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ŠUMARSKI LIST 1-2/1966 str. 9     <-- 9 -->        PDF

SELECTION OF PLUS TREES*
(Introductory paper)
by


MIRKO VIDAKOVIĆ


Forestry Faculty, Zagreb and Institute for Conifers, Jastrebarsko, Yugoslavia.


INTRODUCTION


Plus trees are used as stated by Schreine r (36) for: (1) the establishment
of seed orchards, (2) direct silvicultural purposes, if a clonal propagation by
means of cuttings is possible, and (3) use in hybridization, i.e. in tree breeding
work. In consequence it is understandable that great attention is paid to the
selection of plus trees. Both the success of further work in tree improvement
and the final result depend in many ways on the choice of the initial material.
With the development of forest genetics, methods of plus tree selection have
also been developed. In the initial stages of forest genetics, when work was
done with great enthusiasm but perhaps less knowledge about the heritability
of characters in forest trees, the choice of plus trees was performed mainly
by means of visual estimate. It was believed that such a selection is completely
sastisfactory, and that further use of the material selected in this manner will
yield desirable results. With an ever intensified development of forest genetics
new conceptions were formed. As theortical knowledge and experience with
forest tress, increased it was realized that a good phenotype need not always
be a good genotype. It was established the majority of commercially important
properties is of a quantitative character. It was likewise found that the habit
of a tree is influenced by the site, the silvicultural system, and the environment
under which a tree grows. All this contributed to a more serious approach to
the problem of plus tree selection. If we take into consideration the object of
tree improvement as well as the search for the safest possible criteria for the
selection of trees, we notice all the difficulties with which investigators are
confronted in the solution of these problems. It should be stressed that in this
direction much has already been done.


In this report we are going to treat a number of problems and individual
views about plus tree selection in order to provide a basis for discussion and
explanation of the individual view points. We consider that the following
questions are important:


1.
Intensity (criterion) of selection.
2.
Development and use of mathematical indices in the selection of plus
trees.
3.
Selection for one, two or more characters.
* The author wishes to express his thanks to Professor Howard B. Kriebel who
has read and corrected the manuscript.


ŠUMARSKI LIST 1-2/1966 str. 10     <-- 10 -->        PDF

4. Biology of the species and its variability.
5. Inheritance and environment.
6. Selection of trees in plantations.
7. Selection fcr wood quality.
1. INTENSITY (CRITERION) OF SELECTION
The intensity (criterion )of selection can vary. When making a selection
of plus trees of Loblolly Pine (Pinus taeda L.) Peev y (31) classified the
individual characters by points. In order for a tree to be chosen, it had to
achieve a definite minimum of points. According to Prokazi n (32. 33. 34),
the selection of plus trees should be carried out in seed production areas where
previously an inventory was made. Besides other characters, a plus tree should
surpass in diameter b.h. the stand mean tree by over 1.1. According to Mat thew
s (24) evaluation of trees can be performed a) by the descriptive
method, b) by the method of classification, and c) by the method of
measurement. Usually all three methods are combined. The method of measurement
is least affected by the subjectivity of the selectionist and therefore it
should always be applied if possible. Requiring and practising intense
selection is easier if we are concerned with characters which can be
measured, rather than with those which are expressed in a descriptive manner.
The intensity of selection may vary according to selection objectives. If selection
is performed for one or two characters, the selection intensity can be more
severe than if a greater number of characters are under consideration.
Schreine r (36) recommends that selection be restricted to those trees which
exceed the mean by two or three. M e r g e n (25), on the other hand, states
that the selected tree should lei four standard deviations above the mean. If
selection intensity is expressed in this manner, it is impossible to establish
the same criterion for all important characters, because if the intensity is very
severe for some properties as for instance cylindrical term and staightness of
the stem, it will be impossible to select desirable trees.


2. DEVELOPMENT AND USE OF MATHEMATICAL INDICES IN THE
SELECTION OF PLUS TREES
Ander ss on (1) states that selection of plus trees has been performed
so far on the basis of a relative estimate of the growth rate, stem form, saw-
timber grade, wood quality, seed production and resistance. The criteria for
plus trees are changing in different part of the world. They depend on the
species and local conditions as well as the basis of selection. Selection of plus
trees for superior production has sometimes been carried aut on the basis of
estimated indices. Likewise, selection was based on a multitude of characteristics
which were understandably related to the tree quality. Andersson states that
it was possible to use progeny tests as a basis for the selection of plus trees
only to a small extent. He further states that we need selection indices, as
recommended by L u s h, in order to make the selection of trees more effective.
For obtaining such selection indices it is necessary: (1) to have information
about the additive genetic variance and inheritance of each characteristics, and


(2) to test the genetic and phenotypic covariance between these characteristics.
It is possible to achieve all this on the basis of specifically-planned progeny
tests.


ŠUMARSKI LIST 1-2/1966 str. 11     <-- 11 -->        PDF

Such investigations are becoming more and more topical, and their success
naturally depends on the extent of development of forest tree improvement
in individual countries. Thus, for instance, in the report of the School of
Forestry, North Carolina State College, for the year 1963, it is mentioned that
in a few years it will be possible to determine a selection index by means of
which it will be possible to determine much more accurately the value of a
selected superior tree to serve as a parent for the seed orchard. More has been
done in this direction with coniferous than with breadleaved species. The
tropical species have had the least attention.


