DIGITALNA ARHIVA ŠUMARSKOG LISTA
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ŠUMARSKI LIST 1-2/1966 str. 130     <-- 130 -->        PDF

hybrid P. angulata x P. berolinensis (hybrid between P. pyramidalis x P. laurifolia)
is characterized by intensive growth. Table 2 could be further expanded
by including several P. tremula x P. tremula hybrids many of which do not
show any heterosis at all.


A review of both these tables (1 and 2) leads to the conclusion that the
phenomenon of heterosis occurs more commonly in hybrids whose parental
species come from regions far removed from one another or from stands
otherwise isolated from each other (e.g. the aspen of Dimpfelmeir comes from
various altitudes of the same mountain region).


When the crossing takes place between individuals of the same species
or of closely related species, the phenomenon of heterosis can be observed
only when the parental trees represent distant provenances.


This does not explain the heterosis of P. canescens, since the species P.
alba and P. tremula occur next to each other and their hybrid occurs in many
regions where their ranges of distribution coincide. However, the heterosis


may be the result of the parents being adapted to different ecological
environments.
OBSERVATIONS MADE IN KÖRNIK


Let us compare the hybrids within the subgenus Leuce. In Table 3 are
presented the parental trees, the mean heights of their 5-year-old progenies
(from population samples numbering 30 trees), their standard deviations, and
the standard deviations as percentages of the means.


The lowest mean height was obtained from the cross P. alba from Körnik
x P. tremula from Szczecinek, which can be regarded as an »unsuccessful
cross«. It does not significantly differ from the not much better progeny of
the cross P. tremula x P. tremula of local origin, which has the lowest
variance. P. alba from Körnik which was pollinated from an introduced tree
of the same species has a significantly greater mean height, and its highest
progeny as well as that with the greatest variance is the one which comes
from the cross P. tremula from Körnik pollinated by an introduced P. alba.
The probability of obtaining some individuals characterized by heterosis
increases when the variance is greater. On the other hand, it can be seen
from Table 3 that a greater variance occurs when the mean height is greater.
In order to obtain an objective measure of the variability that would be
comparable, the standard deviation expressed as a percentage of the mean
value is to be used. In the mentioned groups of individuals the greatest
variability was exhibited by the progeny of the cross between a Körnik P.
tremula and an introduced P. alba. The lowest variability was found in the
population of the indigenous aspens.


A similar example for the sub-genus Eupopulus is shown in Table 4. As
before the data refer to the heights of 5-year-old seedlings for 4 hybrid
progenies with 30 sample trees per progeny.


The tallest ones and also significantly differing from the other is the
progeny of the cross P. angulata x P. laurifolia (an intersectional hybrid).
The remaining hybrid progenies do not differ significantly as to their mean
heights. However, the variance expressed as a percentage of the mean places
the progenies in a different sequence (2-3-1-4-). The greatest variability is
shown by the hybrid P. angulata x P. nigra (intrasectional!) which means


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