BOTANICAL AND ECOLOGICAL CHARACTERISTICS
SPECIES: Taxus brevifolia | Pacific Yew 



GENERAL BOTANICAL CHARACTERISTICS : 
Pacific yew is a slow-growing evergreen shrub or tree which commonly
reaches 20 to 40 feet (6-12 m) at maturity [65,68].  On favorable
coastal lowland sites, scattered individuals can grow to 60 feet (18 m)
in height and have diameters of 2 to 3 feet or more (0l6-0.9 m) [5].  On
poor sites, such as those at higher elevations, Pacific yew grows as a
large sprawling shrub [62].  This large shrub or tree can reach maturity
at 250 to 350 years of age [62] and often survives for several centuries
[5].

Pacific yew is characterized by a conical crown and slender, drooping
horizontal branchlets [32,55].  The trunk is limby and often contorted
or malformed [29,32,62].  Twigs are slender, hairless and green, but
become dark reddish brown in the second growing season [32].  Bark is
very thin (approximately 0.25 inch [64 mm]), scaly, with purplish outer
scales covering newly formed reddish or purplish inner bark [30,32,62].
The root system is fibrous [68].

The sharp-pointed leaves are linear to lanceolate, 0.5 to 1 inch (1-3
cm) long, and spirally arranged [32,55,62].  Leaves are dark
yellow-green above and paler beneath [30,55].  Leaves persist for at
least 5 to 6 years [30,62].

Pacific yew is dioecious [30].  Globose, yellowish staminate cones
approximately 0.12 inch (3mm) in length are produced in abundance on
male plants [11,30].  Single, greenish, ovulate cones are borne on the
lower sides of branches [30,62].  Fruit is a red, fleshy, ovoid,
berrylike aril [30,55].  Each fruit is approximately 0.4 inch (1 cm) in
length and matures in one season [62].  The cup-shaped fruit surrounds a
large single, naked seed [30,70].  The seed is reddish, obvoid-oblong,
with a hard bony shell exposed at the apex [29,62].


RAUNKIAER LIFE FORM : 
   Phanerophyte


REGENERATION PROCESSES : 
Pacific yew can establish beneath a closed forest canopy by seed or by
vegetative means [49].  In many areas, layering is the primary mode of
reproduction, but seedlings are also common on some sites [2].

Seed:  Most species of yew (Taxus spp.) produce at least some seed
annually [65].  Seeds can remain viable for 5 or 6 years if properly
stored [65].  Conclusive evidence is lacking, but some researchers have
suggested that seed may be stored in the soil [31].  Seed is commonly
dispersed by birds, and some long-distance transport is possible [50].
Passage through avian digestive tracts may affect seed dormancy [11].

Germination:  Seeds of Pacific yew have a "strong but variable" dormancy
[65] and generally require stratification before germination can begin
[53].  In laboratory experiments, seeds germinated well after prolonged
warm and cold stratification [65].  Seed can be planted 0.4 to 0.5 inch
(10-13 mm) in depth and subjected to alternating day (86 degrees
Fahrenheit [30 deg C]) and night (68 degrees Fahrenheit [20 deg C])
temperatures for at least 28 days.  However, even when properly treated,
some seed may not germinate until the second spring.  Results of an
experimental test were as follows [65]:

  stratification         temp.        duration      germ. capacity
  warm        cold     day  night     (days)        (avg. %)   (range)

  ---         ---      86 F  68 F       60             55       50-99   

In other laboratory tests, average germination ranged from 50 to 60
percent [77].  Under natural conditions, germination may not take place
until the second year [65].

Seedling establishment:  Seedling establishment is generally more
favorable beneath a canopy than in canopy gaps [14].  Means [52]
observed seedling densities of 0.4 per acre (1/ha) in gaps but noted 47
per acre (115/ha) beneath a canopy in the western Cascades of Oregon.
Seedlings are reportedly uncommon in undisturbed situations but are
often abundant in partially cut areas where yew is present [11].

Vegetative regeneration:  Branches and stems of Pacific yew commonly
root when in contact with the soil [14,49].  Plants are generally
unharmed after being flattened by large conifers during canopy break-up
[14,49].  Crushed yews often form a series of layered branches that give
rise to numerous individual plants [14,49].  Regeneration of Pacific yew
is favored by falling debris [49].  Layering enables Pacific yew to
quickly expand into gaps created as senescent conifers fall.  Sprouts
generally develop from cut or broken stumps [14]; epicormic branching is
also common [11].