At present we do not have at our disposal selection indices based on progeny
tests of plus trees. Therefore it is necessary to develop selection methods based
on mathematical calculations and indices, on the basis of which the assessment
of trees will be srfer and the subjective factor eliminated. Cec h (8) came
to the same conclusion on the basis of a comparison of 5 systems for the
selection of superior trees used in the Southern States of the U.S.A. On the
basis of his results he concludes that the system is not so important as the
estimator, and therefore he considers the numerical system to be better than
the less objective descriptive systems.


The attempt by Douglas s (10) to work out a method of selection of
Japanese larch trees which are superior in wood volume production is most
interesting. This selection method is based on the ratio of tree volume to crown
volume. In previous methods for the selection of superior trees there is a lack
of connection with the stand mean, which is necessary, a Douglas s says,
to predict the degree of improvement to be expected. It is clear that a final
evaluation of the superiority of trees would have to be based on testing the
heritability of this property. Such a selection method, when established in
detail, will surely mean a pace ahead compared with these based on the volume
of the selected tree in relation to neighbouring trees, or when taken as the
largest tree of a number of trees or the tree displaying the largest volume
increment or some other good characteristics. Growth rate estimation in
the selection of southern Pines in the U.S.A. as described by Brow n
and Goddar d (4) also merits discussian. According to them, assessment
of growth rate in even-aged stands should be based on the ratio
basal-area increase to crown size (length x mean radius). Rudolp h (35)
recommends use of the ratio of d-h (square of diameter b.h. x tree height)
to D2L (square of crown diameter x crown length) for tree evaluation. By
plotting these data on a coordinate system it is possible to construct curves for
average and plus trees. The most reliable results from the use of this system
are obtained from estimates of trees in even-aged stands.


We should continue such work toward early development of reliable indices
for selection of plus trees, both in eve-aged and selection forests. Clearly
success will be greatest in an even-aged stand where spacing between trees is
equal and the site is of equal quality, and if the selection is made among trees
belonging to the same diameter class.


3. SELECTION FOR ONE, TWO OR MORE CHARACTERS
Selection of plus trees will be much more successful if it is made for one
or two characters (6, 36, 40). The difficulty and the possibility of errors increase
in geometric ratio to the increase of the number of characters, except if these




ŠUMARSKI LIST 1-2/1966 str. 12     <-- 12 -->        PDF

characters are not genetically interrelated (Me r ge n 25). Moreover, it is very
difficult to find a tree which is excellent in many characters. If on the other
hand, we lower the criterion, i.e. the intensity of selection, we cannot expect
any greater improvement in desired characters in the subsequent generation.
Trees are now more and more selected with regard to one or two characters.
Thus we have, for instance, quality-plus trees, volume-plus trees, trees selected
with regard to the technical properties of wood, plus trees selected as resistant
to diseases, insects or non-biotic factors (frost, drought, etc.).


Investigations carried out so far have proved that in forest tree species a
number of characters are genetically bound, i.e. that a correlation exists between
them. For instance, Kiellande r (19) established a correlation in spruce
between bud formation and resistance to spring frosts. K i r s t (20) found that
a correlation exists in Beech between bark shape and wood properties.
L a n g n e r and Ster n (22) found a correlation in spruce between time of
bud bursting on the one hand and height growth and latewood formation on
the other. Hois t (16) found a correlation in Norway spruce between size of
crown and resistance to weevil. Fieldin g (13) found in Monterey Pine (Pinus
radiata) a correlation between the number of whorls in branches which are
formed annually and the abundance of fruiting, and between the lenght and
thickness of branches on the one hand and bole straightness on the other.
Heimburge r (14) considers that degree of facultative parasite attack is
often negative by correlated with host vigor. Therefore the author recommends
that in selection we should aim at those individuals which exhibit an above-
average growth rate on an average quality site. The results achieved by
Ehren berg, Gustafsso n, Plym For shell, and by Si mak (12)
are important. They established that minus trees of Scots Pine have larger
cones than plus trees. Also, form, weight and number of seeds change with
change in cone shape. Although minus trees have heavier seeds and thus a
higher juvenile growth rate, the minus-tree progenies later fall behind in growth,
often by the third year. The authors consider that these as well as other
characters are caused by semidominant or semirecessive factors.


On the basis of the above information concerning the correlation of
characters we can conclude — as stated also by Ma 11 h e w s (24) — that in
selection of plus trees it is necessary to take into account correlation between
individual characteristics.


However, it is specially important to establish a correlation between the
parents and the progeny. Nil s s o n (29) established in Norway Spruce and
in Scots Pine that there exists a clear correlation between the ratio
height/diameter b.h. (H/D) of mother trees and the average height of the
10-year-old progeny. No correlation was found between the volume of mother
trees at constant form and the average height of the progeny. Therefore the
author recomended to select higher trees with equal diameter compared with
the greatest neighbour trees, as mother trees for crossing experiments and seed
orchards.


