What Makes a Tree a Tree? Tree Biology 101 - Up

What Makes a Tree a Tree
Tree Biology 101
• Tree - woody plants
that are large in size
and have a single main
trunk.
• Tree - long lived
woody perennials that
compartmentalize.

Trees are different from
other plants because…
If conditions remain
favorable, they can live
for a very long time

Let’s look at
a tree and
how it might
grow over
time….here’s
how many
seedlings
look shortly
after they
sprout…..

In a few
years……

….at about 10
years or so

…perhaps
in 30 to 40
years

Trees are genetically “wired”
to live for a long time…..This
tree is over 130 Years Old...

The Wye Oak (a white oak) was estimated
to be over 460 years old before it fell…

This is what
a 1700 year
old tree
looks like

This one is over
2,300 years old!

275 ft tall
33 feet in
diameter, that
is 103 ft in
circumference

How Old Is The Oldest Living Thing On Earth
“Methuselah” A Bristlecone Pine
4,777
YEARS
OLD!!!!

What Makes A Tree A Tree
• Tree Biology - the study of structure and
function, and the relationship between
them
– Anatomy - the study of the component
parts of the tree
– Physiology - the study of the biological and
chemical processes within these
components

Basic Tree Anatomy
• All living organisms are made up of
cells, tissues and organs
• New cells form from the division of
existing cells
• In trees, these areas of cell division are
called meristems

Tree Anatomy-Cells and
Tissues continued
• Apical Meristems - primary meristems that
produce cells that result in elongation of
roots and shoots
• Lateral Meristems - secondary meristems
that produce cells that result in an increase
in diameter
– Cambium - a lateral meristem that is a thin,
continuous sheath of cells that divide to produce
the vascular system of the tree:
• Xylem – woody tissue, movement of water
• Phloem – movement of food

Leaves

Tree Anatomy – Deciduous vs
Evergreens
• Evergreen Trees – hold leaves for
more than 1 year
• Deciduous Trees – shed leaves every
year
– Forms an Abscission Zone (A Z)
• The A Z is formed as a result of changes in
cells and growth regulators as temperatures
drop and nights get longer in the Fall
• A Z forms at the base of a leaf stem or
“petiole” every year

Tree Anatomy-Leaves
• Making Food is the Primary Function of
Leaves! (Photosynthesis)
– CO2 + H2O + Light Energy = Carbohydrates (Sugars) + O2
– Occurs in:
• Chlorophyll– green photo-reactive
pigment found in the chloroplasts
• Chloroplasts – specialized organelles
found in plant cells that are the site of
photosynthesis

From micro.magnet.fsu.edu

From ISA Introduction to Arboriculture Tree Biology CD

Stems

Tree Anatomy – Woody Section
Earlywood - xylem produced in Spring
Latewood - xylem produced later in Summer
Growth Ring – visible rings created by the
combination of early and latewood
Ray cells – live parenchyma cells that transport
sugars and other compounds through the trunk
radially

Physiology -
Compartmentalization
• CODIT- Compartmentalization Of Decay In
Trees
-Wall 1-resists vertical spread of decayxylem
plugging (weakest wall)
-Wall 2-resists inward spread of decaycompact
latewood cells
-Wall 3-resists lateral spread - ray cells
-Wall 4-resists outward spread into new
wood that is formed –(strongest
wall)

From ISA Introduction to Arboriculture
Tree Biology CD

Xylem continued
• Ring Porous Wood – form wide vessels early
in the growing season and narrower ones
later
– Elms
– Oaks
– Ash
• Diffuse Porous Wood – form uniform sized
vessels throughout growing season
– Maples
– Sycamore
– Populus

Diffuse
Porous Wood
Ring Porous
Wood
University of Toronto
University of Texas

Tree Anatomy - Phloem
• Phloem – made up mostly of living cells
– Responsible for the transport of food within the
tree (sugars = photsynthates)
• Transport occurs along a gradient
• Transport requires energy
– Comprised of:
• Sieve Cells in conifers
• Sieve Tube Elements and Companion Cells in
hardwoods
– As a tree grows, old phloem is crushed and may be
absorbed into the tree or incorporated into the bark

Tree Anatomy - Bark
• Bark - outer covering of a tree’s
branches and stems composed of
non-functional phloem and corky
tissue that:
-moderates temperature
-offers some defense against injury
-reduces water loss
• Lenticels - small openings in bark
that allow for gas exchange

