Index of Species Information

SPECIES:  Rhododendron macrophyllum

Introductory

SPECIES: Rhododendron macrophyllum
AUTHORSHIP AND CITATION : Crane, M. F. 1990. Rhododendron macrophyllum. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/ [].
ABBREVIATION : RHOMAC SYNONYMS : Rhododendron californicum Hymenanthes macrophyllum SCS PLANT CODE : RHMA3 COMMON NAMES : Pacific rhododendron coast rhododendron California rosebay Pacific rhodendron TAXONOMY : The currently accepted scientific name of Pacific rhododendron is Rhododendron macrophyllum D. Don ex G. Don [32]. There is a white-flowered form, album, in addition to the more typical pink-flowered form, macrophyllum [30,35]. Since the genus has close to 1,000 species, it has been subdivided. The first subdivision is into two large groups: Lepidote with scales on branchlets, leaves and flowers, and Elepidote without scales. These groups do not hybridize naturally and most attempts to create hybrids have been unsuccessful [35]. These groups are divided into 23 Lepidote series and 20 Elepidote series, each named by a representative species. Pacific rhododendron is an Elepidote rhododendron and a member of the Ponticum series and subseries [33,35,50]. LIFE FORM : Shrub, Tree FEDERAL LEGAL STATUS : No special status OTHER STATUS : NO-ENTRY


DISTRIBUTION AND OCCURRENCE

SPECIES: Rhododendron macrophyllum
GENERAL DISTRIBUTION : Pacific rhododendron is found along the Pacific Coast from British Columbia to Monterey County in California [30,41]. It is widely distributed in the Coast and Cascade Mountain Ranges [9,30,43]. Pacific rhododendron is less abundant in the Coastal Mountains of Washington and northern Oregon and more common south of the Siuslaw River [27]. ECOSYSTEMS : FRES20 Douglas-fir FRES21 Ponderosa pine FRES23 Fir - spruce FRES24 Hemlock - Sitka spruce FRES27 Redwood STATES : CA OR WA BC BLM PHYSIOGRAPHIC REGIONS : 1 Northern Pacific Border 2 Cascade Mountains KUCHLER PLANT ASSOCIATIONS : K001 Spruce - cedar - hemlock forest K002 Cedar - hemlock - Douglas-fir forest K003 Silver fir - Douglas-fir forest K004 Fir - hemlock forest K005 Mixed conifer forest K006 Redwood forest K009 Pine - cypress forest K029 California mixed evergreen forest SAF COVER TYPES : 224 Western hemlock 225 Western hemlock - Sitka spruce 226 Coastal true fir - hemlock 229 Pacific Douglas-fir 230 Douglas-fir - western hemlock 231 Port Orford-cedar 232 Redwood 234 Douglas-fir - tanoak - Pacific madrone SRM (RANGELAND) COVER TYPES : NO-ENTRY HABITAT TYPES AND PLANT COMMUNITIES : In the western hemlock (Tsuga heterophylla) zone of northwestern Oregon, Pacific rhododendron is an indicator of cool, dry to mesic environments [22]. Published classification schemes listing Pacific rhododendron as an indicator species or a dominant part of vegetation are presented below. Preliminary plant associations of the Siskiyou Mountain Province [1] The tanoak series of the Siskiyou Region of southwest Oregon [2] A preliminary classification of forest communities in the central portion of the western Cascades in Oregon [12] Vegetation of the Douglas-fir region [14] Ecoclass coding system for the Pacific Northwest plant associations [18] Plant association and management guide for the western hemlock zone: Mt. Hood National Forest [22] Vegetation mapping and community description of a small western Cascade watershed [25] Plant association and management guide for the Pacific silver fir zone: Mt. Hood and Willamette National Forests [26] Plant association and management guide: Siuslaw National Forest [27] Plant association and management guide: Willamette National Forest [28] Riparian zone associations: Dessschutes, Ochoco, Fremont, and Winema National Forests [61]

MANAGEMENT CONSIDERATIONS

SPECIES: Rhododendron macrophyllum
IMPORTANCE TO LIVESTOCK AND WILDLIFE : Pacific rhododendron is one of the shrubs eaten by mountain beaver in the Coast Range [31]. Common rhododendron (Rhododendron ponticum), a closely related species, is poisonous to sheep [29]. Pacific rhododendron may also be poisonous to sheep [52]. PALATABILITY : NO-ENTRY NUTRITIONAL VALUE : NO-ENTRY COVER VALUE : Pacific rhododendron may provide good thermal and hiding cover for big game animals [28]. However, thickets can be difficult for the animals to enter [28]. Presumably, Pacific rhododendron provides cover for smaller animals also. VALUE FOR REHABILITATION OF DISTURBED SITES : Pacific rhododendron offers erosion protection on steep watersheds [42]. It is propagated by cuttings, by layering, and from seed [21]. Air-dried seeds will remain viable for 30 months when stored at 20 degrees F (-7 deg C) [42]. Germination and early growth are good in shaded, moist mineral soil [5]. Detailed information on cultivation of Pacific rhododendron is available [35,38,42,47]. OTHER USES AND VALUES : Pacific rhododendron is the state flower of Washington. Kitsap Peninsula residents host an annual weeklong Rhododendron Festival during spring bloom [33]. Pacific rhododendron is not as widely cultivated as the showier Rhododendron hybrids, but it is used as an ornamental in backgrounds or native gardens and naturalized woodlands [33,35]. Native plants like Pacific rhododendron have potential value for low-water-use landscaping [60]. Both white and pink forms are commercially available [35]. Growing Pacific rhododendron in partial shade to full sun and debudding or pruning encourage compact growth and bloom [33,35]. Wild plants are legally protected from digging [7,33]. OTHER MANAGEMENT CONSIDERATIONS : Pacific rhododendron often indicates low soil nitrogen content. On sites where it is dominant, care should be taken to protect the soil from damage from heavy equipment or fire during logging [22,27]. Conifer planting can be difficult on sites that had dense Pacific rhododendron cover before treatment because of the residual roots. Removing the overstory may stimulate the growth of Pacific rhododendron and other shrubs [22]. If the soil is undisturbed during treatment, it resprouts rapidly [19]. If control is necessary, spraying the foliage in spring with triclopyr ester gives fair to good control [6,54].

