April 2009

Trees

 

 

 

 

 

 

So, what do you think makes a tree a tree?

 

HOW TREES GROW

Trees are the tallest plants in a forest and have woody roots, a trunk, and a "crown" of leaves at the top. Trees grow upward to keep their leaves in the sun.

The tree growth form and lignin, which is the complicated chemical that makes trees woody, have evolved independently several times. A cypress, a palm, and a maple are three species we will discuss. They are unrelated as shown by their classification:

PLANT KINGDOM
     Vascular Plants
        Seed Plants
            Non-flowering (Gymnosperms)
                Cypress Family
                    Taxodium ascendens is pond cypress
                    Taxodium distichum is bald cypress
            Flowering (Angiosperms)
                Monocots (with one seed leaf)
                    Palm Family
                        Sabal palmetto is cabbage palm
                Dicots (with two seed leaves)
                    Maple family
                        Acer rubrum is red maple

We will observe that cabbage palm has a different organization and anatomy than cypress and red maple. From these observations we will hypothesize different advantages and disadvantages.

 

CYPRESS AND RED MAPLE HAVE ONE KIND OF ANATOMY

We start with root hairs, that absorb water and mineral nutrients, and progress to larger and larger roots until we reach the single trunk at the ground surface. Moving up the trunk we reach the largest branches. Each branch has a hierarchy of smaller and smaller branches that end in twigs that bear the leaves. The leaves capture light energy used in photosynthetic production of sugars.

To grow taller and wider crowns, cypress and maple yearly add twigs and leaves at many growing tips. These growing tips are called the apical meristems and are an undifferentiated embryonic tissue that can form any differentiated tissue like leaves or twigs.

The branches, trunk, and roots get thicker as they add new conducting vessels, the xylem and phloem. The xylem brings water and mineral nutrients up from the roots. The phloem brings synthesized organic nutrients down from the leaves to where they are used to grow new twigs and thicker branches, trunk, and roots.

Between the xylem and the phloem at the periphery of roots, branches, and trunk is an embryonic tissue called the vascular cambium. It produces new phloem and bark on the outside and new xylem on the inside. As the xylem ages it dies and becomes the central part of the larger branches, roots, and the trunk. The dead xylem tissue is called heartwood. It provides support.

                                                                                                 red maple roots

Roots of cypress and red maple are very much less deep than the tree is tall. Amazingly, painstaking hand and paintbrush excavation shows that their roots extend beyond the trunk 2-3 times the width of the tree's crown of leaves!! And, about 95 percent of all the roots are in the top three feet of soil.

                palm roots

Palm roots are short, thin, closely spaced, and do not branch many times. They have no root hairs. Instead mutualistic fungi called mycorrhizae take up water and mineral nutrients.


 

PALMS HAVE A DIFFERENT KIND OF ANATOMY

Palms have very compact and non-branching roots and crowns. They have only one growing tip (apical meristem) at the top of the trunk with a cluster of 10 - 30 long leaves called fronds. Each frond has long leaflets arranged like fingers on a hand or teeth on a double-sided comb.

The trunk of a palm never grows in diameter with age. The many vascular bundles (phloem next to xylem) are scattered throughout the trunk. There is no heartwood and the bark is very thin.


AGING PALMS

Palms are difficult to age because they never drop all their leaves at once and their trunk never increases in diameter with age. But there are two ways to estimate the age of wild palms that are not watered or pruned. One method of aging relies on changes in width of the trunk. In droughts the trunk will be more narrow. A second method of aging relies on the number and spacing of marks on the trunk where each frond is shed.

AGING CYPRESS AND RED MAPLE

Cross-section of maple trunk

 

These trees are easy to age because they drop all of their leaves every year and their trunk increases in diameter with age. It is easy to count annual growth rings.

 

 


Cross-section of palm trunk

 

A palm trunk never grows, but it stays alive so it does not show annual rings or dead heartwood.

 

 

LEAF DROP AND FALL COLOR

Before these trees drop their leaves in fall and winter, there are changes in leaf color as chlorophyll is broken down and reabsorbed. Chlorophyll is about 85 percent of a leaf's protein, so it would be a waste to lose it when leaves die and drop. The other yellow, orange, and red pigments always present in red maple leaves become visible when the green chlorophyll is reabsorbed. This accounts for "fall color."

The cartoon below is a conversation between two leaves with several possible captions.

