Pruning wounds and occlusion [2]

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The State of America’s Forests
Michael T. Goergen, Jr.
The Society of American Foresters recently pub-
lished a comprehensive report (available online at
www.safnet.org/aboutforestry/StateOfAmericasForests.pdf ).
Through words, statistics, and graphs, The State of Ameri-
ca’s Forests tells a fascinating story about humankind and its
relationship with the land. It is a story of trial and error, of
consumption and conservation, and of conflict and collab-
oration. But most of all, it is a story of regrowth, renewal,
and abundance.
Indeed, America’s forests have their problems. Insects
and disease, invasive weeds, unmanaged recreation, frag-
mentation, and land conversion are each having an affect
on forests from coast to coast and providing a formidable
challenge to the know-how and talent of the natural re-
sources profession. And yet, as this report suggests, there
remain reasons to be optimistic.
•The United States ranks fourth on the list of most
forest-rich countries, following the Russian Federation,
Brazil, and Canada, with 8% of the world’s primary forest.
•The number of acres of forestland in the United
States has remained essentially the same during the past
century.
•On average, 11% of the world’s forests benefit from
some type of conservation effort. In the United States,
20% are protected by conservation initiatives.
•Assessments of biodiversity on the nation’s forests
have found that the annual rate at which species are listed
as threatened or endangered has declined fivefold.
•Historical trends indicate that the standing inven-
tory (the volume of growing stock) of hardwood and soft-
wood tree species in US forests has grown by 49% between
1953 and 2006.
•Forest management also has been recognized as an
effective means of sequestering carbon over the long term.
In the United States, the total amount of carbon seques-
tered by forests and the creation of wood products during
the 1990s was estimated at almost 200 megatons per year,
an amount equal to approximately 10% of US carbon di-
oxide emissions.
•An estimated 25% of US private forestland is man-
aged in accordance with one of the three major forest cer-
tification schemes (the Sustainable Forestry Initiative, the
Forest Stewardship Council, or the American Tree Farm
System), and conservation initiatives on private land, such
as easements, are becoming increasingly common.
Most encouraging of all, perhaps, is not what has al-
ready been accomplished, but what is likely to be achieved
by forestry and natural resources professionals in the fu-
ture. New scientific discoveries occur almost every day and
these advances are leading to developments in biofuels,
forest conservation, tree farming and production, environ-
mentally sound building materials, fire management, bet-
ter controls for insect and disease outbreaks, and greater
biodiversity—successes that will enhance our efforts to
conserve, regrow, and use the forest more effectively than
we do today.
To that end, this report is designed to inform stake-
holders about the current state of US forestlands, how the
conservation, management, and utilization of US forests
compares with other nations around the globe, and the
problems that threaten to diminish the future health and
productivity of the public and private forests on which all
Americans depend.
SAF will soon make state-specific data available, pre-
sentations that members can use to tell this story, and other
tools and updates that continue to tell this unique story of
success.
Michael T. Goergen Jr. (goergenm@safnet.org) is executive
vice-president and CEO, Society of American Foresters.
LETTERS
Correction
In my June 2006 article, “Guide for
Classifying Lands for Greenhouse Gas In-
ventories,” Kevin MW Hughes (USDA/
NRCS) kindly pointed out an error I made
on page 213, second column: In the sen-
tence containing “1 ac [0.405 ha] for area,
20 ft [6.1 m] for strip width, 10% stocking
for cover, and 13 ft tall [3.97 m] for trees at
maturity,” the strip width should be 120 ft
[36.6 m]. My apologies.
H. Gyde Lund
Gainesville, VA
What’s in a Name?
I read with interest and amusement
Songlin Fei’s article about geographic
place names related to trees and forests
(March 2007). In discussing factors
affecting place names, the author fails
to mention the possibility that those
who selected the names might just have
liked the sound of the name they
chose. I frequently drive by a subdivi-
sion called Hunter’s Ridge. It’s as flat
as a table. So, too, is Michigan’s city
of Mount Pleasant. Then there’s Ice-
land and Greenland. Maybe those who
named places for trees did a better job,
but I’d put my trust in more reliable
sources.
Bill Botti
Eaton Rapids, MI
Pruning Wounds and Occlusion
I don’t find the positioning of the prun-
ing cut as much of a conundrum as does
Kevin O’Hara in his recent well-researched
article “Pruning Wounds and Occlusion: A
Long-Standing Conundrum in Forestry” (J.
For. 105(3):131–138). Alex L. Shigo de-
fined natural target pruning (NTP) through
his understanding of the anatomy of branch
attachment and the process of branch shed-
COMMENTARY
Journal of Forestry • July/August 2007 229

ding. NTP avoids injuring the stem, includ-
ing the branch collar. (The name is unfortu-
nate in that the branch collar is largely stem
tissue).
O’Hara may be presenting a false
choice between making a smaller wound and
leaving a larger defect core (his estimation of
the NPT method) versus making a larger
wound with a smaller defect core (flush or
close pruning method). Flush or close
wounds that injure the stem not only pro-
duce wounds larger in exposed area, but also
injure the previously uninjured stem. These
stem injuries tend to increase the production
and outward propagation of cracks, produc-
ing a larger, not smaller, defect core (A.L.
