DIGITALNA ARHIVA ŠUMARSKOG LISTA
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ŠUMARSKI LIST 1-2/1966 str. 111     <-- 111 -->        PDF

if we compare them with pure P. deltoides and P. nigra. In Europe, the cross
betwoen P. tremula and P. tremuloides is also very promising. This hybrid
displays a vigorous growth, especially in northern Europe. It is assumed that
in this case difference in latitude of origin plays a greater role than the
heterosis Phenomenon itself, by extension of the growing season in comparison
with that of the mother´s parents (26, 40). A good growth rate is also exhibited
by the crosses P. trickocarpa x P. nigra, and P. trichocarpa x P. deltoides.


The genus Quercus is very difficult to study from the aspect of hybridization
and therefore has not been extensively investigated. Piatnitsk y


(41) established the existence of heterosis in the hybrid Q. cerris x Q. suber.
In the genus Eucalyptus heterosis was evidenced in two cases of interspecies
crossing (60).
In the genus Betula, the heterotic effect appears to exist in the hybrid


B. papyrifera x B. lenta. Johnsso n (25) states that the hybrids B.
verrucosa x B. japonica, B. verrucosa x B. papyrifera, and B. pubescens x
B. papyrifera exceed the average stem heights of the parental species at seven
years of age.
Besides these examples of interspacific hybrids exhibiting the phenomenon
of heterosis, we consider it necessary to mention those hybrids which surpass
in growth rate one of their parents or are resistant to diseases. It was
established that the hybrid between Pinus nigra and P. densiflora grows at
a slower rate than P. densiflora but much faster than P. nigra. We also
carried out crossing of P. nigra x P. densiflora and found that two-year-old
hybrid plants did not always grow faster than the Pinus nigra mother tree;
the growth rate depends upon the individual tree taken as a mother tree.
The hybrid between Pinus nigra and P. silvestris appears to grow faster than
Austrian pine but more slowly than Scots Pine (52). The hybrids P. thunbergii
x P. densiflora, P. thunbergil x P. taiwanensis, P. thunbergii x P.
yunnanensis, P. thunbergii x P. massoniana, P. thunbergii x P. nigra austriaca,


P. densiflora x P. thunbergii, P. densiflora x P. silvestris, P. nigra austriaca x
P. thunbergii, P. nigra austriaca x P. tabulaeformis, and P. ponderosa x P.
latifolia grow more rapidly than the first-named female parent The hybrid
P. echinata x P. taeda grows faster in California than shortleai pine (P.
echinata). The hybrid P. rigida x P. taeda outgrows one parent in some localities
and in other localities the other parent. The hybrid between P. monticola and
P. strobus is significant not only because it surpasses one of its parents in growth
rate in several localities, but also because by the selection of parental partners
for this combination it is possible to obtain a blister rust-resistant progeny
(5, 6). With regard to resistance to blister rust, the hybrid between P. strobus
and P. pence is also promising (16).
A spontaneous hybrid between Picea abies and P. sitchensis shows at the


age of 17 years 25% faster growth than Sitka spruce, and has been much less


affected by cold than the latter (47).


The hybrid between Fraxinus excelsior and F. americana likewise exhibits


at 12 years of age a better growth rate than F. americana. In addition, this


hybrid displays great survival capacity (47).


A 1 b e n s k i j (2, 4) crossed Acer negundo with A. platanoides, and Acer
negundo with A. saccharinum. In both instances he obtained progenies more
vigorous than the female parent.


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