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Technology Bulletin 2

Tree Growth in the Urban Forest 

Forestry Report R8-FR 53 - June 1995

Written by:

Dr. Mary L. Duryea School of Forest Resources and

Conservation, IFAS Florida Cooperative Extension Service

University of Florida Gainesville, Florida 32611-0420 and

Dr. Marlene M. Malavasi Department of Agronomy

Federal Rural University of Rio de Janelro Rio de Janeiro,

Brazil

What happens to a birdhouse if you hang it on a tree and come

back in 2 years? Will it be at the same height or out of your

reach because the tree has grown? By the end of this article

you will be able to answer this question and others about how

urban trees grow.

This bulletin is for those who love to plant and grow trees. Of

course, trees grow the same way in the city as they do in the

country (physiologically speaking), but we will emphasize tree

growth in the city and discuss factors in the urban environment

that affect this growth.

Understanding the way trees grow will help in their culturing

and management. For instance, the timing of tree growth

influences the success of such practices as planting and

pruning. Understanding how roots grow may help determine

where to plant certain species such as palms versus oaks.

Knowledge of how trees grow in response to wind helps to

make decisions about staking. And knowing how trees grow as

they age will help to determine future management needs. This

publication will help you answer questions about tree growth

and this understanding may, in turn, help with decisions about

cultural and management practices. Understanding and

promoting good tree growth means a healthier urban forest.

WHAT IS TREE GROWTH?

Trees have six organs: leaves, stems, and roots (known as

vegetative structures) and flowers, fruits, and seeds (known as
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reproductive structures). This bulletin discusses vegetative

growth--that is the growth of leaves, stems, and roots.

Tree growth is the increase in size and number of leaves,

stems, and roots.

WHAT KINDS OF TREES ARE THERE?

Trees are seed plants - that is they reproduce by seed. They are

classified into gymnosperms and angiosperms. Gymnosperms

are trees with seeds that develop on the surface or tip of an

appendage such as a cone. Examples are cypress and pines.

Angiosperms are trees with seeds borne within structures such

as fruits (the seed is enclosed). Examples are oaks, elms,

maples, and palms. This bulletin describes growth of both

gymnosperms and angiosperms. Also, there's one more division

among angiosperms that is necessary. Palms are angiosperms

but they are different than oaks, maples, and elms, which are

dicots, because palms are monocots and are related to grasses.

This bulletin first reviews growth of gymnosperms and dicots;

then because they grow very differently, a special section looks

at palms.

WHAT CONTROLS GROWTH?

Tree growth is a response to the environment and to the tree's

genetic make-up. The environment is made up of factors such

as water, light, temperature, air, pests, and cultural practices.

These factors impact physiological processes such as

photosynthesis and respiration, and the final response is the

tree grows vigorously or slowly. A tree's genetic make-up also

influences the way a tree grows. Genetics means the species,

seed source, or variety (Did it come from south Florida or North

Carolina?), and family (Who were its parents?). These genetic

characteristics along with its environment determine how a tree

grows.

WHERE DOES GROWTH OCCUR?

Growth occurs in meristems. A meristem is a tissue that

contains cells that have the capacity to divide and make new

cells. Some of these new cells continue to function as

meristematic cells. Other new cells grow and differentiate into

structures such as roots and shoots.

Meristems can also produce new meristems--these are called

primordia. For example, an apical meristem in a bud produces

new meristems called leaf primordia - each single primordium

will grow into a new leaf. Root primordia grow inlo new roots.

GROWTH ABOVE GROUND

Tree growth above ground includes shoot elongation, leaf

growth, and diameter growth.

Shoot Elongation

Shoots grow in height or length in apical meristems located at

the tips of branches (figure 1). What happens is

- apical meristem cells divide, elongate, and differentiate. The

steps that we can see are: (1) the bud at the tip of the branch

opens, (2) leaves emerge and enlarge, and (3) the area
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between the leaves expands, namely the stem grows. Lateral

(side) buds grow in the same way but often are dormant and

do not grow until they are triggered to grow. Activities such as

pruning can serve as this trigger.

Figure 1.