4. BIOLOGY OF THE SPECIES AND ITS VARIABILITY
In the selection of plus trees it is important to be well acquainted with
the biology of the individual species. Knowledge of local races and ecotypes
will greatly facilitate selection of the most suitable trees for further improvement




ŠUMARSKI LIST 1-2/1966 str. 13     <-- 13 -->        PDF

work and establishment of seed orchards, or for immediate use for silvicultural
purposes. Norway spruce and Scots pine are examples, with their high and low
elevation races, as is the Norway spruce ecotype occurring in frost localities.
In plus-tree selection, attention should also be paid to phenology B o 11 a n d (2).
For instance, the length of the growing period is an important factor. In beech,
according to Hoffman n (15), there are differences in time of flushing
within provenances and between provenances. It is also very important to
know the range of variability of characters to which we pay special attention
in the selection of plus trees. A determination of the range of variability will
enable us to approach more safely the selection of plus trees. For the purpose
of illustrating this, D o r m a n (9) demonstrated that the distribution curves
of height, diameter b.h. and basal areas of two evenaged stands of Slash Pine
(Pinus elliottii Engelm) are different. 2 u f a (44) has established for stem
straightness of Black poplar from the middle Danubian lowlands that the
range of variability of this character is not the same in all investigated
populations. He also found that the magnitude of variation of this property
is to a large extent genetically conditioned. In contrast, Campbel l (7)
established in Douglas fir that its branching or crown habit is to a very small
extent an inherited character in a broad sense, which points to the fact that
the selection of plus trees for this character is futile.


It is well-known that tree habit varies with age. Therefore this should also
be taken into consideration in plus-tree selection. Because in younger coniferous
trees the micrcecological factors influence their habit, Bouvare l (3)
recommends that in fast growing conifers, one select plus trees among specimens
which have reached or passed their normal age of maturity. In Norway spruce
this would be 100—120 years, and in Douglas Fir over 50 years. Matthew s
and McLea n (23) stated that in selection of plus trees of Scots pine, difficulty
arises from the variation in external appearance of the tree with age and
growth conditions. Mitchel l (26) reported that variability in tree habit of
European Larch at different ages leads to a new problem in tree selection. A
50-year-old tree should have an excellent habit, height and diameter increment.
At the age of 100—120 years such a tree changes its shape, but in spite of this
the decisive factors in selection at this age are large height and diameter
increments, and the quality of the stem. Trees of European larch between 100
and 150 years of age are selected on the basis of bole quality, diameter and
length, which in this case is more important than the total height.


We consider that for the present it is difficult to give a final statement
as to how old an individual should be for selection as plus tree. Both upper
and lower age limits should be neither too high nor too low. At the same time
it is important to know the variabitily of habit of trees in relation to age, as
described by Mitchell for larch.


Just because of this variability in a species we consider it nesessary to
select a great number of plus trees. Only in this manner will it be possible to
cover the variability of a character and reduce risks in further improvement,
as stated by Stern (37).


The work of Open s ha w (30) is very interesting from the standpoint of
improvement. After classifying 140-year-old Scots pine trees into 11 types
according to habit he observed that types with horizontal branches (especially
the broadcrowned ones) have the best volume characteristics from a utilization




ŠUMARSKI LIST 1-2/1966 str. 14     <-- 14 -->        PDF

standpoint. His investigations indicated that the most desirable genotype of
Scots pine is the one with a wide crown and horizontal branches.


These investigations indicate that tree breeders should give more
consideration to economic factors in establishing objectives of tree improvement.
We consider it necessary to intensify such investigations, and to extend them
to other species. In this country, forest industries utilizing oak regard the superior-
trees from the standpoint of money yield to be those which do not have a very
long stem, but instead have a large fork at the base of the crown.


It is understandable that such a manner of assessing forest trees has
repercussions in the choice of plus trees. The question is raised whether or
not plus trees are those which have a narrow and small crown with fine and
thin branches.


5. INHERITANCE AND ENVIRONMENT
In the selection of plus trees the factors of environment should also be
taken into consideration. They influence in many ways the habit and growth
rote of the tree. In Norway spruce, the dimensions of mother trees are to a
greater extent than has been assumed the result of environmental factors rather
than inheritance according to Kiellande r (19). The best trees of European
larch were found in England on very good sites. Where a factor is limiting,
trees have a tendency to concentrate on this factor at the expense of others (26).
Wild e (39) stressed that in the selection of plus trees, the productive capacity
of the soil should also be taken into consideration. Matthew s (24) stated
that in selection of plus trees attention should also be paid to the climatic
conditions, age, origin and life history of the tree or of the whole population.


It is also important to pay attention to the silvicultural treatments which
have been caried out. Microclimatic and microedaphic conditions are also
important Jovančevi ć (18) recommends paying attention in the selection
of plus trees to the ground vegetation and to the lowest phytocenological units
(fades) which can serve as an indicator for microecological conditions, because
in such localities it is possible to find local races of forest tree species. Wc
believe that determination of the plant association in natural stands where the
tree occurs will also give us indications about the ecological conditions of the
population. In a general way it is necessary to know well the external conditions
under which the tree is growing in order to evaluate the individual tree as
well as possible.


If we know the external conditions under which a tree has grown and the
biology of the species, we shall be able to assess approximately in what measure
the observed character is the reflection of the inherited factors, and in what
measure the reflection of the environment, even without full knowledge of
heritability of important characters.


6. SELECTION OF TREES IN PLANTATIONS
Selection of plus trees can be made from non-indigenous stands if these
stands represent the progeny of one uniform population (Langne r (21)). If,
however such a culture represents individuals from different populations in
which many various genotypes of unknown origin are included, selection of
plus trees cannot be made. This statement refers, as also Langne r states,




ŠUMARSKI LIST 1-2/1966 str. 15     <-- 15 -->        PDF

to the selection of plus trees which should have been used for seed orchards.
It is certain that in this case caution should be exercised. We consider, however,
that for further improvement also such plus trees can be used.