Shigo et al. 1979. Internal defects associated
with pruned and nonpruned branch stubs in
black walnut. (US Forest Service Research
Paper NE-440). Of course more severe flush
or close wounds result in visibly greater pro-
duction of woundwood and resin than with
NTP. But is it desirable? Does this increase
the yield of clearwood?
I agree with O’Hara on the need for
research on pruning techniques for specific
wood products. Such research should in-
clude dissection studies and not rely on ex-
ternal indicators or on modeling. Testing
the applicability of NTP for various needs
follows along with Shigo’s legacy. He was
always the first to say that education comes
from resolving our doubts.
Kevin T. Smith
Durham, NH
I am appalled that the article “Pruning
Wounds and Occlusion: A Long-Standing
Conundrum in Forestry” ever made it past
the peer-review stage to appear in the April/
May Journal of Forestry. The Journal, accord-
ing to its editorial policy, is supposed to
present “new and state-of-the-art knowl-
edge, research, practices, ideas, and poli-
cies.” Not only does this article fail to ade-
quately present the current understanding of
accepted principles of tree biology, particu-
larly regarding branch anatomy and re-
sponse to wounding, but nothing new or
state-of-the-art is ever presented. The article
merely rehashes centuries of misunderstand-
ing of pruning. One would hope that forest-
ers are aware that science has indeed ad-
vanced since the days of flush cuts.
The very premise that there is a conun-
drum regarding the proper way to execute a
pruning cut to remove a branch is disingen-
uous. Yes, if you make a flush cut (a cut
parallel with and close to the stem that severs
part of the branch collar), wound occlusion
will occur at a faster rate. The assumption
that the fastest possible wound occlusion is
desirable, upon which this entire article is
based, is never questioned.
Nowhere in the article is referenced
ANSI A300 Part I: Tree, Shrub, and Other
Woody Plant Maintenance—Standard Prac-
tices, Pruning. This is the current American
National Standard on pruning, and it wasn’t
developed and published on a whim. This
standard describes proper pruning in the real
world: the cut is located adjacent to and as
close as possible to the branch collar, when-
ever possible. The author somehow finds it
unusual that this is the only method recom-
mended by pruning guides—which is evi-
dence that he doesn’t understand the tree
biology behind this standard.
To understand why rapid occlusion of
pruning wounds isn’t the sole consideration
when pruning involves a more complete un-
derstanding of the branch collar than the au-
thor provides. The article describes the
branch collar as “a swelling of xylem and
callus tissues.” Actually, there is no callus (a
growth of large, undifferentiated, nonligni-
fied, homogeneous cells at the margin of a
wound; The Dictionary of Forestry, J.A.
Helms, SAF, 1998) in the branch collar
whatsoever. The xylem within the branch
collar consists of alternating layers of branch
xylem and trunk xylem. At the beginning of
the growing season, the branch cambium
begins producing xylem first, before the
trunk cambium becomes active. Then the
trunk begins growth, covering the xylem
produced by the branch. This is why the
branch collar is a swollen area, and it is an
easy matter to remove the bark and see for
yourself that the topmost layer of xylem in
the swollen area of the branch collar is in fact
trunk wood.
To achieve rapid occlusion by cutting
through the branch collar and into trunk
wood opens the trunk to infection by micro-
organisms. It stimulates epicormic sprout-
ing in many species, which defeats the pur-
pose of pruning. It also creates conditions
that favor the development of cracks.
Occlusion of wounds is important, yes,
but not to the exclusion of tree defense sys-
tems. At the most, the difference in clear
wood between pruning properly and cutting
through the branch collar will be the
amount of projecting branch collar that is
removed. This represents significant value?
Proper pruning only removes branch
wood and exposes only the branch protec-
tion zone (the microbe-resistant branch
core, not the branch collar as inaccurately
described in the article) to infection by mi-
croorganisms. Because occlusion generally
occurs rapidly with this method, particularly
if the tree is vigorous, why risk exposure to
damage and defect for a couple centimeters
more clear wood? Proper pruning mimics
what occurs naturally in forests, when trees
shed dead branches. Moreover, properly
pruned living branches are small (an inch or
less), in which case there would be no con-
ceivable reason to prune any way other than
properly.
I find the treatment by the author of the
late Alex Shigo’s work to be irresponsible
and reprehensible. The implication that
Shigo’s books lack scientific merit because
they are self-published is both baseless and
petty. Shigo’s citations are thorough and im-
peccable throughout; he clearly gives credit
where credit is due. To flatly state that “they
apparently underwent no peer review” bor-
ders on libel; on page xiii in Modern Arbori-
culture, Shigo thanks an extensive list of
international reviewers. Moreover, he pub-
lished hundreds of articles in peer-reviewed
journals and his body of work in tree biology
is accepted as one of the most authoritative
in the world, yet this is characterized by
O’Hara as “a small number of observational
studies.” If fifteen thousand tree dissections
with a chainsaw over the course of 30 years is
considered small, I wonder what amount of
data would be considered acceptable?
