|GUIDELINES for||. . .|
|Developing Stand Density Management Regimes|
Tables A2-3 and A2-4 display some of the information generated by TIPSY for the coastal and interior scenarios, respectively. Yields are compared at a constant age which is close to the physical rotation ages given in the second section of the table. Economic analyses for coastal hemlock and interior pine are based on the log and lumber markets, respectively. To see a discussion of minor limitations regarding TIPSY's ability to differentiate between H- and I-grade logs, go to the online documentation, use the `Search for Help On...' option and type in `log grades.' The following summary applies to the pre-commercial thinning (PCT) of hemlock (Hw) or pine (Pl) or, if unspecified, both in these examples:
Table A2-3. Tactical options analysis for four western hemlock stand density regimes
Table A2-4. Tactical options analysis for four lodgepole pine density regimes
In summary, the biological response of the stand to pre-commercial thinning slightly increased the harvest revenue. However, the economic analysis shows that the site value did not increase enough to cover treatment costs when all revenues and costs were discounted to a common point in time. This does not mean that spacing can not be undertaken to meet other management objectives (e.g., maintenance of wildlife habitat).
Rotation ages in Tables A2-3 and A2-4 vary widely depending on the regime and criteria selected. However, the relationships of MAI to age and treatment (Figures A2-1 and A2-2) show that volume production is quite insensitive to both in this range with the exception of the heaviest treatment. TIPSY can draw these figures if each regime file is run in steps of five years from 0 to 160 (Hw) or 120 (Pl) years prior to plotting "MAI (merch) 12.5+" over "Age." Notice that thinning impairs volume production of coastal hemlock until the site is reoccupied, after which it catches up with the untreated stand. Recovery time is related to the intensity of thinning and site productivity. The situation with pine is more complex because utilization limits have a great impact on yield, particularly when the rotations are short.
Figure A2-1. Mean annual increment over stand age for four coastal western hemlock density regimes.
Figure A2-2. Mean annual increment over stand age for four interior lodgepole pine density regimes.
Similar insight into the dynamics of stand development can be achieved by plotting other variables over age (e.g., volume, diameter, number of trees and prime tree statistics). It is also informative to display number of trees (stand table) and volume (stock table) by diameter classes. TIPSY-generated stand tables at age 100 (Hw) and 70 (Pl) show that the unthinned stand has many more trees in the smaller diameter classes. The difference increases with the intensity of treatment. However, thinning produces slightly more trees in the largest diameter classes.
The corresponding stock tables for hemlock (Figure A2-3) and pine (Figure A2-4) are more meaningful than stand tables because they display volume, which is closely related to stand value. Notice that pre-commercial thinning shifts volume toward the larger diameter classes in both examples, which is the intent of this practice. A closer look at the untreated and thinned to 900 hemlock regimes (Figure A2-3) shows that 193 m3 of small wood (mostly in the 20 to 35 cm classes) is replaced with 181 m3 of large wood (mostly in the 50 to 65 cm classes). Spacing pine to 1400 trees/ha (Figure A2-4) converts 77 m3, in the 15 to 20 cm classes into 83 m3, most of which falls in the 30 to 35 cm classes.
Figure A2-3. Distribution of volume by diameter class at age 100 for the coastal western hemlock density regimes.
Figure A2-4. Distribution of volume by diameter class at age 70 for the interior lodgepole pine density regimes.
The site value of all density management regimes is positive over a wide range of ages for both hemlock (Figure A2-5) and lodgepole pine (Figure A2-6). This indicates that, in isolation, all regimes are economically efficient. However, all thinned stands have a lower site value than the comparable untreated regimes, indicating that the untreated regime is the most economically efficient choice in these examples.
The economic outlook for spacing will only be favourable if the increase in tree diameters increases log size and value sufficiently to offset thinning costs compounded to harvest. Notice that DBH distributions shift by about 5 cm in Figures A2-3 and A2-4. TIPSY addresses the premium for piece size through the effect treatment has on log and lumber yields by dimension and the associated prices.
The foregoing analysis indicates that alternative silviculture investment options, including treatments on other sites, should be investigated. Those options which yield higher net economic gains should be considered before investing in any of the thinning regimes in these examples.
Figure A2-5. Site value over stand age (economic efficiency) for four coastal western hemlock density regimes.
Figure A2-6. Site value over stand age (economic efficiency) for four interior lodgepole pine density regimes.
Figures A2-5 and A2-6 illustrate the effect of harvest timing on investment decisions. There is a 10 year plateau of economic advantage when the site value is at or near the optimum economic rotation age. Site value declines rapidly in later years due to the compounding effect of time on silviculture investments.
If, for reasons of resource management constraints (e.g., VQO, ungulate winter range, community watershed requirements), the rotation must be extended or deferred beyond the period of maximum site value, it would be unwise, from a timber optimization perspective, to invest silviculture capital in pre-commercial thinning. Silviculture investments made under these circumstances will likely yield low or negative returns. Similar economic impacts result from spacing stands of low productivity with lengthy investment periods.
Some assumptions used in the preceding analysis may not be based on the best available data for a specific site or area. Thinning costs, for example, were based on the "Linear Equation" option. Use of the "District Average" values raises the costs of thinning pine to 1400 trees by $154 and lowers the economic site value from $831 to $739. Managers should be aware of the relative sensitivity of the various assumptions in their analysis. Examples of sensitivity analysis follow.
Copyright 1999 Province of British Columbia