DIGITALNA ARHIVA ŠUMARSKOG LISTA
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 ŠUMARSKI LIST 1-2/1966 str. 92     <-- 92 -->        PDF other hand, very small plots yield insufficient data on productivity and do notpermit a reconstruction of the population tested.Obviously a compromise is needed. The requirements specified above placesuch extreme demands on the experiment that it is absolutely necessary tominimize them.In order to achieve this minimization of requirements, six questions havebeen asked, and it is the purpose of this paper to find answers to thesequestions on the basis of information available in the literature.Question 1. What is the minimum number of sample trees that will describe a stand?Here recourse must be taken to the experience of the forest mensurationist.According to Tischendorf (1927) for observations of equal weights the meanvalue for these observations will change very little when the number of sampletrees is increased from 14 to 18.A further increase of sample the number of trees will leave the arithmeticmean almost unchanged.An even more extreme point of view is advanced by Hummel (1955). Hehas calculated that about 8 sample trees may lead to maximum errors ofbetween five and ten percent in the estimate of the total volume of a plotwhen his own tariff method of volume estimation is used. To be on the safeside, he recommends the use of 20 trees in calculating the tariff (volume basalarea line) for a stand. His method is good only for conifers and young hardwoods.Its popularity with the British forestry practice is a measure for itssuccess.The volume basal area line is a very good indicator of the productivity ofa stand. If the error in volume estimates is only 10!%, the error in height or indiameter estimates should be considerably less.For practical purposes smaller differences between provenance are of littleconsequence. Thus it can be assumed that Hummel´s minimum size of a sample(namely 8 trees) corresponds to the minimum number of trees that will givea fairly reliable estimate of the value of a given provenance.Both Tischendorf and Hummel refer to uniform stands. In the provenanceexperiments the uniformity of site conditions can be assured, provided theexperimental area is not too large.It can be assumed that the site conditions, age of the trees and the silviculturaltreatments will be more uniform in the experimental plots than is thecase in normal woods. Also the genetic variability within a provenance is likelyto be less than within a seed lot obtained from a commercial seed extractionplant. Thus it is certain that the uniformity conditions specified by Tischendorfand Hummel will be fulfilled. For this reason the adoption of the minimalrequirement of 8 trees would appear permissible.Question 2. At what minimum age will reliable information on productivity perunit area be available?The standard volume tables for Central Europe prepared by Schwappach(1943) start from the respective ages of 25 and 30 years in the better site classesfor pine and spruce. Thus presumably the volume per unit area for the youngerstands cannot be meaningfully estimated.Tyszkiewicz (1961) claims that at the age of 15—25 years pine has alreadypassed the period of its mostTntensive growth in height. This would presumably