Silviculture is the art and science of growing trees. The forester uses his or her knowledge of silviculture, in conjunction with risk assessment methods, to create forests that are productive, healthy and at low risk of sustaining damaging insect outbreaks. The silvicultural prescription is the plan for the development of a particular forest site. The prescription considers pre-harvest treatments such as thinning, fertilization, insect and disease control, and the like, the harvest methods to be used and, finally, the plan for regenerating the next rotation, which includes things like species composition and planting. The prescription is based on a land-use plan for the site. For example, one may have a single-use plan, such as the production of timber, or multiple-use plan which might including timber production, wildlife, grazing, and so on.
EXAMPLES
Fir engraver beetle. White fir (Abies concolor) provenance tests at the Placerville Forest Genetics Laboratory (the provenance refers to the origin or derivation of a seed source). Seedlings produced from seed sources from all over the geographic distribution of white fir were planted at one location near the laboratory. Years later, many of these trees were killed by the fir engraver beetle following a period of severe drought in northern California (see slide). What is significant about this provenance test is that trees grown from local seed sources were more susceptible to engraver attack than those from other geographic regions. This is in direct opposition to classical provenance theory, which states that seed sources should be selected from environments that are similar to those where the trees will be grown. It may be that the insects, or the fungus that they transmit, are more adapted to the defenses of the local trees. As the fir engraver is a major threat to white fir in this area, the forester may want to consider planting trees from non-local seed sources. However, this could create other problems, such as increasing the susceptibility of the stand to the Douglas-fir tussock moth or other defoliators. The conservative approach would be to try some small-scale plantations to test the hypothesis that non-local seed sources would increase stand resistance.
Southern pine beetle. The major threat to pine plantations in the south is the southern pine beetle, Dendroctonus frontalis (slide). The best way to protect stands from this beetle is to maintain health and vigor by periodic thinning (slide), plus sanitation to remove the sources of potential beetle infestations, such lightning struck trees (slide). However, the forester should be aware that when beetle populations are high, healthy stands and even extremely young plantations may become susceptible to attack (slide).
Mountain pine beetle. As a result of large forest fires around the turn of the century, extensive areas of the Rocky Mountain region are covered with lodgepole pine (Pinus contorta var. latifolia) forests. These stands are generally unmanaged and play important roles in supporting wildlife populations and protecting watersheds. Some are also used for summer cattle grazing and timber production. The main insect threat to mature lodgepole pine forests comes from the mountain pine beetle, Dendroctonus ponderosae. In the past 50 years or so millions of hectares of mature and overmature unmanaged lodgepole pine forest have been destroyed by this beetle (slide). However, stands which are thinned at an early age are much more resistant to attack (slide). Even thinning mature stands can reduce or slow down the rate of tree killing (slide).
Douglas-fir tussock moth. Prior to the arrival of Europeans, the forests of the Blue Mountains of southeastern Washington were subjected to repeated ground fires which removed the Douglas-fir (Pseudotsuga menzeseii) understory and maintained park-like stands of large ponderosa pine (Pinus ponderosa) (slide). Attacks on the older and/or weaker trees or small groups of trees by bark beetles, particularly the western pine beetle (Dendroctonus brevicomis), created unevenaged stands which were very resistant to fire and large insect outbreaks. In the first part of this century, these large and valuable ponderosa pine stands were heavily logged and fire suppression reduced the incidence of fires. As a result, dense stands of Douglas-fir have become the dominant forest cover over much of the Blue Mountains, and these stands are highly susceptible to the Douglas-fir tussock moth (slide). Stands can probably be converted back to their original structure by harvesting the fir and the judicious use of controlled fire (slide).
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