BOTANICAL AND ECOLOGICAL CHARACTERISTICS

SPECIES: Rhododendron macrophyllum
GENERAL BOTANICAL CHARACTERISTICS : Pacific rhododendron is an evergreen shrub, normally 6 to 12 feet (1.8-3.7 m) tall at maturity with leaves 3 to 6 inches (7-15 cm) long and 1.2 to 2.7 inches (3-7 cm) wide [35,50]. The flowers are light to rose pink and borne in terminal clusters (racemes) of 20 or more [50]. Shaded plants have elongated branches and may resemble small trees, while plants growing in the open form compact, dense bushes [33]. Pacific rhododendron's mature leaves, like leaves of other rhododendrons in the Ponticum series, have a soft leathery texture and no hairs, trichomes, or other covering [33,50]. Pacific rhododendron is not as cold-hardy as other members of the series [46]. Pacific rhododendron has shallow roots [21]. Good aeration is important for rhododendron roots which often will grow deep into loamy soils [35]. Two studies of Pacific rhododendron in northwestern Washington and northern California describe a large, tuberlike rootcrown growing just under the soil surface with fine, fibrous roots extending into the soil [55,62]. Most rhododendrons have a characteristic type of mycorrhiza that promotes nutrient uptake in nutrient-poor environments and may provide resistance to heavy metal damage [45]. Since Pacific rhododendron grows well on nutrient-poor sites [28], this mycorrhizal relationship seems probable. RAUNKIAER LIFE FORM : Phanerophyte REGENERATION PROCESSES : The flowers are pollinated by bees and plants begin to bear seed when they are 5 years old [21,42]. The fruit is a dry, rusty-brown, pubescent capsule that divides into five parts by splitting lengthwise to release the numerous, minute seeds [41,42]. The seeds, including the wing, are less than 0.1 inch (3 mm) long [41]. Pacific rhododendron seeds germinate without stratification and are viable for up to 2 years. The seeds require light for germination [42]. If aboveground portions are killed, Pacific rhododendron can regenerate vegetatively by sprouting from stem bases and from the rootcrown [3,20]. SITE CHARACTERISTICS : Pacific rhododendron is found in coastal to low montane conifer forests on soils that are moist but well drained and frequently shallow [3,21,52,62]. Many western Oregon sites with Pacific rhododendron as a dominant have soils with low levels of available nitrogen [22,27,26]. Pacific rhododendron grows in several Pacific Northwest forest zones [15]. The Sitka spruce (Picea sitchensis) zone forms a narrow band along the coast. In this uniformly wet and frequently foggy area, Pacific rhododendron is often found on less favorable sites [15]. On old sand dunes and steep slopes facing the ocean Pacific rhododendron forms thickets with salal (Gaultheria shallon) and evergreen huckleberry (Vaccinium ovatum) [15,43]. At the southern end of this zone and extending into northern California, Pacific rhododendron is a dominant under coastal redwood (Sequoia sempervirens) and tanoak (Lithocarpus densiflorus) on soils formed from sandy coastal plain deposits [1,15,16]. It is part of the understory of northern California's pygmy conifer forests on severely podzolized soils formed from similar deposits [55]. While the western hemlock zone is wet and mild, summer drought and temperature extremes are more common than in the Sitka spruce zone. Pacific rhododendron is found on intermediate mesic sites in this zone along the coast [15]. In this zone in the northern Cascades Pacific rhododendron is most dominant on cooler, dry to mesic sites [22]. In the central Cascades it can be found on most types of sites within the western hemlock zone, although it dominates on warmer and drier sites [12,59]. In this zone in the southern Siuslaw National Forest of the Coast ranges, Pacific rhododendron is often a dominant on south-facing slopes and lower elevation ridges with thin soils [27]. In the western hemlock subzone where Port-Orford-cedar (Chamaecyparis lawsoniana) is a dominant, communities containing Pacific rhododendron are typically on better drained soils [24]. It is often found under mesic Port-Orford-cedar - Douglas-fir (Pseudotsuga menziesii) forests on the west slope of the Siskiyou Mountains at low to middle elevations on diorite parent materials [56]. In the higher, wetter and cooler mountain hemlock (Tsuga mertensiana) and silver fir (Abies amabilis) zones, Pacific rhododendron is an understory dominant on relatively warm, dry soils at lower elevations within the type [12,15,28]. In the mixed-conifer zone Pacific rhododendron is found on the moistest sites at mid-elevations in the southern Cascade and eastern Siskiyou Mountains. Average temperatures in this zone are similar to those in the western hemlock zone but the summers are warmer and drier [15]. In the South Umpqua Basin of the southern Cascades in Oregon, Pacific rhododendron is a good indicator of high soil moisture [37]. It occurs on cool, moist sites with soils averaging 40 inches (101.6 cm) deep in the Siskiyou Mountains [1]. On the east side of the Siskiyous, it grows with Douglas-fir where site conditions are most similar to coastal conditions. It is at its environmental limit on these sites and does not extend to drier sites [1]. Elevational ranges in some western regions are [15,28,41,56]: Minimum Maximum feet meters feet meters California sea level 4,000 1,219 c Siskiyou Mts 2,500 670 5,500 1,680 Oregon sea level 5,300 1,615 SUCCESSIONAL STATUS : NO-ENTRY SEASONAL DEVELOPMENT : Throughout its range, Pacific rhododendron blooms between April and July [30]. In southern, coastal Oregon, Pacific rhododendron blooms from April to May [42]. The fruit ripens in August and September and is dispersed during the late summer and fall [42].

