In climax forests, other than those that are fire-driven, decay fungi are a key component in perpetuating multi-aged stands. Decaying mature trees are susceptible to windthrow and breakage. When these large trees fall, they create openings that allow more light to reach the forest floor. Younger trees fill the gaps, as the fallen veterans are decomposed by decay organisms and their nutrients released back into the forest.
Wood decay fungi are also important from a wildlife perspective. Approximately 16 per cent of the birds, mammals and amphibians of B.C. depend on decaying standing trees for habitat. Fallen decayed trees also provide habitat for a great number of species. Wildlife use decaying trees for nesting, food, shelter, roosting and perching.
Related to this principle is the concept of pathological rotation age. Pathological rotation age is the point in time when trees are losing as much volume to decay as they are gaining in annual increment. Prior to the pathological rotation age, trees are adding more wood than the decay fungi are removing. Following this age, the decay fungi are decaying wood faster than the trees are adding it. Climax stands are sometimes referred to as being "decadent" from a timber management perspective. These decadent stands have passed their pathological rotation age.
The internal tree decay in a stand will affect prescriptions in varying ways, depending on the stand management objectives. Usually a combination of values is desired. The following sections discuss how decays may be detected, assessed and evaluated under various silvicultural regimes.
In stands where reserves, riparian areas or other residual stems are retained, those stems should be assessed to determine if they are infected by decay fungi. This will help ensure that the reason for retaining these trees is met. If a goal is to provide suitable wildlife habitat, selecting appropriate trees can help fulfill this. If the goal for leaving trees is to maintain cover, healthy or reasonably decay-free trees should be retained.
Assessing trees for decay is difficult, especially since much remains hidden, and there is little benefit in damaging a healthy tree to find out if it is diseased. Some obvious external indicators of decay can be used to identify potentially decayed trees with a fair degree of success. Conks, blind conks, scars, frost cracks, large areas with no bark, gouges into the sapwood, root injury and broken tops are all good decay indicators (for more detail see Tree Hazards in Recreation Sites in British Columbia, Timber Cruising Manual or similar references).
Tree injury as a result of logging practice is quite common and difficult to avoid. Operator experience is an overwhelmingly important aspect of conducting a successful partial cut. Whether hand-falling, horse-logging or using machinery, an experienced operator can cause less damage to the residual stand. The following findings of Wright and Isaac (1956) derived from studies on western hemlock, Sitka spruce and true firs, may assist in planning a partial cut.
Small western redcedar and western hemlock trees are often suspect, especially redcedar that regenerated by layering rather than from seed. It is quite common for these redcedar trees (usually identified by a sweep in the lower stem) to be infected with the cedar form of Phellinus weirii. Infection by Phellinus leads to a butt rot of the tree and often, as well as being deceptively old, these trees are hollow.
Take care in selecting any species for retention as advanced regeneration. Destructive sampling of a few stems is encouraged to ensure that most of the retained stems are sound.
The most likely avenue of serious decay is damage to the bark and outer cambium from broken tops, pruning wounds, stem and root bark removal, and stem gouging. The latter two are more common in operations that involve skidding, such as commercial thinning, and are broadly covered in the section "Decay fungi and partial cut harvesting systems." The remainder are common stand tending problems in younger stands.
Broken tops can serve as entry points for decay fungi as well as points of stem deformity from forks and crooks. Improper pruning can inflict a wound that takes several years to close and that serves as a point of decay initiation. Cuts made flush with the stem may take longer to close. Branch stubs protruding beyond the branch collar die and take several additional years for the tree to grow over. These stubs form large dead knots in the stem, which often harbor saprophytic decay fungi that gain easy access into the heartwood of the tree. For details on how to prune properly, consult the Pruning Guidebook.
The threat of infecting stumps — created by thinning stands, particularly western hemlock and true fir on the coast — with Annosus root disease may be of concern. Past practice has been to use borax to prevent Annosus spores from colonizing freshly cut stumps and then transferring, via root contact, to adjacent crop trees. Research indicates mixed results, so this is no longer recommended as a blanket treatment. However, in specific situations where values and risk are high, the use of borax may prove worthwhile.
The following recommendations to reduce stand damage during harvest are summarized from several sources. Although intended primarily for thinning operations, many of the principles may be applied to any silvicultural system.
For more information on reducing tree damage, consult the Commercial Thinning Guidebook.
Much of the current decay losses are associated with old growth stands. These losses are predicted to decline in the future, as more stands become managed on shorter rotations and the remaining old growth stands are harvested or set aside for other resource uses.
