Rhododendron macrophyllum

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].

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This species can be found in the following regions of the western United States (according to the Bureau of Land Management classification of Physiographic Regions of the western United States):

1 Northern Pacific Border
2 Cascade Mountains

CA OR WA BC

Rhododendron macrophyllum D. Don ex G. Don:
China (Asia)
United States (North America)

Note: This information is based on publications available through Tropicos and may not represent the entire distribution. Tropicos does not categorize distributions as native or non-native.

Canada

Origin: Native

Regularity: Regularly occurring

Currently: Present

Confidence: Confident

Type of Residency: Year-round

United States

Origin: Unknown/Undetermined

Regularity: Regularly occurring

Currently: Unknown/Undetermined

Confidence: Confident

More info for the terms: resistance, series, shrub

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.

More info for the term: mesic

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

More info for the terms: association, mesic, series

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]

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This species is known to occur in the following ecosystem types (as named by the U.S. Forest Service in their Forest and Range Ecosystem [FRES] Type classification):

FRES20 Douglas-fir
FRES21 Ponderosa pine
FRES23 Fir - spruce
FRES24 Hemlock - Sitka spruce
FRES27 Redwood

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This species is known to occur in association with the following cover types (as classified by the Society of American Foresters):

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

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This species is known to occur in association with the following plant community types (as classified by Küchler 1964):

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

More info for the terms: cover, frequency

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

More info for the terms: cover, frequency

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.

More info for the term: severity

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].

More info for the term: shrub

Tall shrub, adventitious-bud root crown

More info for the term: capsule

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].

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More info for the term: phanerophyte

Phanerophyte

More info for the terms: shrub, tree

Shrub, Tree

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].

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].

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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].

The following is a representative barcode sequence, the centroid of all available sequences for this species.

Barcode of Life Data Systems (BOLDS) Stats
Public Records: 1
Specimens with Barcodes: 1
Species With Barcodes: 1

Canada

Rounded National Status Rank: N4 - Apparently Secure

United States

Rounded National Status Rank: NNR - Unranked

Rounded Global Status Rank: G5 - Secure

More info for the terms: cover, shrubs

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].