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You are here >1Up Info > Wildlife, Animals, and Plants > Plant Species > Tree > Species: Rhododendron macrophyllum | Pacific Rhododendron

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SPECIES: Rhododendron macrophyllum | Pacific Rhododendron
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].

Related categories for Species: Rhododendron macrophyllum | Pacific Rhododendron

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