 

 

1. I'm raising my kids as non-deciduous!

2. I'm leaving you!

3. I told you, your chlorophyll would leave you!

4. I'll be right with you, as soon as I change.


REPAIRED STORM DAMAGE BY RED MAPLE AND CYPRESS

From their anatomy, we hypothesize that these trees have the capacity to repair themselves after damage from a lightning strike or a hurricane. They have many growing tips at the end of branches and so branches can re-sprout if broken or lost. And they have embryonic buds in the trunk so that they can sprout new branches and slowly develop a new crown, if not too much of the trunk is snapped off in a storm.

After the two 2005 hurricanes in our cypress swamp almost all of the damaged cypress and all of the damaged maples regenerated branches and crowns.

And, with the extensive root systems of both red maple and cypress and the buttressing of cypress trunks there were virtually no trees uprooted in the 2005 hurricanes.

 

 


 

 

PALMS RESIST LEAF LOSS AND UPROOTING DURING HURRICANES

From their anatomy, we hypothesize that palms will do well in hurricanes. Their leaves (fronds) are flexible and so should bend in the wind. And the wind should move easily among the leaflets on each frond. And palm trunks should be somewhat flexible since all the tissue is alive.

To test these hypotheses, I have watched both cabbage palms and coconut palms as the wind increased to hurricane force in two 2005 hurricanes. Less than a tenth of one percent of our native palms uprooted, none snapped off, and none with the new green leaves blown off.


 

WEAK RESPONSES OF PALMS TO CHANGES IN LIGHT, WATER AND NUTRIENTS

From its anatomy, we hypothesize that one palm cannot and need not change its leaves, trunks, and roots as the environment changes. The palm crown casts little shade because there are few leaves, they are always moving in the breeze, and there are spaces between leaflets and between fronds.

Our hypothesis is supported by observations of where palms do and do not grow. This is most obvious for coconut palms along an ocean beach but cabbage palms also grow only in the open or in large gaps in a forest with no other trees above them.

Palm roots are probably too short to grow toward patches of soil with higher water or nutrients.

STRONG RESPONSES OF CYPRESS AND RED MAPLE TO CHANGES IN LIGHT, WATER, AND NUTRIENTS

Unlike palms, cypress and red maple have semi-independent parts that allow growth responses to environmental changes. What we observe is that cypress and red maple have differential growth of even adjacent roots and branches. For example, to maximize capture of light for photosynthesis, trees grow more and larger leaves and branches on the sunny side. And to maximize uptake of water and nutrients for organic synthesis they grow more root hairs and both larger and longer roots in ground areas with the most resources.

Here are some examples of this semi-independence of tree parts along with possible advantages.

1. A red maple has very small leaves on a branch with many seed propagules. On an immediately adjacent branch there are large leaves and no propagules. If less energy is allocated to leaves then more can be allocated to reproduction, and vice versa.

2. The same root of a red maple that grew through a patch of sandy soil has long spaces between fine roots with little root branching. But as it continued to grow through a patch of clayey soil it developed short spaces between fine roots and prolific branching. Here the adaptive value is that clay binds more nutrients and holds water better than sand.

3. Large cypress trees have different kinds of leaves on branches that are always in shade, toward the bottom of the tree, and branches that are always in full sun, at the top of the tree. We call these shade and sun leaves.

How would YOU expect leaves in the sun and leaves in the shade to differ and why? Think of the different problems leaves face. In the shade the leaf shape and orientation should maximize exposure to dim light from above. But in the sun the leaf shape and orientation should minimize exposure to bright sunlight with its damaging ultraviolet rays (think of sunburn in humans).

Now, fill in this chart with your hypothesized differences in sun and shade leaves.
 

Leaf characteristic Sun

Shade

Horizontal, flat
Vertical, curly
Thin
Thick
Surface dull, non-reflective
Surface shiny, reflective

How did you do?

           Cypress shade leaves                                    Cypress sun leaves

 

PALM TREES' PROBLEMS DUE TO HAVING ONLY ONE GROWING TIP

With only a single growing tip, palms are at high risk from evolving diseases. In the 1960s we lost all of our Caribbean coconut palms from a virus disease called "lethal yellowing." All of our coconut palms today are a species from Malaysia.

Some of you may have eaten "hearts of palm" which are the core and bud of the single growing tip with developing leaves. Harvesting the "heart" kills the tree since this is its only growing tip.



Hearts of palm being harvested

 

 

REVIEW QUESTIONS
 

a. A cypress tree with shade leaves on lower branches and sun leaves on upper branches is a hybrid between bald cypress and pond cypress.

b. A cypress tree tapers continuously from thick to thinner with height whereas a palm tree trunk can change from thick to thin and back again.

c. "Hearts of Palm" can be harvested many times from a tree.

d. Leaves at the top of a tree in full sunlight can be stressed by "sunburn."