| Douglas-fir Type |
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Reforestation by natural regeneration in the Douglas-fir type is today probably less common than in any other forest type in the Pacific Northwest. In Oregon, the Forest Practices Act virtually mandates artificial regeneration for the area in Northwest Oregon west of the summit of the Coast Range since there must be germinants the year after harvest to produce three-year-old seedlings two years later. The requirements for other areas are less restrictive, but general forestry practice is to employ artificial regeneration measures routinely on all areas currently harvested. Intermittent seed crops, seed- and seedling-eating animals, and the particular susceptibility of newly germinated seedlings to the hazards of often severe microclimates make reliance on natural regeneration too much of a risk for the restocking of clearcuts. Shelterwood cuttings result in microclimatic conditions which often are favorable for establishment of natural regeneration. Although experience with shelterwood cutting in Douglas-fir is still limited, tentative guidelines for obtaining regeneration recommend leaving 100 to 180 square feet of basal area per acre when protection of seedlings from severe environmental conditions becomes necessary. If the remaining overstory consists of trees with large diameters at breast height, greater amounts of residual basal area are required. As a rule, the amount of residual basal area will have to be greater the larger the average d.b.h. of residual trees for providing a given percentage of ground shade. It should be recognized, however, that even a light residual overstory will create sufficient competition for light, moisture, and nutrients that the growth of seedlings will be reduced.
Besides the density of the residual overstory, nature of the seedbed has a profound influence upon amount and kind of natural regeneration. As a rule, disturbance on the forest floor and exposure of mineral soil results in much larger amounts of naturally established seedlings than undisturbed duff and litter. The amount of disturbance of the forest floor accompanying logging operations can be controlled so that sufficient areas of mineral soil will be exposed. Light underburning may also be used in the future for exposing mineral soil. Presently, experience with this practice is still lacking in the
Figure 1. Factors influencing seedling establishment. Natural Reproduction Triangle
Pacific Northwest. Burning, however, should not be considered as an alternative or supplement to mechanical disturbance on sites containing long-lived seeds of brush species whose germination is stimulated by heat from wildfires or prescribed burning. If the overstory contains species other than Douglas-fir, the percentages of Douglas-fir seedlings tend to be much higher on seedbeds of mineral soil than of undisturbed duff. Lack of site preparation on such sites could lead to species conversion in the new stand. The combined effects of residual basal area and kind of seedbed have been summarized in Figure 2 based on work in the Cascades of western Oregon.
Figure 2. Stocking percent related to residual basal area and kind of seed bed. Heavily disturbed: 75-50% in mineral soil. Lightly disturbed: about 50-25% in mineral soil. Williamson, R.L. 1973. Results of shelterwood harvesting of Douglas-fir in the Cascades of western Oregon. Res. Pap. PNW-161. Portland, Oregon. USDA, Forest Service, Pacific Northwest Forest & Range Experiment Station. 13pp.
When environmental conditions are sufficiently favorable to permit the use of the seed tree method, site preparation would be essential for regeneration success unless logging has resulted in enough disturbance of the forest floor.
Reliance upon natural regeneration following clearcutting should be only on those sites which offer a reasonable chance for success. Such sites generally include those with north, northeast, and northwest aspects. Sites with west and east exposures may be suitable if clearcuts are small, that is five acres or less in size, and if they have relatively deep soils with good water holding capacity. Chances for failure with natural regeneration are very high on sites with south, southeast, and southwest exposures. Data in Table 1 represent results of regeneration surveys on clearcuts in Oregon Cascades and Coast Range.
Table 1. Total coniferous stocking by exposure. Lavender, D.P., Bergman, M.H. and Calvin, L.D. 1956. Natural Regeneration on Staggered Settings. Research Bulletin No. 10. Oregon State Board of Forestry, Salem, Oregon. 36pp.
| Exposure | Stocked Plots | Non-stocked Plots | No. of Seedlings | Percent Stocked (Mil-acre) | Estimated Percent Stocked (4 Mil-acre) | Seedlings per Mil-acre Plot |
|
Flat North N.E. East S.E. South S.W. West N.W. |
218 392 189 63 17 43 53 155 300 |
272 242 95 121 63 283 215 297 223 |
614 1,519 727 135 30 67 81 445 910 |
45 62 67 34 21 13 20 34 57 |
77 <95 <95 64 43 30 42 64 95 |
1.25 2.40 2.56 0.73 0.37 0.21 0.30 0.98 1.74 |
| TOTAL | 1,430 | 1,811 | 4,528 | MEAN = 44 | 76 | 1.40 |
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Clearcutting results in large amounts of coarse logging debris, that is pieces with a diameter at small end of 4" or larger, which may range from about 30 to over 200 tons per acre depending on age and size of trees and density of stocking. Fine residues (needles, twigs, and small fragments of wood and bark) may add another 20 to 40 tons per acre to the total debris left. The decision whether and how to remove logging debris is often crucial for success of natural regeneration. Benefits for coniferous seedling establishment to natural regeneration derived from leaving residues depend mainly on five factors. They are:
TABLE 2 Total coniferous stocking by slash classes.Lavender, D.P., Bergman, M.H. and Calvin, L.D. 1956. Natural Regeneration on Staggered Settings. Research Bulletin No. 10. Oregon State Board of Forestry, Salem, Oregon. 36pp.
