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The large amount of slowly decomposing woody debris in western forests has important ecological implications in terms of mineral cycling. nutrient immobilization (Fogel and Cromack 1977, MacMillan et al. 1977). nitrogen fixation (Cornaby and Waide 1973), fire, and wildlife habitat.
In an unmanaged stand, logs are recruited to the forest floor by the fall of either living or dead trees. Living trees fall as a result of natural catastrophic events such as windstorms, snowslides, and floods.
Snags, on the other hand, usually just deteriorate and collapse. The condition of standing live trees and snags can be translated into log classes (fig. 44; tables 19, 20, and 21). When they fall, snags immediately enter one of the first four log decomposition classes (Boyce and Wagg 1953, Wright and Isaac 1956).
No data are available on the accumulation of woody debris or the rate of litter fall in forests of the Blue Mountains. Research in western Oregon, however, has resulted in more complete data than anywhere else in the Western United States. Much of that information is presented to demonstrate the function of woody debris in the forest environment. The accumulation of woody debris and litter in the Blue Mountains is not nearly as great as in western Oregon. The concepts, however, are applicable to the Blue Mountains and to forested areas in other parts of North America.
|
Figure 44. When they fall, trees and snags immediately enter one of the first four log decomposition classes. |
| Table 19. Snag condition translated into log decomposition class. | ||
| Snag stage | Snag condition | Log class |
|
1-3 |
Hard snag Hard snag Soft snag Soft snag, 70% + soft sapwood |
1 |
| Log characteristics |
Log decomposition class | ||||
| 1 | 2 | 3 | 4 | 5 | |
| Bark | intact | intact | trace | absent | absent |
| Twigs <3cm (1.18in) | present | absent | absent | absent | absent |
| Texture | intact | intact to partly soft | hard, large pieces | small, soft, blocky pieces | soft and powdery |
| Shape | round | round | round | round to oval | oval |
| Color of wood | original color | original color | original color to faded | light brown to faded brown or yellowish | faded to light yellow or gray |
| portion of log on ground | log elevated on support points | log elevated on support points but sagging slightly | log is sagging near ground | all of log on ground | all of log o ground |
| Table 20. A 5-class system of log decomposition based upon work done on Douglas-fir (adapted from Fogel et al. 1973, used with permission, see also Minore 1966) | |||||
Natural Accumulation and Decay of Logs
 |
The natural accumulation and decay of logs vary with the type of forest. Preliminary data for a mid-elevation, unmanaged 470-year-old Douglas-fir stand was reported by MacMillan et al. (1977). They estimated the rate of log recruitment to be 1.2 logs per hectare (0.49 log/acre) per year. These data showed that decaying Douglas-fir logs represented from 120 to 595 metric tons per hectare (53.5 to 265.4 short tons/acre). Grier and Logan (1978), working in a 450-year-old Douglas-fir stand, found that as much as 60 percent of the annual litter fall may be woody debris. They estimated that the accumulated logs on the forest floor ranged from 55.2 to 580.6 metric tons per hectare (24.6 to 259 short tons/ acre). In fact, decaying logs in western Douglas-fir forests represent more aboveground volume than is present in the entire aboveground woody debris of typical deciduous forests in the Eastern United States (Day and Monk 1974, McFee and Stone 1966). MacMillan et al. (1977) also estimated the number of logs in various stages of decomposition in unmanaged old-growth Douglas-fir stands (table 22).
The rate at which woody material decomposes depends on climate, species, and the size of the plant material. A study in England indicated that at least 12 to 29 years may be necessary for the decomposition of small hardwood branches 2 centimeters (0.79 in) in diameter (Swift et al. 1976). Fogel and Cromack (1977) found that it may take from 36 to 50 years for Douglas-fir branches, 2.5 centimeters (1 in) in diameter, to reach 95-percent decomposition. In contrast, Douglas-fir needles require only 10 to 14 years to decompose. Litter decomposition models are available for a variety of temperate and tropical forest types (Jenny et al. 1949, Minderman 1968, Olson 1963).
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