Bough Tip Harvest of Pacific Yew Growing in Western Montana
Nan Vance, Ph.D., USDA Forest Service,
PNW Research Station, Corvallis, OR
Cooperator: Rus Willis, Bighorn
Botanicals, Inc., Noxon, MT.
Pacific yew (Taxus brevifolia Nutt.)
continues to be a valuable source of foliar material that
can have a variety of medicinal uses. Bough tips are
harvested, dried and milled to produce products in several
forms: tincture, capsule, herbal tea, or salve. The amount
and kind of bough tip selected is limited by how much
foliage the bough tip bears, as green needles are the
desired resource. Therefore, the bough is clipped only down
to the point where the stem begins to appear woody.
Woodiness begins to occur after new tip growth is about 2 to
3 years old or about 3-4 bud scars from the tip of the
branch. A branch tip can average from
2 to 20 cm or more of growth
depending on branch position, predations, or other external,
such as climatic, variables. The more dendritic the
branching, the more the growth is distributed among the
Harvesting bough tips can be
sustainable if harvest intervals are long enough to allow
replacements and recovery of lost foliage. The Pacific yew
is an important species in western Montana and northern
Idaho for its availability for winter browse by moose, elk
and deer. The species’ growth habit permits considerable
hedging by browsing animals. Because it can easily
withstand repeated pruning and hedging and, in cultivation
settings is stimulated by it, yew species have been
cultivated and bred for use as ornamentals for centuries
(Vance et al 1999). When the tree is browsed it also
responds by vigorous sprouting and branching if the tree has
enough light to support vigorous growth. Many, if not most
yews, that grow in the northern Rockies and in the winter
range of ungulates take on a shrub form because of
persistent browsing (Vance and Rudolf 2006); however, at
higher elevations the trees growing under coniferous
overstory are often stunted and misshapen by heavy snow and
falling limbs and may vegetatively regenerate new yew
sprouts by layering of branches pinned to the ground.
The vigor of regrowth is highly
dependent on the growing environment. Many of the areas in
western Montana and northern Idaho have good soils and
moisture, but through lack of disturbance in the past
several decades, have developed an overstory canopy that
creates deep shade to which even Pacific yew can no longer
reproduce or adapt (DiFazio et al, 1998). The herbaceous
and shrub understory layer often becomes depauperate and
even the shade-tolerant yew becomes spindly from lack of
light. Under these conditions browse or cutting could
seriously set back or eventually kill the tree as the foliar
component upon which its vigor depends is marginal and
reducing it further could even cause mortality. However,
these suppressed trees are not heavily browsed nor are they
desirable for harvest.
The objective of this study was to
determine foliar growth response of Pacific yew trees to
clipping of branch tips. In order to carry out this
objective, we examined the regrowth of harvested bough tips
to determine what interval should be maintained between
cuttings in order for the yew tree to fully recover
harvested foliage. We found that bought-tip length of
well-foliated branches was highly correlated with weight or
biomass, so we used length of bough tips that had been
clipped and length of regrowth from the clipped end as a
measure of recovery.
Two sites were selected for the study:
one site was in the Swan River Valley on Swan River State
Forest land north of Soup Creek Road #554. The other site
was on either side of FS Rd 2741 on steep slopes flanking
the west fork of the Rock Creek drainage in the Cabinet
Ranger District, Kootenai National Forest. The study began
in 1996 and final data were taken in 1999. The study was
discontinued on the Swan River Valley site because heavy
browse by moose and deer compromised the data. Heavy browse
also occurred on trees in the Rock Creek site, but we were
able to collect a full set of data in 1999 on eight trees.
The trees were haphazardly selected and
marked with metal tags. Of eight trees, three were female.
The branches were selected for harvestability and marked.
The bough tips were clipped alternatively by tree number
just above the first woody stem segment or just below the
bud scar of the wood stem.
