|GUIDELINES for||. . .|
|Developing Stand Density Management Regimes|
The interactions between basic tree and wood properties and species, seed source, geographic location, site conditions and management decisions are very complex. As a result, it is difficult to discuss these relationships in detail. However, an attempt is made in this section to outline the more important interactions in a general way so that foresters concerned with maximizing timber value are aware that silviculture decisions can affect both the tree volume and wood quality components of timber value. Tree and wood quality refers to specific characteristics that affect the value recovery chain from harvesting of trees to manufacturing and grade recovery of specific products (Zhang 1997).
Wood quality characteristics depend on the intended products and are usually defined by relative wood density, ring width, microfibril angle, fiber length, knot size and distribution, spiral grain angle and chemical composition (e.g., lignin-cellulose ratios and extractives). Their affect on product quality and value have been discussed in detail by Jozsa and Middleton (1994).
Although very dependent on species, the potentially larger proportion of juvenile wood (also called crown formed or core wood) added to regions of the bole covered by live crown is one of the greatest quality concerns in second-growth stands. The close proximity of growing shoots results in high growth hormone levels in the cambium leading to the production of wood with less desirable properties for most products. As the crowns close and lift, hormone levels below the live crown are reduced and the cambium gradually begins to produce mature wood (also called stem formed wood) of higher quality.
Early stand density affects wood quality largely through its effect on crown development and the subsequent production of wood of differing density and related characteristics. The variation in wood characteristics within a tree is largely associated with radial and, to a lesser degree, vertical position within the stem. The radial pattern of variation differs greatly between species. Figure 12a (after Jozsa and Middleton, 1994) shows the pith-to-bark trends of very rapidly grown trees of six species common to BC. The core wood of many species is lower in relative density than that of the outer shell, but this is not universally true. Wood closest to the pith of most conifers is of high density but this apparent advantage is offset by large fibril angle that contributes to low strength and stiffness and greater longitudinal shrinkage. According to DiLucca (1989), the wood of second-growth coastal Douglas-fir changes from juvenile to mature wood a few metres below the base of the live crown. Using this definition, Figure 12b illustrates how the juvenile wood content of simulated Douglas-fir changes with initial spacing and increasing stand age (after Mitchell et al. 1989). In the early years, all wood is juvenile because trees are open grown. Figure 12c illustrates how these initial spacings affect the proportion and distribution of juvenile wood in prime trees (tallest 250/ha) at harvest. In this example, very wide spacing has 55% juvenile wood relative to total tree volume while the narrow spacing has 45% due to the relatively rapid crown lift in dense stands. Commercial thinnings or late pre-commercial thinnings have less impact on wood quality because crowns usually have lifted.
Tree quality is defined by characteristics such as log diameters, branch diameter and distribution, stem taper and straightness. For example, large tree and log size lowers logging and hauling costs and more lumber is recovered per cubic metre. However, increasing branch diameter for a given log diameter adversely affects structural lumber grade recovery. Silviculture decisions can strongly affect the characteristics defining tree quality. Managing for low stocking density through initial spacing or subsequent thinning operations may increase average piece size. For example, the mean DBH of the prime trees shown in Figure 12c is about 8 cm larger for the very wide as compared to narrow spacing. However, the resulting increase in crown length, particularly in the early years, will produce larger diameter branches and increase both bole taper and the proportion of juvenile wood.
Stand density management decisions affect various links in the value recovery chain, including both tree and wood quality, harvesting and milling costs, product value and financial return. The optimal combination of stocking density and harvest age for each species varies widely with end products produced. The forest manager needs to consider the impact of silviculture treatment on the volume, quality and value at harvest if they wish to maximize their return on investment. The reader is referred to Carter et al. (1986), Ellis (1998), Farr (1971), Kellogg (1989), Jozsa and Brix (1989), Jozsa and Middleton (1994), Jozsa et al. (1998), Middleton et al. (1995), Middleton et al. (1996) and Walker and Johnson (1975) for more in-depth discussions of tree and wood quality of BC species.
Figure 12. a) Average relative density trends from pith to bark in second-growth trees of several species (from Josza and Middleton 1994). b) The proportion of juvenile wood (Douglas-fir) increases with wide spacing. c) Distribution of juvenile wood and mature wood in Douglas-fir trees from stands established at different initial densities.
Managing for high density produces more small slender (larger height/diameter ratio) stems than would occur in stands of lower stocking. These trees have a greater risk of breakage during harvest, and windthrow if left exposed by commercial thinning or partial cutting. Consequently, some of the "extra" volume contained in the smaller diameter classes of the untreated stand in Figure 10c may be lost to breakage, or not utilized because of size, particularly if markets are poor. These losses may be offset by technological changes in the future.
Damaging agents play a major role in the development of young stands, whether those agents are biotic, such as insects, diseases or undesirable vegetation, or abiotic agents, such as wind, snow or abnormal temperatures. For example, in a recent province-wide survey (Nevill 1996), more than 60 different insects and diseases were identified as having some effect on 217 of 234 assessed stands. Of those stands, 74 were experiencing significant damage and were likely to suffer long-term volume production losses. Some of this damage may be accounted for in yield forecasts, but the actual amount is not known. It is likely that damaging agents will prevent stands from achieving their potential production.
The provincial survey indicated the potential risk of damage at the landscape or forest level. However, the occurrence and the impact are highly variable in both time and space at the stand level. This is true whether or not stand density has been reduced. Because of this variability, little is known about the interactions among the occurrence and effect of damaging agents, stand density and density management. There are records of increases and decreases in damage with increasing density (Safranyik and Morrison 1998). Some damaging agents appear to be unaffected by stand density. There is also evidence that the occurrence and effect of some damaging agents are functions of the scale of silviculture treatment over the landscape.
Some types of stand damage, and the agents that cause them, are extremely difficult to detect and may require experts to adequately survey their incidence. This is especially true of many species of decay fungi, particularly in the early years of stand development. Many types of abiotic damage (wind and snow) may be periodic or random in occurrence, and difficult to predict in space and time. Given the complexity of the problem, great care should be taken to avoid creating stand conditions that increase the susceptibility to damaging agents. Resulting damage may partially offset or completely eliminate the intended benefits of stand density management treatments.
Information on specific damaging agents can be found in Forest Practices Code guidebooks.
Copyright 1999 Province of British Columbia