, including R. occidentale . Though the forest was sparse, it was much denser than in areas with lower rainfall farther east. In the Biscuit fire area, near and in the Kalmiopsis Wilderness, the area is generally above 2,000 feet elevation and has a significant rainfall. Some of the fire area can be reached by Forest Service roads. In Fig. 5 is shown a R. occidentale plant three years after the fire. The original long branches of the plant can be seen still attached to the base while the new growth is emerging from the old rootstock. The other ground cover vegetation in the vicinity is recovering much more slowly if at all. The plant pictured was representative of the thousands of azalea plants observed in the burn area.

R. occidentale recovery after the 2002 
Biscuit fire
Fig. 5. A R. occidentale plant in southern Oregon, which was burned to the
ground in the 2002 Biscuit fire, showing much new growth in 2005.
Photo by Michael Oliver

In the 1970s, much of the forest alongside the Rogue River east of Gold Beach, Oregon, was completely logged. There are some serpentine areas with azaleas nearby, but the soil is not serpentine itself. By the 1980s, several of the ravines running down to the Rogue River had azaleas growing in them, especially at the lower levels which were wetter. However, as the replanted forest grew, it choked out the azaleas and by 2000, there were very few, if any, of the azaleas that colonized this area still existing.
There are other areas in California and Oregon that possibly were colonized by azaleas after fires, logging or other disturbance. The azaleas can survive as long as they receive adequate light or water. Among the areas that may fall in this category are Flynn Creek and part of the Navarro River, south of Fort Bragg, California. There are azaleas next to the water, all tall and spindly. Along Flynn Creek, very few of these plants flowered in 2005, indicating that these plants may not be selfstaining and indigenous. Numerous other areas exist where the azaleas appear to have arrived after a disturbance such as a clear-cut or a fire.

Discussion and Conclusions
Within its overall geographic area of southern Oregon and California, Rhododendron occidentale has a natural range determined by the soil and adequate water. This is due to the fact that R. occidentale can live and prosper on serpentine and other mafic soils. One key characteristic of such soils is that many plants are excluded from them, and R. occidentale has less competition for light and water. Rhododendron occidentale may also be able to survive in some undisturbed coastal environments, but now that issue is almost academic since almost the entire coastal area where they now exist has been disturbed by clearing at one time or another.
There are many potentially suitable areas for R. occidentale if one looks at geological maps of the area. However, the specific mineral concentrations in the serpentine rock which are required to exclude many species, including trees such as big leaf maples, Acer macrophyllum , and Douglas fir, Pseudotsuga menziesii , vary from place to place. If competing vegetation is not sufficiently suppressed, there may not be an adequate ecological advantage for R. occidentale to survive. Experience in examining many areas has confirmed this. As a result, when looking for places to explore for R. occidentale , the geological maps are useful guides to focus searches, but not every area labeled serpentine will have R. occidentale .
It appears that a major, if not the major, advantage derived from R. occidentale's ability to prosper in serpentine environments is that competitive vegetation is suppressed. This is also reinforced by its ability to grow, for a span of a few years, in situations on non-serpentine soil where the competitive vegetation has been eliminated by a disturbance. It is useful to note that it does not depend on whether the disturbance is man-made or not. The azalea's lifetime in these disturbed environments is limited by the recovery of the competing vegetation, which can limit access to light and, in some cases, moisture.
Rhododendron occidentale can often begin to grow within a few years following the disturbance of a given region, at least when there already are other populations of the azalea within several miles. It is possible, in extreme cases, that this could produce several plants very far removed from its natural range. It is not understood yet what the likely transport mechanisms for the seeds are.
There are several areas near San Francisco such as Big Basin Redwoods Park and Pescadero State Park where R. occidentale exists but not on serpentine rock. It is not known whether the azalea arrived after the area had once been cleared and has survived to the present or if there is another type of soil that provides good habitat.
A striking example of the association of serpentine and R. occidentale is in the Feather River canyon east of Butte, California, in the Sierra foothills. In that area, in several places strips of serpentine rock, on the order of a mile wide or so, cross through the canyon. In the small draws where there is enough seasonal water, R. occidentale exists. It appears that the draws do not have to have water throughout the year, but apparently the soils underneath stay sufficiently moist to maintain the azaleas. In neighboring draws in non-serpentine soils, there were no R. occidentale in the areas I investigated.
While its beautiful flowers, which can vary a great deal in appearance among plants, have created a great deal of garden interest, its horticultural requirements in domestic gardens can be somewhat demanding. It is well known, for example, that R. occidentale generally grows poorly in the eastern United States, though some diligent people have been able to grow it. However, many hybrid azaleas that have R. occidentale in their background, such as the Exbury azaleas, do well in the same locations. While these limitations are known, there is as yet little scientific understanding as to the mechanisms. This understanding awaits future studies.
While hunting for R. occidentale in northern California and southwestern Oregon, geologic maps have been invaluable for identifying promising areas to search. Also, though, in looking at the geologic maps of southern California, the gabbro areas near Mt. Palomar and Cuyamaca State Parks where R. occidentale has been found are not the only gabbro areas in the region. There are many other areas identified as gabbro in the southern part of the state, but fewer in the north. Other, as yet unidentified, R. occidentale populations may exist on these other areas.
This could also be true for Mexico. If there is an appropriate gabbro area in the Sierra de Juarez just south of the Mexican border, this may be the right place to search.

