Apical control - regulation of the elongation of lateral branches by the terminal (leader) over many seasons.

Apical dominance - regulation of elongation of laterals by the terminal on a current season's growth (or current flush). Apical dominance can be weak or strong. It represents a localized, short-term level of regulation. With apical dominance, there is active involvement of chemical signals from apical meristem to laterals.

Asexual reproduction - vegetative reproduction as opposed to sexual.

Bark Cambium - a layer of meristematic tissue found outside of the phloem in the bark of trees. Responsible for the pruduction of bark tissue. A secondary meristem.

Cambium - a layer of meristematic tissue found between the phloem and xylem of a stem and responsible for the production of xylem elements, phloem elements and ray parenchyma. A secondary meristem.

Cavitation - Because water is pulled from the soil to the leaves to replace what evaporates through stomata, the water inside the xylem conduits (tracheids and vessels) are under tension or a negative pressure (remember the atmospheric pressure outside the tree is positive). At pressures below vapor pressure, liquid water is in a "metastable" state and vulnerable to rapid transpition to the stabel vapor phase (cavitation). The result is a gas-filled (embolized) conduit that does not conduct water (see Sperry, J.S. 1995. Limitations on stem water transport. In: Plant Stems (B.L. Gartner, ed.), Academic Press, San Diego). Cavitation or air entry may occur with drought or with freezing temperatures. With drought, the magnitude of tension in the conduits is critical and plants can close stomata to prevent conduits from reaching critical tensions. There are some species differences in the sensitivity of conduits to cavitation with species growing in wet to mesic habitats being more sensitive than those growing in dry habitats. With freezing temperatures, two things are important: (1) the number of freeze-thaw cycles and (2) the diameter of the conduits. Large diameter conduits (e.g., large diameter vessels are less likely to become functional again after a thaw than smaller diameter conduits). For that reason, the table below helps organize one's thoughts about the tradeoff between conduit efficiency and safety.

Characteristic	Tracheids - all plants have tracheids, but gymnosperms have only tracheids	Vessels - Diffuse-porous where examples include red alder, bigleaf maple, etc.	Vessels - Ring-porous where example include the oaks, hickories, elms, etc.
Efficiency	Size: 25 - 60 µm Rate: 0.25 - 1.0 m/h	Size: 80 - 180 µm Rate: 1.0 to 8.0 m/h	Size: 150 - 450 µm Rate: 5 to 60 m/h
Safety	Very	Moderate to very	Low to moderate

Decurrent - refering to the development of a tree's crown, a central stem is not present

Endodermis - A specialized layer of cells found in the primary root. This layer of cells separates the inter vascualr tissue from the outer cortical cells. The endodermal walls contain the Casparian strip which is composed of suberin, a wax. This wax layer prevents apoplastic water and solute flow along the cell walls and forces water and solutes to take a symplastic pathway between the outside and inside of the root.

Epicormic shoots - shoots (branches) derived from dormant buds in the stem. Typically forms part of a recovery mechanism for a branch or the stem when damaged (mechanically or by an insect). There must be sufficient light to stimulate the release of these buds. Can be a problem in pruning and thinning operations especially with Douglas-fir and Sitka spruce.

Excurrent - refering to the development of a tree's crown, a central stem is present.

Fine roots - The greatest percentage of fine roots are found within the top 15 cm of a soil profile although fine root growth can be observed at much greater depths when surface conditions are sub-optimal. Because of the competion between fine root growth and aboveground growth, fine root growth frequently occurs under conditions less than optimal for root growth. Depending upon the species and environmental conditions, both high and frequent turnover of fine roots can be expected.

Fixed Growth Form - Tree species such as Douglas-fir, Pacific silver fir, white oak, ponderosa pine have over wintering or resting buds which contain all of the future foliar units.

Free Growth Form - Tree species such as red alder, black cottonwood, western hemlock have over wintering or resting buds which contain only a partial number of the foliar units which will unfold the following spring.