I have removed the bark, dissected,
stained, examined, and drawn parts of hun-
dreds of trees myself. By doing so I have
gained a basic understanding of tree anat-
omy and tree defense systems. I certainly
have sufficient understanding of science in
general and tree biology in particular to rec-
ognize misleading information and charac-
ter assassination when I see it.
Clark Beavans, RF, CA
Rock Hill, South Carolina
Kevin O’Hara responds. Although
Mr. Beavans might disagree, his letter pro-
vides ample evidence that a conundrum does
exist. Science should address questions in
objective terms with quantifiable informa-
tion as the basis for testing hypotheses and
(continued on page 276)
230 Journal of Forestry • July/August 2007

Pruning Wounds and Occlusion: A
Long-Standing Conundrum in
Forestry
Kevin L. O’Hara
The proper method for branch removal is a long-standing question in forestry. If the branch is cut flush
with the stem, a larger wound results, but if the branch is cut further from the stem, the resultant
clearwood production will be less. Both alternatives have implications for tree health and wood quality.
A long, international history of forest pruning research generally indicates that a cut parallel and close
to the stem will maximize clearwood production without excessive stem decay. Some exceptions exist
for species where potential for stem decay is high. The method recommended in most guidelines for
all types of pruning is an outside branch collar approach based primarily on a small number of
observational studies. However, maintaining tree health and maximizing clearwood production are not
mutually exclusive and a cut close to the stem can meet both objectives. Alternative approaches to
branch removal may be useful for other objectives.
Keywords: forest pruning, wound recovery, natural target pruning, stand management, occlusion
Foresters have been pruning forest
trees for centuries, having recognized
the potential to enhance wood qual-
ity and affect tree form (Mayer-Wegelin
1936, Curtis 1937). Objectives of this prun-
ing vary, but usually enhancing the quality
of wood sawn from the tree has been a cen-
tral objective. Despite centuries of experi-
ence and thousands of studies on various as-
pects of pruning (Sisam et al. 1940, O’Hara
1989, Nicolescu 1999), there are conflicting
recommendations over a seemingly simple
but central concept in pruning: the appro-
priate technique for cutting the branch.
However, despite the historical conundrum,
a sample of current guides for both forest
and general pruning (Maclaren 1993, Bed-
ker et al. 1995, Emmingham and Fitzgerald
1995, Windell 1996, Iowa State University
1998, Brown 2004, Hanley and Reutebuch
2005, Fazio 2006) and many websites (Ta-
ble 1) include recommendations for only a
single method: a process generally referred
to as “natural target pruning.”
On a properly pruned tree, the wood
formed after recovery from pruning likely
will be free of defects and therefore will
achieve greater strength properties and yield
lumber that earns a higher grade than lum-
ber from a similar location on a comparable
unpruned tree. Occlusion is the process of
trees forming callus and clearwood over
wounds. Usually, the process of branch
wound occlusion begins with formation of
callus over the branch stub eventually form-
ing a new cambium layer and new clear
growth rings. Between the branch stubs and
the clear wood is the occlusion zone where
the tree forms callus wood and new growth
rings occlude the wound. The inner log that
contains the unpruned core and the occlu-
sion zone is the defect core, an irregular cyl-
inder that varies in size and shape with the
pruning regime (Figure 1). The objective of
maximizing clearwood production is achieved
when the defect core size is minimized with
respect to the log size. Reducing the size of the
occlusion zone through the pruning technique
is an important means of meeting this objec-
tive. Therefore, the size of the defect core is the
result of a tradeoff between pruning severity or
timing of pruning operations and tree growth.
An early or severe pruning will reduce the size
of the defect core thereby increasing clearwood
production, but it also may reduce tree growth
having a negative effect on clearwood produc-
tion.
Forest trees are pruned also to meet ob-
jectives such as fuelwood production, en-
Received October 3, 2006; accepted January 24, 2007.
Kevin L. O’Hara (ohara@nature.berkeley.edu), University of California–Berkeley, Environmental Science, Policy and Management, Berkeley, CA 94720-3114.
Helpful presubmission reviews of this manuscript were provided by Dr. Kristen M. Waring of Northern Arizona University, and Yana Valachovic of University of
California Cooperative Extension. The author greatly appreciates the helpful comments of three anonymous Journal of Forestry reviewers.
Copyright © 2007 by the Society of American Foresters.
Journal of Forestry • April/May 2007 131
ABSTRACT
silviculture

hancement of aesthetics, reduction of fuel
ladders, and to remove or reduce susceptibil-
ity to pathogens. Urban trees are pruned also
to modify tree form, remove hazardous
branches, or improve tree health, but not
generally for enhancement of wood quality.
Regardless of the situation or the objective
for pruning, branches are removed and the
techniques used to remove these branches
have implications for tree appearance, tree
health, and wood quality. This article dis-
cusses the current state of our knowledge on
branch removal techniques and wound oc-
clusion for pruning forest trees and how the
transfer of previous research has affected
current practices.