Dick Cavender, the author, 
Frank Mossman and the late Art Stubbs preparing to go on an occidentale field trip in 1994
Fig. 6. Dick Cavender, the author, Frank Mossman and the late Art Stubbs
preparing to go on an occidentale field trip in 1994. Photo by Oliver.
Photo by Marilyn Oliver

I have taken many trips to areas where R. occidentale has grown for almost twenty years. Fellow travelers have included friends and azalea enthusiasts. Most of these trips, though, have been with one or more of three friends, Dick Cavender, Frank Mossman and the late Art Stubbs. These three are pictured in Fig. 6 with me as we prepared for an occidentale trip to southern Oregon and northern California in 1994. I would like to thank them for their friendship, insight and support.

1. Brooks, Robert R., Serpentine and Its Vegetation: A Multidisciplinary Approach , Dioscorides Press, Portland , Oregon, 1987
2 .Dann, Kevin T., Traces on the Appalachians: A Natural History of Serpentine in Eastern North America , Rutgers University Press, New Brunswick, New Jersey, 1988.
3. Deul, Carl, A heat tolerant form of R occidentale, J. American Rhododendron Soc. , 33 (1), 1978, 33.
4. Frisbie, Leonard F., Rhododendron occidentale survey, Rhododendron, April, 1967, l.
5. Kruckeberg, Arthur R., California Serpentines: Flora, Vegetation, Geology, Soils, and Management Problems , U. California Press, Berkeley, 1984.
6. Leiser, Andrew T., Rhododendron occidentale on alkaline soil, Rhododendron and Camellia Year Book , RHS, London, 1957, 50.
7. McCullough, Michael M ., Exploring for the Western Azalea in Southern California, J. American Rhododendron Soc. , 39 (2), 1985, 70.
8. Mossman, Frank, The Western Azalea, Rhododendron occidentale, J. American Rhododendron Society , 29 (2), 1974,96.
9. Mossman, Frank M., The Western Azalea on Stagecoach Hill, Pacific Horticulture , 38 (1), Spring, 1977, 28.
10. Towe, L. Clarence, American Azaleas , Timber Press, Portland, 2004, 20.

Mike Oliver is an engineer who became interested in rhododendrons, especially R. occidentale , when he moved to Oregon in 1980. In addition to searching for and collecting cuttings of unusual and attractive forms, he has started intraspecific hybridizing. For a number of years, he has contributed seed to the ARS Seed Exchange.