Genotype - the genetic makeup of an organism

Hormones - or plant growth substances, a series of compounds, which are produced in specific tissues and are usually present in fairly low concentrations, responsible for the control of plant growth and development (abscisic acid, auxin,ethylene, cytokinins, gibberelins, etc.)

Integration - Both form and function are integrated through space and time. Use the concept of scaling to understand integration (Figure below)

Juvenile - the stage of development during which plants (trees) are not able to reproduce (generally). Characterized by fast growth and often morphological features which are quite distinct. Important in management for cuttings, tissue culture, etc. Hedging is a technique used to maintain trees in a juvenile form.

Mature - The stage of development during which plants (trees) are capable of reproducing.

Meristem - perpetually embryonic tissue capable of regenerating itself plus producing the other tissues in the plant. There are primary and secondary meristems.

Module - module is made up of a leaf, an axillary bud, and the stem section to which they are attached).

Phenology - the specific timing of growth events during a year.

Plant growth substances - see hormones

Phenotype - As the genetic material (genotype) interacts, during plant growth and development, with the environment, a particular individual results.

Prolepsis - lateral buds which do require a rest or dormant period before they elongate

Roots - Roots serve five major functions: they help support and anchor a tree, they are involved in the storage of carbohydrates and other nutrients, they are critical in the absorption of water and nutrients, they form the major link between photosynthetic carbon capture and much of the microbial population belowground, and they, as they grow (or do not grow), produce hormones that can affect aboveground processes.

In order to understand root growth, one must be able to visualized the distribution of roots belowground. This visualization is analogous to examining a crown of a tree and then by using a number of rules of crown development, describe how that crown came about. Unfortunately, it is much more difficult to visualize a whole range of root forms, and the number of rules involving root growth are far fewer than for the crown of a tree. The figure below illustrates the coarse and, to a less extent, the fine roots of a tree. The coarse root system has two parts: a vertical component composed of tap and sinker roots and a horizontal component. Originating along these two components are the fine roots. Three factors appears very important in determining the shape of the root system: Genetics, the soil environment and the length of time a plant has had to develop a root system.

Figure . Idealized diagram of the coarse and fine (insert) system of a tree's roots (modified from Köstler et al., 1968). a. Stem-root colar, b1. tap root or a vertical, skeletal root, b2. Another vertical skeletal root, a sinker root, b3. Horizontal skeletal root, c1 and c2. Fine roots originating from horizontal or vertical roots, respectively, the lateral fine root shown in the insert is infected with mycorrhizae, and d. Root hair.

Below is illustrated a drawing of the actual root system of a middle-aged true fir (Abies spp.)

Figure. A drawing of the root system of a true fir (Abies alba Mill.) (after Köstler et al. 1968). The root system shown is for a tree between 70 and 100 years old.

Root meristem - At the tips of all roots is a meristematic zone. The root meristem is located in the apex of a root and this region is different from the apex of a shoot in a number of different ways. There is a root cap protecting the apex. Three tissue types are produced by the root meristem: epidermis, cortex and vascular tissue. As the root begins to differentiate, the vascular tissue is found in the center, a layer of specialized cells (the endodermis) separates the center from the outer cortical cells. Surrounding the coritical cells is the epidermis. If the root is infected with mycorrhizal fungi, then there are no root hairs emerging from the epidermis and the root cap is either absent or greatly reduced. Most horizontal roots are infected whereas almost no vertical roots are