FIRE ECOLOGY

SPECIES: Rhododendron macrophyllum
FIRE ECOLOGY OR ADAPTATIONS : Pacific rhododendron grows in moist Pacific Coast forests where, historically, major fires were infrequent and more frequent surface fires burned small areas [57]. The main fire adaptation of Pacific rhododendron is its ability to resprout from a shallow, tuberlike rootcrown [3,20,42,55,62]. POSTFIRE REGENERATION STRATEGY : Tall shrub, adventitious-bud root crown

FIRE EFFECTS

SPECIES: Rhododendron macrophyllum
IMMEDIATE FIRE EFFECT ON PLANT : Pacific rhododendron appears to be top-killed by most fires. The shallow rootcrown could be heat-killed during severe fires, thus killing the entire plant. Low severity fires may allow the survival of basal stem buds, accounting for observations of its increased survival following such fires [11,20]. DISCUSSION AND QUALIFICATION OF FIRE EFFECT : NO-ENTRY PLANT RESPONSE TO FIRE : Following fire, Pacific rhododendron sprouts from stem bases or rootcrowns and new seedlings may establish [3,20,62]. There is a marked decrease in cover and frequency immediately after fire followed by a slow, gradual increase [10,11,48,58]. In the western Cascade Mountains of Oregon, Pacific rhododendron is a residual species following light fires but very scarce after more severe fires [11]. Nevertheless in this area, evidence of past fires is shown by brushfields that include Pacific rhododendron [53]. In Asia rhododendron seedlings rapidly colonize open areas after fire [35] and Pacific rhododendron's tiny, winged seeds might allow expansion from surviving plants. DISCUSSION AND QUALIFICATION OF PLANT RESPONSE : Several studies that have used permanent plots to follow vegatation changes after logging and burning in Oregon and Washington demonstrate that Pacific rhododendron is fire-sensitive. Two studies following slash burning compared burned and unburned plots. In the first study, most plots were burned lightly or moderately by fall fires. Samples taken during the first 16 years after slash burning show Pacific rhododendron to be dominant on twice as many unburned plots as burned plots. Where it did attain significant cover on burned plots, Pacific rhododendron had resprouted by the second season after fire [39,40]. A second study found cover of Pacific rhododendron to be 30.5 percent on unburned plots and 4.9 percent on burned plots 11 to 16 years following fire [49]. Results of another postfire study demonstrated a slow increase in frequency of Pacific rhododendron from the first to the fifth and sixth growing seasons [58]. A comparison of old-growth western hemlock - Douglas-fir stands with 2- to 40-year-old stands found mean cover values of Pacific rhododendron decreased from 13 percent to 0.4 percent 2 years after broadcast burning and gradually increased to 6.8 percent at 40 years [48]. Early recovery of Pacific rhododendron on three clearcuts that were treated with medium-intensity fall fires was as follows [11]: Cover (%) Frequency (%) Before logging: 8.5 29.5 Year 1 after logging: 1.0 18.0 Year 1 after slash fire: 0.2 13.1 Year 2 after slash fire: 0.8 11.5 Year 5 after slash fire: 1.8 14.8 FIRE MANAGEMENT CONSIDERATIONS : Since Pacific rhododendron is reduced by fire [20], burning after logging results in better conifer stocking [57]. However, communities with Pacific rhododendron as a dominant are frequently on infertile soils that are sensitive to the effects of fire [27,28]. Moderate to hot slash fires can cause damage to these soils and loss of nutrients, especially nitrogen [27,28].

REFERENCES

SPECIES: Rhododendron macrophyllum
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