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OVERVIEW OF CUTTING PROPAGATION
Kevin Hudson
FY 614, 3/7/97, University
of Auburn,
Editors
note: this article was reprinted with the permission of the
Auburn Dept of Forestry. The article has many references
which are specific to the forestry industry and may vary
from other commercial horticultural practices. The font
layout the original article has been modified to conform to
this web site.
The information presented many be out-dated.
For current information see:
INTRODUCTION
In past years, reforestation
practices have relied heavily on the use of natural seeding,
direct seeding, and nursery-grown stock. Within the past few
decades, genetic improvement of tree species has caught fire
within the forest industry. Research involving hybridization
between superior species, as well as creating orchards of
improved trees, have been examined. One factor of concern is
time. In order to realize genetic goals in tree species,
several generations of tree breeding is required. Usually
each generation requires anywhere from 15 years to 50 years
depending on the species of topic (Ahuja and Muhs 1985). The
dilemma is how to speed up this process. Asexual propagation
has been one answer. The ability to harness superior genetic
traits through asexual reproduction is being examined in
many forest tree-improvement programs. Because of the
shortened time requirement for cuttings of superior trees to
root and grow, this method of reproduction is fast becoming
a very important nursery management tool. Ritchie (1991)
reported that in 1974 only three cutting propagation
programs existed throughout the world. Since that time the
amount of programs has increased dramatically with an
increased realization of potential gains in growth and
quality of trees. In order to maximize the potential genetic
gain in propagation, ways to make this procedure cheaper
must be discovered. Ernst J. Schreiner realized this in 1937
when he stated, "It is essential to find cheaper methods of
vegetative propagation if select hybrids or strains are to
be multiplied and utilized immediately for forest planting."
(USDA 1937).
ADVANTAGES OF CUTTINGS OVER SEEDLINGS
The question of 'why?' is
always an issue when change is discussed. With the topic of
cutting propagation, this is no different. Why should a
landowner or timber company use propagated cuttings instead
of nursery raised seedlings? The answer is mainly quality.
The yield of a tree produced from a cutting can be greater
than a tree produced from a seedling. Characteristics of
boles produced by cuttings are less tapered and potentially
higher quality. Spencer (1987), examined the economic value
of cuttings versus seedlings of radiata pine. It was
discovered that because of the traits cuttings displayed,
they were more valuable in timber production. The volume
recovered was higher in trees produced from cuttings than
from trees produced from seedlings. The taper in the bole of
the seedling produced tree was greater than that of the
cutting. The cuttings also produced 43% more volume of
veneer than that of the seedling tree. The quality
characteristics displayed by the trees grown from cuttings
is, in part, attributed to the selection of trees, but
mostly to the age of the parent stock from which the cutting
was taken.
TYPES
OF CUTTINGS
Cuttings, or 'ramet', are taken
from a portion of the parent stock, referred to as the
'ortet'. Several types of cuttings can be taken from the
parent stock, which depends on the point on the parent stock
the cutting is taken from. The four major categories of
cuttings are: 1) stem cuttings, 2) leaf cuttings, 3)
leaf-bud cuttings, and 4) root cuttings. Brief descriptions
of the four cutting types are discussed in below.
Stem
cuttings
These cuttings are severed
twigs that have been placed into growing medium and
encouraged to develop roots. Speaking in general terms, the
success of rooting of stem cuttings can be achieved by
following a standard guideline. The following table from
(Wright 1976), is a general guideline for propagation of
stem cuttings. The type of tree species used will dictate
variations in these methods:
-
Detach twigs during either
winter or summer months, depending on what type of stem
cutting is being used. Cuttings should be from current
seasons growth.
-
Dip the cutting into a
rooting hormone mix
-
Put the cutting in a
rooting media (bury about 2/3 their height)
-
It is good to maintain a
soil temperature of about 80 to 85 degrees F
-
Keep the above ground
portion of the cutting moist and cool.
Stem
cuttings are broken down into sub-classes consisting of
hardwood, semi-hardwood, softwood, and herbaceous cuttings
(Hartmann, 1975).
Hardwood cuttings
Hardwood
cuttings are taken from the current seasons growth. This
portion of the tree offers young tissue. Cuttings should be
taken during the winter months of November through February.
Two main options are available when considering hardwood
cutting techniques. Some species root easily and are capable
of standing inclement weather sometimes experienced in the
spring months. For these species, propagating without heat
is acceptable. Other species that are sensitive to
temperature should be propagated with the aid of heat.
