Stem decay is decay of wood in the stems of living trees. Such diseases are traditionally called heart rots. Here we deprecate that term because it implies that decay is restricted to heartwood, which is frequently not the case.
Before this page, you should read the page on Wood Decay, which has necessary background information, including wood anatomy and chemistry, types of decay, terms for decays in various positions, disease cycle of decays, and tree resistance.
Rate of growth in trees
How fast does a wood-decay fungus grow in the stem? There are two approaches to finding out: inoculate, wait about five to ten years, then cut and measure; OR find trees with infection courts that can be aged, then cut and measure. What people have found is that decay develops slowly. Results are highly variable, from near 0 to 60 cm/yr. Average is much closer to 0, say 6-8 cm (about 3″). There is reason to believe that most figures overestimate the long-term average growth rate, because initial growth may be faster, perhaps due to better aeration.
Factors affecting stem decay in stands (epidemiology)
Age of Stand
It is important to understand that stem decay increases with age of a stand. Obviously, the longer a tree stands, the more likely it is to become infected, and the longer the fungus has to do its thing. Stem decay is not like a cold, it is more like arteriosclerosis: once you get it you’ve got it the rest of your life and it only gets worse. But there are other reasons that make it more serious in older stands:
- more infection courts in older trees (stubs, breaks, wounds, etc.)
- more heartwood in older trees
- wounds heal more slowly in older trees so more likely to be infected
- older trees grow more slowly, so the fungus grows faster relative to the tree.
Examples: An example of a sound stand is Sitka spruce in BC:
- 200 yr: 0.7% decay of merchantable vol.
- 750 yr: 27%
At the other extreme, aspen in the Lake States:
- 20 yr: 8% decay of total volume
- 80 yr: 34%
But stands may vary tremendously. E.g., hemlock in Washington and Oregon:
|stand A||180 yr||14% decay of total volume|
|stand B (only 4 miles from stand A)||214 yr||61%|
So 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?
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.
- basal scars
- Ice and snow storms
- top breakage, top rot
- felling wounds, basal scars from skidders and logs
- This factor 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 infection courts are not created. 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.
- Remember ungulates can create infection courts, particularly when the stand is young. Therefore the history of animal management on the site can influence heart rot levels.
As mentioned, species vary greatly in their susceptibility to decay, so stand composition is an obvious factor determining the level of decay. The percent of defect 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 leave an understory of hemlock and true firs, which are 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.
Relationships of stem decay to site are not very consistent, and no sweeping generalizations can be made. For root and butt rot, there are some, but for trunk rot there are few. There is a tendency for levels of stem decay to be greater in the drier parts of a tree’s range. For example, Porodaedalea pini is more common in Douglas-fir in central and southern Oregon (dry) than in other parts of the Pacific Northwest; also more common on southern slopes than northern. The higher temperatures may be more important than moisture in these cases. Dichomitus squalens is much more serious on ponderosa pine in Arizona and New Mexico (10-50% cull) than in California. This is thought to be because the first states get summer rains with high humidities, allowing infection in the dead branches. In pine regions of California, summers are dry.
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.
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.
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 main goal to remove such defective 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 significant genetic component, so in doing that you reduce the quality of the next stand and future ones.
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 di.
Other infection courts
For cases where larger branch stubs are infection courts, maintain stand density to reduce their size, consider prunging. I’m not aware of any operational tests of this or actual cases where it was used, but it should be effective.
For cases where animal wounds are damaging, exclude or reduce cattle or deer.
For cases where fire scars are important, protect against fire.
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.
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 (or what would be merchantable without decay).
In addition, the practical length of time a stand should grow for timber production (rotation age) may be influenced by decay. This is referred to as pathological rotation. The concept was first presented in 1916 by Meinecke (®), although he did not present supporting data. The data of Kaufert (®), using balsam fir, provide a good example (data are graphed at right). Pathological rotation may be defined variously:
- 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, and the stand is at maximum net volume. 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 is decreased by decay. This is somewhere between the peak net growth rate (also known as maximum net periodic increment) and peak net volume.
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:
|yellow birch||Nova Scotia||120|
|balsam fir||New York||70|