Branch Anatomy
The anatomy of a living tree branch is
similar to the stem: xylem and phloem are
separated by the vascular cambium. All of
these are connected to their counterparts on
the stem. Water and nutrients flowing up
the functioning xylem or sapwood continue
to flow out the branches and carbon pro-
duced in photosynthesis flows from the
leaves through the branch phloem to the
stem phloem. At the base of the branch,
where it joins the stem, often there is a
branch collar in some species (Figure 2), par-
ticularly on larger branches. The branch col-
lar is a swelling of xylem and callus tissues
but does not break the link between con-
ducting tissues in the stem and branches.
Apparently, its function is as a support struc-
ture and as a barrier to fungal infection in
the main stem.
A typical dead branch has no living con-
nections to the stem and no live tissues but
may have a branch collar. The stem cam-
bium essentially forms a ring around the
dead branch that moves outward with for-
mation of each annual ring as long as the
dead branch is in place. The disconnection
from the cambium and the stem xylem
formed after branch death leads to loose
knots in wood that is cut from this part of
the tree. Because of the differences in anat-
omy and function of living and dead
branches, the wound occlusion after prun-
ing is profoundly different. Pruning a live
branch severs the living xylem, phloem, and
cambium tissues, whereas removing a dead
branch typically will affect no living tissues.
The conundrum over pruning primarily in-
volves live branch pruning. However, stud-
ies involving dead branch pruning provide
some relevant insights into wound occlusion
for both types of branches.
The Conundrum
The conundrum over pruning tech-
niques involves the branch collar or any ad-
ditional living tissues at the base of the
branch. Branch diameter decreases quite
dramatically in a short distance from the
stem (Herring et al. 1958). If the branch is
cut flush with the stem (i.e., a flush or close
cut), the resulting wound is larger than if the
branch is cut outside the branch collar. If the
branch is cut farther from the stem (i.e., an
outside branch collar cut), the resulting
wound at the end of the branch stub is
smaller but the defect core will be larger be-
cause branch stubs are longer. The conun-
drum is whether a larger wound and smaller
defect core are preferable to a larger defect
core and a smaller wound.
Solutions to this conundrum are related
to the objectives of pruning. For forest prun-
ing, minimizing the size of the defect core
Figure 1. Schematic of pruned bole showing the occlusion zone and the clearwood formed
outside the knotty or defect core. This example shows a bole pruned in two lifts. The defect
core includes both the knotty core and the occlusion zone. Schematic is not to scale.
Table 1. Sampling of websites presenting pruning recommendations for forest trees.
www.dnr.wa.gov/htdocs/rp/stewardship/bfs/EASTERN/pruning.html;
www.dnr.wa.gov/htdocs/rp/stewardship/bfs/WESTERN/pruning.html
muextension.missouri.edu/explore/agguides/forestry/g05160.htm;
www.for.gov.bc.ca/tasb/legsregs/fpc/fpcguide/pruning/pruntoc.htm;
www.forestry.about.com/od/arboriculture/ss/why_prune_5.htm
www.extension.umn.edu/distribution/horticulture/DG0628.html
www.dnr.state.mn.us/fid/october98/10019808.html
www.ume.maine.edu/⬃woodlot/prune.htm
www.maine.gov/doc/mfs/pubs/htm/fpminfo/pruning.htm
All websites last accessed Aug. 21, 2006.
132 Journal of Forestry • April/May 2007

and maximizing clearwood production is a
common objective. The flush or close cut
therefore would minimize the defect core if
there was not a corresponding increase in the
occlusion zone, fungal infection, or the time
required for occlusion. A different set of so-
lutions would be appropriate for objectives
other than maximizing clearwood produc-
tion such as aesthetics, hazard branch re-
moval, or fuelwood production.
Natural Target Pruning
Building on his work with compartmen-
talization of decay in trees (Shigo and Marx
1977, Shigo 1984a, 1985a), Shigo developed a
“natural target pruning” approach (Shigo
1984b) that removes branches by cutting out-
side the branch collar (Figure 3). Natural tar-
get pruning severs the branch to preserve the
branch collar but emphasizes that no addi-
tional stub length should be left. Shigo
(1985b) described the branch collar as provid-
ing a protection zone that forms as branches
begin to die. According to this work, removal
of the branch collar leaves the wound suscep-
tible to internal spread of discoloration and de-
cay (Green et al. 1981, Shigo 1985b).
Shigo (1984b) recommended a prun-
ing approach that identified the intersection
of the branch and the branch collar on the
upper and lower sides of the branch. These
“targets” form the points of initiation and
termination of the cut. By preserving the
branch collar and not leaving any additional
branch stub, this method was designed to
meet the singular objective of maintaining
tree health. Therefore, a tree’s potential to
occlude quickly and prevent infection is fun-
damental to this pruning method.
Pruning Wound Occlusion
The process of occlusion of a pruning
wound is central to this conundrum because
occlusion must occur before clearwood is
formed and rapid occlusion is a key defense
against infection by stem decay fungi (Mayer-
Wegelin 1936, Boyce 1961, 373; Shigo 1986
Chap. 38; Biggs 1992). Previous research on
pruning wound occlusion indicates that this is
not a new conundrum: Mayer-Wegelin
(1936), in probably the most comprehensive
review of pruning at that time, cites scores of
studies on wound occlusion from Europe dat-
ing back to the 18th and 19th centuries. The
most significant of these early studies included
the detailed branch anatomy studies of Kienitz
(1878), who examined orientation of branch
and stem fibers. He concluded more rapid oc-
clusion would result from a cut through the
Figure 2. Branch collars of coast (A) Douglas-fir, (B) radiata pine, (C) E. globulus (Labill.), and (D) northern red oak (Q. rubra L.). Branch
collars are highly variable between species and generally are smaller in young trees with small branches.
