Operational Adjustment Factors for use in TIPSY

Description for TFL/TSA Data Package prepared by:

Albert Nussbaum, Ministry of Forests, Research Branch (1998)

The objective of this section is to describe what Operational Adjustment Factors (OAFs) are, why they are needed, and how to determine OAFs for planning purposes. To understand why OAFs are needed , one must understand what type of yields TASS generates for use in TIPSY.

The yield tables generated by TASS for use in TIPSY reflect the growth relationships observed in research plots established by the Ministry of Forests and industry. Research plots were generally located in fully-stocked, even-aged stands of uniform site, and in forests with little or no pest activity. The influence of stand density on yield is reflected in the yield tables, but full stocking is assumed. As a result, TIPSY yields reflect the potential yield of a specific site, species and management regime given full stocking.

Why Operational Adjustment Factors (OAFs) are needed

OAFs are applied to potential yields, with inherent assumptions, to make them reflect an operational environment.

In timber supply planning, operational adjustment factors are used to net down potential yields; otherwise yields would be overstated.

OAF Development

Use an OAF for any operational condition that causes yield to be less than the potential reported by TIPSY. Some conditions are influenced by management. The impact of some conditions might change over the rotation, and others may be constant.

To determine the OAFs required to generate operational yields, one needs to understand the assumptions behind the generation of TIPSY's (TASS) potential yields. Then these assumptions can be used as a reference point for yields realized in actual operational conditions. Assumptions relate to specific 'elements' of a stand such as spacing and disease. Here is a list of elements, TIPSY's assumptions about the elements, and conditions that require an OAF.

The default stem map for planted stands (Fig. 1) is nearly uniform while the stem map in Fig. 3 has more variation in planting location. The default stem map for natural stands (Fig. 2) has a random distribution of seedlings covering the entire site while Fig. 4 has a more clumped distribution of seedlings. The planted stand in Fig. 3 produced no significant yield reduction compared to the default (Fig. 1). The yield of the natural stand in Fig. 4 was about 4% lower near culmination of MAI than the default (Fig. 2).

Elements other than these may be evident, requiring additional OAFs.

Type of OAFs in TIPSY

Two types of OAFs are available in TIPSY to account for elements that reduce potential yields. The two OAFs are referred to as OAF 1 and OAF 2. OAF 1 affects the magnitude of the yield curve and is constant across all ages, whereas the impact of OAF 2 accelerates with age. Changing both OAFs affects the magnitude and shape of the yield curve.

OAF 1 - Constant reduction over the entire rotation

Note: For discussion purposes OAF 1 is broken into OAF 1a and 1b. When using TIPSY enter a single OAF 1 adding both the OAF 1a and 1b reductions.

OAF 1a - Portion of OAF 1 not influenced by management

Of the elements outlined above, 'Holes' is best addressed in this OAF category. Small non-productive areas incapable of supporting tree growth must be netted out of the productive land base. The best way to approach this element is to consider how much of your productive land is incapable of growing trees. If it is 10%, then a reduction of 0.10 or an OAF 1a of 0.90 (i.e 1-0.10) is appropriate. This is equivalent to saying "I have 90% of the stand available for growing trees and thus receive 90% of the yield". This OAF 1a adjustment will not change from rotation to rotation as 'Holes' cannot be influenced by management.

The magnitude of an OAF 1a must reflect the level of information you have. If a Silviculture Prescription (SP) is available for a stand, then mapping of non-productive areas will be at a higher resolution than that of a TFL or TSA inventory. The OAF 1a reduction for 'Holes' would be smaller for a stand with an SP than for one without.

OAF 1b - Portion of OAF 1 influenced by management

Elements named 'Espacement' and 'Non-commercial competition' are best addressed in this OAF category. Irregular spacing relative to that assumed in TIPSY will impact yield, although it takes a substantial difference to generate a small OAF 1b reduction (see Fig. 1 through Fig. 4). Yields are affected by irregular spacing only when crowns take a long time to close, as is the case in the clumped stand (Fig. 4) If non-commercial cover competes persistently for site resources, then an OAF 1b can be used. Management practice influences these OAFs; therefore they can change from rotation to rotation.

OAF 2 - Increasing losses towards maturity

Elements named 'Solid wood' and 'Disease and pests' are best handled by OAF 2. The impacts of pests, disease and decay may be small for many years, then increase significantly. These OAFs could be influenced by management practice and thus can vary from rotation to rotation. Efforts to model OAF 2 impacts such as root rot and spruce weevil attack are underway. This will reduce the number of elements that need to be dealt with using an OAF 2 in the future.

The reduction for OAF 2 starts at zero and increases with age. This is in contrast to OAF 1 where the reduction is constant across all ages.

 Table 1. Comparison of both OAFs set at a factor of 0.90 (10% reduction)

* index age for OAF 2 reductions

Notice that the OAF 2 reduction increases by 0.001 per year (1-0.90 / 100 yrs), reaching the 0.90 specified at index age 100. Had an OAF 2 of 0.95 (5% reduction) been applied, it would increase by 0.0005 per year (1-0.95 / 100yrs).

It should be noted that OAF 1 and OAF 2 are applied sequentially. If, for example, OAF 1 is 0.85 and OAF 2 is 0.75, then the combined OAF is 0.64 at 100 years.

Combined OAF=

Historical OAF values

OAF 1 was historically handled by a 15% reduction, or a factor of 0.85 and OAF 2 has generally been handled by a 5% reduction, or a factor of 0.95.

The OAF 1 of 15% was based on the findings of a few studies where the yield of research plots was compared to that of the surrounding stands. No attempt was made, in these studies, to quantify the various elements that made up the reduction. It is likely, however, that the elements 'Holes', 'Espacement', 'Non-commercial competition' and a small amount of endemic ‘Disease and pests’ contributed to the 15% reduction.

The OAF 2 of 5% was based on inventory data quantifying the impact of decay, waste and breakage in second growth stands. No insects and disease problems were part of the 5% reduction.

These historic OAF values of 15% and 5% number can easily be improved with local knowledge, data, professional judgment, or a combination of these and therefore should be used only if no better information exists.

Ongoing projects to refine OAF estimates

It is well recognized that OAF estimates need to be improved. Two projects are underway. The first addresses the development of a methodology to determine an OAF 1a estimate of 'Holes', the non-productive portion within the productive land base, for a TFL or TSA. The method should be available in 1998.

The second project will develop a survey technique to determine a combined OAF 1, both a and b, for the current rotation of a given stand. This will be a stand specific OAF 1 estimate. If the project is successful, it will provide OAF 1 data as regenerated stands are surveyed.

Beyond these projects, special case studies could be carried out if a particular problem is identified in an area. An example of this might be root rot in fir stands. This could lead to the creation of a special case OAF separate from those listed above.

Until more information is available on OAFs, a great degree of professional judgment is required.

Other potential yield models

All models calibrated on potential yield data require some form of operational reduction. Examples of other models requiring adjustment are the Forest Projection System (FPS), Stand and Tree Integrated Model (STIM) and Y-Xeno.