Factors affecting stem decay in stands (epidemiology)

Age of Stand

Stand age is such an important and complex factor affecting the amount of decay, it gets (part of) its own page!

But there are obviously factors other than age that influence level of stem decay. What accounts for such variation among stands in the amount of decay?

Stand History

Events and conditions in the life of a stand, especially those that create infection courts, can cause dramatic increases in the level of stem decay.

• Fire causes basal scars.
• Ice and snow storms break off tops and branches.
• Logging often causes felling wounds and basal scars from skidders and logs.
• Density is not so obvious. Assume a fungus gets in through large branch stubs. If the stand is dense, branches are shaded out while they are still small, and making infection by many decay fungi less likely. An open stand will have larger branch stubs. But remember that some fungi use small twigs as infection courts, so the dense young stand may favor them.
• Animals, especially ungulates can create infection courts, particularly when the stand is young. Therefore the history of animal management on the site can influence decay.

Stand Composition

As mentioned, species vary greatly in their susceptibility to decay, so stand composition is an obvious factor determining the level of decay. The amount of decay in a mixed stand should be roughly a composite of that expected for the species represented.

This has obvious management implications: we can influence the amount of decay in a stand by influencing composition. In the West, Douglas-fir is often clear cut. Clear-cut stands can come back to Douglas-fir, which generally has relatively low levels of decay. If we engage in partial cutting, as many argue we should for esthetic reasons, we favor an understory of shade-tolerant hemlock and true firs, which are much more susceptible to decay than Douglas-fir. Worse, the understory is often wounded by the logging. Obviously, a manager has many factors to consider, and decay is just one of them.

Site

Relationships of stem decay to site are not very consistent, and no sweeping generalizations can be made. In an enormous study of red ring rot, caused by Porodaedalea pini in Pseudotsuga menziesii (Douglas-fir), Boyce & Wagg found that decay begins at a younger age and reaches higher levels on good sites compared to poor sites ​[3]​. However, they also found it more common in central and southern Oregon (dry) than in other parts of the Pacific Northwest; also more common on southern slopes than northern. There is a general tendency for levels of stem decay to be greater in the drier parts of a tree’s range. On the other hand, Dichomitus squalens is much more serious on ponderosa pine in Arizona and New Mexico (10-50% cull) than in California and Colorado. This is thought to be because the first states get summer rains with high humidity, allowing infection in the dead branches. In the latter states, summers are relatively dry.

Inoculum

Here’s another concept. We generally feel that the factors outlined above are more important in determining infection than the availability of spores. We assume that the spores will usually be there. However, in many cases, we do not have hard evidence for this.

Indicators

Indicators are the external signs and symptoms of the presence of decay. They are very useful in diagnosis, but also in management. Knowledge, skill, and experience in recognizing and understanding indicators is essential in working with and managing stem decays.  Indicators vary among tree species and decay fungi.  Traditionally, forest pathologists and/or mensurationists would conduct a cull study in a given forest region to quantify losses associated with given categories of indicators.  These were then used to produce local cull manuals that foresters could use when cruising a stand to calculate defect and estimate net merchantable volume. Cull guides or manuals (e.g., ​[1]​) provide valuable information to foresters, especially cruisers, in managing forests with stem decays.

Fruiting. It often happens that only a small portion of trees with decay have fruiting, but it’s a great indicator when it occurs. Depending on the fungus, this may be big beautiful conks, small conks, mushrooms, or fruiting flat on the surface, with or without pores. Some fungi fruit only after the tree or infected branches fall to the ground. A few produce so-called “sterile conks”, which are not conks at all, but black fungal masses protruding from the stem.

Punk knots. Not many fungi result in punk knots, but they include Porodaedalea pini, the most important cause of stem decay in conifers in the northern hemisphere. A punk knot is a knot where at least the inner wood is decayed and replaced by fungal tissue that looks like the context (flesh) of the conk. The tissue is not visible at the surface, but only when the bark is shaved away. Such knots may be swollen or not. Boyce termed those “swollen knots” vs. “punk knots”, respectively ​[2, 3]​. Since they are the same except for swelling, it is perhaps less confusing to refer to the former as swollen punk knots ​[10]​. Less commonly, punk knots may be sunken.

Exposed decay. Decay may be exposed at branch stubs, wounds, and cavities. That’s a pretty good indicator of decay, don’t you think?

Sounding. When decay is advanced near the base of a tree, typically one can hear the low-pitched, hollow sound when striking the bark with a heavy tool. This can be considered an indicator.