Lenticels in Cherry Bark

Tree Anatomy - Stems
• Hierarchy of stem composition:
– Trunks support branches
– Branches support other branches or twigs
– Twigs support leaves, flowers and fruit
• Buds occur:
– At the tip of each twig (terminal or apical buds)
– Along the twig (lateral or axillary buds)
– At the base of each leaf ( flower and leaf buds)
– Just under the bark (adventitious buds)
• Dormant until released
• When released they give rise to epicormic shoots

Tree Anatomy-Branches
• Branches - are autonomous and usually
produce and store enough carbohydrates to
sustain themselves
• Branch Collar-shoulder area where a branch
joins another branch or trunk created by the
overlapping xylem tissues
– Strong attachment on bottom of branch to the parent stem
– Weaker attachment on the top
• Branch Bark Ridge-area of a branch union
where the growth and development of the two
adjoining limbs push the bark into a ridge
• Included Bark-bark that is pushed inside a
developing branch union, causing a
weakened structure

From ISA Arborists’
Certification Study
Guide, Figure 8.2,
1 st edition

From ISA Arborists’ Certification
Study Guide, Figure 8.3, 1st edition
From ISA
Arborists’
Certification
Study Guide,
Figure 8.5, 1 st
edition

Roots

Mycorrhizae – the symbiotic association
consisting of fungi and roots

Critical Root Zone

Physiology – Water Movement
• Water is used for:
– Maintaining cell turbidity
– Transport of essential elements
– Growth and Metabolism
• Most of a trees water is lost through
Transpiration
• Pathway of water into roots = Osmosis:
– Water goes from area of high water potential to
area of low potential
– Usually moves into roots from the soil, but can
move out of roots into soil if potential is lower in
the soil

Tree Physiology
• Transpiration:
– It is the loss of water vapor through the
stomata
– Cools leaves
– Creates “transpirational pull”
– Temperature, humidity & available water
affect rates

The Cultivation
of Trees
is the
Cultivation of
the Good…

…the
Beautiful…

…and the Ennobling
in Man…
- J. Sterling Morton

QUESTIONS

WE NEED TO PLANT THE RIGHT
TREE IN THE RIGHT PLACE!!!

We Cannot Control The
Weather

Tree Physiology –
The Vascular System
• Phloem Transport:
– Movement of photosynthate in the phloem
• From areas of high sugar concentration to areas of lower sugar
concentrations
• Requires energy
– Source - leaves, areas of high sugar concentration
– Sink - plant parts that use more energy than they produce
• Most plant parts can be sinks at some point in their life
• Phloem transport occurs from leaves to roots, but most
photosynthate is either used or stored in close proximity to where it
is created
• Radial Transport - horizontal movement of water and
nutrients primarily through ray cells

From ISA
Arborists
Certification
Study Guide,
Figure 1.12, 1 st
edition

Physiology – Growth Regulators
• Growth & Development are results of the
interaction of a tree’s genetic potential and
the environmental conditions where it is
growing
– Plant Growth Regulators:
• Naturally occurring substances that act to
regulate plant growth and development
• It only takes small quantities to affect growth
• Classified as plant hormones:
– Auxins
– Cytokinins
– Gibberellins
– Abscisic acid
– Ethylene

Physiology – Growth Regulators
• Auxins
– Produced primarily in shoot tips, but may be produced at other sites
of cell division
– Instrumental in root development
– Involved with “tropisms”
• Geotropism
• Phototropism
• Cytokinins
– Produced primarily in the roots
– Instrumental in shoot initiation and growth
• Gibberellin
– Growth regulator involved in cell elongation
• Abscissic Acid
– Plant growth substance that triggers leaf or fruit drop
• Ethylene (gas)
– Naturally occurring growth substance that triggers fruit ripening

Physiology – Growth Regulators
• Tree form can be strongly directed by
growth regulators
– Decurrent tree form (rounded crown form)
• Result of new lateral shoots outgrowing the original
terminal shoot year after year
• Indicates weak apical control
– Excurrent tree form (upright crown form)
• Results from strong apical control so that terminal shoot
remains dominant as lateral shoots form
• Most juvenile trees start out with excurrent form

Excurrent Habit
Decurrent Habit
from urbantree.org