There are four interrelated factors to consider in managing tree decay:
The management objectives set the tone for future stand activities, be they timber-oriented or otherwise. The silvicultural system influences the level of decay hazard and risk. Species susceptibility to wounding and decay may constrain management objectives. Most significantly, the retention period determines the amount of decay that can be expected, given the silvicultural system and species involved.
This management section of this guidebook is in four parts. The first section, "Species susceptibility groupings," puts tree species into groups that react similarly to decays. The second section, "Management objective regimes," details a series of management regimes, from short-term retention to true uneven-aged management. The third section, "Damage criteria by management regime," is a detailed species group and management regime matrix. Using this matrix, you are guided to select acceptable tree damage levels to better achieve the prescription's objectives. These damage criteria are detailed in the fourth section, "Applying stand damage criteria."
Table 3. Species susceptibility to decay, ranked from greatest to least
|2||B, H, Lw, Ss, and Cw under 60 years|
|3||Cy, Sx, and Cw over 60 years|
Regime A (short-term retention)
Apply this regime where the stand will be re-entered for a final harvest within 20 years. Examples include seed trees, poling and commercial thinning.
Stands managed under this regime can withstand considerable damage, regardless of species (with the possible exception of broadleafs). The remaining stems will be removed before decay pathogens have a chance to cause significant decay. Some incipient stain is likely to occur, but serious loss of volume or quality is not likely. Due to the short retention period, even the most decay-prone species may be managed successfully under this regime.
Regime B (long-term retention)
This regime applies where the residual stand is maintained for a longer period or there is a long period between stand entries (i.e., 20 to 40 years). Examples of this regime include partial cutting with retention, shelterwood systems and commercial thinning.
Stands managed under this regime are moderately susceptible to loss, depending on a combination of degree of wounding, species composition and retention. Using the damage criteria for this regime may result in some loss in volume and quality. These losses, however, should not be significant.
Regime C (true uneven-aged management)
This regime applies where the stand is managed in a true uneven-aged state. Examples include selection systems.
Stands managed under this regime are very susceptible to loss from decay. Harvesting across a profile of stem sizes and ages results in the prolonged retention of some trees, and may allow decay to progress to a level where losses become serious. The damage criteria for this regime are set to restrict these losses to an acceptable level.
Regime D (special management areas)
Where this regime is prescribed, only minimal tree wounding is acceptable. Examples include trees in recreation sites or other high hazard areas.
Stands to be managed under this regime are susceptible to serious decay loss, are of high value for non-timber purposes and may pose a risk to public safety (i.e., recreational areas). Where possible, sensitive sites such as campgrounds should not be set up in stands consisting primarily of broadleaf trees. For more information on assessing hazard trees, consult Tree Hazards in Recreation Sites in British Columbia.
Table 4. Damage criteria matrix
|Stand management objective|
|B, H, Lw, Ss, and Cw <60 years||B||C||D||E|
|Cy, Sx, and Cw >60 years||B||C||C||D|
Use the following criteria when assessing partial-cut sites for satisfactory residual crop trees. Trees that exceed the specified criteria are unsuitable, because the damage will:
Definitions: A wound is an injury that removes a portion of the bark and cambium from the tree but does not penetrate into the sapwood. A gouge involves penetration into the sapwood or deeper. A supporting root is one that originates from the base of the tree and maintains a diameter greater than 2 cm.
Based on biological considerations, a tree is not acceptable as a residual crop tree if it meets or exceeds the following levels of damage, as determined by the stand management regime defined in Table 4.
Damage Type A:
Damage Type B:
Damage Type C:
Damage Type D:
Damage Type E:
Under a timber management objective, leave-trees should be as defect- and disease-free as possible. Leave-trees should not have defects such as forks, large wounds, dead or broken tops, rotten branches or frost cracks. Leave-trees should also be free of diseases causing stem cankers, mistletoe brooms and conks. Under other management objectives, diseased or damaged trees may be retained for a particular purpose, such as wildlife trees.
Additional acceptability criteria may be found in the guidebook specific to your silvicultural operation (e.g., Commercial Thinning Guidebook, Silvicultural Systems Guidebook).
It is recommended that district managers set their own threshold levels for unacceptable damage for individual stands. This allows for flexibility in tailoring damage limits to reflect individual site characteristics, operator experience, timing of operations, stand management objectives and other factors. These damage thresholds may be set using some standard of measure such as number of stems per hectare, per cent basal area, or by identifying specific species in an area that fail to pass the individual test for acceptable tree damage.