|GUIDELINES for||. . .|
|Developing Stand Density Management Regimes|
Commercial thinning occurs later than pre-commercial thinning when the trees are larger and less vigorous. Depending upon tree size and thinning intensity, it can create relatively large holes in the stand canopy that are reoccupied slowly by the crowns of leave trees.
When thinning commercially in BC today, it is common to carry out a single entry not long before the final harvest. This entry often removes a wide range of tree sizes, with most coming from the lower diameter classes. In Europe, it is common practice to conduct a series of frequent, light, low thinnings intended to capture wood that would be lost to mortality if untreated. One typically compares the harvest volume of the untreated stand with the total volume (final harvest plus all thinnings) taken from the treated stand. Lower utilization limits in Europe also increase the merchantable volume available from thinnings. A limit of 7 cm is common for both DBH and top diameter.
If we look at the mortality in an unthinned plot of Douglas-fir from BC we can see how much volume is potentially available from a series of frequent, light, low thinning entries. That is, we will assume we are able to harvest and utilize each overtopped tree just before it dies. In Figure 7, the uppermost gross production curve is what we get if all wood is salvaged by thinning without regard for merchantability limits. The middle gross merchantable curve shows what is left if tops (< 10 cm), stumps (< 30 cm), and small trees (< 12.5 cm) are not merchantable. The lowermost curve displays the standing merchantable volume if the stand was not thinned. If all merchantable mortality in Figure 7 is captured through repeated, light, low thinnings, it is possible to increase the harvest volume (12.5 cm+) by 20% at age 85 years. This increases to 30% with the harvest of all trees (0.0 cm+), tops and stumps. In this example, there is little doubt that mortality, merchantability standards and the number of stand entries affect the volume of wood available from commercial thinning, particularly beyond age 50.
A series of light entries increases the total harvest because the space vacated by small-crowned trees is small, and frequent entries maximizes the opportunity to salvage trees before they die. A single heavy entry, timed well before the final harvest, will likely decrease the total yield because tree removals create large openings, resulting in less than full site occupancy by the residual stand. Furthermore, only one opportunity to harvest impending mortality will result in lost volume between thinning and final harvest.
Figure 7. Utilization standards affect the yield from commercial thinning.
Figure 8 depicts research data from the control (T0) and three of four levels of thinning, most of which were quite light in terms of the volume removed (T1=14%, T2=21%, T3=29% and T4=41%) as reported by Omule (1988). Figure 8a shows how the standing volume of the untreated and thinned plots developed over time. Regime T2 is not displayed for reasons of clarity.
Figure 8b displays the total harvest volume (standing plus thinnings). The response to treatment was small in terms of merchantable volume (T1=5%, T2= 3%, T3=7%, T4= -6%), and the differences were not significant.
Figure 8. Light to moderate thinning regimes a) reduce standing volume but b) have little impact on total harvest volume.
Figure 9a, a hypothetical scenario that builds on the preceding example, displays the impact of removing 12, 38 and 62% of the stand volume as thinnings, with most coming from the lower crown classes.
The total harvest volume, including thinnings, of the very light entry surpasses that of the untreated plot in Figure 9b, while the heavy regime falls considerably below. The yield of the moderate thinning and the untreated control coincide.
Figure 9. Heavy thinning regimes reduce a) standing volumes, b) total harvest volume and c) crown cover.
Also note that the MAI culminates later in the thinned stands. Figure 9c illustrates the rate at which crowns close following the very light, moderate and heavy commercial thinning regimes described above. Notice that the heaviest thinning regime is unable to reoccupy all of the available growing space, which accounts for its lower volume production.
Commercial thinning will not likely produce extra volume unless a regime of frequent, light, low entries is implemented. But it will provide an earlier, interim harvest by moving wood forward in the scheduling queue. It may also delay the final entry.
Copyright 1999 Province of British Columbia