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Arboriculture Diploma
Tree Botany week 3
Roots and the rhizosphere
The primary functions of a root are:
• To absorb and transport water with
dissolved nutrients
• Anchorage
• Storage of the sugars that have been
manufactured in the leaves
• Associating with soil microbes in a
symbiotic relationship.
Older roots develop bark and do not absorb water
and dissolved nutrients. They provide anchorage.
Bark protects these exposed roots from drying out.
Young roots, the root tips, are the
primary location for absorption of water.
The epidermis has no (or very
little) cuticle, so water can be
absorbed through all areas
covered with epidermis.
Stomates in the young root
epidermis cannot close. They
stay open for gaseous exchange
and absorption.
At the tips of roots some
epidermal cells modify into root
hairs.
Cross section through a young root.
tip
Root hairs only occur at the
tips, behind the zone where
cells elongate.
Root hairs are an extension of epidermal
cells. They are up to 8mm long.
Root hairs
increase the surface
area for more
absorption. They are
the most absorptive
part of a root.
Root hairs are short
lived, typically
around 3 weeks, and
are replaced as the
root grows. They do
not grow into roots.
A root cap protects the root tip as it
pushes through the soil.
Root cap
Outer cells of the root
cap are worn off and
replaced by new cells
from the inner layer.
This leaves a layer of cell
slime that helps the new
root push through soil.
New cells produced in
the meristematic zone
= growth
The cortex is much wider in a young
root than it is in a stem.
It is made of
parenchyma tissue
with many air spaces.
The parenchyma cells
store starches.
The air spaces allow
oxygen to move
through the root.
Roots need oxygen
for respiration.
Cross section through a young root.
The inner layer of the cortex is
called the endodermis.
Water can enter the
root easily through the
epidermis and
stomates. It freely
moves across the
cortex, both through
the parenchyma and
along the parenchyma
cell walls.
The endodermis acts
as a filter. Its cells are
closely packed with
thickened walls.
Endodermis cells are
closely packed together.
Endodermis of a monocot root
The thickened area of the cell walls of the
endodermis are called casparian strips.
The casparian strips
force water and its
dissolved nutrients
(solutes) to move
through the
endodermis cell.
The cell membrane
stops undesirable
solutes, such as
salt, from entering
the vascular tissue.
The casparian
strip contains
suberin, the
waterproof
material found in
cork cells. It
seals the cell
wall and the
middle lamella
between cells.
Inside the endodermis is the area
of the root called the stele.
stele
The outer layer of
the stele is called
the pericycle.
Lateral roots
originate here.
Roots do not have
nodes and buds.
They do not branch
in an opposite or
alternate pattern.
Lateral roots grow from the pericycle
which is close to the vascular tissue.
Pruning
most roots
will
stimulate
branching.
Root pruning is done
to prepare plants for
transplanting. It
produces a smaller
densely branched
rootball including new
root tips.
By pruning in alternate
sections you leave
enough young roots to
support the tree until
new root tips are
formed.
The xylem and phloem of a young root
is not arranged in vascular bundles.
In a root the xylem is in the centre with radiating arms.
Phloem is in patches between the radiating arms of xylem.
Dicot roots, and roots of gymnosperms,
typically have four arms of xylem
Companion cell
Sieve tubes
of phloem.
Monocot roots have up to 20 xylem
arms, and pith in the centre of the root.
pith
xylem
phloem
The junction
between the
root and stem
of a young
plant shows an
interesting
rearrangement
of the vascular
tissue.
The junction will flare
out at the root collar,
where the many lateral
roots connect to one
stem.
Trunk flare should be
visible at the surface
of any woody plant.
Roots should be close
to the surface. If they
are not, the soil level
may have been
altered.
These Magnolia trees were to be protected when
a home was replaced with shops. They show
signs of stress, branch dieback, and epicormic
shoots.
Mosman, June 08
There is no trunk flare or roots close to the
surface. The roots have been buried. They have
also had paving installed over the outer roots,
and probably some roots cut for the wall footings.
Two years later and these trees are struggling.
They are still alive, but unattractive, in 2015.
Dicot and
gymnosperm roots
have a vascular
cambium between
the xylem and
phloem.
As a root develops
secondary growth its
vascular tissue
forms rings (like a
stem).
A cork cambium
forms in the
pericycle, and the
epidermis, cortex
and endodermis are
destroyed.
Periderm = cork,
cork cambium and
cortex
Roots of dicotyledons
and conifers can
become woody, but
they do not grow as
thick as trunks and
main branches. They
do not develop
heartwood. All the
xylem is able to
transport water.
These woody roots have been
used ornamentally to create a
rootery (also called a
stumpery), a very Victorian era
garden feature.
This rootery is at Highgrove Palace
in England
3
Roots do not
grow as thick as
branches, but
they can grow
longer and more
numerous. More
roots produce a
greater surface
area for
absorption.
We classify roots into three
different types.
fibrous
taproot
adventitious
When a seed germinates it produces either
a fibrous root system or a tap root system.
Pictures from ‘Botany for
Gardeners’ by Brian Capon
A fibrous root system
Fibrous root system of a palm.
• Is grown by
monocotyledons.
• Produces roots of equal
diameter from the seed.
• Roots may or may not
branch, usually only once
or twice.
• New roots continue to grow
from the base of the plant
stem / crown all its life. Old
roots die. Palm roots may
live for around 3 years.
• Fibrous roots do not
become thicker and woody.
Monocots with fibrous root systems, such as
palms, are always producing new roots from the
base of the stem. This usually makes them easy
to transplant.
Washington
palms being
transplanted
at Dee Why
beach
Some palms are harder to move than others. They
may have fewer longer roots, or their roots may die
when pruned. Bismarkia nobilis has a reputation
for being hard to move. Its roots
mostly grow down, and it needs
a larger deeper rootball.
Transplanted Bismarck Palms in Singapore
Cocus nucifera on Gilli Trewangan
In general, bigger
rootballs, 0.6 to 1m from
the trunk, have more
success.
After care is critical,
especially irrigation.
Trunks will need
stabilisation for up to 1
year.
Pruning leaves, or tying
them up, has variable
success. Most palms do
better if not pruned.
Plant palms at their original soil depth, or at
the point where roots originate on the trunk. If
they are planted to shallowly, they may snap.
Too deep and they may be stunted.
This Phoenix at
Alpine
Treemovals
snapped when
being moved. It
has been
planted like a
giant cutting,
and has
developed new
roots.
This Phoenix canariensis could be
planted deeper than soil level.
A tap root system
Ground
Level
Primary root
Lateral
root
• Is grown by dicotyledons
and conifers
• Produces one primary (tap)
root from the seed.
• Lateral roots branch from
the tap root.
• The tap root may grow
deep, but usually it doesn’t.
• Roots may become thicker
and woody, unlike a fibrous
root system.
The root system of a tree is much wider than it is
deep. This is because the upper soil is warmer, more
fertile and has more air than subsoil. Most roots are
in the top 1 to 1.5m. It is called a root plate.
Young roots with root hairs are produced mostly
towards the perimeter of the root zone, but they can
also grow close to the trunk. The heart roots provide
anchorage.
Cutting roots close to the
trunk will compromise the
trees stability.
Lateral roots extend out
from the trunk, sharing
the soil with roots of
other trees. The dots in
this diagram of a red
maple root represent
tree trunks.
Intermingled roots in
a mixed hardwood
stand
http://arborcaresolutions.com.au/
treerootfacts.pdf
Roots are opportunistic. They proliferate in
areas conducive for growth.
• Roots in dry or low nutrient soils will grow
longer with less branching. Roots in moist
fertile soils are more likely to be densely
branched and closer to the trunk.
• Where there is a high water table, roots
will remain shallow. They will only grow
where oxygen is available.
Trees that develop deep roots may be adapted to
deep sandy soils, or windy environments where
they need extra anchorage. They may also be
searching for water.