Leaf Growth

On the surface of the apical meristem in the bud, a new

meristem is formed. This new meristem is called a leaf

primordium, and this is where cells divide and grow to form a

leaf. The number of meristems on a leaf influence leaf shape

(figure 2).

Soon after leaves develop, a new bud primordium (meristem) is

formed at the base of each leaf stem. Once formed, this axillary

bud has the capacity to become a branch, but may lie dormant

for many years.

Diameter Growth

Between the wood and bark is a thin layer of dividing cells

called the vascular cambium. This vascular cambium is a

meristem which is only a few cells thick. This meristem divides,

creating wood on the inside and bark on the outside (figure 3).

Layer by layer these new cells increase the diameter of the

trunk and branches. The wood cells are called xylem, which

means wood in Greek; they carry water and minerals up from

the roots. The bark cells are called phloem, meaning bark in

Greek. These cells carry sugars and other materials produced

by the plant.

Annual Rings

New layers of wood are added each year between the bark and

the previous year's wood. These are called growth rings or

annual rings and may be used to age a tree. Annual rings vary

in size and thickness according to the season and

environmental conditions when they are formed. Cells produced

in the spring are larger with thinner cell walls. These are the

light-colored rings, and the wood is called early or spring wood.

Cells produced in the summer are smaller, and this late or

summer wood has a higher density and darker color.

Bark

Trees have bark that protects the tree from pest attacks and

environmental impacts such as fire and mechanical injury. Trees

with thick bark are more resistant to injury than those with thin

bark. As the tree grows in diameter, the bark has to give,

forming ridges and cracks. Eventually the outer bark sloughs

off.

GROWTH BELOW GROUND

Roots can grow in length (root elongation) and diameter, and

new lateral roots can form.

Root Elongation
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At the tip of a root is the root cap (figure 4). This cap protects

the root, and it must be constantly replaced as it pushes

through the soil. Behind the root cap is a meristem, which

produces new cells for the root cap and for root elongation.

These new cells elongate, divide, and differentiate into root

parts. The root grows in length and pushes through the soil.

Root Diameter Growth

Many people don't think that roots grow in diameter, but they

do! And root diameter growth is similar to

growth in the stem with the vascular cambium producing wood

(xylem) and bark (phloem). A couple differences between root

and shoot diameter growth are: (1) diameter growth is much

more irregular in roots, resulting in roots that are oval or

irregular shape, and (2) there is greater variation in root

diameter with age and with horizontal roots comparcd to

vertical roots.

New Root Growth

Roots can also grow new lateral roots that branch off the main

root. Some cells located in a layer inside the root produce a

new root primordium (figure 4). This new meristem divides and

elongates, pushing the new root out through the parent root

and into the soil.

PALMS

There are two major points to be made about palm growth: (1)

palms only have one terminal growing point, and (2) palms do

not grow in diameter as they age. This section shows how

palms grow taller and grow roots and how the one bud

influences their form compared to other trees.

Shoot Elongation

The apical meristem lies in a depression at the crown of the

stem (figure 5). Within this meristem is a soft mass of leaves

and leaf bases. In this crown there is continuous production of

leaf meristems (primordia). There may be up to 50 meristems

in the apex. The leaves grow in sequence with one leaf

emerging as a spear at a time. The meristem at the base of the

leaf causes the leaf to elongate, grow bigger and thicken and

eventually equal the diameter of the palm stem.

Root Growth

Root growth occurs near the root cap just as in gymnosperms

and dicots. However, just like shoot growth of the stem, there is

no diameter growth of palm roots. The roots remain the same

diameter for long distances (giving the appearance of a

spaghetti-like root system). Descending roots are for

anchorage. Most of the horizontal roots are in the top 40

centimeters (15.7 inches) of soil and grow out from the palm

about 4 meters ( 13 feet).

SEASONAL GROWTH, LIFESPAN AND AGING

Different parts of trees grow at different times of the year, and

the pattern varies by species and climate. In one typical pattern

trees begin with shoot growth in the sprlng, followed by trunk
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diameter growth and then root growth. However, this growth

can occur in any order, and root growth may be year-round or

have two growing periods per year. Young trees grow longer in

the growing season than older trees. Length of active growth is

determined by temperature, day length, and water.