Infection courts. These include lightning scars, fire scars, frost cracks, and mechanical injury from trees falling, wind damage such as broken tops, logging damage, etc. Dead tops, broom rusts that infect the stem, and old dwarf mistletoe cankers can also be infection courts. They are not definite evidence of decay, only that decay is more likely than in a tree without indicators. The larger the infection court, the more likely is decay. Likelihood also depends greatly on the tree species.

Stem decay impacts

Because losses increase with stand age, they were generally highest in the days of cutting old-growth stands and lowest in young, managed stands.

In the 1940s, it was estimated that stem decays caused an annual loss of 1.5 billion board feet in commercial forests of the US ​[4, 11]​.

Management to reduce decay

Decayed trees should be removed

In thinnings and other partial cuts, decayed trees should be removed. This is less an issue of reducing inoculum than of improving the residual stand and giving healthy trees more room to grow.

An exception is in cases where wildlife, particularly endangered species, are a consideration. Many animals nest in decayed trees. For instance, red cockaded woodpecker in southern pines. One reason for their endangerment is the shortage of such trees in intensively managed forests.

Cuts whose goal is to remove such diseased trees are called sanitation cuts. Those marking the trees, in fact for any cut other than clear-cut, should be trained to recognize indicators of decay in the particular tree species.

Also, salvage cuts after a fire or storm damage can be done to remove trees that have fresh infection courts that will certainly lead to decay.

A common practice in old days and no doubt still done in some cases today, was to leave decayed, defective trees as seed trees. The idea was that they weren’t worth harvesting anyway, let’s get some use out of them. But susceptibility to decay probably has a genetic component, so in doing that you could reduce the quality of future stands.

Wounding should be prevented

There are many approaches that can be used to reduce wounding:

• Clear-cutting, where otherwise possible is advantageous in this regard.
• Plan logging roads carefully to avoid damage to residual stand. Their are techniques for this. Use trees to be harvested as bumper trees, then harvest them last.
• Keep stand entries to a minimum, avoid frequent light cuts.
• Keep vehicles away from trees.
• If pruning is practiced, do it early, when branches are less than few inches diameter.

For cases where animal wounds are damaging, exclude or reduce cattle or deer. For cases where fire scars are important, protect against fire.

Other infection courts

For cases where larger branch stubs are infection courts, maintain stand density to reduce their size, consider pruning early. I’m not aware of any operational tests of this or actual cases where it was used, but it should be effective.

Sprouts

Stump sprouts can be infected via heartwood through the stump. In pruning excess sprouts, favor lower sprouts that are less likely to be infected this way.

Favor sprouts that are alone. When sprouts are large (>3″) clumps of very close sprouts should be treated as a unit, either all cut or all left.

Pathological Rotation

Yield tables are often used in forestry to predict wood volume produced in a stand of given species, age, and site quality. They usually predict gross yield. However, yield may be much less when you account for decay. At right is an example, shown in terms of both total volume and merchantable volume.

The practical length of time a stand should grow for timber production (rotation age) may be reduced by diseases that increase with age, especially stem decays. This is referred to as pathological rotation. The concept had been bubbling up in the literature, but Meinecke in 1916 was apparently the first to use that term ​[6]​. He stated “. . . the upper limit of this range [of cutting ages] should not lie beyond the period at which the gain from increment is offset by loss from decay . . .” This is the point at which you are losing as much wood as you are gaining, and net volume is all downhill from there. The data of Kaufert ​[5]​, using balsam fir, provide a good example (data are graphed at right). Pathological rotation may be defined variously:

• The absolute upper limit, as Meinecke wrote in the quote above, is the age at which volume added by growth = volume lost to decay. At this point, the slopes of the decay/cull and gross curves at right are equal, the stand is at maximum net volume, and net periodic increment is essentially 0. Carrying the stand longer will result in reduced sound volume.
• In practice, it is the age beyond which carrying the stand is not economically feasible because net volume growth (also known as net periodic increment) is flat or decreased by decay. Schmitz recommended a rotation for aspen to peak net increment, which depended on degree of decay allowed in the products ​[8]​. Similarly, Meinecke’s estimate for aspen in Utah was based primarily on the age at which net increment became flat ​[7]​. Other recommendations for pathological rotation are somewhat beyond the culmination of net increment, but certainly before peak net volume ​[5, 9]​.

In our example, Kaufert recommended 80 years as a pathological rotation for balsam fir, based on merchantable data ​[5]​. This is roughly midway between the maxima of net increment and net volume. He noted that net periodic increment is still positive at 90 years, but, agreeing with Schmitz ​[8]​, he felt that rotation ages should probably be shortened further due to mortality from butt rot and snapping, which is not accounted for by such studies.

Pathological rotation may need to be factored into a manager’s decision to determine the actual rotation based on all considerations. In some species, such as aspen, pathological rotation is usually limiting. Here are some examples of fairly short pathological rotation ages: