Download Tree Structure and Biology

TREE STRUCTURE AND BIOLOGY
Introduction. Sarasota County lies along a transient tension zone line. In this
region, climate cycles cause periods of warmer weather, when normally tropical trees find
their northern-most reach here – and then colder periods, when trees native to Northern
Florida and Georgia migrate somewhat and find their southern most reach in our County.
Our region contains 7 basic forest communities (and 57 native tree species):
Type/Community
Tree Indicator Species
Scrub
Sand pine, myrtle oak, turkey oak
Bay Swamp
Loblolly bay, sweetbay, swamp bay, slash pine
Mangrove Swamp
Red mangrove, white mangrove, black mangrove, buttonwood
Hardwood Swamp
Tupelo, Florida elm, red maple, dahoon holly
Hardwood Hammock Live oak, laurel oak, cabbage palm
Flatwoods
Slash pine, longleaf pine, saw palmetto
Pine prairie
Slash pine, longleaf pine
In terms of our urban forest, it is comprised of street and park trees, residential yard trees,
trees growing in street medians and in parking lot islands, as well as trees found school
grounds, golf courses, reserves and agricultural lands.
You may not think there is a great connection between your everyday life and the forests
that surround us, but that couldn’t be further from the truth. Your urban forest exchanges
carbon and greenhouse gas compounds for oxygen. It moderates temperatures, surface
water runoff, and ambient (up to 10 feet above the ground) ozone. It provides natural
corridors that support species and genetic diversity among wildlife. Tree root systems can
filter toxic metals from ground water and soil, using a process called phytoremediation.
Trees buffer noise, and shield us from harmful sun glare. Well-planted and well-maintained
trees along the southern and western exposures of your home can lower your energy costs
by 1/3, and act as a storm buffer for your home and roof.
And, although most of you know that many items we use on a daily basis are extracted or
processed from trees, you may not be fully aware how much we rely on trees for products,
medicines, chemicals and food. (See Appendix I for a partial list of tree by-products and
derivatives.)
The Basics. Trees are perennial plants that grow year after year after year. They have one
stem (trunk), and are generally considered either hardwoods or conifer. Hardwoods trees
experience seasonal loss of all of their leaves. Their leaves are usually broad and thin,
which is why they are sometimes referred to as Broadleaf trees. Conifers – also called
Evergreens – keep their either all or a part of their leaves year-round and usually grow in a
conical or pyramid-like shape.
Tree shapes differ as well, frequently based on the sunlight and water
conditions of the area in which they grow. Some trees grow quite tall and straight. Some
smaller trees, referred to as understory trees, collect indirect or fragmented sunlight through
openings in the overhead canopy of larger trees. Nearer to the equator, the noontime sun is
almost directly overhead year-round, and so tall trees with flat treetops (or crowns) are very
common. The flat shape helps expose more leaves to the direct, overhead sun. Nearer to
the Artic circle, however, the sun lays close to the horizon. Trees in that region tend to be
cone-shaped, in order to make the most of the available light. Artic trees also sport needles
(rather than flat leaves), which have a higher capacity to retain water.
Physiology. All trees have three major organs:
1)
2)
3)
trunk/stems, which provide support, transport, and storage regulation;
roots, which provide support, transport and storage; and
leaves, which provide food for living cells.
THE TRUNK. (serves to store food and manage chemical regulation) The trunk
supports the tree crown, gives the tree its shape and strength, and produces the bulk of a
tree’s useful wood. The trunk consists of four layers of tissue, and each of these layers
contains a network of tubes that runs between the roots and leaves. Some transport water
and minerals from the roots to the leaves, and others distribute sugar (glucose) from the
leaves to the branches, trunk and roots.
Heartwood – As a tree grows, older xylem cells in the center of the tree
become inactive and die, forming heartwood. Because it is filled with stored sugar, dyes
and oils, the heartwood is usually darker than the sapwood. The main function of the
heartwood is to support the tree.
Xylem/Sapwood – The xylem, or sapwood, represents the youngest layers
of wood. Its network of thick-walled cells carry water and nutrients from the roots to leaves
and other parts of the tree through a tubular network inside the trunk. With age, these cells
decline and die, to become part of the heartwood. Sapwood is further divided into “early
wood,” the clear yellow area of rapid cell growth; and “late wood,” as cell growth slows down
and compresses to form the dark area of the ring (not necessarily on an annual basis).
Cambium -- Cambium is a very thin layer of growing tissue that generates
new cells, which will have the capacity to become either xylem, phloem, or additional
cambium. Every growing season, a tree’s cambium adds a new layer of xylem to its trunk,
producing a visible growth ring in most trees. The cambium is what makes the trunk,
branches and roots grow larger in diameter.