SITE CHARACTERISTICS : 
Pacific yew grows in a variety of cool and moist shaded habitats in
coastal lowlands and mountains [5,30,32].  It occurs in canyon bottoms,
on moist forested flats near streams, and scattered at various upland
sites [14,30,32].  At middle elevations in northern Idaho, it forms a
dense tangle of shrubs approximately 10 to 15 feet (3-5 m) in height.
Elsewhere, small groups or scattered individuals are more common [5,11].
Pacific yew grows on dry, rocky sites and in avalanche chutes west of
the Cascades [5].  However, it is commonly found in warm, humid
concavities [82].  Pacific yew is the most shade tolerant tree in the
Pacific Northwest [62].  In less humid climates, it may actually require
shade [41].

Plant communities:  Pacific yew commonly grows beneath the dense shade
of western hemlock, Douglas-fir, and Pacific silver fir (Abies amabilis)
forests [5,68].  Although most often associated with relatively moist
plant associations dominated by western hemlock, Sitka spruce (Picea
sitchensis), and Pacific silver fir, it also occurs in relatively moist
microsites beneath species more typically associated with drier sites
such as ponderosa pine (Pinus ponderosa), incense cedar (Libocedrus
decurrens), Oregon white oak (Quercus garryana), Jeffrey pine (Pinus
jeffreyi), and knobcone pine (P. attenuata) [11,55,68].  In parts of
eastern Oregon and California, Pacific yew is a prominent component of
white fir forests [11,54].  In northern California and southwestern
Oregon, it is common in mixed evergreen forests dominated by white fir,
Douglas-fir, canyon live oak (Quercus chrysolepis), chinkapin
(Castanopsis chrysophylla), and Pacific madrone (Arbutus menziesii)
[6,7,55,82].  At the extreme southern edge of its range, Pacific yew
grows beneath sequoia (Sequoia sempervirens) [11,27].  In the northern
Rockies, it is associated with grand fir and western redcedar forests
[11,56].  Pacific yew also occasionally grows in warmer subalpine fir
(Abies lasiocarpa)-Engelmann spruce communities [11].

Plant associates:  Common plant associates in coniferous forests of the
northern Rocky Mountains include pachistima (Pachistima myrsinites),
northern twinflower (Linnaea borealis), menziesia (Menziesia
ferruginea), Rocky Mountain maple (Acer glabrum), blue huckleberry
(Vaccinium membranaceum), wild ginger, queencup beadlily, one-sided
wintergreen (Pyrola secunda), western rattlesnake plantain (Goodyera
oblongifolia), oneleaf foamflower (Tiarella unifoliata), and bunchberry
(Cornus canadensis) [6,13,25,81].  In southwestern Oregon and
California, vine maple (Acer circinatum), dwarf Oregon grape (Mahonia
nervosa), northern twinflower, salal (Gaultheria shallon), and hazel
(Corylus cornuta) grow with Pacific yew [6,67,82].  Elsewhere in the
Northwest, Pacific rhododendron (Rhododendron macrophyllum), vine maple,
salal, western swordfern (Polystichum munitum), Oregon oxalis (Oxalis
oregana), Pacific dogwood (Cornus nuttallii), and oceanspray (Holodiscus
discolor) are common associates [11].

Climate:  Pacific yew grows in cool temperate and mesothermal climates
[40,41].  Abundance increases with increasing precipitation and
decreases with greater elevation and latitude [40].  Average annual
precipitation ranges from 18 to 116 inches (47-294 cm) [11].  Sites are
generally characterized by mild wet winters and warm dry summers [14].
Pacific yew is moderately tolerant of frost, but the protection offered
by a layer of snow is necessary in continental climates [41].  This
plant is resistant to flooding and survives temporary inundation [41].

Soils:  Western yew commonly grows on deep, moist, well-drained soils
[62,77] and is well adapted to acidic conditions [41].  In British
Columbia, it tends to be most productive in alluvial habitats where
soils are nutrient-rich [41].  A study conducted in the Bitterroot
Mountains of Montana and Idaho indicated that sites dominated by Pacific
yew have high levels of nitrogen [51].  Pacific yew grows on soils
derived from a variety of parent materials including granite, diorite,
gabbro, serpentine, pre-Cambrian metasediments, schists, and gneiss
[14,42,67,79].