| Slash Percent | Stocked Plots | Non-stocked Plots | No. of Seedlings | Percent Stocked (Mil-acre) | Estimated Percent Stocked (4 Mil-acre) | Seedlings per Mil-acre Plot |
|
0 0-25 25-50 50-75 75-100 - |
64 829 327 147 63 0 |
125 935 416 211 115 9 |
298 2,700 935 450 145 0 |
34 47 44 41 35 00 |
63 81 77 73 65 00 |
1.58 1.53 1.26 1.26 0.81 0.00 |
| TOTAL | 1,430 | 1,811 | 4,528 | MEAN = 44 | 76 | 1.40 |
Heavy concentrations of slash are usually detrimental to natural regeneration of Douglas-fir because they intercept too much light and may prevent seeds from reaching the ground. By contrast, a light or medium cover of slash will create enough shade to moderate extremes of temperature without interfering with growth of seedlings.
When disposal of logging debris is desirable or necessary, the principal methods are burning, chipping, or mechanical removal from the site. Each of these procedures has advantages and disadvantages.
Burning greatly reduces fire hazard, exposes mineral soil, removes diseased advanced reproduction where present, and usually reduces vegetative competition for several years (unless seeds stimulated to germinate by heat are present). Generally, burning also increases the amount of available nutrients in the surface soil with the possible exception of nitrogen.
However, the benefits of burning have to be weighed against other effects adverse to regeneration. Broadcast burning destroys advanced regeneration and new seedlings or seed from the most recent seedfall. Destruction of seed and seedlings can be largely avoided by burning logging debris after piling it. However, burning of piled residues produces a hot fire which usually results in severely burned soil underneath the pile. Hot fires tend to reduce the moisture holding capacity of the soil and often leave a non-wettable soil.
Patches of severely burned soil will also be found over sites that have been broadcast burned. Usually, severely burned spots account for less than 8 percent of the typical broadcast slash burn. Broadcast burning, however, will consume much of the light slash, duff, and litter and thus lead to losses of 300 to 500 pounds of nitrogen per acre. This loss would significantly reduce the amount of nitrogen available for the next crop, particularly on sites that are already low in soil nitrogen.
Table 3. Total coniferous stocking by degree or burn. Lavender, D.P., Bergman, M.H. and Calvin, L.D. 1956. Natural Regeneration on Staggered Settings. Research Bulletin No. 10. Oregon State Board of Forestry, Salem, Oregon. 36pp.
| Burn Class | Stocked Plots | Non-stocked Plots | No. of Seedlings | Percent Stocked (Mil-acre) | Estimated Percent Stocked (4 Mil-acre) | Seedlings per Mil-acre Plot |
|
No burn Light Medium Heavy |
948 161 277 44 |
787 283 555 186 |
3,222 354 874 78 |
55 36 33 19 |
92 66 62 41 |
1.86 0.80 1.05 0.34 |
| TOTAL | 1,430 | 1,811 | 4,528 | MEAN = 44 | 76 | 1.40 |
Available information concerning growth of seedlings on severely burned soil is scarce and conflicting. Measurement of one and two year old Douglas-fir established naturally on burned and unburned sites did not indicate any adverse effects of burning on growth. In another study, first-year seedlings of Douglas-fir grown on burned soil were considerably smaller, contained much less nitrogen but higher concentrations of potassium, phosphorus, and sulphur than seedlings grown on unburned soil. These results may create a nutrient imbalance more critical to growth than the loss of nitrogen.
Perhaps the most serious consequences of burning are encountered on steep slopes with shallow soil. On such sites,
The forester pondering the pros and cons of burning may be well advised to follow Isaac's ideas of promoting natural regeneration and still providing adequate protection against accidental fires. His suggestions include: 1) spot burn high hazard areas instead of broadcast burning entire units; 2) avoid burning southerly aspects where slash provides needed shade; 3) restrict broadcast burning to those areas where reasonable protection from fire requires hazard reduction or where heavy brush development or debris will hinder regeneration; and 4) minimize the fire intensity by burning after early fall rains as soon as the fine slash dries enough to ignite but while the duff layer is still moist. Miller et al. considered these general guidelines still valid today but emphasized that their use requires an area by area appraisal and decision about the need and most feasible means for reducing fire hazard and securing regeneration.