7. SELECTION FOR WOOD QUALITY
In tree improvement work more and more attention is paid to wood quality,


i.e. anatomical and physical properties of wood. According to Mitchel l (28),
the most important properties are: density, ratio between earlywood and
latewood within the growth ring, number of growth rings per centimeter, fibre
length and thickness, fibril orientation and the occurrence of abnormalities.
Mitchel l (27) considers that these anatomical and physical properties,
especially the position of fibrils and wood density, are genetically controlled.
Zobe l (41) reported in 1954 that specific gravity of the wood of Loblolly
Pine (Pinus taeda L.) is under close genetic control. Later works of Zobe l
and his collaborators on improvement of southern Pines in the U.S.A. strengthened
our knowledge of inheritance of specific gravity. Z o be i (43) says in
general that the length of the tracheids and specific gravity in Conifers are
under genetic control.
Investigations have shown that there are also variations in branching,
growth, stem straightness and other morphological characteristics. Therefore
in the improvement of wood quality it is possible to select individuals with
desired characteristics. Thus, for instance, Du f fiel d (11) reported variation
in trachcid lenght in Pseudotsuga menziesii (Mirb.) Franco. Similar types were
found among trees within the same stand. By means of a graphical method
it was possible to estimate the extreme plus and minus variants. Va n
B u i j t e n e n (5) found significant differences in wood density among clones
of Loblolly pine (Pinus taeda L.). He also found differences in wood density
in progenies originated from controlled pollinations. According to Mitchell (28),
the well-established ratio between wood density and pulp content provides a
method of evaluating trees on the basis of wood density. Trees can be classified
as below average, average, above average or outstanding with respect to wood
quality. We believe that in the improvement of wood quality the mathematical
indices will be determined first, for use in the selection of plus trees. Substantial
progress has already been made toward achievement of this objective.


SELEKCIJA PLUS STABALA


UVOD


Plus stabla koriste sc, ´kako navodi Schreine r (36), za podizanje sjemenskih
plantaža, za uzgoj ukoliko je moguće klonsko razmnažanje putem rožnica te za korišćenje
kod hibridizacije, tj. rada na oplemenjivanju. Prema tome razumljivo je da
se velika pažnja pridaje izboru plus stabala. Od izbora početnog materijala u mnogome
ovisi i uspjeh daljnjeg rada na oplemenjivanju kao i konačni rezultat. Razvojem
šumarske genetike razvijale su se i metode izbora plus stabala. U početnim
stadijima šumarske genetike kada se radilo s velikim entuzijazmom, ali možda s manje
znanja o nasljeđivanju svojstava kod šumskog drveća, izbor plus stabala vršio
se uglavnom okularnom procjenom. Smatralo se da takav izbor u potpunosti zadovoljava
i da će daljnje korišćenje tako izabranog materijala sigurno dati željene re




ŠUMARSKI LIST 1-2/1966 str. 16     <-- 16 -->        PDF

zultate. Sve intenzivnijim razvojem šumarske genetike dolazilo se do novih spoznaja.
Obogaćena su teoretska znanja i praktična iskustva o šumskom drveću, došlo se dc
saznanja da dobar fenotip ne mora biti uvijek i dobar genotip. Utvrđeno je da je većina
privredno važnih svojstava kvantitativnog karaktera. Isto je tako ustanovljeno
da na izgled jednog stabla utječe i stanište, način uzgoja te sredina u kojoj raste.
Sve je to pomoglo da se još serioznije pristupi problemu izbora plus stabala. Ako se
uzme u obzir još i cilj oplemenjivanja kao i pronalaženje što sigurnijih mjerila kod
izbora stabala, očigledno je kakve sve poteškoće stoje pred istraživačima u vezi s rješavanjem
svih tih problema. Mora se naglasiti da se u tom pravcu već i dosade,
mnogo uradilo.


U ovom referatu iznijet ćemo neke probleme kao i pojedina mišljenja o izboru
plus stabala kako bi taj prikaz mogao poslužiti kao baza za diskusiju i za razjašnjenje
pojedinih stavova. Smatramo da je kod toga važno slijedeće:


1. intenzitet (kriterij) selekcije,
2. pronalaženje i korišćenje matematskih indeksa kod izbora plus stabala,
3. selekcija na jedno, dva ili više svojstava,
4. biologija vrste i njezina varijabilnost.
5. nasljednost i okolina,
6. izbor stabala u plantacijama,
7. selekcija na kvalitetu drva.
1. INTENZITET (KRITERIJ) SELEKCIJE
Intenzitet (kriterij) selekcije može biti različit. Peev y (31) vrši bodovanje pojedinih
karakteristika kod izbora plus stabala Pinus taeda. Da bi jedno stablo bilo
izabrano, treba postići određeni minimum bodova. Prema Prokazin u (32, 33, 34)
izbor plus stabala vrši se u sjemenskim bazama gdje je prije toga provedena inventarizacija.
Plus stablo treba da osim ostalih karakteristika svojim prsnim promjerom
nadmašuje srednje stablo sastojine preko 1,10. Prema Matthews u (24) ocjena
stabala može se vršiti: a) deskriptivnom metodom, b) metodom klasiranja i c) metodom
mjerenja. Obično se kombiniraju sve tri metode. Metoda mjerenja ne podliježe
subjektivnosti selekcionera i zbog toga je treba primjenjivati uvijek kad je to
moguće. Postavljanje zahtjeva na intenzitet selekcije kao i njezino provođenje lakše
je ako se radi o svojstvima koja se mogu mjeriti, a ne o onima koja su opisno izražena.
Intenzitet selekcije može biti različit, što ovisi o njezinu cilju. Ako se selekcija
vrši na jedno ili dva svojstva, njezin intenzitet može biti stroži, nego kada se uzima
u obzir veći broj svojstava. Schreine r (36) preporučuje da se selekcija ograniči
na takva stabla koja se nalaze za 2 ili 3 standardne devijacije iznad prosječnog stabla.
M e r g e n (25) pak iznosi da se odabrano stablo treba nalaziti za 4 standardne
devijacije iznad prosjeka. Kada se intenzitet selekcije izrazi na taj način, onda se
ne može odrediti isti kriterij za sva važnija svojstva jer se u takvom slučaju — ako
je intenzitet jako strog — za neka svojstva kao što je npr. prunodrvnost i pravnost
debla ne bi mogla izabrati željena stabla.


2. PRONALAŽENJE I KORIŠĆENJE NUMERIČKIH INDEKSA
KOD IZBORA PLUS STABALA
Andersso n (1) navodi da je odabiranje plus stabala dosta vršeno na bazi
relativne procjene rasta, oblika stabla, kvalitete deblovine, strukture drva, uroda
sjemena i otpornosti. Kriteriji za plus stabla varirali sxi u različitim dijelovima
svijeta. Oni ovise o vrsti drveća, lokalnim uvjetima kao i o postavljenoj osnovi za
selekciju. Selekcija je prema kvaliteti produkcije plus stabala provedena u nekim
slučajevima na temelju procijenjenih indeksa. Isto tako ta je selekcija bazirana na
mnoštvu karakteristika koje su, razumljivo, u vezi s kvalitetom stabala. Autor napominje
da je samo u manjem opsegu bilo moguće da se koristi testiranje potomstva
kao baza za selekciju plus stabala. Andersso n osim toga navodi da su nam potrebni
selekcioni indeksi koje preporučuje Lus h jer bi tako selekcija stabala bila
efikasnija. Da se dobiju takvi selekcioni indeksi, potrebno je: 1. đa se znadu rezultati
o aditivnoj genetskoj varijanci i nasljednosti svake karakteristike i 2. da se
jspita genetska i fenotipska kovarijanca između tih karakteristika. Sve je to moguće
postići na bazi specijalno planiranih testova potomstva.


Takva istraživanja sada su sve aktuelniija a to, naravno, ovisi o stepenu razvoja
i dostignućima oplemenjivanja šumskog drveća u pojedinim zemljama. Take




ŠUMARSKI LIST 1-2/1966 str. 17     <-- 17 -->        PDF

se npr. u izvještaju Šumarskog fakulteta, Sjev. Karolina za 1963. godinu navodi da
ie se za koju godinu moći odrediti selekcioni indeks pomoću kojega bi se mnogo
tačnije odredila vrijednost selekcioniranog stabla koje će služiti kao roditelj za sjemensku
plantažu. U tom je pravcu općenito više -učinjeno na koniferama, nego na
listačama. Najmanje su u tom pogledu obrađene tropske vrste drveća.


Budući da zasada uglavnom još ne rapolažemc selekcionim indeksima koji se
baziraju na testiranju potomstva plus stabala, potrebno je pronalaziti takve metode
selekcije koje će se osnivati na računskim procjenama i indeksima na osnovi kojih
će ocjena stabala biti sigurnija, a subjektivni momenat eliminiran. To mišljenje zastupa
i Cec h (8). On je do takvog zaključka došao na temelju komparacije pet sistema
za izbor boljih stabala koji se koriste u južnim državama SAD. Na osnovi
rezultata koje je dobio zaključuje da sam sistem nije toliko važan koliko procjenjivao,
te zbog toga smatra da je numerički sistem bolji nego manje objektivni deskriptivni
sistemi.


Smatramo da je u tom pravcu interesantan pokušaj Douglass a (10) na izradi
metode selekcije stabala japanskog ariša koja su bolja u produkciji drvne mase.
Ta metoda selekcije bazira se na odnosu obujma stabla prema obujmu krošnje. Kod
dosadašnjih metoda nedostaje pri izboru boljih stabala veza s prosjekom sastojine
koja je potrebna da, kako Dougla s kaže, predskaže stupanj poboljšanja koji se
može očekivati. Jasno je da bi se konačna ocjena boljih stabala osnivala na ispitivanju
nasljednosti spomenutog svojstva. Osim toga ovakva metoda selekcije, kada.
se detaljno utvrdi, bit će sigurno jedan korak naprijed u odnosu na one kod kojih
se obujam odabranog stabla uspoređuje sa susjednim stablima, ili se uzima kao najjače
stablo od izvjesnog broja stabala, ili kao stablo koje ima najbolji volumni prirast
i neka druga dobra svojstva. Isto je tako vrijedno naglasiti i procjenu rasta
kod selekcije južnih borova u SAD koju su opisali Brow n i Goddar d (4)
Prema njima procjenu rasta u jednodobnim sastojinama treba bazirati na odnosu
između prirasta temeljnice i veličine krošnje (duljina x srednji radius). Rudolp h