Ngilgi Caves have been carved by water out of
limestone in SE West Australia. Living tree roots from
the Marri trees, Corymbia calophylla, are growing
through the cave roof, 45m below the surface.
Some trees have a
persistent deep taproot,
with less lateral root
development. Nyssa
sylvatica does not
transplant easily,
because of its deep
taproot. Brachychiton
spp. have a deep water
storing taproot, but it
can be cut if the tree is
transplanted.
Most trees develop other downward growing
roots, like sinker roots or water roots, and do
not retain a dominant taproot. The overall
pattern is influenced by species and soil.
tracheid
Lateral roots mostly
transport water to the
branches above the
root, though some
trees, such as
conifers, have a more
spiralling pattern of
xylem. This may be
because water in
tracheids moves
sideways into the next
tracheid through pits.
If the roots on one side
of a tree are damaged,
often the branches on
that side will die.
The roots of this Eucalyptus had lifted the footpath. The root
was cut out and the footpath restored. After showing signs
of dieback in the canopy the tree was removed.
The lateral roots extend well beyond the drip
line, continuing to branch and produce new root
tips. They radiate in a more or less symmetrical
pattern. The root cap appears to be involved in
orientating the root growth. It can sense light,
pressure and perhaps gravity.
The finer feeder roots, sometimes called fibrous
roots, are mostly in the top 150mm of the soil,
particularly of the soil is protected by mulch.
Electric cables were run under
the feeder roots of Norfolk
Island Pines at Manly.
Unfortunately, the
roots were later
severed as the new
paving was installed
at a level lower than
the roots.
The trees have shown
no sign of dieback.
Possibly they had
developed a second,
lower layer of lateral roots. This may happen
with conifers in sandy soils.
Roots may grow at or
above the soil surface.
They will be attracted to
the regular water and
fertiliser applied to
lawns. They can be
damaged by herbicides.
This company will
slice through the
top 600mm of roots
to protect lawns.
They cut to the drip
line.
Roots will also grow at the
surface if soil is compacted or
has a high water table.
Compacted soils are low in
oxygen, restricting respiration.
These 17 year old
spotted gums have
grown very slowly
in a primary school
playground.
Soil can be compacted by people and machinery.
Compaction is a problem where cars park under
trees. It is affecting the figs and Holm Oaks in
Centennial Park, some of which will need replacing.
To raise money to replace and care for trees,
there is a program of tree sponsorship.
In Sydney’s Royal Botanic Gardens fences keep
visitors off the root zone of older trees. This
reduces soil compaction (and also stops people
from climbing on the trees).
On building sites, the root zones of existing
trees should be protected from compaction.
Tree Protection Zone radius
= at least 10 x the trunk
diameter at 1.4m.
‘Canopy’ treetop
apartments, Victoria Rd
Ryde
Nov 2013
This Eucalyptus pilularis has
its TPZ protected, but it may
be affected by changes in
hydrology and wind.
Feb 2015
The whole root zone should be protected from
changed soil levels, not just the area close to the
trunk.
Roots that have been
buried will not get enough
oxygen for respiration.
In compacted soil,
tree roots will follow
cracks in soil or
rocks or paving to
get oxygen and
water.
Image from www.earthporm.com
They may be
attracted to pipes
for the same
reasons.
Image from www.drainrus.com
Here at Kew Gardens in London, soil is being
aerated with high pressure nitrogen gas to help
the older trees get enough oxygen in their roots.
Providing well aerated soils can be a dilemma in
urban areas, where soils are compacted for stability
of buildings. This structural grid will be filled with
uncompacted soil mix for health root growth
From ‘Living in a Garden – The Greening of Singapore’.
If a root is bent or distorted when a
plant is potted up or planted out it may
function well at first, but as it gets
thicker and woodier the plant will loose
vigour and may die.
These roots are called kinked or girdling
roots
Kinked and girdling roots are a common
cause of sudden death in plants.
Roots may become
kinked when plants
are potted up into
containers too small
for the roots.
Never bend or fold
roots. It is better to
cut them so they fit
into the container.
