Stand density decision making requires a thorough understanding of the concepts of stand development, the rigorous application of appropriate economic and investment principles, and careful consideration of the objectives of the forest owner. These biological, economic and forest-level factors must be integrated by the prescription writer to determine the need and opportunity for density management, and the priority of stand-specific treatments. The resulting prescription is a co-ordinated, stand-level plan aimed at meeting specific forest management objectives.
This document advocates a knowledge-based, structured, decision-making approach. A decision framework is described which helps to organize the decision process for the prescription writer. The following concepts, principles and considerations are fundamental components of the decision framework. They provide information useful in all aspects of stand density management planning, and all practices including initial espacement, and pre-commercial and commercial thinning. The principle focus of this guideline, however, is pre-commercial thinning within the context of timber production.
Biological concepts of timber production
Professional experience, scientific data and mathematical models support the following general biological relationships between density management and timber production. A more detailed discussion of each relationship, and possible exceptions, are provided in the main body of the document.
- Increasing establishment density elevates harvest volume and reduces biological rotations, but at a rapidly diminishing rate - particularly at high densities.
- Repression of height and/or volume increment may occur in lodgepole pine stands established at high initial densities. The impact of repression on height and diameter increment is evident in stands with densities as low as 10 000-20 000 sph, beginning before trees reach 2 m in height.
- Pre-commercial thinning may prevent repression and loss of volume production if carried out when trees are less than 2 m in height; repressed stands taller than this have been found to respond erratically to juvenile spacing.
- Stands regenerated or spaced to relatively high densities (e.g., 1500-10 000 sph) have small differences in volume and diameter at harvest.
- Stands regenerated or spaced to relatively low densities (e.g., 250-1000 sph) have, at harvest, larger piece sizes because more growing space is available to each tree, lower volumes per hectare because of slow site occupancy following treatment, and longer biological rotations. That is, there is a trade-off between tree diameter and stand volume which is most clearly reflected in stand and stock tables, as opposed to volume per hectare and average diameter. The diameter benefits of spacing diminish the longer treatment is delayed beyond crown closure.
- Commercial thinning moves future yields forward in the harvesting sequence, but does not greatly affect the total (thinnings + final harvest) volume. In general, a sequence of frequent, light, low entries can increase total yield if merchantability limits are very low. Infrequent, heavy entries can decrease yield substantially, particularly if concentrated in the upper crown classes long before the final harvest.
- Increasing stand density improves wood quality by influencing the development of smaller knots, narrower annual rings and greater lumber strength.
- The yield of mixed-species stands is usually intermediate between pure species of the same density when trees of a second species are substituted for trees of the first species. Stand yields of mixed species may increase if trees of the second species are added to the stand, thereby increasing stand density, or if they exploit resources not utilized by the first species.
- Density management practices influence other forest resource values (e.g., wildlife habitat, livestock and wildlife forage, visual quality, water) and may interact with natural stand processes, such as the susceptibility to pest damage.
- Stand density decisions should incorporate predictions of stand response. Any growth and yield decision aids used in the analysis should be consistent with the biological concepts of timber production.
A stand-level economic analysis must be undertaken to determine the relative efficiency of proposed treatments or treatment regime options. Generally, unless the harvest ages are identical between treatments, the soil expectation value (SEV) should be calculated for each option. The option with the highest SEV at harvest should be selected.
Because of the usually lengthy time period between density management treatments and final harvest, current utilization limits, product values, timber market conditions and harvesting costs may not be useful. Key elements and variables in stand-level economic analyses include:
- stand models and data sets that are consistent with the biological concepts of timber production
- anticipated product (log, lumber or chip) values at the time of harvest, including any anticipated future real price increases (these values and prices should be based on estimates of future product markets)
- harvest costs anticipated at the time of future harvest, 30 to 80 years hence
- silviculture treatment costs, if comparing rotations of different length
- selection of an appropriate social discount rate
- a sensitivity analysis of silviculture options.
Forest planning considerations
All stand density decisions, whether for timber or non-timber objectives, should be consistent with forest-level objectives (such as those identified in TSA or TFL management plans). Without a forest-level context, local stand density management interventions may result in:
- costly silviculture investments with little or no forest-level benefits
- cumulative forest-level effects that exacerbate a forest management problem
- large opportunity costs if actual forest management problems are ignored.
It is recommended that identification of silvicultural opportunities that help meet forest-level objectives follow a two-step process:
- obtain and assess existing forest-level analyses
- conduct additional forest-level modelling analyses.
The first step is essential. Completion of the second step may not always be possible, nor is it always necessary.
The decision framework and process
A stand density management decision framework is described which consists of assessing management objectives and determining strategic opportunities and tactical options. These elements are interdependent and hierarchical. Management objectives represent the highest level decisions, upon which strategies and tactics depend.
Within this decision framework, a decision process is presented which defines the appropriate sequence of queries and analyses. This process will assist planners and silviculturists in identifying appropriate density management strategies, and biologically and economically feasible stand-level prescriptions.
Decision support tools
- A number of stand growth and yield models and decision aids are available that are based on the biological concepts of tree and stand production. These tools are useful for predicting the response of stands to density manipulation (espacement, juvenile spacing, and commercial thinning). Appendix 2 "Tactical analysis and design" provides an illustration of the benefit and use of a stand-level model (TIPSY) in evaluating the suitability of a particular density management action.
- Various forest estate models may be used to estimate the flow of goods and services from managed forests. They may be used to compare the supply of timber and other commodities and amenities resulting from different stand management interventions.
- Computer models, which incorporate accepted economic principles, simplify the procedures of financial and sensitivity analyses.
Users of decision support tools should always be familiar with the specific capabilities and limitations of the models they use. Of particular importance is an understanding of the quality and quantity of data used in the analysis.
The language of this document represents terminology from the disciplines of forest science, economics and planning. A glossary is provided to assist readers in understanding unfamiliar terms or usage.