Removal of logging debris by chipping avoids many of the drawbacks of burning, however, germination and initial survival of Douglas-fir is generally poor in organic seedbeds. Thus, chipping is not an ideal solution when Douglas-fir is the desired species. Furthermore, a layer of chips may result in an undesirable C/N ratio necessitating addition of nitrogen fertilizer to enhance microbial activity and prevent depletion of the available nitrogen on sites of relatively high native fertility. These disadvantages do not obtain, of course, when chips are removed.
Wind-rowing of slash creates a mineral seedbed, but care has to be taken that not too much of the surface soil is removed. Otherwise, the most fertile part of the soil would be removed and seedlings may stagnate after initially rapid growth.
Mechanical treatment of slash by mean other than chipping or wind-rowing is not commonly practiced, and effects of such other methods of slash treatment on natural regeneration are largely unknown.
The decision whether or not to rely on natural regeneration should also depend on an assessment of the potential for vegetative competition and the feasibility of its control. The general nature of vegetation in secondary succession following clearcutting in the Douglas-fir region has been described repeatedly. However, considerable variation within this broad pattern of secondary succession may occur depending upon soil, aspect, elevation, and severity of disturbance.
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In general, cover by herbaceous vegetation will be the same on the burned and the unburned clearcuts, although composition of species tends to differ considerably between burned and unburned sites. Fireweed and willow herbs (Epilobium species) and wood groundsel (Senecio sylvaticus L.) often form an extremely dense cover in the first two years after burning and virtually disappear thereafter. Shrub cover, however, tends to be far more abundant on unburned than burned sites.
The effect of herbaceous vegetation on establishment of natural regeneration of Douglas-fir appears to vary considerably. Degree and type of herbaceous cover did not affect germination and initial survival of Douglas-fir in a study conducted in the Tillamook Burn. However, mortality of first-year seedlings of Douglas-fir tends to be high in dense stands of wood groundsel. Here, seedlings are heavily shaded and develop poor root systems. Often this places seedlings at a severe disadvantage in the competition for moisture soon after the start of the growing season. This inability of first-year Douglas-fir to compete successfully for moisture in dense stands of herbaceous vegetation is also reflected in the difficulties which were encountered in using mustard (Brassica juncea) as a nurse crop for Douglas-fir.
The large acreage, approximately 4.4 million acres in 1974, of commercial forest land in the Pacific Northwest occupied by brush fields would suggest that brush is the principal obstacle to the establishment of regeneration. Table 4 and 5 summarizes the relationship of plant cover to Douglas-fir regeneration on a range of clearcuts in western Oregon. Factors such as a succession of poor seed crops may well be the primary reason in many instances that invading shrubs succeed in taking over the site. Residual vegetation, in contrast to species invading the site after clearcutting, usually spreads slowly after logging and is unlikely to hinder successful regeneration of Douglas-fir. Notable exceptions are salmonberry (Rubus spectabilis Purch) communities characteristic of open Douglas-fir stands in the Coast Range and swordfern (Polystichum munitum (Kaulf.) Presl.) - Oregon oxalis (Oxalis oregana Nutt. ex T & G.) communities on moist sites in the western Cascades. These two communities tend to increase rapidly in vigor after clearcutting. Vegetation management is mandatory for securing coniferous reproduction on such sites.
The amount of seeds required for adequate regeneration will vary greatly depending upon prevailing combinations of environmental factors determining stand establishment. As a broad average for the Douglas-fir region, Isaac suggested that eight pounds per acre (at 40,000 seeds to the pound) naturally disseminated over a period of 6 to 8 years would result in satisfactory restocking. If seed trees are left, 8 or more trees per acre are usually needed for satisfactory stocking. Mature seed trees may produce as much as 1 pound of seeds in a good seed year. On a site with scattered seed trees in British Columbia, amount of Douglas-fir seedfall ranged from 3 to 5 pounds per acre in 3 good crop years. Viability of the seeds ranged from 23 to 40%.