As kinked roots get thicker they will cut off
the xylem, like a hose being folded. Plants
can die suddenly on hot days.
A kinked root may come up out of the soil
then back under. Sometimes you cannot
see kinked roots until the plant dies and you
pull it out.
Girdling roots occur when a plant is left to grow
too long in a container. The plant becomes root
bound. As the roots become thicker and woodier
they may eventually ringbark the trunk.
Girdling roots should be cut through when the
plant is young, when planting or potting up.
NATSPEC
Standards, to ensure
trees from nurseries
have healthy well
formed roots, were
released in 2003.
Roots on the surface
of the root ball are
shaved off, to
stimulate root
branching and avoid
girdling roots.
Sometimes circling roots grow near the surface
and around the trunk after planting. They may be
the result of growing in dry compacted soils.
They may
also be
produced
in
response
to root
pruning at
planting
time.
Girdling roots may
also result in trees
falling over.
Tree roots may die back in dry times, and put
on new growth when water becomes available.
These dead roots add to soil organic matter,
and help aerate soil. Trees also leak phloem
sap and other soluble
compounds into soil, to
support soil microorganisms, such as
bacteria and nematodes.
This creates a
rhizosphere, an area of
intense biological and
chemical activity.
The rhizosphere is only narrow, around 5mm
wide, and is associated with root tips.
The sap may contain hormones, such as
strigolactone, that attract and stimulate growth of
mycorrhizal fungi. The fungi act as an extension to
the feeder roots, bringing nutrients to the root
tissues. The roots support the fungi with sugars.
Mycorrhizal fungi on Spotted Gum
The hyphae of the fungus may form a sheath or
mantle around root tips, or may penetrate and grow
between the cortex cells.
http://mycorrhizas.info/ecm.html
Some plants produce compounds like flavonoids in
their rhizosphere, to attract nitrogen fixing bacteria to
their roots. Plants in the family FABACEAE attract
Rhizobium bacteria. Frankia colonises roots of some
Casuarina spp. and Alnus spp.
Frankia on Alnus roots
Rhizobium nodules
Proteoid roots, or cluster roots, are root
modifications that produce an increased surface area
for absorption of nutrients like phosphorus.
They are not the same as root hairs. Proteoid roots grow when
conditions are favourable, then die back. Plants with proteoid roots
include PROTEACEAE plants, plus 30 species from BETULACEAE,
CASUARINACEAE, ELAEAGNACEAE, FABACEAE and MORACEAE.
Tree roots tend to avoid each other when young, but
as they grow they may be forced together and form
a graft union. These grafts can conduct diseases,
such as Armillaria, from one tree to another.
Treatment for
Armillaria may
include root
barriers.
Roots are
exposed to
sunlight, to try
to kill the
fungus.
Some plants secrete
allelopathic chemicals
from their roots,
which discourage the
growth of other plant
roots in their root
zone. These chemicals
may also be in leaves,
and accumulate in leaf
litter. Examples
include Casuarina,
Pines and Camphor
Laurels.
Natural herbicides
made from
allelopathic chemicals
are being researched.
In America, the Black
Walnut chemical
Juglone is used
successfully.
This new product
contains Pine Oil
The third type of root system is
called adventitious roots.
• Both monocots and
dicots can produce
adventitious roots.
• Adventitious roots do
not grow from a
seed.
• They grow from
other parts of the
plant, such as stems.
• Adventitious roots
Some climbing plants, like
can develop above
Monstera, use adventitious
or below ground.
roots to cling to rough surfaces.
Adventitious roots are important for vegetative
(also called asexual) propagation. Examples
include cuttings, division, layering and tissue
culture.
Stem cuttings of
Cordyline fruticosa
Layering is when a stem produces adventitious
roots while it is still attached to the plant. This
Magnolia has branches that touched the ground.
New trunks have grown where roots have formed.
We can
manipulate a
plant to do
this by making
a shallow cut
in the stem
and pegging it
down in
contact with
soil.