Journal of Forestry • April/May 2007 133

branch collar because the angle of severed
branch fibers favored more rapid occlusion
than branch cuts outside the branch collar.
Mayer-Wegelin’s (1936) assessment of prior
research led him to recommend not only a
close cut, but also cutting into the bark and
outside the annual ring to minimize the length
of the branch stub. An extreme example of a
close-to-the-stem cutting approach was rec-
ommended by Des Cars (1900, 35) in his clas-
sic book on pruning: branches should be
“evenly cut and shaped as nearly as possible to
the trunk of the tree.”
Similar questions were asked and simi-
lar studies were undertaken in the 20th Cen-
tury. Adams and Schneller (1939) com-
pleted a detailed comparison of pruning
through the branch collar and outside the
branch collar in eastern white pine (Pinus
strobus L.). They looked at resin flow and
callus formation on dead and live branch
wounds 1 year after pruning. Resin flow is
an initial defense to infection and during the
growing season may occur within minutes of
cutting. The chemical composition of resin
changes after wounding to be more resistant
to infection (Gref and Ericsson 1985, Cook
and Hain 1987). Cutting branches through
the branch collar increased both resin flow
and callus formation. Additionally, resins
are concentrated in the branch base: Ko¨ster
(1934) reported resin concentrations in the
branch bases in Norway spruce (Picea abies
[L.] Karst.) 10 times greater than those in
the branch or bole. Adams and Schneller
(1939) found over 97% of branches sawn
through the branch base showed callus for-
mation after 1 year compared with 18.9% of
branches cut outside the branch collar. Paul
(1938) concluded pruning cuts should be
“well into the collar at the base of the
branch” in eastern white pine and red pine
(Pinus resinosa Ait.), as did Barrett and
Downs (1943) with eastern white pine in
the southern Appalachians.
Helmers (1946) compared close cut-
ting with cutting outside the branch collar in
both live and dead branches of ponderosa
pine (Pinus ponderosa P.&C. Lawson) and
western white pine (Pinus monticola Dougl.
ex D. Don). He found greater occlusion
with the close cutting than with the outside
branch collar cutting on both live and dead
branches, greater occlusion on smaller
branches than on larger branches, and
greater occlusion in open stands, despite
these stands having larger branches, than in
denser stands. Olsen and Paul (1948) also
reported more rapid initiation of occlusion
in ponderosa pine when cuts were closer to
the stem. Huey (1950) found the greatest
increase in lumber quality came from con-
fining knots to the “smallest possible core”
in ponderosa and western white pines.
Childs and Wright (1956) found close
cuts occluded more quickly in coast Douglas-
fir (Pseudotsuga menziesii var. menziesii [Mirb.]
Franco) despite their larger size and—like
Mayer-Wegelin (1936)—also suggested oc-
clusion might be hastened by injuring the
cambium. In studies in the United Kingdom,
Donald (1936) and Anderson (1937) advo-
cated a close cut approach in Norway spruce,
Sitka spruce (Picea sitchensis [Bong.] Carr.),
Scots pine (Pinus sylvestris L.), and coast
Douglas-fir. Similarly, Henman (1963) rec-
ommended “close pruning” with Norway
spruce and Sitka spruce.
There are fewer studies of wound occlu-
sion in broad-leaved species; historically,
they probably have been pruned less often
for clearwood production. Meyer-Wegelin’s
(1936) recommendations to use close cuts
based on research to that date in Europe in-
cluded broad-leaved species. With European
beech (Fagus sylvatica L.), Zimmerle (1943)
found greater occlusion with close cuts. Oc-
clusion was more rapid with cuts that dam-
aged the branch collar in red alder despite
these cuts leaving larger wounds (Brodie and
Harrington 2006, DeBell et al. 2006). In
eastern North America, Roth (1948) exam-
ined occlusion and defects in four oak spe-
cies (Quercus spp.) and recommended close
cuts for live branch removal and cutting into
the living tissues at the base of dead
branches. Similarly, faster occlusion was
found with close cuts in several broad-leaved
species in New England (Moss 1937). Skill-
ing (1958) also found leaving even a small
stub would increase time to occlusion in
sugar maple (Acer saccharum Marsh.) and
American elm (Ulmus americana L.). For
black cherry (Prunus serotina Ehrh.), Grisez
(1978) recommended cutting close to the
stem after observing no serious problems af-
ter pruning. With black walnut (Juglans
nigra L.), Shigo et al. (1978, 1979) recom-
mended close cuts only for small branches.
However, for (larger) branches on older trees
they recommended not cutting branch col-
lars because dead and dying branches have
already compartmentalized (see Shigo and
Marx [1977]), creating a potential for for-
mation of ring shakes and discolored wood.