TABLE 4. Total Douglas-fir stocking by living plant cover class.Lavender, D.P., Bergman, M.H. and Calvin, L.D. 1956. Natural Regeneration on Staggered Settings. Research Bulletin No. 10. Oregon State Board of Forestry, Salem, Oregon. 36pp.
| Cover Class | Stocked Plots | Non-stocked Plots | No. of Seedlings | Percent Stocked (Mil-acre) | Estimated Percent Stocked (4 Mil-acre) | Seedlings per Mil-acre Plot |
|
No plant cover Herbaceous 0-33% 33-66% 66-100% Woody 0-33% 33-66% 66-100% |
32
168
99 |
53
217
189 |
43
399
288 |
38
44
34 |
68
76
64 |
0.51
1.03
1.00 |
| Total Herbaceous | 548 | 790 | 1,185 | Mean = 41 | 72 | 0.89 |
| Total Woody | 506 | 1,312 | 947 | Mean= 28 | 54 | 0.52 |
|
Total Herbaceous & Woody 0-33% 33-66% 66-100% |
.
267 |
.
406 |
.
687 |
.
40 |
.
71 |
.
1.02 |
TABLE 5. Total west coast hemlock stocking by living plant cover class. Lavender, D.P., Bergman, M.H. and Calvin, L.D. 1956. Natural Regeneration on Staggered Settings. Research Bulletin No. 10. Oregon State Board of Forestry, Salem, Oregon. 36pp.
| Cover Class | Stocked Plots | Non-stocked Plots | No. of Seedlings | Percent Stocked (Mil-acre) | Estimated Percent Stocked (4 Mil-acre) | Seedlings per Mil-acre Plot |
No plant cover Herbaceous 0-33% 33-66% 66-100% Woody 0-33% 33-66% 66-100% |
9
117
47 |
76
268
241 |
14
325
119 |
11
30
16 |
27
58
35 |
0.16
0.84
0.41 |
| Total Herbaceous | 414 | 924 | 1,179 | Mean= 31 | 59 | 0.88 |
| Total Woody | 291 | 1,527 | 730 | Mean= 16 | 35 | 0.40 |
|
Total Herbaceous & Woody 0-33% 33-66% 66-100% |
.
164 |
.
509 |
.
444 |
.
24 |
.
48 |
.
0.66 |
Total seedfall within an old-growth stand of Douglas-fir was much greater for the same crop year. But based on the number of contributing trees, seedfall was only 30% of that under scattered seed trees, and the seed had lower viability, namely 11 to 36%. These data suggest that high light levels over the entire crown favor production of viable seeds. But as Isaac has also pointed out, the leaving of seed trees does not provide positive assurance of complete natural regeneration.
Stands of seed bearing trees along the edges of clearcuts probably provide the most reliable source of seed. Cuttings should be laid out in such a fashion that no part of them will be farther than about a quarter of a mile from a seed source because adequate regeneration, even under favorable conditions, cannot be expected for a greater distance than a quarter of a mile from the edge of a body of seed-bearing timber. This advice becomes readily understandable when one considers that most seed falls within 1,000 feet from the margin of the uncut stand. Seeding distances of less than a quarter of a mile are advisable on severe sites.
Garman's observation in British Columbia that lack of natural regeneration in the Douglas-fir type is largely a matter of cutting practice, such as clearcutting of large areas by progressive cuts, holds probably true for the entire Douglas-fir region. Slow restocking after logging and burning may reflect an inadequate supply of viable seed rather than low survival of seedlings. Longterm observations have shown a distinct relationship between viability and size of crop, that is, proportionately more light or empty seeds fall from small crops of cones than from large ones.
Foresters have little if any control over populations of seed-eating animals and insects, but they do have control over another major source of seed losses, namely slash burning. Comparison of areas where slash was burned in mid-August prior to a heavy seedfall, and in October after seedfall, showed that October burning reduced stocking to half of that following August burning. The obvious conclusion is to burn in spring when a good cone crop is in sight. Usually the crop is poor in years preceding a good crop. Consequently, there is no seed in the ground to be destroyed by slash burning in spring of a seed year. Garman has summarized the advantages of a spring burn, which are not associated with the fall burn, as follows:
It is apt to do a minimum of damage to the seedbed, which will be ready for any seed falling in early September and perhaps in late August of the same year. Relict species from the ground cover that preceded logging will almost certainly have sprouted soon after the slash fire and will provide moisture-conserving shade for seedlings that germinate the following spring. On a spring burn, tree seedlings grow fast and have no trouble competing with the second year sprout growth and with the first crop of annuals.
The major difficulties in deciding whether a spring burn is appropriate will be a reliable forecast of the cone crop. Allen developed a technique for crop forecasting by using the ratio of reproductive to vegetative buds six months before flowering. Allen's method was further refined by Silen. Four reliable forecast samples must be taken in the upper part of the crown. Obviously, such a procedure is impractical in ordinary practice. However, there is a compromise that still permits a fairly reliable forecast. This compromise consists of the checking of sunlit branches blown out on the crown by the first winter storm. If half or more of these branches have reproductive buds, a good cone crop may be anticipated.
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