Phloem/Inner Bark – The phloem is located between the cambium and the
outer bark, and acts as a food (sugar and nutrients dissolved in water) supply line from the
leaves to the rest of the tree.
Bark – Bark is the visible outer covering of a tree’s trunk, branches and twigs.
It is actually a layer of dead phloem cells, shed outward to act as a shield against insects,
disease, storms and extreme temperatures. In some species, the outer bark is effective as
protection against fire.
THE ROOTS. A tree’s roots absorb water and elements from the soil, store food and anchor
the tree upright in the ground. All trees have lateral roots that branch into smaller and smaller
roots, and usually extend horizontally beyond the branch tips. Some trees have a tap root that
reaches down as far as 15 feet. Each root is covered with thousands of root hairs that facilitate
water and mineral absorption from the soil. The majority of the root system is located in the
upper 12-24 inches of soil, because the oxygen roots require to function property is most
abundant and accessible at these depths. REMEMBER that, underground, a tree’s root
system normally spreads 2-3 times the height of the tree.
THE CROWN/LEAVES. The crown, which consists of the leaves and branches at the top of a
tree, plays an important role in filtering dust and other partricles from the air. It also helps cool
the air by providing shade, intercepts rainfall, and reduces erosion.
Leaves function as a tree’s nutritional factory through a process known as PHOTOSYNTHESIS
What is It? Photosynthesis is the process by which (green) plants (including trees)
convert light energy to chemical energy and store it as food in bonds of glucose (or sugar). The
conversion takes place primarily in plant leaf structures, specifically in chloroplasts, where the
plant processes light energy with CO2 (carbon dioxide) and H2O (water). This self-sufficient
means of generating food is known as “autotrophic nutrition.”
Are Trees and Shrubs the Only Place Where We Can Find Photosynthesis? NO.
Because chlorophyll – the green pigment of plants – is necessary to the process, we also find
photosynthesis among Protista and Moneran Kingdom algaes. In fact, because the earth’s
surface is predominated water, global photosynthesis takes place more in the seas than on
land.
How Do Tree Leaves Manage Photosynthesis? In trees, photosynthesis occurs
mostly in leaves, which are composed of upper and lower epidermis, the mesophyll, vascular
bundles of veins, and stomates.
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The upper and lower epidermis act as protective layers, and do not contain
chloroplasts
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Stomates are concentrated in the lower epidermis, appearing as a network of holes that
allow for gas exchange – taking in CO2 and releasing O2 (oxygen)
Vascular bundles form a leaf’s mobility system for the internal transport of water and
nutrients. This circulatory system is broken into xylem, (which transport water and
minerals from the roots, through the stem, and into leaves) and phloem (which distribute
glucose that is synthesized through photosynthesis)
The mesophyll houses chloroplasts and is thus the site for photosynthesis
What About Chloroplasts? Chloroplasts are made of outer and inner membranes,
intermembrane space, stroma, and thylakoids that are stacked in structures called grana.
Chlorophyll gives plants their green color by absorbing red and blue light. It reflects the
color green, and so this is the color that our eyes register. This absorption and reflective
mechanism is not random, for it is the red and blue light energy transfer that plants use for
photosynthesis.
CLIMATE, WEATHER AND PHOTOSYNTHESIS Plants adapt and photosynthesis cycles
change in response to climate changes. For example:
Summertime -- Hot summer weather increases the amount of water a plant evaporates.
To conserve water in hot weather, leaves will close their stomates, but this also restricts the
exchange of carbon dioxide for oxygen, and photosynthesis decreases. This explains summer
browning and wilting of plants and trees
Desert Heat -- Certain plants, such as cactus, have developed biological strategies to
cope with hot, dry or desert climates. They open their stomates only at night, and store
captured CO2 in organic compounds until daybreak, when those compounds can be processed
with light energy
Fall Colors –- As summer moves into autumn, days get shorter and shorter, leaving
trees and plants less exposure and access to sunlight. In winter, trees must survive with far
less light and water, and resort to living off nutrients stored during the summer. Chlorophyll
disappears from leaves as they shut down nutrient-generating processes. Yellow, gold and
orange tones that are normally concealed by the abundance of chlorophyll-saturated green, rise
to the visible surface. In some tree species, leaves also turn red from glucose trapped in the
leaves after photosynthesis stops. The brown color noticed in trees, such as oaks, is made
from waste left in the leaves.
Human Benefits of Photosynthesis –- Photosynthetic organisms remove CO2 from
our atmosphere and exchange it for oxygen. In addition, leaves give off cooling vapor in the
course of their photosynthetic processes. If we continue to stress or damage the ecosystems
where photosynthesis occurs – oceans as well as forests - increased CO2 levels may promote
global warming.