|GUIDELINES for||. . .|
|Developing Stand Density Management Regimes|
The effects of density management (espacement, pre-commercial and commercial thinning) on stand volume and tree diameter can be evaluated using four types of stand information: stand tables, stock tables, stand mean diameter and prime-tree mean diameter. Each is subsequently described and compared using an example based on data from a well-designed experiment of thinned and unthinned Douglas-fir research plots. In 1972, plots were established in a plantation which had been planted at a density of 3000 trees/ha. Note, however, that the treated stand was pre-commercially thinned from below to a residual density of 750 trees/ha when it was 16.6 m tall, which is very late by current standards.
Stand tables indicate the number of trees, by diameter class, at a particular stage of stand development. Stand tables of comparable thinned and unthinned research plots at heights of 16.6 m and 31.6 m are displayed in Figures 10a and b, respectively. In this example, Figure 10a indicates that the proportion of trees removed from five diameter classes ranged from 0 (25 cm class) to 100% (5 cm class), relative to the unthinned plot. Figure 10b illustrates that, as both stands matured (top height approached 32 m), the diameters of the largest trees in the thinned stand moved slightly ahead of the diameters of comparable trees in the unthinned stand.
A stock table displays volume, by diameter class, and enhances stand table data by identifying the diameter classes that contain the bulk of the volume. It is important to focus on the upper and middle diameter classes of each plot since they contain the largest trees. Figure 10c illustrates a stock table comparison of the thinned and unthinned plots at 31.6 m of top height. Note that the extra wood in the unthinned stand is concentrated in the smallest diameter classes.
Figure 10c also illustrates how stock tables can be used to evaluate the impact of pre-commercial thinning on future commercial thinning opportunities. In this example, considerably less wood will be available in the 15 to 30 cm diameter classes, but a little more can be removed from the larger classes. The benefits of early stocking control, in terms of later thinning opportunities, will depend on the diameter classes targeted and the volume to be removed.
Figure 10. Thinning has the greatest impact on number and volume of small trees.
Stand average DBH or volume
The average diameter or volume of all trees in the stand provides a useful but narrow view of a stand, compared with a stand and stock table summary of stand structure. For example, in Figure 10a thinning from below instantly raises the average diameter of the plot (15.7 to 19.0 cm) in what is known as the chainsaw effect. This is caused by the removal of small trees during thinning inflating the arithmetic average diameter of the remaining trees.
Average diameter also fails to show the large spread in tree diameters in the thinned (15-50 cm) and unthinned (10-50 cm) stands in Figure 10b. Similarly, a heavily thinned stand has a larger stand average diameter than a lightly thinned stand, particularly during the sapling stage. Neither of these examples represents a biological response to thinning treatments, merely the presence or absence of many small trees that influences calculation of stand average diameter. These non-biological differences persist as the stands grow, although they tend to decrease towards maturity as competition and mortality eliminate the smaller trees from the unthinned stand. Since the average diameter of all trees does not adequately assess response to density management, analysts also look at other measures such as the average diameter of the larger and more valuable trees in a stand. These statistics include all trees above a particular diameter limit, or a fixed number of the largest trees. For example, one might trace the development of the average diameter of the largest 400 trees/ha if this is the cohort most likely to reach harvest age.
Prime tree average DBH
If there is a wide range of establishment densities, it is useful to compare the development of prime trees (largest 250 trees/ha) because these trees will likely survive to harvest in all stands. Furthermore, prime trees are independent of the chainsaw effect in stands thinned from below. For example, the average diameter of the prime trees in the thinned and unthinned stands is 38.3 and 37.5 cm, respectively, in Figure10b. The prime trees in the thinned stand outgrew those in the unthinned stand, but the difference is small because prime trees do not suffer from the same intensity of competition as smaller trees in the stand. Prime tree diameter is largely insensitive to stand density, unless the inter-tree distance is quite large. For example, the stand in Figure 10 was thinned late (16.6 m) to a residual density of 750 trees, which will only stimulate the growth of prime trees for a short period.
In summary, while average diameter of prime trees and stand average diameter are informative statistics, they must be used cautiously when assessing stand response to density management. Both statistics have shortcomings in portraying stand structure, and neither should be used in isolation of other relevant information (e.g., the range of tree diameters or volumes, the average diameter of non-prime trees, and stand and stock tables).
Copyright 1999 Province of British Columbia