Tree species have a wide range of lifespans. For example, while

peach trees may live only 30 years, oaks may live several

hundred years, cypress 1,600 years, and bristlecone pine up to

5,000 years in an undisturbed site in the forest (Kramer and

Kozlowski 1979). Forest tree species may undergo stress when

planted in the city: The average tree in the downtown may live

only 13 years, one tenth the lifespan of a tree at a rural site

(Skiera and Moll 1992).

What happens to trees as they age? Some of the likely

occurrences are the following: (1) growth slows down, (2) trees

are more susceptible to diseases and insects, (3) the tops of

the trees are more likely to die back, (4) wounds heal more

slowly, (5) there are fewer leaves relative to size of the tree,

and (6) the number of dead branches increases. All of these

characteristics can also be seen in urban trees as they begin to

die and/or show signs of poor health.

GROWTH AND TREE FORM

The way trees grow affects their form. The apical bud is the bud

at the tip of each branch; lateral buds are on the sides of the

branches. Apical dominance means that the apical blld at the

tip of the stem controls and inhibits growth of lateral buds.

Most palms only have one apical bud. Therefore palms usually

grow straight with no branching; they are an extreme example

of apical dominance. The main stem or leader of pines is

prominent, but branches formed by lateral buds change the

form, resulting in less apical dominance than palms. An oak

tree has very little apical dominance, with several branches

dominating the

crown.

EXAMPLES OF THE URBAN ENVIRONMENT'S INFLUENCE

ON GROWTH

Urban trees are often subjected to a tremendous amount of

stress. Soils are compacted, trees are over - or incorrectly -

pruned, roots are given very little space to grow, trees are

improperly staked, and on and on. A few of the stress-induced

practices and environmental conditions are discussed briefly

along with how they affect growth.

Pruning

What happens when you prune a tree? If you prune the apical

buds, the lateral buds are released and new shoots grow. If you

prune older, large-diameter stems, two things may happen: (1)

dormant buds, produced long ago during primary growth, can

now grow into new branches, and (2)new meristems arise and

produce new buds and shoots. This growth of new branches is

most likely to occur in angiosperms - everyone has seen

sprouts from the stump of a freshly cut oak, maple, or elm.

Pine trees, however, do not have these dormant buds and will

not sprout. Of course, if you remove the apical bud from a
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palm, it will die--remember? most palms only have one bud at

the top of the tree.

Staking

How does staking affect growth? When trees are planted, they

are often staked. However, studies have shown that trees that

are allowed to sway in the wind are sturdier. Staked trees tend

to be less stable and sometimes topple when stakes are

removed and trees are exposed to wind. This can be explained

by looking at growth differences. When trees blow in the wind

there is a redistribution of the amount and nature of diameter

growth on the leeward and windward sides of the tree trunk.

Reaction wood is formed, and this wood has larger and thicker

walled cells than other wood. Hence when exposed to wind a

tree becomes sturdier and can tolerate wind better. For this

reason, do not stake young trees unless they absolutely will not

stand up without a stake. Remember palms do not have

cambial (diameter) growth, so staking palms is okay and often

necessary until new roots stabilize the palm in the landscape.

Compacting the Soil

Soil conditions in urban plantings are often ignored, resulting in

many tree health problems. Trees often have limited rooting

space and are similar to large potted plants. Soil compaction

can occur due to such things as construction equipment,

sidewalks and streets, and footsteps. What happens to growth

when tree roots are in compacted soil? Studies show that trees

grown in highly compacted soil have less root volume and

weight because roots cannot penetrate compacted soil. And this

translates into reduced shoot height and overall growth.

Compacted soil in the city means roots cannot grow, water

cannot permeate into the soil, and nutrient uptake is reduced,

resulting in poor tree growth. Of course, the ultimate effect is

poor tree health and death. Some ways to avoid compaction

are: (1) mix organic matter into the soil, (2) prevent vehicles

from operating beneath the tree canopy, and (3) when

construction traffic must work around trees make sure it is

when the soil is dry (wet soil is more easily compacted) or

spread a thick layer of coarse mulch beneath the canopy.

