| Ponderosa Pine Region |
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Forests in the ponderosa pine region of Oregon consists of three major kinds:
The common presence of advance reproduction in stands of ponderosa pine may give the impression that large numbers of seedlings become established almost every year. Actually, establishment of appreciable amounts of natural regeneration occurs at rather infrequent intervals. Ponderosa pine is not only a variable producer of seed, but a good crop and climatic conditions favorable for seedling establishment seldom coincide. Advance reproduction is usually the result of a slow accumulation of surviving seedlings from good and fair seed years.
In Oregon, good cone crops can probably be expected at intervals ranging from 3 to 6 years. Cone crop records from eastern Oregon for a 17-year period (1912-1928) show that 1912, 1917, 1923, and 1928 were good seed years. The years in between were fair and poor, including 4 years in which cone crops were classified as failures. Ponderosa pine in the Pringle Fall Experimental Forest in central Oregon produced 5 good cone crops during the 22-year period 1952-1974. The good seed years were 1958, 1964, 1968, 1971, and 1974. However, accurate forecasts of seed availability are difficult at best because several adverse but often unpredictable circumstances can drastically reduce the supply of seed within a short time. They include heavy rainfalls during the pollination period, injury of strobili by frost, destruction of seed by cone and seed insects, and consumption of seed before and after seedfall by birds and small mammals.
As a rule, formation of a new stand of ponderosa pine through natural regeneration is a process which takes many years. For this reason, reliance on natural regeneration should probably be limited to stands which contain a substantial amount of advance reproduction. This advance reproduction will form the nucleus of the new stand which will be augmented gradually by establishment of more seedlings subsequent to harvest cuttings.
The group selection method is perhaps the silvicultural system best suited when the management goal is stand renewal through natural reproduction. This form of timber harvest guarantees a permanent supply of seed and offers a good chance for gradual establishment of reproduction even in those parts of the pine region with a pronounced arid and continental climate. The canopy of the remaining trees provides a more favorable microclimate than in the open and protects seedlings at a time when they are most vulnerable to environmental stress. However, the selection method requires very careful logging and slash disposal to avoid often severe damage to established reproduction.
Shelterwood cutting is well-suited for the harvest of even-aged stands. This has been demonstrated in the extended even-aged stands of ponderosa pine in the Black Hills. Shelterwood cuttings have also been employed successfully in western Montana and northern Idaho. Weidman suggested a 2-stage shelterwood cut for ponderosa pine in eastern Oregon and Washington. He felt that the first cut should remove 30 percent of the stand and that 20 to 25 years would be required for establishment of regeneration and removal of the overwood. Apparently, the soundness of his prescription has not been tried out much in practice. Judging by the lack of documentation, the shelterwood system has been used rarely for the management of pine forests on Oregon. Presumably, the shelterwood method can give satisfactory results in Oregon's pine region if the seed cut is followed by good or fair seed crops and care has been exercised in selecting the trees comprising the residual stand. The most vigorous and full-crowned dominants should be left after the first shelterwood cut because these trees are the best seed producers. Studies in northern California demonstrated that almost 90% of the ponderosa pine seed in a stand was produced by the dominant trees. If a substantial overwood is reserved, waiting several years for a seed crop should be an acceptable practice. In case natural regeneration fails, underplanting of the residual stands remains a feasible alternative.
Clearcutting is the least desirable choice when establishment of a new stand is being attempted by natural regeneration. Most of the sites in the ponderosa pine region are sufficiently severe so that clearcutting creates a highly adverse environment for growth and survival of newly germinated seedlings. Furthermore, advance scheduling of cuts for natural regeneration is difficult because crops do not occur at regular intervals and absolutely reliable methods of forecasting are not available. Presence of developing cones indicates chances for a good cone crop a year ahead of its maturity. But such "advance notice" is not an iron-clad guarantee because even potentially heavy crops do not always mature.
If natural regeneration is desired, clearcuts must be kept small because dissemination of ponderosa pine seed occurs only over short distances. Surveys of cutover areas in eastern Oregon during the 1920's showed that open sites larger than 2 acres were generally understocked. Barrett suggested that clearcutting in central Oregon should be done in small patches or narrow strips less than 5 chains wide. Seedfall decreases rapidly with increasing distance from timber edge and so does the number of sound seeds reaching the ground.
Use of the seed tree method has been recommended for natural regeneration on cutting areas wider than 260 feet. The seed tree method has been used extensively for regenerating ponderosa pine forests in Oregon before World War II. Often this did not reflect any planning for regeneration. Rather, the seed trees were residual trees left from diameter-limit cuttings, usually to diameters of 15 to 16 inches. Weidman concluded that the seed tree method offers good prospects for adequate stocking, even in areas with scant or no advance reproduction, within 10 years after cutting. His opinion was based on a survey of stocking on 10 cut-over areas located throughout the pine region of central Oregon. Notwithstanding Weidman's view, the seed tree method has probably more often resulted in failure than success in the pine region of central Oregon.
The number of seed trees required for provision of an adequate supply of seed will vary according to differences in site. Fowells and Schubert estimated that 3 or 4 ponderosa pines 20 to 26 inches in diameter in Dunning tree classes 1 to 3 would produce enough seed for adequate reproduction if rodent control and ground preparation have prepared the way for regeneration. Foiles and Curtis feel that 8 to 12 dominant trees 16 to 24 inches dbh will provide enough seed in an average seed year.
These estimates may be taken as guidelines, although the number of seeds required to produce an acceptable crop of seedlings varies from site to site. Estimates of seed-to-seedling ratios, that is the number of seeds required for establishment of acceptable stocking, have varied from 55:1 to 120:1. Actually observed ratios for ponderosa pine in northern California range from 14:1 to 100:1. Anticipation of the combination of environmental factors favoring seed establishment is virtually impossible. Occurrence of good seed-to-seedling ratios is largely a matter of chance.
Mortality of first-year seedlings of ponderosa pine is usually very high. Fowells and Schubert assumed that even under favorable conditions, only 1 out of 4 seedlings germinated would survive. Observation of sample plots over 15 years in central Oregon showed average mortality of first-year seedlings to be over 90 percent. Shade provided by a light cover of slash has a beneficial effect on germination and survival. Heavy concentrations of slash are undesirable because they prevent seeds from reaching mineral soil or cause injury to advance reproduction.
Ponderosa pine germinates best on mineral soil. Seedbeds consisting of burned soil and ashes can be adverse or beneficial to seedling establishment depending on degree and extent of burn and nature of soil. Organic materials such as duff and litter are generally poor seedbeds for ponderosa pine. However, once seedlings are growing in seedbeds of litter or other organic debris, mortality tends to be less than on mineral soil because a soil cover of organic material acts as a mulch preventing frost heaving and preserving soil moisture. This may also be the reason that seedlings germinate and grow in soil covered with squaw carpet (Ceanothus prostratus). In general, however, scarification is probably the most effective way of aiding germination and of increasing effective distance of seed dissemination by increasing the seed-to-seedling ratio.
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