Here
is an image of tracheids and annual rings on a cross-section of
longleaf pine (Pinus palustris). Click on
photo to learn more.Trees are long-lived, sessile, highly integrated, modular, plastic organisms (sensu Harper).
"The growing tree is an integrated system with water, minerals, amino acids, carbohydrates, growth regulators, and other organic substances moving freely, though often in phases, between different plant modules."
~Dickman and Pregitzer
One very important characteristic of modular organisms such as trees is that they are plastic and can adjust their component parts (modules--leaves, branches, roots) in response to changing environmental conditions. For example, trees exhibit seasonal phases of growth, and tree growth can be plastic in terms of carbon allocation to different modules. Trees also store sugars and starches in the bole and belowground in roots during the winter and then remetabolize these stored reserves the following spring. Many trees can reproduce vegetatively by producing ramets. This can happen many different ways; for example, layering (black spruce), stump sprouts (red maple), root suckers (trembling aspen).
Aboveground modules
Belowground modules
Leaves, auxillary buds and internodes comprise the shoot. Shoots are responsible for fixing carbon and collectively, all the shoots on a tree represent the maximum capacity of the tree to fix carbon during photosynthesis. The beginning of shoot growth/leaf expansion in deciduous trees is primarily controlled by temperature in the spring. Thus, trees can adjust their date of leaf-out based on flucuations in spring temperatures. In contrast, the date the leaves drop in the autumn appears to be controlled by photoperiod (hours of daylight). Both these mechanisms help the tree avoid frost damage.
Phenology
Woody plants in the temperate zone exhibit very programmed growth. They grow according to "pre-programmed" rules using environmental cues to maximize their fitness. The study of temporal aspects of plant growth is known as phenology.
Phenology: The study of the temporal aspects of plant
growth and development, and their relation to climate.
To understand the significance of genetic differences among populations
of trees we must first understand the basics of shoot growth.
Shoot Types
Woody plants are constructed of repeating units, the annual shoots, which are made up of leaves (needles), and buds (vegetative and reproductive) attached to the stem.
The stem portion of the shoot soon becomes woody and is termed
a twig.
The collection of twigs plus older stems makes up the branch system
of the tree.
Illustration (fresh material)
The annual shoot is the repeating unit of which the branch system
is composed.
Patterns of shoot growth give each species its peculiar form and
structure.
Temperate woody plant species exhibit three shoot types:
Short, spur-like or dwarf shoots
*Preformed inside certain lateral vegetative buds
*Make little growth in length because there is little or no interleaf
elongation
*Examples: Betula, Larix, Prunus, Malus
Determinate long shoots
*The most common type of shoot in temperate trees
*Totally preformed within the terminal bud and vegetative lateral
buds; these shoots develop during a brief period in the spring
and early summer.
Indeterminate long shoots
*Basal portion of the shoot is predetermined in terminal buds
*After the predetermined portion elongates in the spring, a new
section of shoot continues to grow
*"Indeterminate" portion of the shoot grows in response
to environmental conditions
*Example: Betula, Populus.
Dormancy
Many buds and seeds go through a physiological stage of resting during which rates of respiration are very low and little or no growth occurs. Dormancy is typically "broken" by some environmental que(s). For example, seeds may germinate following warmer soil temperatures or a period of heating induced by fire.
Dormancy defined:
(1) A state of relative metabolic quiescence;
(2) A state in which viable seeds, spores or buds fail to germinate
under conditions favorable for germination and vegetative growth.
The buds and seeds of most temperate woody species require a period
of winter chilling before growth is resumed in the spring. After
shoots fulfill their dormancy requirement, their development in
the following spring is controlled by temperature sums.
Temperature or heat sum: The product of temperature
above a certain base or threshold level (e.g. 0
degrees C) and the time of that temperature. Temperature
or heat sums are usually calculated in a cumulative fashion, and
cumulative heat sum can be used to predict when shoots will begin
to flush in the spring.
Thus, the growth of shoots is highly programmed.
*Shoot flush in spring depends on cumulative heat sum.
*Bud set in fall and leaf drop in deciduous species are regulated
by photoperiod.
The stem provides support for the crown and also transports water, minerals and other substances between the roots and the crown of the tree.
Important structures and concepts:
Cambium: meristematic sheath of cells that surrounds the stem, shoots, and roots.
Xylem: Wood cells inside the cambium that are responsible for conducting water. They are the major support structures in the stem. The xylem elements that conduct water in conifers are known as trachieds; in angiosperms they are known as vessel elements.
Tracheids: Found in gymnosperms, these xylem cells are responsible for transporting water, nutrients, and growth regulators from the roots to the leaves.