CONCLUSIONS

Trees grow in response to their environment and genetic makeup. Envirorimental factors such as high temperature or soil

compaction influence physiological processes such as

photosynthesis and, in turn; impact growth. Meristems are

areas within the tree where growth occurs. Most trees grow in

shoot length and diameter and root length and diameter. Palms,

however, have only one growing tip and do not grow in

diameter. A tree's form is controlled by branch orientation and

by the amount of apical dominance. Altering the environment

or management practices to decrease stress, and selecting the

right tree for the right place will promote growth and longevity.

And,finally, as the tree ages and grows, does the birdhouse

move up the tree and out of your reach? No! Why not?

Remember a tree only grows in height at apical meristems at

the tips of branches. At the height of the bird-house only trunk

diameter growth is occurring.
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SOUTHERN REGION USDA FOREST SERVICE 1720

PEACHTREE RD., NW ATLANTA, GA 30367

REFERENCES

Kramer, Paul J.; Kozlowski, Theodore T. 1979. Physiology of

woody plants. New York: Academic Press. 811 p.

Skiera, Bob; Moll, Gary. 1992. Trees in the red. Urban Forests.

12: 9-11.

FOR MORE READING ABOUT TREE GROWTH

Bell Adrian D. 1991. Plant form: an illustrated guide to

flowering plant morphology. Oxford (England); New York:

Oxford University Press. 341 p.

Corner, E.J.H. 1966. The natural history of palms. Berkeley and

Los Angeles: University of California Press. 393 p.

Fahn, A. 1991. Plant anatomy. Fourth edition. Oxford

(England); New York: Pergamon Press. 588 p

Harltey, C.W.S. 1977. The oil palm. (Elaeis guineensis Jacq.).

Second edition. London; New York: Loogman. 806 p.

Harris, Richard W. 1992. Arboriculture: integrated management

of landscape trees, shrubs, and vines. 2nd edition. Englewood

Cliffs, New Jersey: Prentice-Hall, 674 p.

Kozlowski, Theodore T. 1971. Growth and development of trees.

Volume 1. Seed germination, otogeny, and shoot growth. New

York: Academic Press. 443 p.

Kozlowski, Theodore T.; Kramer, Paul J.; Pallardy, Stephen G.

1991. The physiological ecology of woody plants. San Diego:

Academic Press. 657 p.

Laetsch, Watson M. 1979. Plants: basic concepts in botany.

Boston: Little, Brown and Company. 510 p.

Mosbrugger, Volker. 1990. The tree habit in land plants. A

functional comparison of trunk constructions with a brief

introduction into the biomechanics of trees. In: Bhattacharji,

Somdev; Friedman, Gerald M, Neugebauer, Horst J., Seilacher,

Adolf, eds., Lecture notes in earth sciences, Vol 28.

Berlin,Heidelberg; New York; London, Paris; Tokyo; Hong-Kong:

Springer-Verlag. 161 p

Uhl, Natalie W.; Dransfield, John. 1987. Genera Palmarum. A

classification of palms based on the work of Harold E. Moore,

Jr., The L.H. Bailey Hortorium and The Internationl Palm

Society: Lawrence, Kansas: Allen Press. 610 p.

Wilson, Brayton F. 1984. The growing tree. Amherst: The

University of Massachusetts Press. Amherst. 138 p.

This bulletin is also available in an electronic format.

The United States Department of Agriculture (USDA) prohibitsdiscrimination in its programs on the basis of race, color,

national origin, sex,, religion, age, disability, political beliefs and

marital or familial status. (Not all prohlblted bases apply to all
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programs.) Persons with disabilities who require alternative

means for communication of program information (braille, large

print, audiotape, etc.) should contact the USDA Office of

Communications at (202) 720-5881 (voice) or (202) 720-7808

(TDD). To file a complaint, write the Secretary of Agriculture, U

S Department of Agriculture, Washington, DC 20250, or call

(202) 720-7327 (voice) ot (202) 720-1127 (TDD). USDA Is an

equal employment opportunity employer.

JUNE 1995 FORESTRY REPORT R8-FR 53

SOUTHERN REGION /USDA FOREST SERVICE 1720

PEACHTREE RD.. NW ATLANTA. GA 30367

Last Updated: November 05, 2001

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