DIGITALNA ARHIVA ŠUMARSKOG LISTA
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| ŠUMARSKI LIST 1-2/1966 str. 33 <-- 33 --> PDF |
candidate« and from a number of check trees have been plotted, over the mean width of the annual rings of the wood samples. In this case, the proposed plus tree has a substantially higher basic density than the mean basic density for sample trees in the same habitats of given latitude and height above sea level; wood samples also indicate that the basic density of the plus tree is greater than the annual ring width would suggest. Measurement of the check trees has several purposes. These measurements, with those for the plus trees, form the basis for the calculation of the regression functions. In addition, they are a means of checking that the function is valid for the habitat in question; and they make it possible to determine whether the basic density of the proposed plus tree has changed, as a result of thinning, fertilization, and other changes in the environment of the stand, in the same way as that of the check trees. In order to make such a historical review possible, the wood samples, which are composed of cores, have been divided into sections consisting of ten annual rings, beginning from the outside. The mean width of the annual rings and the basic density are determined, for each section of the core, with the instrument described by E r i c s o n, 1959. The percentage ratio between the observed basic density, and that calculated by means of the regression function is termed the relative basic density (see Ericson , 1960 b and Fig. 6 in this report). The basic density has been determined for a number of plus trees and for grafts from these trees. Despite the fact that the grafts were grown under a different temperature climate than were the original trees, and that the mean width of their annual rings varied widely, the relative basic density of the original trees and grafts are in good agreement with each other, (see Ericson , I960 b, Fig. 7, 9, and 13). Consequently, we can expect a high degree of heritability in respect to relative basic density. Routine investigations of the basic density, of all the proposed plus trees are now carried out. The measurements are processed by means of a computer. As a rule only tree whose relative basic density exceeds 100, are approved as plus trees. Relative basic densities down to about 95 per cent are tolerated exceptionally, if in other important economic properties a tree is eminently superior to other plus trees. Several plus trees have already been found whose relative basic density is over 115; and what is perhaps equally important: several proposed plus trees with a relative basic density of 85 per cent or less have been detected and their inclusion in clonal seed orchards has been forbidden.« Another property of plus trees that has attracted more and more attention is their seed production capacity and seed quality. Climatic tolerance during the course of meiosis, seed setting, and seed development, is especially desirable in the extreme high altitudes of northern Sweden. Only a low per cent of the Norway spruce have, in these climatic regions, a good fitness or adaptability with regard to seed germination capacity and seed production. These properties vary widely. Consequently, the generative adaption of the trees to the climatic conditions of their habitats plays an important role in certain areas. Repeated observations on individual plus trees during a succession of seed years can furnish, useful information on characters such as seed quality, seed yield, and repeatability of these traits (see Anderss o n, 1965). The evaluation or assessment of certain complex characters is both difficult and uncertain, this is especially so, when several characters are being selected |