Elevation:  Pacific yew grows at elevations ranging from 2,000 to 8,000
feet (610-2,438 m) [62].  In Oregon, it occurs at low to middle
elevations [74], and in British Columbia, it occurs from submontane to
subalpine habitats [41].  Elevational ranges by geographic location have
been reported as follows:
                                               Reference
from 3,200 to 7,000 feet (975-2,134 m) in MT   [17]
     350 to 4,350 feet (104-1,329 m) in CA     [11]
     < 7,000 feet (2,134 m) in CA              [55]
     200 to 4,450 feet (60-1,350 m) in OR & WA [11]


SUCCESSIONAL STATUS : 
Pacific yew is present in many climax or near climax communities of the
Pacific Northwest and northern Rocky Mountains [9,51,59,81].  It is a
particularly common component of old-growth grand fir, western redcedar,
and Douglas-fir-western hemlock communities [23,49,51,68].  Pacific yew
increases in cover up to a stand age of at least 500 years in
northwestern old growth Douglas-fir forests which are characterized by
long fire-free intervals [68].  This fire-sensitive species is absent
from areas characterized by high fire frequencies.

Pacific yew does occur on disturbed sites, including previously logged
stands [11], but reaches greatest abundance in undisturbed areas
[19,68].  Plants often grow as suppressed individuals in undisturbed
stands [26].  After timber harvest, this residual species expands as the
overstory develops [5], but where residual plants have been removed,
such as by broadcast burning, plants do not generally develop until a
protective overstory canopy has formed [50].

Pacific yew was common in mature stands 230 years or older but was
absent in second-growth communities (50- to 80-year-old stands) in
Washington [57].  Similarly, it represented 15 to 20 percent cover in
various old-growth stands in the northern Rocky Mountains but was rare
(1.4 to 2 percent cover) in immature stands (7 to 16 years old and 30 to
90 years old) [2,3,4].  Percent cover of Pacific yew in different aged
stands in western hemlock-Douglas-fir forests of the western Cascades
was documented as follows [69]:

stand age
 (years)    2      5      10     15     20     30     40     undist. old growth
 % cover   0.05   0.18   0.16   1.51   0.66   2.26   0.49    9.56

Mesic old-growth forests in canyons of the Bitterroot Mountains in Idaho
and Montana are commonly dominated by Pacific yew, western redcedar,
and/or grand fir [49].  Pacific yew typically establishes after the
initial colonization period, and is described as the only "relay"
species not colonizing these sites in early seral stages [49].  The
ultimate composition of these forests (dominance by Pacific yew, grand
fir, or western redcedar) is largely attributable to random events which
occur during stand establishment rather than to a sequential replacement
process [49,50].

In parts of the northern Rocky Mountains, the short tree, Pacific yew,
"expresses climax sociological dominance over tall conifers" such as
grand fir [14].  This situation differs from the classical pattern in
which progressively taller taxa gradually assume dominance over shorter
forms [14].  The successional role of Pacific yew in these forests has
been subject to a number of interpretations.  However, Pacific yew is
generally considered the climax dominant because, in the absence of
disturbance, it successfully replaces itself "to the near exclusion of
tall conifers" [14].  Evidence suggests that grand fir may be slowly
eliminated where Pacific yew is replacing itself successfully [14,34].
"At climax, other tree species occur primarily as a result of gap-phase
replacement in the yew canopy and definitely do not have as great an
influence on the community as [Pacific yew]" [14].  Gap phase
replacement or microsuccession prevents the development of an exclusive
canopy of Pacific yew because other conifer seedlings tend to outcompete
yew in the canopy openings.  Elsewhere, grand fir and other conifer
seedlings may be eliminated by dense yew competition [68].  However, on
sites where Pacific yew occurs only sporadically, grand fir often
reproduces more successfully.


SEASONAL DEVELOPMENT : 
Fruit of Pacific yew matures in a single season.  Seeds ripen in
September and October [30], and the fruit generally falls from the plant
in October [77].  Flowering and fruiting has been documented as follows:

Location            Flowering        Fruit ripens        Reference

CA                  April-May         ----               [55]
n ID                ----              August-October     [58]
WA                  June              August-October     [65]
Pacific Northwest   April-June        ----               [30]

Related categories for Species: Taxus brevifolia | Pacific Yew