Site selection is important
when propagating hardwood cuttings without heat. This method
often displays greater success in regions exhibiting mild
winters. Protection from winds is also important. New tissue
may become wind burned, or even killed. Windrows such as
fences or native trees planted in rows will help reduce the
risk of wind damage. Soils should have adequate drainage to
ensure proper aeration. After cuttings are taken from stock
hedges, they are prepared for planting. Cuttings should be
between 200 and 250 mm in length. The cuttings are detached
just below the node at the bottom and just above the node at
the top. The cuttings are then stored in healing areas until
soil conditions improve. As soon as the ground is capable of
being planting, you should proceed. Once the cuttings have
started producing top growth, they are easy to damage. Once
in the ground, weed control, and irrigation should be
administered. The following fall, cuttings are lifted.
Semi-hardwood cuttings
Cuttings of
this type are produced from woody, broadleaf evergreens, and
leafy summer cuttings. They are taken from partially matured
portion of the plant, usually taken during the summer
growing months just after new shoot development, and
partially matured (Hartmann 1975).
Softwood cuttings
Softwood
cuttings are taken from new, soft, succulent spring growth
from either deciduous or evergreen species. Although
softwood cuttings usually root easier and quicker than other
cuttings, they also require more labor and equipment. This
is because the cuttings are made with their leaves still
attached. Therefore, they must be handled more carefully
than hardwood cuttings. This type of cuttings generally
utilize the aid of rooting hormones faster that other stem
cuttings (Hartmann 1975). When choosing material for this
type of cutting, care must be taken. The fast growing, soft,
tender shoots are not desirable, because they have a greater
chance of deteriorating before rooting occurs. The best
material has some degree of flexibility, but will break if
bent sharply (Hartmann et al. 1990).
Herbaceous cuttings
Herbaceous
cutting are taken from succulent, herbaceous plants. This
type of cutting roots fast, but is not used in forestry
practice.
Leaf
cuttings
Though not
used extensively in forestry applications, this cutting
method warrants a brief mentioning. This form of propagation
utilizes the leaf to promote new plant growth. A root and
shoot will form and develop, from the leaf cutting, into a
new plant. The original leaf cutting does not remain as part
of the new formed plant.
Leaf-bud cuttings
This type
of cutting propagation also is not used extensively in
forestry applications, but a brief description is warranted.
The leaf-bud cutting includes the leaf itself, petiole, and
a small piece of stem with the axially bud. This form of
cutting propagation is useful when material is scare,
because the same amount of stock will produce twice as many
new plants as that of stem cuttings (Hartmann et al. 1990).
Root
cuttings
Root
cuttings, which are used in forestry propagation, should be
taken from the young plant stock during the winter and
spring months to ensure that they are saturated with stored
foods. This time frame also prevents cutting during the time
the parent plant is rapidly expanding shoot growth. Cutting
during active expansion will take food stores away from the
root system (Hartmann et al. 1990). Another consideration
when dealing with root cuttings is to make sure the polarity
of the root is correct before planting. The portion of the
root that was located nearest the crown of the plant should
be planted up. A uniform system of identification of the top
and bottom of the root is a good idea to insure correct
directional planting.
TYPES OF PROPAGATION
Often several
methods of propagating cuttings will be used in combination to
optimize growth of the fastest and healthiest cutting. Below,
are some common forms of propagation used for production of
various species.
Hydroponics
Although not widely used on
large scale forestry application, the use of hydroponics
circulators is sometimes utilized for small scale research,
as well as larger scale agricultural production. The
hydroponics system cycles oxygen into the water where the
cuttings are submerged. Nutrients are introduced into the
solution to ensure no deficiencies occur. Although there is
no immediate commercial use in the forest industry for large
scale hydroponics systems, potential exists for its large
scale utilization.
Mist
The use of mist in the
propagation of plants was developed in the 1950's and has
since revolutionized plant propagation. Because mist
propagation is such an efficient and economical system to
produce large quantities of rooted cuttings, it is one of
the most widely used means of propagating cuttings in
nurseries. This method has aided in the rooting of difficult
species and also helps to decrease the rooting time of more
"friendly" propagated species (Hartmann et al 1990).