No significant decay was noted and close
pruning was recommended in young trees
with fast growth rates (Shigo et al. 1978,
1979).
An alternative approach to this conun-
drum are studies that dissect pruning wounds
and develop models to predict either time to
occlusion or the size of occlusion zone from
branch stub length and diameter. Studies us-
ing this approach in slash pine (Pinus elliottii
Engelm.), red pine, sugi (Cryptomeria japonica
D. Don.), radiata pine (Pinus radiata D. Don),
Norway spruce, ponderosa pine, and coast
Douglas-fir have found stub length was far
Figure 3. Cutting approaches for a natural target pruning approach (left) and a close cut
parallel to the stem but not quite flush (right). The “close cut” leaves a larger wound but
previous research indicates it would occlude more quickly than the natural target pruning
cut and would reduce the size of the defect core.
134 Journal of Forestry • April/May 2007

more important in delaying occlusion than
stub width (Hogan 1957, Lohrey 1963,
Takeuchi 1981, Gosnell 1987, Vadla 1989,
O’Hara and Buckland 1996, Petruncio et al.
1997). Bauger and Orlund (1962) used a sim-
ilar but graphical analysis to reach similar con-
clusions for Norway spruce, Sitka spruce,
grand fir (Abies grandis [Dougl. ex D. Don]
Lindl.), and western hemlock (Tsuga hetero-
phylla [Raf.] Sarg.). DeBell et al. (2006) report
similar conclusions for red alder (Alnus rubra
Bong.) based on correlation analysis. The sen-
sitivity of occlusion to stub length relative to
stub diameter in these studies indicates the ad-
vantages of close cuts when the pruning objec-
tive is to minimize the size of the occlusion
zone.
Another common finding in pruning
studies with both conifers and broad leaves is
more rapid occlusion in trees with faster ra-
dial growth rates (Mayer-Wegelin 1936,
Roth 1948, Lohrey 1963, O’Hara and
Buckland 1996, Petruncio et al. 1997).
More rapid radial growth rates indicate
greater xylem production and result in more
rapid occlusion. Similarly, Rapraeger (1939)
found both greater branch size and more
rapid occlusion higher in the bole of western
white pine. In Scots pine, Pietila¨ (1989)
found both faster growth rates and a smaller
occlusion zone with increasing height. Be-
cause occlusion is the result of callus/xylem
formation over the branch stub, more rapid
radial growth rates—as often are found in
the upper stem of pruned trees (Mayer-
Wegelin 1936, Staebler 1963, Keller
1968)—result in more rapid occlusion.
Many studies have noted the more
rapid occlusion of live branches than dead
branches. For example, Paterson (1938) and
Vadla (1989) in Norway spruce, Adams and
Schneller (1939) in eastern white pine, Ro-
mell (1940) in Norway spruce and Scots
pine, Henman (1963) in Norway spruce and
Sitka spruce, Smith et al. (1988) in pon-
derosa pine, and Miler et al. (1990) in Scots
pine reported slower occlusion in dead
branches. This slower occlusion probably is
related to lower radial growth rates on the
stem in the area of dead branch attachment.
Results from cutting method studies with
dead branches resemble those for live
branches. Curtis (1936) reported that when
the branch base of eastern white pine was
cut, exposing a larger wound, 63% of cuts
showed callus development compared with
7.8% for outside the branch collar. Similar
results, by Adams and Schneller (1939),
were obtained 1 year after pruning eastern
white pine. Several studies have recom-
mended that the branch base be injured dur-
ing cutting to stimulate occlusion (Meyer-
Wegelin 1936) because there is the potential
for the tree to fail to occlude a dead branch
wound. Collectively, these studies suggest
that wounding the cambium is not necessar-
ily bad for the tree and it may stimulate more
rapid occlusion.
Although the objective of maximizing
clearwood production generally is unique to
forest pruning, similar results with wound
occlusion have been reported in the arbori-
culture literature. Neely (1988a, 1988b)
found faster occlusion with a “conventional
cut” that removed part of the branch collar
than with a cut outside the branch collar in
several broad-leaved species including pin
oak (Quercus palustris Muenchh.), American
sycamore (Plantanus occidentalis L.), and
Norway maple (Acer platanoides L.). Despite
wounds that were as much as 51% larger
using the conventional cut, by the end of the
2nd year after pruning, 44% of conventional
cuts were occluded compared with 36% for
the outside the branch collar cut.
Pruning Wounds as Infection
Sites
Infection of fungal decay organisms al-
ways has been a concern with pruning and is a
primary factor in assessing branch cutting
techniques. Meyer-Wegelin (1936) reported
problems with decay in the 1800s that were
attributed to poor pruning practices: leaving
long stubs and ax pruning. Once these were
corrected, decay was not a problem. Subse-
quent work with Norway spruce has produced
mixed results. Paterson (1938), Romell
(1940), and Henman (1963) reported no is-
sues with fungal infection, whereas Bauer
(1945) and Risley and Silverborg (1958) re-
ported severe problems. In Norway, Vadla
(1989) found 12% of pruned Norway spruce
trees with some decay or staining, but only ap-
proximately 1% of branch wounds were in-
fected in all trees and this decay was attributed
to poor pruning practices. In coast Douglas-
fir, infections of heartrot fungi occurred after
pruning, but Childs and Wright (1956) found
these infections were small and died soon after
occlusion. Others have reached similar conclu-
sions with coast Douglas-fir (Anderson 1951,
Stein 1955, Lukert 1959, Henman 1963), ra-
diata pine (Roche and Hocking 1937), Scots
pine (Romell 1940, Henman 1963, Vuokila
1976), Sitka spruce (Henman 1963), loblolly
pine (Mann 1952), and ponderosa pine
(Roche and Hocking 1937, Andrews 1954).
