How Harvest Methods Affect Tree Genetics

By George E. Howe

Forest landowners are practicing geneticists each time they select a tree to harvest or leave. Selection is a powerful tool for good or bad, and negative selection may be even more powerful than positive selection. Forest landowners cannot take lightly the genetic effects of their selections of forest trees any more than ranchers can the selection of quality wheat plants or bulls as parents of the next generation.

A Short Course in Selection

Plant and animal breeders predict how much their selection will improve the next generation in a particular trait. They call this improvement genetic gain (see Figure 1). Genetic gain is a function of the heritability of the trait and the breeder's selection intensity, or what proportion of the population he or she has selected as parents of the next generation (the cross-hatched portion of Figure 1). The selection differential is the difference between the mean of the overall population and the mean of the selected portion (Fig. 1), and genetic gain (*G) equals selection differential (SD) multiplied by heritability (H), or *G = SD X H.

The variation among trees in any trait is caused by environment and genes. If that variation is determined strongly by genes that are passed to offspring, the trait is said to be strongly heritable. For a given selection differential, genetic gain will be greater the greater the heritability of the trait. Disease susceptibilities tend to be strongly heritable while many growth traits tend to be only moderately heritable.

It may be easier to achieve genetic loss than genetic gain from selection in traits like height growth rate and disease resistance that have been important in adaptation. It may be difficult to make gains in the direction Mother Nature's been selecting for millennia, but it's easy to achieve losses by selecting in the opposite direction.

Choice of Silvicultural System

Is your silvicultural system consistent with the evolutionary history of the ecosystem? The even-aged system is the choice that is consistent with ecosystems having a history of one or two age classes, while the uneven-aged system is consistent with a history of multi-aged structure. Major departures from the natural condition will have evolutionary effects.

Even-aged System Using Natural Regeneration in Even-aged Ecosystems

Shelterwood Regeneration Method. Some have suggested that the shelterwood method may encourage inbreeding. Figure 2 shows a two-family stand—the pink family and the green family. In a shelterwood, most of the short trees are harvested because it has been taught that they may carry inferior genes. In this case most members of the green family have been harvested (Fig. 3), leaving predominantly the pink family. Trees begin mating with one another, but because many of them are close relatives, their offspring are likely to suffer from inbreeding depression.

Strong family structure like this may occur in stands originating from the two or three veterans on the ridge that survived the last wildfire, but the phenomenon is probably not common. But if you suspect that a stand originated from few parents, plan a regeneration method other than natural at the end of the rotation.

Seed-tree Regeneration Method. In a seed-tree harvest, you may achieve some authentic genetic gains, particularly in strongly heritable traits, because the selection intensity is higher. In a seed-tree harvest you leave the best 5 trees out of 120, rather than the best 40 out of 120 in a shelterwood. The seed-tree method does not appear to increase inbreeding.

However, the seed-tree method assuredly creates strong family structure. As the seed trees reproduce, the offspring tend to cluster around the parent tree (Figure 4). As the offspring become reproductive they tend to mate with trees in their proximity, many of which are close relatives, and would establish the next stand with many inbred seedlings. So again, you may want to consider some alternative to natural regeneration at the end of the rotation.

Clearcutting Regeneration Method. The clearcutting method likely will be genetically neutral if the trees in the seed wall (the first tier of trees surrounding the clearcut) are comparable to those harvested. Some genetic loss may be avoided in the regeneration if the "sick, lame and lazy" in the seed wall are harvested, so if you must choose, spend your limited time and money getting rid of "the sick, lame and lazy" rather than selecting the very best to leave.

Even-aged System Using Planting in Even-aged Ecosystems

Planting offers at least three benefits that generally cannot be achieved by natural regeneration.
• Excludes Inbreds. Planting stock derived from quality seed orchards will be free of inbred seedlings because their parentage has been controlled. There is virtually no naturally regenerated stand that is free of inbreds.
• Predictable Genetic Gains. If the planting stock comes from a quality seed orchard that is part of a sound tree improvement program, you can reliably predict gains in whatever trait(s) the tree improvement program focused on.
• Higher Gains from Thinnings. Planting followed by thinnings offers opportunities not achievable from natural regeneration. Planting generally ensures that the trees are quite even-aged and more uniformly spaced than in a naturally regenerated stand. So when that plantation is thinned, you can be more confident that the tallest and healthiest trees have some genes for those traits rather than just being older or grown under less competition.