Mist units usually are programmed to run continually or
intermittently to ensure cuttings do not dry and wilt. Tests
have shown that intermittent or interrupted misting is
superior to continually misted cuttings. Hess and Snyder
(1954), found that cuttings subjected to interrupted misting
rooted faster and developed a better root system than
cuttings subjected to continuous misting. This increase in
performance gained by using the interrupted mist is due to
the physical condition of the growing medium. The reason
mist propagation works so well is that it simulates the real
growing environment of the plant. The optimal growing
conditions of plants is a cool moist leaf area, and a warmer
soil area. Interrupted mist uses less water, which keeps the
medium warmer and closer to the optimum conditions required
for good rooting. Constant mist lowers the temperature of
the medium below this optimum point because of the large
amounts of water used.
The main advantage of misting is that this system keep
leaves cool, and moist. This maintains turgor pressure and
prevents them from wilting. Because of the lack of roots,
cuttings must be keep moist to ensure survival. Priapi,
(1993) formulated four criteria to determine if a plant
species can be propagated using mist. Those criteria are:
Plants that are not adversely affected by excessive misting
Plants
requiring normal patterns of misting
Plants
needing to be kept fairly dry
Plants
requiring special attention in methods of overwintering.
Misting can be done in outdoor
areas, or in closed greenhouses. In outdoor misting two key
factors contribute to the success of a program. A well
designed and drained bed, and plant species that are capable
of being propagated using the misting method. In outdoor
misting, windbreaks should be used to ensure uniform
moisture coverage over all cuttings.
Indoor misting programs can be used in combination with
other propagation techniques to further decrease the amount
of time required for rooting and the quality of rooted
cuttings produced. Bottom heating of the beds, which is
discussed later, can help cuttings root faster. This system,
in combination with misting, is especially useful during
winter months. It is critical to maintain adequate
ventilation with indoor misting. Without ventilation,
excessive heat can build-up causing damage to the cuttings.
Table 1 shows the rooting times of general categories of
plant types using misting.
Table 1: Time
required for mist-propagation to produce rooting
|
Type of
cutting |
Rooting
period |
A |
Evergreens, difficult rooting
(semi-ripe) |
Approximately 8 weeks |
B |
Conifers, easy rooting (semi-ripe) |
Approximately 8 weeks |
C |
Softwoods, forced stock plants |
Approximately 4 weeks |
D |
Softwoods, normal method |
Approximately 4 weeks |
E |
Conifers, dwarf, if not grafted (semi
ripe) |
Approximately 8 weeks |
F |
Evergreens, easy rooting (semi-ripe) |
Approximately 8 weeks |
G |
Conifers, difficult rooting
(semi-ripe) |
Approximately 8 weeks |
(Stanley and Toogood, 1981)
Fog
Fog units are another form of
supplying water to cuttings. Fog units produce very fine
water vapors (<20m). One difference between foggers and mist
units is the fog stays in the air long enough for
evaporation to occur. Evaporation causes the relative
humidity to rise to between 93 and 100 percent. Moisture
from mist units lose suspension and fall to the leaf surface
and medium below. When the mist drops to the surface, it can
leach nutrients out of the medium, in addition to,
over-wetting the medium. One disadvantage to fog units is
the cost related to them. Mist units usually cost less
(Hartmann et al. 1990).
Greenhouses
The term 'greenhouse' is a
fairly generic term. This structure is basically a
protection from the outside environmental conditions such as
wind and temperature. The house is a framed unit covered
with material capable of utilizing solar heat. Materials
used to cover these houses are glass, plastic, polyethylene,
polyvinyl fluoride, fiberglass, and many other materials.
One key aspect to remember when dealing with greenhouses is
ventilation is absolutely essential for survival. Excessive
heat build-up, which can happen fast, will kill cuttings
(Hartmann et al. 1990). Greenhouse units are often used in
conjunction with one or many other propagation techniques
such as mist.
FACTORS AFFECTING CUTTING
PROPAGATION
Propagation Media
No propagation method is going
to work if the right media for growth is not used. In
propagation, the air content of your media should be between
20 and 45 volume percent to promote root formation and
growth. The volume percent in media should not drop below 15
volumes percent (Gislerod, 1983). This ensures adequate
oxygen availability for the developing root systems.
Increases in air within the media increases the oxygen
diffusion rate (ODR). This increase is what will aid the
root systems in acquiring the optimum amount of oxygen
needed. The contents of media can vary for different regions
and different species. Most mediums contain combinations of
sand, peat, sphagnum moss, vermiculite, pearlite, compost,
and shredded bark/sawdust (Hartmann 1975). The following is
a guideline to follow to help obtain good results from your
media:
-
The media should be firm
and dense enough to hold the cutting without movement
during rooting. Excessive shrinkage, of media, after
drying is not desirable.
