Introduction to trees, stands, communities and ecosystems. Processes and lingo

I. Trees - in this case, we treat tree as the whole-organism.

A. Made up of parts (these may be organs [leaves, stem, roots] or tissues [xylem, phloem], etc. Leaves are responsible for gas exchange including carbon dioxide (CO2) and oxygen for light capture and conversion to carbon products via photosynthesis and water vapor via transpiration. Stems and branches provide the architectural support fot the leaves and connect the leaves to the roots. Stems and branches have two major tissue types, xylem and phloem. Xylem conducts water and nutrients from the roots to where needed (most often the foliage). Phloem conducts nutrients and carbon products both up and down in the plant. Roots are responsible for anchoring, supporting, and acquiring water and nutrients.

Idealized two year old black cottonwood tree showing stem, branches, leaves and roots. Two different type of branches are shown: (1) sylleptic branches grow without a pause or rest whereas (2) proleptic branches rest (usually over-winter).	Douglas-fir shoot showing main axes (solid white line) and lateral branches (dashed line). Along the main axes and branches are buds (future branches).

B. Parts react to environment

1. What is the environment of a plant or a tree: Includes the resources of light, water and nutrients, it includes factors that affect the ability of a plant to utilize these resources (e.g., temperature, wind, snow, ice, soil oxygen, pollutants, insects, pathogens, etc.). One can think of the environment has a long laundry list of factors and these factors vary temporily and spatially or one can think of the environment as a composite of features.

2. Sum (mostly) of the reaction to the environment = the reaction of the whole

3. Operational environment: the environment normally "seen" by an organism and to which the organism responds positively or negatively. When some individual or component part of this environment becomes limiting, a stress results (e.g., temperature or drought stress)

4. Plants are characterized by an inherent ability to withstand harsh environments (viewed as the stress [brick] placed on the wire [plant which is now under strain] in the figure below. If the environment is harsher (or the stress greater) then picture B. If too harsh, then C. But plant has two options (can avoid stress, D) or tolerate stress (E).

5. Response of individual to stress, environment is dictated by genotype and the individuals previous history.

C. Types of stress

1. Drought and its definition (see definitions of stomata and water movement below)
2. Temperature (cold or hot)
3. Wind
4. Nutrient
5. Light
6. Air pollution
7. Salt

D. A plant's reaction to the environment changes with

1. Time of day
2. Season
3. Stage of Development

E. The important physiological processes occurring in a tree are: growth and development, water uptake and loss, nutrient uptake, use, re-translocation and loss, carbon fixation (photosynthesis), use (respiration, metabolism) and circulation (translocation or allocation).

F. Ultimately, it is the individual organism (trees) that reacts to climate change, ozone, drought, etc.. The individual is the unit upon which natural selection acts.

G. Examples

Illustration of environmental stress (drought) or resources (water). Figure above illustrates a juniper-pinyon pine woodland in New Mexico. Please note how the trees are distributed on the slope (variable density, with the greatest density in the draws). Trees are located in the draws because there is more moisture. Go to the following link to see another way droughts are detected and monitored (http://earthobservatory.nasa.gov/Drought/NAmer_anomaly_200008.html). Droughts typically develop in the United States during the summer -- in western US a combination of warm, long days and low precipitation contribute to the formation of a drought whereas in the eastern part of the US, summer rainfall can compensate for the warmer, longer days of summer (see animation of the drought of 2000; to stop the animation, click on the map of the US, to resume, hit return).

H. In addition to stress, trees (stands, ecosystems, landscapes, regions, etc. can be affected by disturbances or the lack of disturbances. Illustration of what happens when a disturbance factor is changed (fire suppression). Figure below illustrates an eastside Ponderosa pine stand. Before fire suppression, most of the understory trees would not be present (i.e., much more open in appearance). The following is excellent site describing the fire history of the SW: http://geochange.er.usgs.gov/sw/impacts/biology/fires_SOI/.

II. Stands

A. Define - localized group of plants with very similar characteristics (age, species, structure).

B. Composed of plants, animals, microbes, invertebrates, individual organisms and species.

C. Stages of Development (see book by Oliver and Larson): stand initiation, stem exclusion, understory re-initiation, mature, old-growth.

