|GUIDELINES for||. . .|
|Developing Stand Density Management Regimes|
A specific target stand density can be achieved at an early age by means of initial espacement or pre-commercial thinning. The silvicultural benefits for both practices, in terms of average diameter growth, are due to the extended period of free growth before crown closure. High regeneration density followed by pre-commercial thinning provides unique silvicultural advantages over espacement because of the opportunity to select the best trees. For example, those left after spacing a stand from 5000 to 1000 trees, as opposed to planting 1000 trees, would likely to be:
If the goal of forest management is to maximize stand volume production, crop tree selection criteria should focus on factors that reflect crop tree vigour, such as the rate of height growth, total tree height and diameter. The potential response of the residual stand to pre-commercial thinning will be compromised and could lead to the degradation of stand vigour and performance if:
Pre-commercial thinning immediately reduces the number of trees, the occupancy of growing space and the standing volume per hectare. The magnitude of the reduction is related to the intensity of treatment. The subsequent development of the stand is more complex.
Crown cover normally increases at a diminishing rate until complete canopy closure occurs, and then levels off. The number of trees in the spaced stand remains fairly constant until the onset of crown closure, competition and mortality. Volume increment/ha is reduced until the vacant growing space created by spacing is fully utilized by the residual stand. The corresponding volume curves of the spaced and untreated stands will initially diverge and later parallel one another. Convergence of the curves usually starts shortly after mortality begins in the spaced stand. The duration of each phase depends on the intensity of pre-commercial thinning and the level of utilization.
If only overtopped trees are removed and 100% crown cover is maintained, convergence will begin immediately without any divergence. On the other hand, a very heavy spacing will initiate a lengthy phase of divergence that could extend until the stand is harvested. Since one phase tends to dominate, volume curves can be described in terms of decreasing, constant or increasing departure as illustrated in Figure 5.
Figure 5. The intensity of pre-commercial thinning affects future volume.
The response of pre-commercial thinning is also affected by the product objectives and the stage at which the impact of treatment is assessed. An unspaced stand can produce more or less wood depending on the merchantability standard applied and when it is evaluated (Figure 6). A lower diameter limit (12.5 cm+) accounts for the many small trees in the untreated stand once they reach pole stage and attains greater volume per hectare than a spaced stand (Figure 6a). However, thinning provides more growing space for the leave trees, permitting greater diameter growth, allowing trees to cross diameter thresholds more quickly. The advantage is evident when the minimum diameter is increased to 17.5 cm (Figure 6b). Notice that the volume benefits of pre-commercial thinning diminish as an increasing proportion of the trees in the unspaced stand exceed the minimum diameter. Consequently, treatment response should not be assessed at an early age using a relatively large diameter limit. Methods of evaluating the effects of density management practices are discussed later.
Figure 6. a) A minimum diameter limit of 12.5 cm gives unspaced stands a volume advantage, but b) increasing the limit to 17.5 cm gives an advantage to the thinned stands that diminishes over time.
Thinning and fertilization
In many cases the impacts of thinning (pre-commercial and commercial) on stand production can be modified by nitrogen fertilization. The nature of interactions between spacing and nutrition, and the tree- and stand-level biological responses are summarized in five steps:
Fertilization may compensate for the loss of stand volume increment caused by incomplete site occupancy immediately following thinning alone. However, due to the complexity of interactions between tree, stand and site factors, the actual response of combined thinning and fertilizer treatments can be quite variable.
For example, the timing of fertilization, relative to thinning, affects the efficiency of fertilizer uptake. Thinning usually leaves a large amount of fresh slash on the forest floor. During the first one to two years, the mineralization of this material can significantly improve the nutrient status of the forest floor. Elevated spacing slash can impede the penetration of fertilizer materials increasing nitrogen volatilization losses from the stand. Fertilization should therefore be delayed in most cases for one to two years following thinning in order to maximize the beneficial effects of treatment interaction. See the Fertilization Guidebook for more detailed recommendations.
Pre-commercial thinning and commercial thinning
Can pre-commercial thinning prepare a stand for a later commercial entry? Logging contractors report that access is improved considerably by the absence of many small trees that would have survived had they not been removed earlier. Long-term data from pre-commercial thinning trials suggest that operations which favour the removal of trees in the lower diameter classes also favour the growth of trees in the larger diameter classes. When pre-commercially thinned stands are ready for a second entry, the distribution of volume by diameter class tends to have shifted towards the larger diameters relative to stands that were not thinned earlier. The link between early and late treatment is illustrated in a later section concerning the use of stock tables to evaluate the effects of density management practices.
Copyright 1999 Province of British Columbia