Forest Measurement and Modelling.

Introduction In forest mensuration, the term site generally refers to the:
totality of environmental conditions that exist at a specified location.
Site is an abstract concept which combines a multitude of environmental factors affecting tree growth into a unified classification.

Diagram of factors affecting site

The environmental factors that influence growth include:

  1. soil factors, e.g. physical and chemical properties, soil moisture, soil microorganisms;

  2. topographic factors, e.g. slope, elevation, aspect;

  3. competitive factors, e.g. other trees and lesser vegetation, animals, man.

  4. climatic factors, e.g. air temperature, humidity, radiant energy, precipitation, wind;
Defining and mapping site (survey and classification) is useful for several reasons:
  • Estimates of site can be used to identify land that is more (or less) appropriate for different uses. Hence better land use policies and practices are encouraged.

  • Management planning can be improved when homogeneous land areas are delineated on maps. Better documentation of these areas will also reduce the potential loss of information that follows the movement or retirement of experienced field staff.

  • Estimates of yield can often be improved if the quality of the site is known.

Site Quality Site Quality is used as an indicator of site when a specific management objectve is being pursued. It may be defined as:
an indicator of the productive capacity of a specific area of land for a particular species.
In the context of timber management, site quality can be defined as:
the potential of the site to produce timber given a particular species or forest type.
Site quality is not easy to assess. The factors of the site and the plants themselves are interacting and interdependent making it difficult to assign cause and effect relationships. There is no single measure of site quality that has been found to be entirely satisfactory.

Some methods used to describe or measure site quality (Based on Frankcombe, 1997)

Description Advantages Disadvatages
Soil Evaluation Classification of a site using soil characteristics. Individual or collective soil properties that correlate with site qualiy are chosen and measured. Permanent (soil depth and texture, etc.) and dynmic (nutrient evels, structure, pH, etc.) soil charactersitics can both influence site quality. Looks at causal factors of site productivity Expensive, time consuming and difficult to carry out
Climatic Evaluation Based on the assumption that climatic factors like rainfall, temperature, length of growing season, wind and light all affect plant survival and growth and are therefore related to site quality. Easy to obtain data from external sources (Met. Bureaus, etc.) Accurate data not typically available. Arbitrary ratings often used
Topographic Evaluation Based on the assumption that growth can be correlated with landform variables like aspect, gradient, position on the slope, length of slope and the geometric shape of the site. The Terrain Shape Index (TSI) is one example of using topographic measurements to correlate with site (McNab, 1989). Accounts for local variation Can be complex to carry out
Stand Pognosis A forecast of stands which is based on vegetational and environmental features in a model. These features can include plant density, topography, habitat type, soil type, moisture, soil depth and humus content. Good to forecast stand growth Requires alot of data which is exensive and time consuming to collect
Wood Production Timber volume at a given age or mean annual increment as an integrated expression of all biological and environmental influences. Typically well researched. Relatively easy to measure Problems associated wih measuring tree volumes. Trees must already exist on the site
Tree dimensions / size Based on the assumption that trees grown on a good site will be bigger (eg taller, larger average diameter, greater natural basal area, etc.) than those on a poor site of the same age or structure. Widely applicable. Easy to measure. Intensive measurements may be costly
Species Composition Indentification of native vegetation (or even weed species) which are indicative of site characteristics. Related to causal factors and very sensitive Requires skilled people. Restricted to simple composition forests.
Ordination An arrangement of species composition ordered against a gradient such as climatic conditions or parent material. Simple to implement. Integrates vegetative and environment factors Subject to an understanding of environmental interactions
Stand Appearance A description of characteristics like general vigour, form, crown density, needle length and colour, bark colour and tightness, green level and canopy formation. Easily carried out Subjective
Relative Stocking Index A ratio of measured stand density in a fully stocked stand to the predicted mean tree size. (See Berguson et al, 1994) Close association with causal factors Requires fully stocked stands
Site Index The mean height of a specified number of dominant (and co-dominant) trees at an index age. The number and selection of trees and the index age varies. See {Code of Forest Mensuration Practice} for details. Easily implemented Only suitable for even-aged stands of known age
Site Form The average or modelled height of trees with a nominated reference diameter at breast height. (See Vanclay and Henry, 1988) Easily implemented Intensive measurements may be costly
Modelling using Geographic Information Systems Use the data handling and modelling capabilities of a GIS to extrapolate site quality measurements from areas with similar environmental conditions. Fast and efficient means of managing and interpreting data Cost and difficulties with use of many current software systems
Remote Sensing Determine the amount of intercepted photosynthetically active radiation. The more radiation intercepted and used efficiently, the higher the productive capability or site quality of the land. Able to cover large areas effectively and efficiently Cost of data collection and analysis software. Current wariness of true ability

Site Quality Maps Accurate and reliable site maps are essential under intensive forest management. These maps are the basis of:
  • Evaluation of land for purchase.

  • Siting of species.

  • Forecasting the productivity capabilities of planted stands (yield forecasting and yield regulation).

  • Forecasting thinning, pruning, fertiliser application and other tending and protection strategies.

When site maps are used to determine thinning and related logging priorities, they must be made before the time of first thinning, i.e. at age approximately 9-10 years.

[temp.htm] Revision: 6/1999