Vessels: These xylem cells are only found in angiosperms, and they serve the same function in angiosperms as tracheids in conifers.
Phloem: Part of the inner bark, these 'tubes' conduct the products of photosynthesis, hormones and other compounds between the crown and the roots.
Heartwood: It is the central core of the tree, composed of dead xylem cells, but heartwood also can contain phenolic compounds in some species. The heartwood provides support for the tree and is not involved in any physiological processes.
Sapwood: Surrounds the heartwood, composed of young xylem cells. The sapwood also contains the living, ray parenchyma cells, which are important storage sites for non-structural carbohydrates and nutrients.
Annual Rings: These are composed of layers (rows) of xylem cells deposited periodically by the cambium. There can be considerable annual variation in the boundaries of xylem production. The varying width of the rings can be used to determine changes in climatic conditions (e.g. drought can decrease the width of the ring). Dendrochrology is the study of tree rings.
Bark: The bark of a tree is composed of tissues that lie outside the cambium and have undergone secondary thickening. These cells include the cork cambium, phloem cells, cortex, and periderm (protective tissue). Bark forms with the formation of the periderm outside of the phloem cells.
Here
is an image of tracheids and annual rings on a cross-section of
longleaf pine (Pinus palustris). Click on
photo to learn more.
The root system is of primary importance to the tree, and often, at least 50% or more of the photosynthates produced within the leaves are shunted belowground and used to construct and maintain the roots and their associated mycorrhizae. The root system provides support, is the primary absorption surface for water and nutrients, and can also store carbohydrates. Deciduous and conifer root systems can differ greatly. For example, deciduous species sometimes form a more complex and branched root system than conifers. Also, different environmental conditions (e.g. soil texture, nutrient availability, or presence of a hardpan in the soil profile) can cause the tree to modify the structure of the root system. Another important aspect of tree root systems is the symbiotic relationships between the fine roots and mycorrhizal fungi. Almost all tree species have mycorrhizal fungi associated with their root systems. Mycorrhizae can be very important to tree growth. In return for carbon from the tree, the fungus greatly increases the surface area of the root system and can increase the amount of water and nutrients absorbed.
Coarse roots
These woody roots function primarily as support structures and provide stability for the tree. The coarse root network is comprised of a large tap root with several attached, vertical sinker roots. These roots can become very large and can be long-lived. The coarse, or 'structural', roots are also important sites for storage of non-structural carbohydrates and nutrients during the dormant season.
Fine root system
Fine roots are usually those nonwoody roots <1.0 mm in diameter, and they are relatively short-lived. They have a very large surface area and are the major site for absorption of water and nutrients from the soil solution. Fine root systems can be very extensive and are usually concentrated within the first 20 cm of the soil. Root hairs extending from fine roots can increase surface area and aid in the absorption of water and nutrients.
Here
is an example of the fine roots of sugar maple (Acer saccharum). Click on photo to learn more.
Mycorrhizae
Myccorhizae are beneficial fungi that infect the fine root systems of most tree species. The fungus forms a symbiotic relationship with the tree by increasing the absorption of water and nutrients from the soil in exchange for carbon from the tree. They can also help protect the root system from pathogens in the soil. There are 2 types of mycorrhizal fungi: ectomycorrhizae (EM) and vesicular-arbuscular mycorrhizae (AM). These two types of fungi differ in how they infect the root. Ectomycorrhizae do not penetrate the cell, but instead form an extensive hyphal network between the root cells, and this network of hyphae extends outward into the soil. Vesicular-arbuscular hyphae do penetrate the root cells, and vesicles and arbuscles actually proliferate within the cells.
Here
is an example of ectomycorrhizal fungi on red pine (Pinus resinosa)
roots. Click on photo to learn more.
The growth and development of modules (shoots, roots) can be periodic and very dynamic and can respond to changes in environmental conditions. For example in cool temperate and boreal forests, root and shoot growth follow distinct seasonal patterns. Shoot growth is concentrated primarily in the early spring and ceases by mid-summer. In contrast, root growth starts at about the same time as shoot growth, but roots continue to grow throughout the growing season and cease growth only with the onset of lower soil temperatures at the beginning of the dormant season.
There can also be a trade-off between different modules of the tree. For example, Burton and Pregitzer (1991) found that when sugar maple (Acer saccharum Marsh.) produced a heavy seedcrop, there was a decrease in the amount of leaf area.
Another important aspect of the tree's modularity is the ability of some species to produce ramets (stems genetically identical to the parent tree) during vegetative reproduction. This can increase the amount of growing space a tree occupies, increase the water and nutrients available to the tree, and help prolong the life of the tree, and perhaps in time, increase the success of sexual reproduction.