Productive forest lands are obviously capable of supporting vigorous tree growth. However, it is equally true, though perhaps not so obvious, that they are capable of supporting luxuriant stands of competing vegetation. Many of these "weed" species are characterized by much more rapid juvenile growth—elderberry, for example, commonly extends shoots of over 4 meters in a single growing season—than that of the most rapidly growing crop species. And it is also equally true that any piece of ground, no matter how fertile, is capable of supporting only a finite quantity of vegetation. In essence, the role of the reforestation specialist is to modify the sequence of plant succession, working with nature where possible, to achieve a stand of commercially valuable species.
This unit is concerned with the range of techniques, collectively termed "mechanical site preparation," which foresters have developed to physically manipulate plant succession.
Procedure
This study guide will describe, in general, mechanical site preparation techniques and discuss the relative efficiency of such methods in establishing plantations of desired crop species. Lectures will discuss in greater detail factors governing the choice of mechanical site preparation techniques.
Upon completion of this study guide and assigned material, you should know the advantages and disadvantages of mechanical site preparation and be able to answer questions such as the following:
Timber Harvest
Both rubber tire or track laying vehicles have been shown to cause significantly more soil compaction than do moving cable logging systems. Losses of seedling survival and growth in areas of soil compaction may be severe. At least one major land management organization compensates for such compaction damage during logging by scarifying ("ripping") all cat trails and landings after harvest. The forester should be aware that the relative costs of several harvest systems should not be compared solely on the basis of timber removal, but should include costs required to regenerate the stand.
Objectives
Click me for a view of Mac Forest (North Zone) timber age class maps
Click me for a view of Mac Forest (Central Zone) timber age class maps
Click me for a view of Mac Forest (South Zone) timber age class maps
The system employed in timber harvest may affect the site preparation necessary to establish a new stand in several ways. For example, partial cuts prior to final harvest may salvage dying timber and may increase the growth of final crop trees or facilitate advanced natural regeneration by stimulating seed crops and providing favorable seed beds, but such practices also favor establishment of understory species which will proliferate rapidly after final harvest.
The choice of logging method, i.e., skyline vs. cat, will affect greatly the proportion of the cutover area which has exposed mineral soil. Mineral soil favors either seeding or planting Douglas-fir or other conifers, but if there is a seed source of a competitor such as red alder nearby, exposure off mineral soil can lead to virtually "instant" alder thickets which are expensive to remove.