(35) isto tako preporučuje da se kod ocjene stabala koristi odnos d2h (kvadrirani
promjer stabla u prsnoj visini x visina stabla), prema D2L (kvadrirani promjer krošnje
x duljina krošnje). Nanošenjem tih podataka na koordinantni sistem mogu se
konstruirati krivulje za prosječna kao i za plus stabla. Razumljivo je da se i u tom
slučaju najsigurniji rezultati dobiju kod ocjene stabala iz jednođobnih sastojina.
Zbog toga smatramo da takve i slične radove treba nastaviti kako bismo što
prije došli do sigurnijih mjerila (procjenbenih indeksa) za selekciju plus stabala u
jednodobnim kao i u prebomim sastojinama. Razumljivo je da će uspjeh biti najveći
u jednodobnoj sastojini gdje bi razmak između stabala bio jednak i gdje je
jednako stanište te ako se selekcija vrši između stabala koja´pripadaju istom deb-
Ijinskom razredu.


3. SELEKCIJA NA JEDNO. DVA ILI VIŠE SVOJSTAVA
Selekcija plus stabala bit će mnogo uspješnija ako se vrši s obzirom na jedno
ili dva svojstva (6, 36, 40). Stupanj poteškoće i mogućnost pogreški povećava se u
geometrijskoj progresiji s povećanjem broja svojstva izuzevši ako nema genetske
povezanosti između spomenutih svojstava. Osim toga vrlo je teško naći stablo koje
m a više odličnih karakteristika. Ako se pak kriterij, tj. intenzitet selekcije snizi
ne možemo očekivati u narednoj generaciji veće poboljšanje s obzirom na tražena
svojstva. Možemo reći da se stabla sada sve više selekcioniraju uzevši u obzir jedne
ili dva poželjna svojstva. Tako npr. imamo kvalitet-plus stablo, volumen-plus stablo,
plus stabla odabrana s obzirom na tehnološka svojstva drva, plus stabla izabrana
kao otporna na bolesti ili štetnike odnosno na abiotske faktore (mraz, suša
ltd.).


Dosadašnja su istraživanja pokazala da su kod šumskog drveća neka svojstva
genetski vezana, tj. da između njih postoji korelacija. Tako je npr. Kiellande r


(19) za smrču ustanovio da postoji k^ralacija između tjeranja izbojaka i otpornosti
na proljetne mrazove. Kirs t (20) je utvrdio da kod bukve postoji korelacija između
oblika kore i nekih svojstava drva. Langne r i Ster n (22) su pronašli kod
smrče korelaciju između vremena otvaranja pupova s jedne strane te visinskog rasta
i stvaranja kasnog drva s druge strane. Hols t (16) je kod obične smrče našao
korelaciju između veličine krošnje i otpornosti prema insektima (pipama). Fielt ding
(13) je za Pinus rađiata ustanovio da postoji koralacija između broja pršljenova
kod grana koje se godišnje stvore i obilnosti uroda češera, zatim duljine i deb15




ŠUMARSKI LIST 1-2/1966 str. 18     <-- 18 -->        PDF

ljine grana te pravnosti debla. Heimburge r (14) smatra da je stepen napada
fakultativnih parazita često u negativnoj korelaciji s bujnosti domaćina. Zbog toga
autor preporučuje da se kod selekcije treba usmjeriti na one primjerke koji na prosječnim
staništima imaju iznadprosjecni rast. Vrlo su interesantni i važni rezultati
do kojih su došli Ehrenberg, Gusta fsson, Plym Forshell i Simak (12).
Oni su ustanovili da minus stabla običnog bora imaju veće češere od plus stabala.
Paralelno s ohlikom češera mijenja se i oblik, težina i broj sjemenaka. Premda minus
stabla imaju teže sjeme, a uslijed toga i veći juvenilni porast, potomstvo minus
stabala često već i u trećoj godini starosti počinje zaostajati u rastu. Autori smatraju
da su ta kao i druga svojstva uzrokovana semidominantnim ili semirecesivnim
faktorima.


Na osnovi izloženoga o korelaciji svojstava možemo zaključiti, kako i Mat thew
s (24) navodi, da je kod izbora plus stabala potrebno voditi računa o korelaciji
između pojedinih karakteristika. Međutim posebno je važno ustanoviti korelaciju
između roditelja i potomstva. Nilsson (29) je ustanovio kod smrče i običnog
bora da postoji jasna korelacija između odnosa visina (prsni promjer majčinskog
stabala i prosječne visine potomstva. Nikakva korelacija između obujma majčinskih
stabala kod konstantnog oblika i prosječne visine potomstva starog 10 god. nije nađena.
Zbog toga autor preporučuje odabrati za majčinska stabla viša stabla i istog
promjera komparirano sa najvećim susjednim stablima, koja majčinska stabla će se
koristit; za križanje i sjemenske plantaže.


