BOTANICAL AND ECOLOGICAL CHARACTERISTICS
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