II.B._TREE AUTECOLOGY: The Concept of Environment
Note about the assigned reading:
Kimmins’ textbook organizes the information on tree autecology into chapters on separate plant resources or environmental factors (i.e. solar radiation, temperature, wind, water, and soil-related factors). For each of these chapters you should be able to answer the following questions:
1. To understand the variation in the availability of each resource. How does availability vary spatially and temporally?
For example, in what manner does solar radiation available to a plant vary? Does it vary in intensity, duration, and/or quality (wave lengths)?
2. To know the general significance of the resource to the plant. How does variation in the quantity or intensity of the resource affect plant growth and survival?
For example, why do plants need water? What are the consequences of too little or too much water?
3. How are plants adapted to different levels of the resource? How are plants adapted to obtain enough of a particular resource or to tolerate excessive amounts or intensities of a resource?
For example, what plant adaptations prevent injury associated with high temperatures?
--THE CONCEPT OF ENVIRONMENT--
1. Approaches to the analysis of a plant's environment.
A. Holocoenotic approach.
The holocoenotic approach to a plant’s environment stresses that a plant responds simultaneously to all environmental factors that impinge upon it. This is a holistic approach to the environment in the sense that it recognizes that the environment as a whole affects the plant.
Holocoenotoic is derived from holocoen a term coined by the marine ecologist Karl Frederick in the 19th century to emphasize the interdependence and interconnections of organisms with their environment.
In forestry the concept of site (see Kimmins pages 363-364) is a holistic or holocoenotic approach to the tree’s environment.
The practical value of the holistic approach is that it reduces the chances that we will over-emphasize a single environmental factor in trying to explain plant distributions or plant performance. An example of an old-fashioned single-factor approach in ecology is C.H. Merriam’s life zone classification based exclusively on temperature.
B. Factorial approach.
In a factorial approach to the plant’s environment (such as the Concept of the Operational Environment described below) we identify a number of parameters (i.e. environmental factors) to try to better understand the relative importance of different aspects of the environment in determining plant distribution and performance. For example, foresters often identify numerous site factors (e.g. elevation, soil depth, precipitation, etc.) in their attempt to quantitatively describe the concept of site.
Although at first glance the holocoenotic and factorial approaches may seem contradictory, actually they are compatible approaches. We will use a factorial approach in discussing the plant’s environment, but the holocoenotic approach will remind us that the plant actually responds to all environmental factors. This is important in reminding us that individual factors (e.g. temperature, moisture, etc.) are not independent variables (i.e. they interact among themselves).
2. Concept of critical factors
Whenever an environmental factor (resource) approaches a threshold for the growth, survival, and/or reproduction of a plant, its relative effect on the plant is critical .
PLANT RESOURCES AND THE USE OF THE CONCEPT OF THE OPERATIONAL ENVIRONMENT
General resources required by plants are: light, heat, carbon dioxide, oxygen,
water, and mineral nutrients. When we consider the environment as both providing these needed resources and presenting a range of conditions that the plant must tolerate to survive, we will use the factorial approach of the Operational Environment.
The Operational Environment
Definition: The operational environment of a plant includes the environmental factors which directly impinge upon a plant. This concept allows us to factor or breakdown much broader concepts such as Aclimate@ or Asoil@ into numerous factors which may be critical to the growth, survival or reproduction of a plant. Although most of these factors are interdependent (e.g. solar radiation affects air temperature), they may be measured separately.
In the field, the factors of the operational environment serve as a checklist to remind you which aspects of the plant=s environment may be critically affecting that plant at a particular site and time.
I. Abiotic factors (i.e. physical-chemical factors)
A. Mainly climatic factors
1. Solar radiation
2. Air temperature
3. Atmospheric gases and particulate matter
4. Atmospheric humidity
5. Mechanical force (for example, wind)*
B. Mainly edaphic factors (i.e. soil-related)
1. Soil moisture available to plant roots
2. Soil temperature
3. Soil chemicals in liquid and solid phases
4. Soil gases
II. Biotic factors
1. Symbionts
2. Pathogens
3. Herbivores
4. Direct human action
Should competition be included as a factor in the operational environment?
1. General definition of competition
Competition is an indirect interaction between two or more plants involving their utilization of a common resource in short supply and resulting in mutually adverse effects.
2. Definition of competition in the context of the operational environment:
Competition is the modification of one plant's operational environment by a second plant so that the operational environment becomes less favorable to the growth, survival and/or reproduction of a second plant. This definition stresses the mechanisms by which competition occurs.
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How to apply the concept of the operational environment:
1. At high elevations on south-facing slopes, regeneration of Engelmann spruce often fails after logging. What factors of the operational environment are likely to play a critical role in this regeneration failure?
2. On north- versus south-facing slopes at 7000-8000 feet elevation in Boulder Canyon, forest type and structure are typically different. Dense stands of Douglas-fir occur on the north-facing aspects, and open stands of ponderosa pine occur on the south-facing slopes. How do variations in factors of the operational environment probably explain these differences?