Scaling - It is important to define both scaling and integration. When one examines the hierarchical structure of biological systems (i.e., scales or levels such as the organelle are characterized by being small and having processes that occur rather rapidly. In contrast, watersheds are larger, have processes that occur over much longer time periods), one finds that physiologists, whether working on agronomic or ecologically important species, tend to focus on biological processes at the organ or associated sub-levels. In contrast, ecologists focus on whole organisms or groups of organisms or even landscapes. This is where socially-relevant and evolutionarily-important issues are played out. For example, issues of global climatic change, air pollution, plant yield and management, all tend to be at canopy, ecosystem or higher levels of biological structure. In addition, issues of evolutionary importance, such as how do traits with high survival value (e.g., water-use efficiency) become fixed and permit adaptation to variable environments, can only be addressed at population levels. Scientists tend to integrate or scale to these higher levels through models, the use of indicators (e.g., stable isotopes, remotely sensed parameters), by summing all the parts, or by actual measurement. Each of these steps has its own set of weaknesses. Integration may be defined as the summation of a process of interest, which has been measured at a particular space-time range, to a greater time-space range. Scaling is defined as the mechanisms by which a change, whose effect occurs at a lesser time and space scale, is propagated up through scales or levels of biological organization of increased space and time. For example, when relative humidity decreases during the afternoon and stomata close in response, how does this change affect the loss of water from the plant canopy to the atmosphere? Integrating one would simiply multiply the average stomatal conductance by the leaf area of the stand, whereas in scaling one would also consider the temporal and spatial variation in leaf conductance, and the effects of canopy structure and wind speed on canopy boundary layer conductance.

Although important and revealing issues emerge at each scale of biological organization, it is critical to retain the perspective of Allen and Hoekstra (1992. "Toward a Unified Ecosystem." Columbia University Press, New York. 384 pp.): "For any level of aggregation, it is necessary to look both to larger scales to understand the context and to smaller scales to understand mechanism; anything else would be incomplete." I have tried to illustrate some of these concepts in the table below:

Structure or Level of Biological Organization	Process under study	Human impacts on	Consequences
Leaf	Carbon exchange: Including photosynthesis and respiration	Elevated CO2 Higher temperatures Ozone	Increased Productivity Decreased Productivity Decreased Productivity
Leaf	Water loss or transpiration	Elevated CO2 Higher temperatures Ozone	Stomatal closure & decreased water loss Increased water loss Stomatal closure or damage
Whole organism or indvidual tree	Carbon balance Water loss Sensitive Genotype	Same as above	± Productivity ± Water loss Elimination
Species	Population dynamics Natural Selection Competition	Elimination or introduction of exotic species Altered selective pressures	Competition, pest Altered biodiversity
Community	Succession Disturbance Composition	Climate change Air pollution	Change in rate or pattern Increased fire, floods, etc. Loss of species, links, keystone species
Ecosystems	Net productivity Nutrient cycling	Climate change Air pollution	Loss of productivity Increased nutrient losses

Shoot meristem - At the tip of all shoots and branches is a meristmatic region; however, there may be either latent (visible) or dormant meristematic regions in the axils of all current and former foliage units. When you can see the region is called a bud and it may be either a covered (i.e., with bud scales) or a naked bud. Within the bud is the meristematic cells and within the resting bud are either a partial complement of future foliar units (free growth form) or the entire complement (fixed growth form).

Stems - The stem of the plant links the roots, branches and leaves. The stem provides support and conducts water, plant growth substances or hormones and nutrients upwards in the xylem and carbohydrates, nutrients and plant growth substances upwards and downwards in the phloem. The stem is produced by a secondary meristem called the cambium.

Syllepsis - lateral buds which do not require a dormant period to elongate. They grow out in the year they are formed.

Tracheids - a vascular conducting element found in gymnosperms and angiosperms. Diameters range from 30 to 80 µm with Thuja having the smaller diameters and Larix having the larger. Water moves laterally from one tracheid to the next through bordered pits.

Vessels - a vascular conducting element found only in angiosperms. Diameters range from 80 to 180 µm in diffuse-porous species (e.g., red alder) to 150 to 400 µm in ring-porous species (e.g., white oak). Water moves from one vessel to another through either perforation plates (image below was provided by Professor Paul Schulte, Ph.D. UW 1985, Biological Sciences, UNLV) in the end wall or open end walls.