Introduction
Trees need tremendous amounts of water on a daily basis. Even though it may not rain every day, a trees' roots spread through the ground absorbing water. A mature oak tree needs 40-60 gallons of water every day. Trees have ways of conserving water, because water is very precious to a tree. One way in which a tree helps to conserve water is to develop two kinds of leaves. There are sun leaves and shade leaves. Sun leaves are small, with less surface area, which reduces the amount of exposure to the sun and wind. A shade leaf is large, with greater surface area, which increases the amount of area exposed to the sun. Remember, it is important for a tree to have its leaves exposed to the sun so that photosynthesis (food making) can take place, but not so much that it loses too much water.

Questions

1. Where would you expect a shade leaf to be found? On the top or bottom of a tree crown?

Hypothesis
     Students should make their own before continuing.

Materials
     Hand pruner
     Metric ruler
     Ladder (optional)
     Extension pruner (optional)
     Pencil and paper

Procedure

1. Locate a small tree in an open field. Obtain three leaves from high on the south side of the tree. Measure the length and width of all three leaves and record them in your notes under the category of top leaves.
2. From the lowest part of the tree crown on the north side, obtain three leaves that appear to be growing underneath other branches. Measure the length and width of all three leaves and record them in your notes under the category of bottom leaves.
3. Compare the measurements. Which leaves were consistently larger? The top or the bottom? What would these leaves be called?
4. Which leaves were consistently smaller? The top or the bottom? What would these leaves be called?

Conclusions
Why does a tree have sun leaves that are small and shade leaves that are large. What are the relationships between needing sunlight, needing to reduce water loss, and the size of a leaf?

CONCLUSION
Use format discussed by teacher.

Background Information
Every tree or plant has a daily need for water. The cell, as the basic unit of life, is 75% water. Therefore, if a tree is to live, its cells must have enough water. A tree loses water by a process known as evapo-transpiration. Evaporation of water is increased by heat and wind. Transpiration is the movement of water from the roots through the stem to the leaves where evaporational losses can be high. The leaves must have a continuous supply of water to avoid dehydration and to carry out photosynthesis.

The effect of heat and wind on leaf water loss is greatest at the top of the tree. A tree, or any other plant, has several strategies to reduce the inevitable loss of water. There is a waxy covering (cuticle) on the leaf to reduce desiccation. Stomata (leaf openings which are necessary for gaseous exchange, but do enhance evaporation) are concentrated on the underside of the leaf so as not to be directly exposed to the sun. Stomata guard cells close when evaporation conditions are most intense.

Shade leaves and sun leaves are different. Surface area is a key consideration in reducing water loss. The less surface area that is in contact with wind or heat, the less water is lost. In other words, small is good when in direct contact with the sun. When considering the entire set of leaves on the tree, one notices that some of the leaves receive direct exposure to the sun, and other leaves receive indirect sun because of shading by other leaves. Sun leaves are found on the top part of the crown. Shade leaves are found on the bottom part of the crown especially on the north side, and have a larger surface area.

Determining a ratio between shade leaves and sun leaves on a tree helps a forester or arboriculturist determine its tolerance or intolerance to shade. A tree that has a high ratio of shade leaves to sun leaves indicates it is tolerant to shade. In other words, the tree does not mind growing in the shade and is a species that is able to grow as a sapling under a dense forest canopy.

Target Group
Sixth through eighth grade.

Timeline
This activity can be done between 30 and 45 minutes.

Student Learning Objectives
Students will be able to:

1. Differentiate between a shade leaf and sun leaf on a tree.
2. Understand a tree's strategy for reducing water loss.
3. Measure and compare leaves from the lower part of a tree and the upper part of a tree.

Procedure
A 20-30 foot high tree can be used for making a visual observation and comparison between the sun and shade leaves. Make sure the tree is out in the open (in direct contact with the sun), so that the tree is more likely to produce both shade and sun leaves. Shade leaves will be easier to find on the north side and on interior branches. If you can walk through a fairly dense forest, try to locate a young seedling who has just a few starter leaves on it and note that the leaves may be significantly larger than those found on a more mature tree of the same species. Locate a small tree (10-15 feet high) and obtain three sun leaves and three shade leaves. Measure the length and width of each leaf and then compare as a group for marked differences between them.