4. BIOLOGIJA VRSTE I NJEZINA VARIJABILNOST
Kod izbora plus stabala važno je da se vrlo dobro pozna biologija dotične vrste.
Poznavanje lokalnih rasa i ekotipova u mnogome će olakšati rad u odabiranju najprikladnijih
stabala za daljnji rad na oplemenjivanju ili podizanju sjemenskih plantaža
ili pak za neposredno korišćenje u uzgojne svrhe. Dovoljno je spomenuti kao
primjer običnu smrču i obični bor s njihovim visinskim i nizinskim rasama ili ekotip
obične smrče koji dolazi na mrazištma. B o 11 a n d (2) naglašava da kod izbora
plus stabala treba obratiti pažnju i na fenologiju. Tako je npr. jedan od važnih faktora
duljina vegetacione periode. Kod bukve postoje prema Hoffmann u (15) razlike
u dobi listanja, kako individualne unutar jedne provenijencije, tako i između
provenijencija.


Isto je tako vrlo važno poznavanje raspona varijabilnosti svojstava na koja
obraćamo posebnu pažnju pri izboru plus stabala. Utvrđivanje raspona varijabilnosti
u pojedinim populacijama omogućit će nam da sigurnije pristupimo izboru
plus stabala. Za ilustraciju toga prikazuje Dorm an (9) da frekvencije distribucijskih
krivulja za visine, promjere i temeljnice dviju jednodobnih sastojina Pinus
elliottii nisu jednake. Zuf a (44) je ustanovio da za pravnost stabla crne topole iz
srednjeg Podunavlja raspon varijabilnosti toga svojstva nije jednak u svim ispitivanim
populacijama. Osim toga je utvrdio da je raspon variranja toga svojstva u dobroj
mjeri uvjetovan genetski. Nasuprot tome Campbel l (7) je pronašao kod duglazije
da je grananje odnosno krošnja u vrlo maloj mjeri nasljedno svojstvo u širem
smislu, što ukazuje na to da je izbor plus stabala na to svojstvo uzaludan.


Poznato je da postoji varijabilnost izgleda stabala s obzirom na starost. Zbog
toga i taj momenat treba uzeti u obzir kod izbora plus stabala. Budući đa kod mladih
stabala četinjača mikroekološki faktori utječu na njihov izgled, predlaže B o uvare
l (3) da se za plus stabla brzorastućih četinjača odabiru takva koja su dostigla
ili prešla normalnu sječnu zrelost kao npr. za običnu smrču 100 do 120 godina, duglaziju
preko 50 godina. Matthew s i McLea n (23) konstatirali su da kod izbora
plus stabala običnog bora čini poteškoću variranje vanjskog izgleda stabla sa
starosti i uvjetima rasta. Mitchel l (26) iznosi da varijabilnost u izgledu stabala
evropskog ariša kod različite starosti upućuje na novi problem kod selekcije stabala.
Stablo od 50 godina starosti mora imati odličan izgled, visinski i debljinski
prirast. To isto stablo u starosti od 100—120 godina izmijeni svoj oblik, ali su unatoč
tome odlučujući faktori za selekciju u toj dobi jak visinski i debljinski prirast
kao i kvaliteta debla. Stabla evropskog ariša između 100 i 150 godina starosti odabiru
se na bazi kvalitete, debljine i duljine debla, što je u ovom slučaju važnije nego
totalna visina.


Smatramo da je zasada teško iznijeti kategorično mišljenje koliko star treba
biti individuum koji će se odabrati kao plus stablo. Gornja i donja granica starosti




ŠUMARSKI LIST 1-2/1966 str. 19     <-- 19 -->        PDF

.stabala ne smije biti previsoka odnosno preniska. Osim toga važno je poznavati varijabilnost
izgleda stabala jedne vrste s obzirom na starost, kao što je to prikazac
Mitchell za ariš.


Baš zbog toga variranja koje postoji kod jedne vrste smatramo da je potrebno
izabrati veliki broj plus stabala jer će se samo na taj način moći najbolje obuhvatiti
varijabilnost jednog svojstva, a umanjiti riziko kod daljnjeg oplemenjivanja,
kako kaže Stern (37).


Sa stajališta oplemenjivanja vrlo su interesantni rezultati do kojih je došao
Opensha w (30). Klasiranjem stabala po habitusu u 11 tipova prema raspoloživom
materijalu kod običnog bora starosti od 140 godina ustanovio je da tipovi s horizontalnim
granama, posebno oni sa širokom krošnjom, imaju drvnu masu koja
prema upotrebljivosti daje najveći financijski efekt. Prema tim istraživanjima tip
običnog bora sa širokom krošnjom i horizontalnim granama je najpoželjniji genotip.


Ta istraživanja ukazuju da oplemenjivači moraju kod postavljanja cilja oplemenjivanja
voditi još više računa o ekonomskom efektu. Smatramo da bi bilo potrebno
da se takva istraživanja prodube i prošire na druge vrste šumskog drveća.
Kad kod nas u Jugoslaviji poduzeća koja prerađuju hrastovinu preuzimaju stabla
u šumi, smatraju kvalitetnijima (tj. da donose bolji financijski efekt) ona stabla
koja nemaju vrlo dugo deblo, ali imaju jednu jaku rašlju odmah na početku krošnje,
a krošnja im je prilično velika.


Razumljivo je da takvo promatranje drveća ima svojih reperkusija i na izbor
plus stabala. Postavlja se pitanje da li su prava plus stabla ona koja imaju vrlo
lijepu, usku i malu krošnju s finim i tankim granama ili ne.


