DIGITALNA ARHIVA ŠUMARSKOG LISTA
prilagođeno pretraživanje po punom tekstu
| ŠUMARSKI LIST 1-2/1966 str. 76 <-- 76 --> PDF |
because (1) the performances expected at older ages are still not known, (2) inferior individuals have not yet been rogued and they contribute inferior genes to the population. However, in the case of gene preservation plantations, these points do not apply. Here the objective is to collect seeds from the registered plantations with the minimum loss of genes. Therefore, the seeds should be collected as soon as the stands reach full sexual maturity. Besides the four major conifers, we have several tree species which are harvested in natural forests and are desired in the market but are not planted. Some of them supply rather large quantities of timber, such as Abies firma, Tsuga sieboldii, Castanopsis cuspidata, and Machilus thunbergii. Others, such as Sciadopitis verticillata, Torreya nucifera, Betula grossa, Quercus gilva, Zelkova serrata, Cercidiphyllum japonicum, Cinnamomum camphora, and Fraxinus sieboldiana, are rather scarce but produce timber of good quality. Natural stands usually show much smaller increments of standing crops than artificial plantations, so they are being rapidly replaced with the latter. Although there are still fairly large stands of naturally regenerated second growth, it is quite seldom that superior specimens of these species are able to establish their offspring. Because of this, the preservation of the gene pool is urgently needed. In fact, it is already too late for some species such as T°rreya nucifera, of which almost all of larger specimen has been felled selectively. For those species such as Abies firma or Castanopsis cuspidata which appear more commonly in the form of almost pure stand, the same method of sampling as for planted stands can be employed. For the species of rare occurrence, it is necessary to select a sufficient number of seed trees throughout a wider region in order to avoid considerable inbreeding. It is well known that an isolated group of a few individuals have a high degree of inbreeding, which increases generation by generation, until little genetic variation is shown within the group (Sewall Wright efect). In such cases, only a small area is needed for the preservation of each small population, but the desirable procedure is to obtain wide genetic variation by mixing several populations into a single plantation. The species which are not often planted are usually slow-growers and require large dimensions for their commercial maturity. Therefore, they have to be retained far longer than the usually planted conifers.Such slow growth is the main reason why these valuable species have not been artificially propagated, and it makes it difficult, at the same time, to get an area of forest land to plant the seedlings for gene preservation. In the authors opinion, the best place to grow them is on the protection belts which separate commercial plantations and consist of natural vegetations. The belt is used as a safeguard of the plantation against fire or pests and, in total, amounts to very large areas. It is, therefore, favorable not only from the genetic point of view but also from the commercial one that the seedlings of these valuable species are planted mixed with natural second growth, because the operation will give increased quality products. Unfortunately, we have not yet been able to start such a program, but there are good prospects to do so soon. Finally, we have to discuss the analysis of the genetic variation of the populations. From the view point of gene preservation only, it is not necessary to distinguish seeds from individual seed trees, but it is not difficult to collect |