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 not permit a reconstruction of the population tested. Obviously a compromise is needed. The requirements specified above place such extreme demands on the experiment that it is absolutely necessary to minimize them. In order to achieve this minimization of requirements, six questions have been asked, and it is the purpose of this paper to find answers to these questions 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 mean value for these observations will change very little when the number of sample trees is increased from 14 to 18. A further increase of sample the number of trees will leave the arithmetic mean almost unchanged. An even more extreme point of view is advanced by Hummel (1955). He has calculated that about 8 sample trees may lead to maximum errors of between five and ten percent in the estimate of the total volume of a plot when his own tariff method of volume estimation is used. To be on the safe side, he recommends the use of 20 trees in calculating the tariff (volume basal area 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 its success. The volume basal area line is a very good indicator of the productivity of a stand. If the error in volume estimates is only 10!%, the error in height or in diameter estimates should be considerably less. For practical purposes smaller differences between provenance are of little consequence. 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 give a fairly reliable estimate of the value of a given provenance. Both Tischendorf and Hummel refer to uniform stands. In the provenance experiments the uniformity of site conditions can be assured, provided the experimental area is not too large. It can be assumed that the site conditions, age of the trees and the silvicultural treatments will be more uniform in the experimental plots than is the case in normal woods. Also the genetic variability within a provenance is likely to be less than within a seed lot obtained from a commercial seed extraction plant. Thus it is certain that the uniformity conditions specified by Tischendorf and Hummel will be fulfilled. For this reason the adoption of the minimal requirement of 8 trees would appear permissible. Question 2. At what minimum age will reliable information on productivity per unit 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 classes for pine and spruce. Thus presumably the volume per unit area for the younger stands cannot be meaningfully estimated. Tyszkiewicz (1961) claims that at the age of 15—25 years pine has already passed the period of its mostTntensive growth in height. This would presumably |