Blowouts

1. Find a 10-12 foot Sugar maple (Acer saccharum) growing under the canopy of a forest. Have the students get a ratio of shade to sun leaves on the tree. Find a 10-12 foot willow sp., cottonwood or oak sp., and have the students do the same. A sugar maple is very tolerant of shade while a willow is very intolerant. A shade tolerant tree commonly exhibits a high ratio between shade and sun leaves. It might have as many shade leaves as sun leaves. On the other hand, a shade intolerant tree will commonly exhibit a low ratio between shade and sun leaves. It might have very few, if any shade leaves, developing all sun leaves to trap as much sunlight as possible regardless of the amount of water that it might lose.

Glossary
evapotranspiration: the movement of water from the roots to the leaves of the tree with the eventual loss of water through the stomata of the leaves.

desiccation: loss of water by heat, wind, or a combination of both.

sun leaf: a leaf on a tree that is almost always in direct contact with the sun; usually on the upper crown.

shade leaf: a leaf that is entirely or partially shaded by other leaves on the tree; usually on the lower portion of the crown.

shade tolerance: the ability of a tree to endure shade.

References

1. An Outline of Forest Hydrology. Hewlett, J. and Nutter, W., University of Georgia Press, Athens, Georgia. 1969.
2. Teacher, Why Are WE Planting These Trees? Environmental Education Activity Sampler from Project Learning Tree, Project Wild and CLASS Project.

example data

CONCLUSION:
The leaf size is smaller at the top than at the bottom. Every species of tree may have a different ratio of sun and shade leaves. These ratios can be used as extensions to the lab exercise for the class, or as projects.

The teacher will need some experience to understand how this relates to a particular site.

REFERENCE INFORMATION ON SHADE TREE TOLERANCE

Shade Tolerance of Trees:
Definition - the ability of a plant to endure shade.      Plants may also be drought tolerant, salt tolerant, etc.

Factors that affect shade tolerance:
Age (young>old)
Site quality (good>poor)
Available moisture
Available nutrients
Geographic portion of range

Some probable causes:
High photosynthesis rate under low light
Photosynthate used for development and maintenance of plant body - especially leaf area.
Modifications which increase efficiency of light absorption.
Maintenance of vigor at low metabolic rates.
Ability to minimize respiratory losses

Characteristcs of Tolerant and Intolerant Species:

Characteristic	Tolerant	Intolerant
Compensation point	Low intensity	High intensity
Saturation point	Low intensity	High intensity
Net assimilation	Low	High
Ratio: Shade/Sun leaves	High	Low
Tolerance to high temperatures at high light intensity	Low	High
Total foliage biomass	High	Low
Self-pruning	Poor	Good
Leaf area	High	Low
Juvenile growth rate	Slow	Fast
Response to release	Good	Poor

REFERENCE INFORMATION ON SHADE TREE TOLERANCE^1,2

EASTERN CONIFERS

Very Tolerant
Red spruce
Eastern hemlock
Balsam fir
Northern white cedar
Atlantic white cedar

Tolerant

Red spruce
Black spruce
White spruce
Northern white cedar
Atlantic white cedar

Intermediate

Eastern white pine
Eastern redcedar
Pitch pine
Loblolly pine
Slash pine
Virginia pine
Baldcypress

Intolerant

Eastern redcedar
Red pine
Pitch pine
Shortleaf pine
Loblolly pine
Slash pine
Virginia pine
Baldcypress

Very Intolerant

Tamarack
Eastern redcedar
Pitch pine
Jack pine
Longleaf pine
Shortleaf pine
Baldcypress

EASTERN HARDWOODS

Very Tolerant
Eastern hophornbeam
American hornbeam
American beech
American holly
Sugar maple
Flowering dogwood

Tolerant

Rock elm
Sycamore
Red maple
Silver maple
Box elder *
Basswood
Tupelos
Persimmon ***
Buckeyes ***

Intermediate

Hickories
Yellow birch
Sweet birch
American chestnut
White oak
Red oak Tupelos
Black oak
Other oaks *
American elm
Rock elm
Hackberry ***
Magnolias **
Sycamore
Black cherry
Silver maple
Basswood
Persimmon **
White ash
Green ash
Black ash

Intolerant

Black walnut
Butternut
Pecan
Hickories
Paper birch
Red oak
Black oak
Yellow poplar
Sassafras *
Sweetgum
Sycamore
Black cherry
Honey Locust
Kentucky coffeetree **
Tupelos
Persimmon ***
White ash
Green ash
Black ash
Catalpas ***

Very Intolerant

Willows (S. bebbiana tolerant)
Quaking aspen
Bigtooth aspen
Osage orange ***
Cottonwoods
Gray birch
Black locust

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*Very common
**Not common
***Floodplain
¹In cases where considerable preference for other categories was expressed the name appears underlined.
²Baker, F.A. 1949. Journal of Forestry 47 (3): 179-181.

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