DIGITALNA ARHIVA ŠUMARSKOG LISTA
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ŠUMARSKI LIST 1-2/1966 str. 93 <-- 93 --> PDF |
correspond to the time when the trees have reached a period of steady increment in growth, and therefore the future performance is predictable. From the juvenile/mature correlations established so far it appears that at the time corresponding to 1/3 of the rotation age the quality and growth of trees can be predicted for the rest of the rotation. This is the criterion used in Sweden for the evaluation of the genetic value of different progenies (Johnsson 1964). Thus it would apear that not earlier than at the respective ages of about 30 and 35 years for pine and spruce it is possible to obtain information about the quality and productivity of the different provenances. Question 3. What is the minimum number of trees needed to reconstruct a population for production purposes? The most obvious way of reconstructing a population is by means of establishing a seed orchard from the available trees. All seed orchards already in existance are too young to give reliable information on the optimum number of clones for commercial seed production. To provide sufficient genetic variability in the resultant population the number of clones should not be too small. Andersson (1963) suggests that there should be 36—60 clones per plantation. Matthews (1964) considers 20 or 30 trees as an acceptable minimum number of trees in a seed orchard composed of plus trees. These are only estimates and they may be too optimistic. The more clones in a seed orchard, the greater the variability in the resultant population. Since seed orchards established from plus trees are likely to yield more elite seed than those established from trees remaining on the experimental plots of the best provenance, there must be some scope left for the thinning of clones in the seed orchard. Thus I would suggest that 40 is the minimum number of trees that have to remain per provenance at the time of establishment of a seed orchard from the provenance trial. Question 4. What is the maximum forest area that can be assigned to a provenance experiment? The two basic criteria that have to be considered before answering such a question are site uniformity and conformance with sound silvicultural practices. In Polish forestry regulations (Ministerstwo Lesnictwa 1961) it is specified that clear fellings should cover no more than 6 ha in one belt with a maximum width of 80 m. Generally, compartments in Polish forestry are of the order of 25 ha. These are divided into sub-compartments on the basis of site conditions. As far as possible every difference in site requiring a different silvicultural treatment or a different species composition is delimited into a sub-compartment. A provenance experiment should be located within one sub-compartment, and it should be rectangular in shape. Thus it is not likely that more than 5 ha. could be found for one trial area, but usually less. |