|GUIDELINES for||. . .|
|Developing Stand Density Management Regimes|
Theoretically, near-maximum production in monocultures is realized when a stand fully occupies the site quickly, and performs to its potential throughout the rotation. Actual stand production is lower if crown closure and site occupancy are delayed by low establishment density. Similar yield reductions occur if portions of the site remain unoccupied because of factors such as inadequate stocking, brush competition and pests.
Full utilization of stem wood is achieved if all mortality is harvested as is illustrated by the gross production curve in Figure 2. The difference between gross production and standing volume represents the volume lost to mortality.
Maximum theoretical stand production must be tempered by consideration of economic merchantability. Merchantability standards, such as top diameter, stump height and minimum diameter at breast height (DBH) are economic constraints that reduce the yield of the stand, particularly when the trees are small. The difference between the curves for standing volume and close utilization (Figure 2) shows that tops, stumps and trees less than 12.5 cm reduce the volume by a fairly constant amount throughout the rotation.
Note that the impact of minimum DBH limits (close, intermediate and rough utilization) diminishes over time as an increasing proportion of the trees exceed a particular diameter. These limits also delay the culmination of mean annual increment (maximum MAI).
Relative to monocultures, the yield relationships of stand mixtures are less well defined because of the numerous possible combinations of constituent species and their relative proportions (Kelty et al. 1992). The yields from stand mixtures can be either greater than or less than the yield from corresponding single-species stands depending on the component species, stand density, height stratification patterns and site quality.
Figure 2. Harvest volume is affected by merchantability standards.
In order to better understand the complexity of mixtures and how they relate to monocultures, it is helpful to visualize mixed-species stands from the perspectives of substitution and addition. In the substitution perspective, a mixture is derived from a monoculture by substituting trees in the monoculture with trees of another species, keeping the total stand density constant.
In Figure 3, for example, the monoculture of species A (open circles in Figure 3a) at 600 trees/ha can be transformed into a mixture by substituting 300 trees of species B (shaded circles in Figure 3b). The yield of mixtures created by substitution are almost always intermediate between the yields of the two species in separate monocultures. Notable exceptions are combinations in which one species enhances the growth of another, such as some mixtures of Douglas-fir and red alder. Otherwise, the stand yield of the mixture is usually close to the average production of the component monocultures when weighted by the species proportions in the mixture.
In the addition perspective, a mixture is derived by adding to a monoculture some trees of a different species. From the monoculture of species A at 600 trees/ha (Figure 3a), we can create a mixture by adding another 600 trees/ha of species B (Figure 3c), bringing total stand density to 1200 trees/ha. Yields of mixtures created by addition can be greater than yields of the monoculture simply because there are more trees.
Yields will also increase if the competition between species is less intense than the competition within species. This will occur when the two species differ in important silvical traits like shade tolerance and rate of height growth. A common example is where a slower-growing, shade-tolerant species forms the lower stratum beneath a canopy of a faster-growing, shade-intolerant species.
If the lower canopy species takes advantage of resources unused by the overstorey trees, the yield of the lower stratum species may be completely additive to that of a monoculture of the overstorey species.
Where the demands of each species for site resources are not completely independent, the lower canopy species may reduce the growth of the overstorey somewhat through below-ground competition. Nevertheless, there is frequently a net additive effect where the production of the lower canopy species outweighs the reduction of the overstorey production.
Figure 3. a) The density and productivity of a monoculture can be altered by b) substituting or c) adding trees of another species.
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