5. NASLJEDNOST I OKOLINA
Kod izbora plus stabala treba imati u vidu i faktore okoline. Ona u mnogome
utječe na formiranje habitusa i prirast stabla. Kiellande r (19) je ustanovio za
običnu smrču da su dimenzije materinskih stabala u mnogo većoj mjeri, nego što
se pretpostavljalo, rezultat činilaca sredine, a manje činilaca nasljeđa. Najbolja
stabla evropskog ariša nađena su u Engleskoj na veoma dobrim tlima. Tamo gdje
jedan faktor ograničuje, stabla imaju tendenciju da se svode na taj faktor što ide
na uštrb drugih faktora (26). Wild e (39) naglašava da kod izbora plus stabala treba
imati u vidu i produktivnu sposobnost tla. Matthew s (24) donosi da kod izbora
plus stabala treba obratiti pažnju i na klimatske prilike, starost, porijeklo i
historiju stabla ili čitave populacije.


Važno je da se obrati pažnja i na uzgojne zahvate koji su vršeni, na mikroklimatske
kao i na mikroedafske prilike. Jovančevi ć (18) preporučuje u vezi s time
da je kod izbora plus stabala ponekad potrebno voditi brigu i o prizemnoj flori odnosno
o najnižim fitocenološkim jedinicama (facies), što nam može poslužiti i kao
indikator za mikroekološke prilike, budući da se na takvim lokalitetima mogu naći
i lokalne rase šumskog drveća. Smatramo da će nam i određivanje biljne zajednice
u prirodnim sastojinama gdje stablo dolazi dati indikacije c ekološkim prilikama
populacije. Uglavnom, treba dobro poznavati vanjske uvjete pod kojima stablo raste
kako bi se moglo što bolje ocijeniti pojedino stablo.


Ako poznajemo vanjske uvjete pod kojima je stablo raslo kao i samu biologiju
dotične vrste, bit ćemo u mogućnosti da — iako i ne poznajemo u punoj mjeri nasljednost
važnijih svojstava — približno ocijenimo u kojoj je mjeri promatrano svojstvo
odraz nasljednih faktora, a u kojoj mjeri odraz okoline.


6. IZBOR STABALA U PLANTACIJAMA
Prema Langner u (21) kod odabiranja plus stabala iz umjetno podignutih
sastojina izbor se može vršiti ako takve sastojine predstavljaju potomstvo jedne jedinstvene
populacije. Međutim, ako takva kultura predstavlja individue iz više različitih
populacija u k>ju su uključeni mnogi različiti genotipovi nepoznatog poriiekla,
selekcija plus stabala ne može se vršiti, to manje što su populacije različitijc.
Ova postavka se odnosi, kako i Langne r kaže, na izbor plus stabala koja bi se
koristila za sjemenske plantaže. Sigurno je da je u tom slučaju potreban oprez.
Međutim, smatramo da se za daljnje oplemenjivanje mogu koristiti i ovakva plus
stabla.




ŠUMARSKI LIST 1-2/1966 str. 20     <-- 20 -->        PDF

7. SELEKCIJA NA KVALITETU DRVA
Kod rada na oplemenjivanju sve se više pažnje pridaje i kvaliteti drva. Pod
kvalitetom u tom slučaju podrazumijevamo anatomska i fizikalna svojstva drva.
Prema Mitchel l (28) posebno važna svojstva su specifična težina drva, odnos
ranoga i kasnog drva u godu, broj godova na 1 cm, duljina i debljina vlakanaca,
orijentacija fibrila i pojava nekih abnormalnosti. Prema Mitchel l (27) ta anatomska
i fizikalna svojstva, a posebno položaj fibrila i specifična težina drva su pod
genetskom kontrolom. Zobe l (41) je još 1954. godine iznio mišljenje da je specifična
težina drva Pinus taeđa pod strogom genetskom kontrolom. Kasniji radovi
Zobe l a i njegovih suradnika na oplemenjivanju južnih borova u SAD s obzirom
na kvalitetu daju vrijedne informacije u tom pravcu. Zobe l (43) kaže općenito za
četinjače da su kod njih duljina traheida i specifična težina nasljednog karaktera.


Istraživanja su pokazala da isto tako postoji varijabilnost u anatomskim i fizikalnim
svojstvima kao što postoji varijabilnost u grananju, rastu, pravnosti debla
i drugim morfološkim karakteristikama. Zbog toga je kod oplemenjivanja na kvalitetu
drva moguće izlučiti individue sa željenim karakteristikama. Tako je npr. Dul fiel
d (11) ustanovio kod Pseudotsuga menziesii tipove kod kojih variraju duljine
traheida. Slični tipovi su nađeni među stablima unutar iste sastojine. Grafičkom
metodom mogu se procijeniti ekstremne plus ili minus varijante. Van Buijtene n


(5) utvrdio je signifikantne razlike u specifičnoj težini drva između klonova Pinus
taeda. On je također konstatirao razlike u specifičnoj težini drva i unutar potomstva
kontroliranog oprašivanja te vrste. Prema Mitchel l (28) dobro postavljen odnos
između specifične težine drva i sadržaja celuloze omogućuje procjenjivanje. Stabla
se mogu Masirati na ispod prosjeka, prosječna i na iznad prosjeka, ili opet takva
koja su naročite kvalitete. Vjerujemo da će se kod oplemenjivanja na kvalitetu najprije
ustanovit numerički indeksi koji će se koristiti kod izbora plus stabala. Kao
što smo spomenuli, taj se postupak već najavljuje.
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