-
The media should be able to
hold moisture so that excessive watering is not needed.
-
Adequate existence of pores
for the purpose of draining of excess water. This will
permit sufficient aeration.
-
The media should not
contain weeds, unwanted seeds, nematodes, and other
noxious organisms.
-
Salinity levels should not
be excessively high.
-
Media should be capable of
being sterilized with steam.
-
The availability of
nutrients for plant growth should be adequate
(Richards, et al., 1964)
HORMONE TREATMENTS
Indole butyric acid (IBA) and
naphthalene acetic acid (NAA) are two synthetic rooting
chemicals that have been found to be reliable in the
promotion of rooting in cuttings. IBA is widely applied in
general use because it is non-toxic to most plants over a
wide range and promotes root growth in a large number of
plant species. Both of these chemicals are available in talc
or in liquid formulations.
EFFECTS OF CUTTING SIZE ON SUCCESSFUL PROPAGATION
Very thin cuttings do not have
the food reserves thicker cuttings have. This may lead to
mortality before the cutting has a chance to root. The
length of the cutting may vary depending on the species.
Cutting sizes range from 2 inches to over 16 inches
(Frampton and Hodges 1989, Foster 1990, Edson et al. 1991).
Smalley and Dirr (1988) discovered that a two inch long
cutting of red maple grew straighter than the larger
cuttings, and the rates of growth after out planting was
similar.
SHADING
Shading of cuttings was once
thought not to benefit the cuttings in their task of root
development. It was believed that shading cuttings hindered
the photosynthesis process, which would reduce the rooting
activity. Photosensitize is responsible for the production
of carbohydrates which will aid cuttings in the formation of
roots. There are several reason associated with shading. It
prevents leaf scorch, and aids in the prevention of excess
buildup of carbohydrates, which will actually hinder the
development of rooting systems (Stanley and Toogood 1981).
An adequate amount of shading is around 20% from the
beginning of spring to the end of fall.
WATER
QUALITY AND AVAILABILITY
The issue of water potential is
extremely important with any type of cutting propagation.
Since the cuttings have no root system, they cannot maintain
turgor pressure in the absence of water, even for short
periods of time. This makes the availability of water
critical to cuttings survival. When the cutting is taken
from the parent plant, the turgor pressure is broken. This
pressure is similar to a rubber band, in that as water
stress becomes higher, the force exerted from the top of the
plant to get water from the roots increases. As water
availability increases this pressure is reduced. Under
normal conditions a plant can adapt, or adjust to situations
of lower water availability through what is termed osmotic
adjustment, in order to keep from wilting and dying. Osmotic
adjustment occurs when the osmotic potential, at turgor
loss, of a plant is lowered through the active accumulation
of solutes in the cells (Turner and Jones 1980, Fan et al
1994). Cuttings do not have this luxury because they have no
root system. Instead of adjusting, the cutting will wilt and
die without constant water. As the root system develops, the
plant will be able to take up water from the soil more
efficiently.
HEAT
Thermo-chemical reactions,
which means the speed of plant activity is directly related
to the temperature, it one of the most important aspects of
successful propagation (Wells 1955). As temperature rises,
the respiration of a plant will also raise. In common terms;
higher temperatures, to some point, will generate higher
activity levels in the plant.
This section will examine the propagation programs in
several countries. Other countries obviously have tree
improvement programs, but these few countries comprise a
large part of the active programs.
PROPAGATION IN DIFFERENT COUNTRIES
JAPAN
Japan is
the single largest producer of cuttings. Although
propagation of sugi plants has dated back
as far as 500 years ago, the planned application for genetic
improvement of trees was anticipated after the end of World
War II (Toda 1973). In 1985 the Japanese planted a total of
about 31 million trees on both public and private lands,
that were created from cuttings. The field performance of
sugi cuttings is similar to sugi seedlings. The initial
growth of the cuttings is slightly slower than the
seedlings, but the cuttings displayed resistance to needle
blight, which has been a problem with seedlings. The
Japanese plan to continue programs steered toward the
improved resistance to destructive agents (Ritchie 1991).
AUSTRALIA AND NEW ZEALAND
The only conifer propagated on a large scale in these two
countries is radiata pine (Pinus radiata). Over ten million
cuttings are produced by several industries for the purpose
of increasing the limited supply of genetically altered
seed. Controlled cross seeding is the main method of
propagation. Both countries report 90-95% success of
directly rooted juvenile cuttings (Ritchie 1991).