In eastern white pine, Cline and Fletcher
(1928) observed no decay problems in small
limbs but Spaulding et al. (1935) found prob-
lems after pruning larger limbs. Similarly, Paul
(1938) found no decay problems in eastern
white pine and red pine after pruning. Chou
and MacKenzie (1988) used artificial inocula-
tions and found greater probability of infec-
tion with increasing wound size in radiata pine
in New Zealand.
For broad-leaved trees, there have not
been many reports of stem decay after pruning
in the northern hemisphere. Nylinder (1955)
described the risk of rot after pruning pedun-
culate oak (Quercus robur L.) in Sweden as
“slight.” Skilling (1958) found no evidence of
decay in sugar maple and American elm and
found less decay in pruned trees than un-
pruned trees. Similarly, there have been no sig-
nificant studies indicating fungal decay prob-
lems after pruning in silver birch (Betula
pendula Roth.; Vuokila [1976]) or red alder
(DeBell et al. 2006). In Taiwan, wood decay
was infrequent in smaller branches of zelkova
(Zelcova serrata Hay.; Chiu et al. [2002]). In
European beech, Volkert (1953) found decay
and discoloration but reported that fungal ac-
tivity essentially ceases by approximately 9
years after pruning.
Exceptions to the minimal fungal decay
problems associated with pruning are in Eu-
calyptus and Acacia species. Glass and Mc-
Kenzie (1989) reported decay problems in
Eucalyptus regnans (F. Mueller) after prun-
ing in New Zealand and similar problems
have been observed in other eucalypts (see
Montagu et al. [2003] and Barry et al. [2005]).
These problems were more common in large
branches (Glass and McKenzie 1989).
Pruning also led to a greater incidence
of decay in Acacia mangium (Willd.) in Asia
(Lee et al. 1988, Barry et al. 2005). No stud-
ies, including these studies, with eucalypts
and acacias, report decay spreading into
wood formed after pruning.
Although there are reasons to be cau-
tious regarding potential infection of trees
by fungal decay organisms when developing
a pruning regime, the evidence that close
cutting contributes to these problems is
minimal. With the notable exception of the
problems in eucalypts and acacias, pruning
does not appear to be a major factor in af-
fecting stem decay or related defects in the
great majority of pruning studies. Where
stem decay infection is a potential problem,
pruning in younger stands where branch size
is minimized is recommended or avoiding
Journal of Forestry • April/May 2007 135

pruning species in locales where problems
are significant. It is also interesting that sev-
eral studies noted the potential of pruning to
reduce infections in both conifers and broad
leaves (Andrews 1954, Skilling 1958, Grisez
1978, Barry et al. 2005).
Forest Pruning
Recommendations
An old idiom says “there’s more than
one way to skin a cat.” There also are many
ways to prune tree branches and—contrary
to many sources—more than one may be
correct. Correctness is primarily a function
of management objectives and species. For
plantation forestry regimes, the two objec-
tives of maintenance of tree health and max-
imization of clearwood production are not
mutually exclusive. It follows that for pro-
duction of clearwood on conifers, particu-
larly in plantations, a cut parallel and reason-
ably close to the stem is a logical means to
minimize the size of the defect core and
maximize clearwood production. For broad-
leaved trees, a similar recommendation
should be successful except in the situation
where branches are excessively large, result-
ing in many years for occlusion to occur.
The expectation in most situations is that
this will not lead to significant fungal infec-
tion. There will be exceptions, e.g., euca-
lypts and acacias that have a high probability
of fungal infection after pruning. Close cuts
of conifer branches that occur in whorls with
branch collars or nodal swelling may girdle
the tree (Figure 4). Close cuts also may ex-
acerbate other problems such as Douglas-fir
pitch moth (Stynanthedon navaroensis [Hy.
Edwards]) attacks in coast Douglas-fir
(Briggs 2000). Additionally, available
knowledge is almost exclusive to temperate
forests and may not apply to tropical species.
Pruning should be integrated into stand
management regimes that include careful
density management to control branch sizes.
Stand density affects branch length and di-
ameter and radial growth rates; therefore,
there will be a tradeoff between minimizing
branch size and maximizing growth rates.