The Uneven-aged System: General

Where uneven-aged silviculture is practiced in the United States the age of every tree potentially to be harvested is rarely known, so diameter is used as an indicator of age. It is assumed that the trees biggest in diameter are the oldest, so only trees whose diameters lie above a certain limit (diameter-limit cutting from above) may be harvested.

Diameter-limit Cutting from Above: What's the Risk? If you want to harvest the oldest trees but use only diameter as the criterion for harvesting, you may be cutting a fast-growing young tree instead of an old, large tree. This practice has been likened to "destroying the first, second and third place finishers in every horse race, and putting all the last place finishers out to stud!"

Uneven-aged System Followed by Natural Regeneration in Even-aged Ecosystems

In the 1980s, forest managers tried lots of ideas in response to deepening public disapproval of even-aged management. Most ecosystems became candidates for uneven-aged management. But many of these ecosystems were naturally even-aged, with an evolutionary history of infrequent, large-scale, stand-replacing wildfires followed by regeneration in one or a few years. These ecosystems were to be converted to uneven-aged even though most of the genetic effects were unknown. These practices have not entirely disappeared. Here are some effects.

Exclusion of Shade-intolerant Species. Uneven-aged silviculture widely practiced in even-aged ecosystems may exclude shade-intolerant species like larch, lodgepole pine, or in some cases, Douglas-fir naturally dominating these ecosystems. Excluding shade-intolerant species over large acreages reduces interspecific diversity, one component of biodiversity.

Vulnerability to Natural Disasters. Species conversions and altered stand structures can make ecosystems vulnerable to insect and disease epidemics and fire events outside the evolutionary history. Trees having genetic resistances to these selection agents within their normal range may perish at the extremes. For example, the thick, insulative bark of veteran ponderosa pine or Douglas-fir trees may protect them from low-intensity ground fires but won't protect them from the intense fires in fuels characterizing uneven-aged stand structure.

Increased Inbreeding. Conversion of even-aged to uneven-aged structure may increase inbreeding in much the same way as the seed tree method can, described earlier.

The Uneven-aged System in Uneven-aged Ecosystems

Ecosystems which have an evolutionary history of uneven-aged structure are best suited for uneven-aged silviculture. A good example is the dry Douglas-fir and ponderosa pine types historically dominated by scattered large veteran trees with two or more distinct age classes underneath, sustained by frequent, low intensity ground fires. By perpetuating the distinct age classes it is less likely that a fast-growing young tree mistaken as a tree in an older age class will be harvested. But most important, the tree to be left should be carefully examined to make sure it's healthy, vigorous and well-formed. In fire-adapted ecosystems prescribed fire should be used—if fuels aren't too heavy—as a hedge against genetic effects of fire exclusion. Finally, if the genetic quality of natural regeneration is dubious, you should plant.

Other Silvicultural Practices

Other cutting practices result in the sick, lame, lazy or maladapted trees contributing more than their fair share to the regeneration. Fair share is something in the normal range of natural variability. If you are regenerating hundreds of acres from trees left after earlier high-grading, that's outside the normal range of natural variability and strongly dysgenic. The same is true of leaving predominantly green culls to regenerate a site.

Fire exclusion this century has potential evolutionary effects. For example, cone serotiny in lodgepole pine is strongly heritable and very sensitive to the selection effects of altered fire regimes. Populations arising from this altered selection will be maladapted to a natural fire regime. Other traits, like ponderosa pine bark thickness, which appears to be strongly heritable, are likely to be lost over time in the face of altered fire regimes.

Summary

Selection is a powerful tool that may be more powerful in the negative than the positive direction. Your choice of silvicultural system generally should be consistent with the evolutionary history of the ecosystem. Regeneration methods should recognize stand ancestry, treatment history, regeneration method at the end of the next rotation, and probable genetic quality of the trees that will be the parents of the regeneration.

George E. Howe is owner of Howe Forest Genetics Consulting in Missoula, Montana.

This article appeared in the Northwest Woodlands magazine, Winter 2000 - Published quarterly by the World Forestry Center for the Oregon Small Woodlands Association, Washington Farm Forestry Association, Idaho Forest Owners Association and Montana Forest Owners Association.