Australians are moving in the direction of producing all
trees from cuttings. They are preparing for this transition
by developing controlled pollination orchards in conjunction
with nurseries to grow the cuttings. In 1982, the Tasmania
forest began producing radiata pine from rooted cuttings on
a commercial basis (Arnold and Glued 1985). New Zealand's
methods are being adopted by Tasmania forestry. They are
also currently researching methods to arrest the maturation
of plant stock (Retch 1991).
SCANDINAVIA
Norway spruce is a major cutting of Scandinavia, with about
8 million rooted cuttings produced each year. Within this
region, Norway is the largest producer. Northern portions of
Norway have had little success with seed orchards programs.
Cuttings have been used to supplement the low supply of
seeds. In the southern regions of Norway, seed orchards have
met with greater success than their northern counterparts.
In the south, cutting programs are used to harness the best
quality trees, in an effort to reduce the rotation age of
spruce (Ritchie 1991). Finland started experimenting with
cuttings of Norway spruce in 1962, at the Foundation for
Forest Tree Breeding. The hope of this project, and larger
ones set up later, are to find ways to produce cuttings on a
large scale for practical use (Lepisto 1973). Throughout the
different regions of Scandinavia, various rooting programs
exist; however, the future looks bright for the expansion of
cutting propagation in this region. Research priorities
include the quest to mechanize various steps in the process
of propagation, and improved rooting (to decrease production
costs)(Ritchie 1991)
NORTH AMERICA
The North American cutting programs, while smaller than
previously discussed programs, encompasses a larger variety
of species, goals, and production. Canada comprises the
majority of cutting propagation in North America. The
Canadians feel that, while a relatively small part of the
total reproduction program, rooted cuttings are the most
viable option to introduce improved trees into these
programs. The black spruce seed source is preferred in
different regions, but is in short supply. Cape Breton
Island produces rooted cuttings to increase the supply of
this species. Canada also produces smaller amounts of white
spruce, Norway spruce, and hybrid larch. In the United
States, two companies have begun propagation programs. The
International Forest Seed Company and Weyerhaeuser Company
both are attempting to enhance and improve existing seed
sources of loblolly and Douglas-fir respectively. Cuttings
in the North American region are obtained differently by
each region (Ritchie 1991).
GREAT BRITAIN AND IRELAND
Sitka spruce and, to a smaller degree, a hybrid larch are
produced in these two countries. With sitka spruce being the
most important conifer species propagated, Great Britain's
objective with their program is simply to increase this
scarce seed, while the main objective of Ireland is for
clonal testing. Cutting production is currently at 150,000
specimens. Although production is expected to reach about 10
million cuttings by the year 2000, recent governmental
fiscal policy could reduce the demand for cuttings. Prior to
March of 1988, full tax credit was allowed for the cost of
reforestation, and forest management. After March 1988, a
fixed amount is deferred from reforestation and management
costs. This loss of aid has decreased planting and interest
in costly planting stock. Because of this situation,
research has focused on examining ways to decrease
production costs. Also, interest is focused on producing
hybrid larch and several other important species (Ritchie
1991).
WESTERN EUROPE
The western European community is currently incorporating
large cutting programs in Germany, France, and Belgium. The
main species of concentration are Norway spruce and maritime
pine. In Germany, the cutting program attempts to reproduce
clones that have undergone extensive selections. These
cuttings are put in nursery environments and grown for
several years to examine the affects of donor maturation on
the vigor of cuttings. A second reason for German's
propagation program is to protect a disappearing genetic
resource. Due to pollution, many trees no longer produce
seed. Propagation is a way to ensure the continued genetic
traits of these trees. While Germany is interested in
preservation and genetic gains, France is focused on the
commercial production of maritime pine cuttings. A goal of 1
million rooted cuttings per year has been established.
Although Belgian programs are considerable small that either
Germany or France, production of rooted cuttings is expected
to double in the near future. Future production of cuttings
is expected to increase during the next several years in
Belgium, as well as many countries in Europe (Ritchie 1991).
EASTERN EUROPE
Czechoslovakia is currently trying to propagate Norway
spruce that display tolerance to air pollution for use in
high altitude forests. Poland currently has programs
examining:1) re-construction of damaged lands by air
pollution and tree species to aid this task, and 2)
production of fast growing species. Future propagation in
Eastern Europe will be centered around growth and wood
quality. Specific qualities strive for are resistance to air
pollution, frost, and wind damage. Technological propagation
techniques must be made and monetary resources must be
available if this region of the world is to continue its
genetically path (Ritchie 1991).