Pruning early in a rotation often is recom-
mended because it minimizes the size of the
defect core; additionally, early rotation
pruning is likely to involve smaller branches
that are still alive and associated with more
rapid occlusion, thinner bark, and less de-
cay. In situations where stem decay is a po-
tential deterrent to pruning, light pruning
regimes or multiple pruning lifts can result
in “earlier” pruning that removes smaller,
live branches; these treatments led to fewer
infections in radiata pine artificially inocu-
lated with Diplodia pinea (Chou and MacK-
enzie 1988). Season of pruning also may af-
fect infection potential; however, there is no
consensus across species on optimal season
to prune (Uotila 1990).
Science and the Conundrum
It is logical to question why science has
not solved this conundrum when it has been
recognized for so long. Science often works
this way. It is not linear and it is not always
correct, but it is usually directional. The sci-
entific method involves developing hypoth-
eses; making predictions from hypotheses;
testing these predictions with observable,
empirical, and measurable evidence; and re-
fining these hypotheses into new theories. In
this case, the issues are clouded from a myr-
iad of variables that affect pruning wound
responses. These include species, regional
differences in pathogen virulence, season of
pruning, age of trees, and genetic variation
within species and affects on occlusion.
There have been three general ap-
proaches to this problem: (1) the extension
of observational studies of wood anatomy to
recommendations on pruning, (2) compar-
ative treatment studies of different pruning
methods with dissections and either direct
comparisons of pruning methods or infer-
ences to pruning methods from observations
of occlusion, and (3) models of occlusion
based on radial growth rates and the length
and diameter of the branch stub. The first
approach is a useful means of developing hy-
potheses that might later be tested with com-
parisons of alternate approaches. In this case,
the extensive international research that pre-
ceded the development of the “natural target
pruning” method and demonstrated the
merits of close cutting approaches should
have—at a minimum—warranted thorough
testing of alternative hypotheses. The latter
two approaches are the quantitative means
generally accepted by science for hypothesis
testing.
The emphasis on the “natural target
pruning” approach in current guidelines also
establishes the persuasive power of books in
our society. The common sources in prun-
ing guidelines (e.g., Bedker et al. [1995], Ta-
ble 1) and textbooks (Kozlowski et al. 1991,
500; Nyland 2002, 470–471) for the out-
side branch collar approach are a series of
books written and published by Shigo
(1986, 1989, 1991). Not only were all three
of these books self-published, they appar-
ently underwent no peer review. Although
these books were built, in part, on the au-
thor’s substantial body of previous re-
search—some of which was published in the
peer-reviewed literature—the thrust of this
work is primarily observational descriptions
of tree responses to injury; there is a general
Figure 4. Ponderosa pine whorl after pruning showing cuts generally parallel to the stem
but not flush. In this case, flush cuts would nearly girdle the tree.
136 Journal of Forestry • April/May 2007

lack of quantitative data and no statistical
comparisons of pruning methods.
Science also should build on previous
knowledge, but in the case of pruning re-
search, there has been a disconnect between
the extensive research in the past and the ob-
servational studies that led to “natural target
pruning.” An additional problem has been the
assumption that pruning techniques should be
consistent regardless of the pruning objective
and species. Although a close cut may be ap-
propriate for maximizing clearwood produc-
tion, it may produce an unattractive wound on
an ornamental tree, it may increase costs for
pruning to remove diseased branches, or it
may form an infection site in some species. A
search for a universal model for tree wound
response for all species, objectives, and situa-
tions is flawed and probably hinders our scien-
tific progress in this area.
Finally, this conundrum leaves room
for more long-term studies of pruning
wound occlusion after different branch re-
moval methods. Studies that compare prun-
ing techniques in trials that follow individ-
ual wounds and then dissect them will yield
the strongest conclusions. Observational
studies of dissected branch wounds without
information on their origin and subsequent
development of conclusions without appro-
priate controls or alternative treatments is
not likely to solve this problem. Planned tri-
als of different branch removal methods also
will be most amenable to statistical compar-
isons that permit testing of hypotheses.
Conclusions
There is a considerable international
body of literature on forest pruning and much
of it pertains to methods of branch removal
and subsequent wound occlusion. This branch
removal literature includes designed studies to
compare occlusion response with different
pruning methods, quantitative assessments of
dissections of previous pruning wounds, and
observational and pictorial evidence of prun-
ing wound response. The clear majority of this
work indicates that a cut close to the stem is
effective for pruning forest trees to enhance
wood quality without detrimental infections
of fungi or bacteria. This body of work indi-
cates that pruning early when branches are
small will minimize problems with infections.
Other work recommends an outside-the-
branch collar method of branch removal and is
based on observations of wound response.
One consensus that is apparent in this litera-
ture is that forest pruning is most likely to be
successful in young, fast-growing trees with
small branches that occlude quickly and are
less likely to be infected by stem decay fungi.
This is consistent with objectives in forest
pruning of minimizing the defect core and re-
ducing costs of pruning branches.
There is also a theme in recent forest
pruning guidelines to recommend only a
single method of branch removal regardless
of the pruning objective. Branches are re-
moved for a variety of objectives in forests
and different methods probably would be
more suited for some objectives than others.
Likewise, pruning in nonforest situations
may require a diversity of approaches de-
pending on objectives. Foresters and ar-
borists must integrate their knowledge of
branch wound response with the suitability
of different branch removal methods rather
than assuming that a single method is best in
all situations.
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