D. Stand structure

E. Types of Stands: Even and uneven aged.

F. Important processes: stand development or succession, competition, gap dynamics, disturbance, natural regeneration vs. artificial regeneration.

III. Communities

A. Definition: The assemblage of plants and animals living together in a common environment. Lists and definition of "common environment." Often defined by likely dominant climax overstory and understory species. (Populations of organisms of different species that interact with one another. )

B. Potentially only transitory.

C. Stages of succession: pioneer, climax

D. Important processes: succession, competion, disturbance, reproduction, establishment, dispersal, herbivory, keystone species.

IV. Ecosystem (Any geographic area with all of the living organisms present and the nonliving parts of their physical environment. Involves the movement and storage of energy and matter through living things and activities. )

A. Tight definition: a stand or community at a particular stage of development/succession. Old-growth ecosystem, plantation ecosystem.

B. Very loose definition: forest ecosystem. Study question: Why is this weak?

C. Important processes: Energy capture, storage, movement, exchange, water cycling, nutrient cycling. Usually process oriented.

D. Important difference: ecosystem management vs. ecosystem based management. Study question: What is difference?

Definitions

Biodiversity - The range of variation found among microorganisms, plants, fungi, and animals. Also the richness of species of living organisms. (see web page maintained by Ecological Society of America: http://esa.sdsc.edu/biodiv2.htm)

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 (see Figure below under tracheid)	Vessels - Diffuse-porous where examples include red alder, bigleaf maple, etc. (see Figure below under tracheid)	Vessels - Ring-porous where example include the oaks, hickories, elms, etc. (see Figure below under tracheid)
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

Gene - A unit of inherited material. An organism's collection of genes determines what it is, what it looks like, and often how it behaves.

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.)

Meristem - perpetually embryonic tissue capable of regenerating itself plus producing the other tissues in the plant.

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

Needle - a unit of foliage.

Upper left is a cross-section through a conifer needle. Around the needle are a group of cells, the epidermis, that are covered with wax. This wax reduces water loss. Occasionally along the surface of the needle, one sees pores or stomata (see upper right, stomata on the surface of a black cottonwood leaf). Stomata can open or close, stomata control gas exchange between the atmosphere and the inside of the needle. Within the needle are cells and within the cells are organelles. Chloroplasts (contain chlorophyll) are the organelles in which photosynthesis occurs. Lower left is a diagram of part of one of the membrances inside a chloroplast showing some of the structures responsible for the light reaction part of photosynthesis.

Organism: An individual living thing

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

Phloem - a group of living cells that in woody plants is outside of the cambium (cambium is a meristematic tissue that produces xylem to the inside, that is the annual ring of wood growth) and phloem and bark to the outside. Sieve-tube members are associated with companion cells (see diagram below) and the sieve-tube member is responsible for moving food (from storage or photosynthesis) up and down the plant.

Sieve-tube member and associated companion cell. These cells would be located in the middle circle to the right (labeled transport-storage).	An idealized plant system showing foliage, branch, stem and roots. At three levels, additional detail is shown within circles. Bottom(close-up of the root showing the path of water movement from the soil particles to a root surface and into the root), middle (stem radial view with the bark to the outside, xylem with tracheids to the inside; between the xylem and the phloem is the cambium).

Population - A group of individuals belonging to one species living in an area.

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.

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

Species - A group of populations of similar organisms that reproduce among themselves, but do not naturally reproduce with any other kinds of organisms (e.g., Haliaeetus leucocephalus--bald eagle; Quercus rubra--red oak tree).

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. Water is transported in the stem under tension. When the tension becomes too great or the water in the stem freezes, then cavitation can occur. The stem is produced by a secondary meristem called the cambium.

Stomata - Pores in the surface of leaves and needles by which gas exchanged between the atmosphere and the inside of the leaf (two major gases are water vapor and carbon dioxide; three processes are transpiration and photosynthesis and respiration, respectively). See Movie

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.

Tracheids - dead cells that conduct water and soil nutrients up the plant (from roots to foliage). Each tracheid has a closed end wall so that water has to go through the small pits or holes in the walls.

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.

Water movement - water moves from the soil to the atmosphere following the cohesion-tension theory (see movie).