OTHER FORMS OF PROPAGATION
In the forest industry,
propagation of cuttings is one of two major forms of genetic
improved propagation. Grafting is the other form of
propagation extensively used in forestry. In this form of
propagation, seedling or cuttings are grown until the
appropriate size is achieved. These seedlings or cuttings
must be closely related to the species that will make up the
top of the graft, or scion. The scion is attached to the
under stock utilizing several possible methods and cared for
until the two parts have grown together. Once this union has
successfully been accomplished, the top of the under stock
is removed and the graft is complete (Wells1955).
SPECIES RESPONSE TO PROPAGATION
HARDWOODS
Willows and cottonwoods are
rooted from dormant season cuttings. Cutting programs with
these two species is performed on a commercial scale. These
species are considered extremely easy to root (Wright 1976).
Oaks, chestnuts, beeches, ashes, and walnuts are all
considered extremely hard to root. When these species
undergo vegetative propagation, grafting is usually the
preferred method (Wright 1976).
CONIFERS
Monterey pine will usually root
fairly easy is kept under shaded conditions. Cost for this
species to be propagated is between 5 and 10 times greater
that for producing seedlings (Wright 1976).
Radiata pines are often rooted directly in the nursery bed
from cuttings taken from hedges and nursery stool beds.
Rooted cuttings are bare root transplants. The performance
of radiata cuttings seems to be closely related to the
maturity of the stock plant. Cuttings from older stock (3
years +) has fewer branches, thinner bark, and generally
exhibits traits of more physiologically mature trees. The
cuttings taken from younger stock displayed slower growth
characteristics of younger seedlings (Ritchie 1991). Radiata
pine can be rooted using stem cuttings. Cuttings detailed
from an experiment conducted by Arnold and Gleed (1985),
state that cuttings were taken during the months of May
through July. Cuttings were taken from younger parent stock
(<5 years). In this experiment, cuttings were placed in
outside beds and watered frequently. Rooting often took
several months. Cuttings were significantly superior as
branch size decreased. Cuttings showed less taper throughout
the entire log than seedlings.
Norway spruce was first propagated by cuttings in 1830 by
Oberforster Pfifferling in Hessen. Since this time genetic
propagation programs have started to examine and develop
various improvements to this species (Kleinschmit 1973).
This species rooting response is greatly influenced by the
medium and treatment. Rooting success is greater in young
plants, and in some instances success decreases as age of
parent stock increases. Fertilization increases the ease of
rooting. Hormone treatments influences rooting differently
at seasons, age of the cutting, and its position on the tree
(Kleinschmit 1973).
FUTURE OUTLOOK
Although seedlings still cost
less in most regions of the world, rooted cuttings and other
forms of genetic enhancement through propagation could be
the key to the future. Growth performance of cuttings is
almost identical, if not better, than that of seedlings. In
addition, with cuttings, often the end product is utilized
more efficiently at the mill due to less taper and general
growth characteristics of more mature trees. In addition,
cuttings are capable of producing a higher percent of
veneer, and higher quality lumber. Propagation also produces
uniform, cream of the crop trees consecutively, where
seedlings may vary, even within the same family. In the near
future, a widespread commercial production of cuttings could
be the replacement of current seedling production. Through
research, a reduction in the cost of commercially sold
seedling, and the ability to sale and grow higher quality
trees is not far in the horizon.
REFERENCES
Ahuja,
M.R., and Muhs, H.J. 1985. In vitro techniques in clonal
propagation of forest tree species. In: In vitro techniques,
propagation and long term storage. Edited by A.
Schafer-Menuhr Martinus Nughoff/Dr W. junk publishers. 1985.
P. 41-49.
Arnold, R. and Gleed, J.A. 1985. Raising and managing
radiata pine cuttings for production forests. Australian
Forestry. 48 (3):199-206.
Edson, John L., Wenny, David L., Fins, Lauren. 1991.
Propagation of western larch by stem cuttings. Western
Journal of Applied Forestry 6(2): 115-125.
United States Department of Agriculture. Better plants and
Animals. 1937.
Fan, S., Blake, T.J., and Blumwald, E. 1994. The relative
contribution of elastic and osmotic adjustments to turgor
maintenance of woody species. Physiologia Plantarum
90:408-413.
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