Aerial layering or
marcotting is a way of
propagating a tree that is
not easy to grow from
cuttings. The stem is
wounded or ringbarked.
Sphagnum moss is
wrapped around the
wound, protected by
plastic. New roots grow
into the sphagnum. The
top can then be cut off
and planted.
Generally roots can only grow more roots. Only
a few plants can make new stems from their
roots. These new stems are called suckers.
Shoots from the base of Shoots from the roots.
the stem / trunk, growing They develop their own
from epicormic buds.
root systems.
Plants with roots that sucker can be a problem in
gardens. Examples include Poplars, Wisteria,
Robinia, some Casuarinas, and grafted Pears.
Populus alba
Using systemic herbicides on suckers will
damage the roots of the original tree.
Recommendations include:
• Continuous removal, such as mowing, so
the suckers run out of energy.
• Digging down to the point where suckers
originate, and cutting the sucker off close
to the root.
• Avoid damaging roots of trees that sucker.
• Avoid stressing trees with root that sucker.
Buttress roots
are
adventitious
roots that
develop on
large rainforest
trees, the
emergents of
the canopy.
Their ‘I’ shaped profile is strong, like a steel beam.
They provide stability for tall, top heavy trees that
may be exposed to monsoonal winds.
Stilt roots develop from
the lower nodes of a
stem. They give additional
support, mostly to
monocots. Pandanus spp.
have stilt roots
Some Mangroves have stilt roots, to resist strong
tidal forces. The seeds produce a long root
before dropping vertically into the mud.
Rhizophora apiculata
Prop roots grow from branches or higher on the
trunk. They thicken up and act as supports to
long, heavy branches.
This is a
Banyan Tree,
Ficus
bengalensis.
The Great
Banyan at
Calcutta
spreads over
four acres of
ground and
has more than
1775 prop
roots.
As the roots are
growing the root
tips are
vulnerable to
drying out.
Sometimes the
roots dry out and
don’t make it to
the ground.
At the Sydney Botanic gardens they have
protected prop root tips in moist hessian cloth.
Epiphytic roots help an
epiphyte to cling onto its
support. The roots do not
take water and nutrients
from the host plant. There
are many epiphytic
orchids, ferns and
bromeliads.
Strangler figs are epiphytes that produce roots
that encircle host plants, eventually cutting the
flow of phloem to roots and killing the host.
Looking up through the centre of a
strangler fig at Nambucca Heads,
after the host tree has died and rotted
away. Photo by Wayne
Parasitic roots grow
into the tissues of
other plants and
take water and
nutrients. This is
mistletoe on a
Scribbly Gum. The
point where the
roots penetrate the
host is called the
haustorium.
Mistletoe is spread
from plant to plant
by birds who eat
the fleshy fruits.
Some mistletoes, like Dodder, have multiple
haustoriums.
When mature Dodders are
totally dependant on the
host plant with no roots or
green leaves of their own.
Some mistletoes have haustoria that produce
long strands within the host. They mostly grow
down through the phloem, and may produce
shoots that break through the bark. Just cutting
off the terminal clump will not remove them.
Mistletoe may take
so much of the water
and nutrients that the
host plant dies,
especially in dry
times.
Evergreen Brachychiton
populneus, Kurrajong,
struggling with a heavy
infestation of mistletoe.
Mistletoe is killed by
bushfire, but is a problem
for isolated paddock trees.
Mistletoe on
deciduous plants
shows up clearly in
winter. This is a
good time to try to
cut the mistletoe
out.
Mistletoe on Platanus x hybrida
It may have multiple
haustoriums, requiring
long sections of branches
to be removed.
Some plants are hemi-parasites. They have a
small root system of their own but need to take
water and nutrients from the roots of other trees.
West Australia’s Christmas Tree, Nuytsia
floribunda, is a hemi-parasite.
This tree can send out
parasitic roots for up to
100m, looking for host
plants.
The local plant, Exocarpus cupressiformis, is a
root hemi-parasite. It looks like a conifer, but will
not survive if surrounding trees are removed.
Root carvings