The branches were placed in marked zip
lock bags, and transported back to the lab in a cooler. In
the lab, the length of all branching segments (branchlets)
for each branch was measured and the number of branchlets
tallied. If the tree was female, the number of fruit on the
clipped branches was also counted.
An attempt was made to clip about 36
bough tips from each tree; however in 1996, a few more than
36 were clipped, and in 1999 some of the clipped boughs were
not found because of browsing or other damage. In 1996 the
average number of boughs measured per tree was 43 and total
number of measured bought tips 349; in 1999, average bough
tips was 31 and total 245. Analysis was by ANOVA.
Results: The mean length of bough tips
was not significantly different between 1999 and 1996 (37.2
cm and 42.5 cm, respectively). However, trees 7 and 8 which
were located in a deep gully were heavily mite-infested.
Mites invade the buds and the result is damage to the
growing tip which, if severe, can cause defoliation and
branch tip death. The average stem length of these two
trees was lower than all the other trees. The mean number
of branchlets or rate of branching was significantly higher
in the bsranches harvested in 1999 than in 1996 (p=0.003).
The mean stem count in 1996 was 1.5 stems per branch and in
1999 was 2.7 stems per branch. This indicates that cutting
branch tips stimulates further branching and that the length
is distributed among the stems that form the branchlets.
The total number of fruits counted in comparable number of
branches in the three female trees in 1996 was 43, 53, and
107, and in 1999 was 24, 4, and 0, respectively. The tree
that had 0 fruit in 1999 was one of the trees heavily
damaged by mites.
It should be added the female strobili
are particularly targeted by mites possibly because of their
size (personal observation). A spreading infestation of the
yew big bud mite Cecidophyopsis psilaspis, an
introduced mite, has been reported by the Canadian Forest
Service in British Columbia (Canadian Forest Service 2002).
It is impossible to determine if bough tip harvest increased
mite infestation and damage or if their increased effect was
due to other factors. These trees had also been browsed to
a lesser extent, and the tearing of the foliage and branches
would appear to damage the yew as well and may contribute to
the spread of mites. None of these and other factors were
addressed by this study.
The study indicated the importance
of monitoring, if for no other reason than to estimate the
level of use by ungulates and associate that use with
climatic conditions that change from year to year.
Restricting bough tip harvest to only the ‘green’ portion of
the stem, the interval for regrowth , is calculated by
determining the number of years of replacement plus one
year. At this particular site, we recommend four years
between a return to these trees for harvest based on
allowing an additional year as a conservative margin to
allow for variability in other factors that affect growth.
Recommended for sustainable harvest of bough tips would
require collecting bough tips only from trees that have a
sufficient foliage and growth to respond vigorously to
harvest; should take no more foliage than can be replaced by
the tree without reducing its vigor or no more than 20% of
the total current and previous 2 year foliage in the crown,
and requires monitoring of harvest trees and comparable
reference trees for foliar growth. This could be done at
the time of harvest by tagging branches of representative
harvested trees and comparable unharvested (reference)
trees, and measuring annual branch stem growth and
Canadian Forest Service
2002. The yew big bud mite. Natural Resources Canada.
Canadian Forest Service,
Pacific Forestry Centre, Victoria, British Columbia, CA.
DiFazio, S.P.; Wilson,
M.V.; Vance, N.C. 1998. Factors limiting seed production of
western Oregon. American Journal of Botany 85: 910-918.
Vance, N.C.; Kelsey, R.G.;
Copes, D.L. 1999 Comparing biomass and taxane concentrations
yield in rooted
cuttings of Taxus brevifolia Nutt. In Rose, R.,
Haase, D.L. , editors.
Proceedings Native Plants Propagating and Planting (Oregon
State University Press,
Corvallis, OR pp
Vance, N.C. and Rudolf,
P.O. 2006. Taxus L. yew in Bonner, F.T. and Nisley, R.G.,
Seed Manual (revised edition). On line URL: