E. GIORDANOERVEDO GIORDANO is research coordinator at the Centre for Experimental Agriculture and Forestry, Rome, Italy, and assistant professor at the University of Bari.
FEW INVESTIGATIONS deal with the interaction between breeding and intensive cultural practices. This subject, in fact, has not in the past held much interest, either for geneticists or for silviculturists. How-ever, the problem exists, and it must receive more attention in the future. Wright (1962) pointed to the importance of the problem when he wrote that forest breeding is highly effective for cultivated species, while it proves of minor importance for naturally reproducing ones.
FAO (1970) estimated that forest plantations in 1965 covered a surface of some 80 million hectares. The yearly production of round timber from such stands when mature will be about 500 million cubic metres. Breeding alone can increase such production substantially, but breeding done with an eye to intensive cultural practices will lead to the greatest gains in productivity.
This paper has, therefore, two parts: the influence exerted on plant phenotypes by intensive cultural practices in the nursery and in the field; and the most valuable means of achieving the best results with improved stock.
Most of the present observations refer to fast-growing species, such as eucalypts, poplars, Pinus radiata, Pseudotsuga menziesii and Cryptomeria japonica. For these the cultivation experience is greatest. However, other species deserve more attention.
EFFECTS OF CONTAINERS
The use of containers to grow young forest trees has spread rapidly in recent years. It has expanded from dry to temperate regions. This practice ensures less damage and desiccation of the root system than in bare-rooted plants.
Rigid containers, such as plastic bags or tubes, tin boxes, and terracotta pots, present some inconveniences. The root system, once it reaches the container bottom, winds up in a spiral and becomes malformed. After growing seedlings 9 months in polyethylene bags Giordano (1967) proved root malformation of 67 percent in Eucalyptus bridgesiana, 61 percent in E. globulus, 54 percent in E. trabutii, and 47 percent in E. viminalis.
The influence of root malformation on tree habit is clearly evident in all soil types. In fact, some years after the plantation establishment, the area around the root collar shows a swelling due to callus tissue and due to the joining together of lateral roots. Moreover, Pinus radiata usually records a diameter decrease at the tree base; in this zone the bark loses its characteristics and becomes smooth and gray-black. The most serious alterations are found in the area of connexion between the callus and the roots; in many cases this connexion consists only of a small basal crook just a few centimetres thick. Under such conditions, both water and mineral supplies to the stem are restricted, and in the long run growth is markedly arrested.
The danger of using planting stock with a distorted root system was stressed by Toumey and Korstian (1942) and confirmed by Gruschow's (1959) research on P. taeda. Little and Mergen (1966) reported that external and internal changes associated with basal crook formation in P. rigida and P. echinata affect tree habit for a period of 10 to 12 years.
This problem cannot be neglected in plantations employing selected stock or those set up to study the performance of different genetic materials. As an illustration, 11 percent of Eucalyptus dalrympleana trees were broken off by the wind owing to defective nursery growing. This made it difficult to compare six provenances field-planted on the outskirts of Grosseto, Italy, for their resistance to low temperatures. The result was a great inconvenience to an international trial (Gemignani, 1969). It is indispensable to give plants normal growth conditions. Distorted roots should be eliminated by proper pruning or by using decomposing pots made of peat, wood fibres, or paper.
Species vary in their adaptability to being raised in containers. It is important to determine the degree of adaptability and to consider for container growing only those types or forms with a suitable root system.
EFFECTS OF TAP-ROOT PRUNING
Root pruning is widely practiced to produce stock having a desirable stem/root balance and to stimulate differentiation of lateral roots. In some species, for instance in Picea excelsa, the stem/root ratio does not vary with age before transplanting. In other species it rapidly increases.
When carried out at the proper time and in the right way, pruning roots in conifers generally produces positive effects. It does not have a negative influence on tree habit or characteristics, at least in the juvenile stage. However, young plants can obviously suffer from lack of nutrients when more than 50 percent of the root system is pruned.
In broadleaved trees this practice is limited to the species grown in nurseries for over a year. Root pruning prevents the stem from excessive growth. The effect is the same as in conifers, except that special attention has to be paid to soil mineral content. For instance, in the case of Liriodendron tulipifera (Thor, 1965), root pruning did not give very effective results when the nitrogen content of the soil was low.
When fast-growing species, particularly eucalypts, are raised in containers, the taproot grows so rapidly that it is necessary to prune it two or three times. These repeated cuts markedly affect root success and tree growth; according to Mangieri (1961) failures can reach 20 to 30 percent. Moreover, the capacity for other roots to replace the taproot varies considerably from species to species. This occurs more easily in Eucalyptus camaldulensis and E. trabutii than in E. globulus and E. viminalis. In the latter species such behaviour can compromise height growth and plant stability. Thus, rapidity of taproot development is a characteristic to be promoted in many species.
EFFECTS OF TOPPING SEEDLINGS
Seedlings are topped in many nurseries to prevent excessive height growth and to establish a good stem/root balance before outplanting.
Apical buds markedly dominate in conifers, in which active apical meristems hinder or delay lateral bud development. Besides such a morphological polarity, clear evidence also exists for physiological polarity. Carbohydrates and other metabolites tend to move toward active zones of growth, such as the apical meristems (Kramer and Kozlowski, 1960). Imbalance between apical and lateral buds may give rise to bifurcate or ill-shaped plants. For example, examination of Pinus radiata seedlings 9 months after topping showed that a single lateral bud was apically dominant in only 42 percent of the subjects while, in the remainder, three or more buds developed at the same time (Giordano, 1968).
Topping of seedlings provokes no serious inconvenience in species lacking a well-defined polarity, such as eucalypts. Root success proves higher in topped trees (Goes, 1962). However, marked differences occur between species; Eucalyptus globulus and E. viminalis have less reaction to this treatment than E. botryoides, E. camaldulensis and E. trabutii.
Topping should be done only when it is indispensable, and then it should be carefully performed. Whenever trees are topped there is a risk of producing trees having modified phenotypes.
EFFECTS OF SELECTING SEEDLINGS
Seedling selection prior to planting is usually practised in most forest nurseries. The principles governing it relate to plant morphology and size and phytosanitary conditions. Elimination of poor subjects, those that are unhealthy, damaged or showing bent, bifurcate or multiple stems, favours the creation of stands with a lower number of failures and more uniform trees.
The influence of seedling size on subsequent tree growth is not yet well known, because complex external factors mask genetic differences. Sweet and Wareing (1966), studying growth of one-year-old seedlings. of Betula pubescens, Larix leptolepis and Pinus radiata, showed that the considerable discrepancies recorded were due to some influence at the moment of seed germination or soon after it. Further, while the absolute growth rate is important in determining size in first-year seedlings, there is no evidence yet that it will have equal importance during the later stages of plant development. Thus, while the breeder could if requested select for faster growth rate in seedlings, there is no present guarantee that this will give larger trees at rotation age.
However, early selection, as it is practiced in many nurseries, offers some advantages. Callaham and Duffield (1963) observed that progenies of P. ponderosa selected according to height growth at two years were superior in height at age 15. Bethune and Langdon (1966) pointed out that the largest seedlings of P. elliottii in the nursery were 35 percent taller 6 years after planting than middle-size nursery stock.
Nursery selection is very important for species with a high degree of heterogeneity, like eucalypts. A concrete example is provided by the results attained in Brazil by double selection, that is by selecting both when transplanting the seedlings into the containers and when outplanting. In some plantations of Eucalyptus saligna set up with nursery-selected stock, the number of nonmerchantable trees (diameter less than 5 centimetres) went down to 8 percent, while it reached 36 percent for unselected controls. At the end of the rotation, the yield of trees from selected stock proved 27 percent higher than that from unselected stock (De Andrade, 1961).
In a comprehensive analysis of results obtained over the last few years, Nanson (1968) underlines the positive influence of mass selection which eliminates the smaller seedlings.
Establishing homogeneous plantations is of the first importance for many countries. For this purpose, strict measures have been envisaged by European Economic Community countries for production and sale of forest trees having special features of height and diameter. The species considered are: Albies alba, Fagus silvatica, Larix decidua, L. leptolepis, Picea abies, P. sitchensis, Pinus nigra, P. sylvestris, P: strobus, Pseudotsuga menziesii, Quercus borealis, Q. pedunculata, Q. sessiliflora and poplars. The importance of selecting seedlings for superior height growth is clearly evident for poplars, for seedlings are to be propagated vegetatively and trees are to be grown on very short rotations.
VEGETATIVE PROPAGATION
Propagation by cuttings or grafting is the most rapid method to obtain improved stock. Grafting is largely adopted when establishing seed orchards. Propagation through cuttings has been resorted to for centuries, mainly for poplars in Europe and for Cryptomeria japonica in Japan. Selected trees obtained from cuttings are more uniform than those from seed and show a more rapid initial growth. The number of species to be propagated by cuttings on a large scale has increased over the last few years. Remarkable advances have been recorded in propagating eucalypts, cypresses (Franclet, 1969), and Pinus radiata (Thulin, 1969).
The type of material from which cuttings are taken exerts a considerable influence upon the phenotype. Thulin observed that cuttings from 7-year-old P. radiata trees retain the morphological stage of development reached by parent trees. Repeated vegetative propagation also can give rise to some inconveniences. Miyajima (1969) reported that the repeated propagation by cuttings of Cryptomeria japonica can bring about a decrease in growth energy.
Therefore, the creation of mono-clonal plantations makes it advisable to create gene banks to avoid the danger of the reduction in frequency or elimination of genes useful for further improvement.
EFFECT OF PLASTIC GREENHOUSES
Polyethylene greenhouses and plastic coverings have been widely diffused during the past few years for the production of forest planting stock. In northern Europe, mainly in Finland, a cultural practice has been perfected for raising plants from seed collected from plus trees. The results are very promising, since sowing under plastic cover allows the seedlings to develop rapidly. Planting stock for comparative trials between different genetic materials is being produced under plastic cover.
Plastic greenhouses are also proving particularly useful both in propagation of cuttings and in grafting. In southern Europe they are extremely useful in revealing genetic differences.
Plastic greenhouses create problems. Special attention has to be paid to soil sterilization, in order to avoid the diffusion of weed and fungal parasites. As a rule, the seedlings grown in greenhouses are less lignified than those raised in open fields, and such trees demand a longer period of adaptation when passing from the moist environment of the greenhouse to usually drier outside conditions.
EFFECTS OF SOIL WORKING
The depth of soil available for root development exerts a remarkable influence on plant habit and growth, mainly during the first years. A characteristic instance of this is provided by Pinus halepensis. On rocky substrata or shallow soil this species develops a sinuous stem and irregular ramification. It presents on the contrary a well-differentiated and straight stem on deep soil. This behaviour is typical of many other species, but it is more evident in fast-growing ones, mainly eucalypts. Therefore, the importance of deep soil ploughing to allow trees a regular development is clearly evident.
INFLUENCE OF SPACING
Spacing is one of the factors that affects both height, and, still more, diameter growth. Branch formation. and development too depend in part on it. Thus spacing influences the whole tree habit.
Plant reaction to phototropism is greatly influenced by spacing. Most euramerican poplar hybrids are very susceptible to phototropism. If planted too close, stems bend toward the light. Thus, phototropism must be given great attention when hybrid poplar production is meant for pulp and panels. Conversely, Populus nigra var. pyramidalis, chiefly the provenance from Turkey, grows perfectly straight, even when spaced very closely (40 to 50 centimetres).
Spacing can influence other tree characteristics, such as specific gravity, basic density, and fibre features. Observations in Turkey by Currò et al. (1965) on a stand of Populus x euramericana outplanted at four different planting distances showed that neither specific gravity nor basic density varied significantly with different espacements. Conversely, fibre length decreased as spacing decreased, ranging from 1178 microns at 5 x 8 metres spacing to 984 microns at 3 x 4 metres spacing, while fibre diameter increased with less space between trees (25.6 microns at 5 x 8 metres spacing and 28.5 at 3 x 4 metres spacing).
EFFECTS OF SELECTIVE THINNING
Thinning is particularly important for young plantations, since they respond vigorously to the release from competition. Trees are usually chosen according to size, habit, potential growth and state of health.
Trees to be favoured are those producing the highest wood quantities as compared with the space occupied (Matthews, 1963). Hence, it is necessary to take into account not, only morphological features, but also physiological influences. In physiological terms, thinnings prove efficacious when they increase the growth processes. It is, therefore, exceedingly important to provide for the best development of crowns. Some plants produce many cones and utilize their carbohydrate reserves to the detriment of growth. According to Matthews, such trees should be eliminated in the juvenile stage.
INFLUENCE OF PRUNING
Strong competition develops between the main axis and lateral branches in many species. When not eliminated, lateral branches develop considerably and give trees an irregular shape. This phenomenon is evident in fast-growing species. For some species, such as Pseudotsuga menziesii, Pinus radiata, and Chamaecyparis lawsoniana, pruning is closely related to the type of timber utilization and to the economy of the operation. For eucalypts and poplars pruning appears to be indispensable in order to achieve desired tree shapes. The effect of pruning on eucalypt habit is clearly evident. It fosters the development of trees with a well-differentiated stem and superior height (Gemignani, 1967). Pruning during the first four years favours the development of a regular stem in poplars, which are usually planted at wide spacings. In fact, in the Mediterranean region, selected poplar clones have big branches under the apical bud which tend to prevail over the leader from the second year after planting. When such branches are not removed, the young poplars show a moderate height growth and a flattened crown. To promote height growth, poplars must be pruned very early, from the second year of growth in the nursery.
INFLUENCE OF COPPICING
Coppicing is extensively practiced with broadleaved trees, mainly chestnut, beech, alders and willows. Eucalypt coppicing with rotations of 12 to 14 years is widely adopted in many countries. Influences of coppicing on plant habit are evident. As a rule, Eucalyptus camaldulensis and E. trabutii show straighter stems and more regular ramification after the first coppicing. Stands too appear more uniform. This improvement is observed also in some species, such as E. stellulata, which usually presents a crooked stem.
Finally, it is interesting to note that coppicing repeated twice at short intervals for some bushy-shaped seedlings of E. viminalis made it possible to obtain trees of normal form.
Improved stock should be given all necessary cultural assistance to prevent external causes from decreasing its potential productivity. Such practices entail, of course, larger expenses. It is up to breeders to detect plant forms that reduce demands for cultural care to a minimum or that react successfully to them.
CHOICE OF SITE
The most favourable sites should be planted to improved stock. In exceptional cases the purpose may be to produce plants suitable to difficult site conditions. In any case, soils with improper reaction and anomalous physiochemical conditions are to be avoided.
SOIL WORKING
Site preparation, including thorough soil cultivation, is indispensable for plantation success.
The present trend to abandon localized working of small patches in favour of strip or complete ploughing is justified. Better tilling will supply roots with a greater amount of soil to explore. Most species react positively to the new practice. Cayford (1961) found in Manitoba that success of transplants of Picea glauca (2 + 3), Picea mariana (2 + 2) and Pinus banksiana was higher on strip-worked soil than on uncultivated soil. Stoeckeler (1962) in the hills of Wisconsin worked the ground in terraces 3.5 metres long and 1.80 metres apart and achieved very high survival (90 to 95 percent) for 25000 plants of 11 species. The important afforestation work carried out in Spain, France, Israel and Italy is further evidence of the positive influence of complete ploughing. Investigations conducted in France by Duchaufour et al. (1961) on terra rossa revealed that pines and cedars show a higher growth in fully cultivated plantings than in hole plantings, owing to a greater moisture content during the summer.
Thus, initial growth of trees is positively affected by soil working. Smith and Smith (1963) proved that the initial growth of P. palustris was markedly higher on worked soil.
Deep soil working promotes plant growth for a long period of time. Trials performed in the Ukraine by Drjucenko (1963) pointed out the positive effect of deep ploughing on P. sylvestris and P. nigra var. caramanica up to 11 years after plantation establishment.
Deep and complete ploughing is increasingly being practiced with soils destined to specialized cultures, i.e., poplar cultivation, since hole planting on compact soil can bring about serious consequences. In fact, if hole planting is performed by means of mechanical borers, soil compaction hinders regular root development.
ELIMINATION OF SPONTANEOUS VEGETATION
Soil working has two main purposes: eliminating the competing vegetation and making soil nutrients productive (Huygh, 1957). In practice, it is difficult to find a compromise between complete weeding and the maintenance of maximum soil fertility.
Remarkable progress has been recently achieved in this regard by means of herbicides. Most research on this subject concerns weed removal in nurseries, but in some countries promising results have also been obtained in achieving natural and artificial regeneration and in improving the growth of young plantations. In New Zealand, in particular, elimination of competing vegetation is fundamental to the success of P. radiata (Little, 1966). In the Federal Republic of Germany, Günther (1966) investigated the utilization of herbicides (2,4,5T and dalapon) on a large scale. Herbicides proved extremely useful in promoting plant growth in the steppe region of the Ukraine (Tarasov, 1967).
The use of herbicides on a large scale needs to be improved further, since damage may sometimes result for plantations (Oldenkamp, 1966). Root collar injuries can occur, for instance, and residual effects of treatments on soil are still barely known.
Breeding trees able to withstand weed competition would represent a great success.
THINNING
Use of improved stock demands the elimination of excessive competition between plants, in order not to reduce growth. Early thinnings or wide initial spacings are of prime importance for the future development of stands. Many factors support the validity of this cultural practice. The use of small-size stock by papermaking industries has given early thinnings a new practical interest.
Utilization of fast-growing poplar clones, such as Populus x euramericana I 214, makes thinning indispensable. In a poplar plantation established near Rome with cuttings at 1.5 X 1.5-metre spacing, a marked decrease in diameter growth can be noticed since the third year (Giordano and Avanzo, 1968).
As to eucalypts, thinning is usually limited to stands meant for the production of saw timber. It is done to remove suppressed trees having diameters less than 15 centimetres; in Brazil, it is carried out in the tenth year after planting.
Early thinnings prove of primary importance for P. radiata raising in New Zealand.
Initial spacing and thinning schedules will have to be determined and adjusted for each improved variety. Wherever possible, initial spacings should be wide enough to permit rapid growth to continue until the first thinning can be made at a profit.
PRUNING
Pruning is the most direct method for further improvement of stem quality in selected clones. It is indispensable for poplars to produce knot-free timber for plywood and matches.
Pruning aims at obtaining in a very short time clear stems up to 6 to 8 metres in height without compromising growth. Lack of pruning or badly performed treatments can annul the benefit of employing clones selected for high productive capacity or for special timber properties. This has recently been confirmed in Europe; owing to a crisis in the poplar market, the price of timber from trees not submitted to regular prunings has considerably decreased.
Pruning of eucalypts is limited to the first two years of growth. It favours differentiation of the main axis and lightening of the crown. In some species, for instance Eucalyptus globulus, wide crowns develop after the second year, and trees are easily broken off by the winds. According to Goes (1962), over the first two years basal branches should be left unpruned up to a height of 1.30 metres.
Intensive culture treatments can increase the number of branches produced. Thus it can increase requirements for pruning. In some cases intensive culture leading to more branching may offset successful breeding to reduce branch numbers or to speed natural shedding of branches.
FERTILIZATION AND IRRIGATION
Use of fertilizers in forests is relatively recent, and there is still much to do to derive the greatest benefit from them. Investigations in Norway, Sweden, New Zealand and other countries indicate that the application of fertilizers provokes a growth increase lasting 5 to 10 years. Besides increasing wood production, fertilizers make plants more vigorous and more resistant to the attacks of some insects, bacteria and fungi. Effects of fertilization become more pronounced when nutrients are added to relatively deficient soils.
Fertilizing is advisable when using improved stock. Euramerican poplars, for instance, respond appreciably to it. In the Po valley of Italy mean tree weight recorded at the end of the 13-year rotation proved to be 12 quintals for fertilized trees and 4.5 for nonfertilized trees. In both Portugal and Brazil fertilization of eucalypts gave satisfactory results in areas with high precipitation, but it was less promising in regions with low rainfall. Given that the majority of tree improvement programmes aim at increasing wood production, the need to enable plants to exploit soil productive capacities fully is evident.
Irrigation is of' prime importance for the success of plantings established with poplar stock selected in most of the Mediterranean and Near East countries.
Intensive cultivation trials have been recently conducted also in the United States, by fertilization and irrigation of selected poplar material. Furthermore, the selection of more drought-resistant clones is under way in different Mediterranean countries, owing to poor water resources.
As breeding progresses, more will be learned about the requirements of varieties for nutrients and water. Undoubtedly tree breeding will parallel breeding of crop plants and produce varieties having relatively low requirements for nutrients and moisture.
CONTROL OF INSECTS AND DISEASES
Much research dealing with tree improvement has as, its main object the detection or production of planting stock resistant to disease (Schreiner, ] 966). In some cases, selection also seeks species and forms less susceptible to insect attacks. Callaham (1966) emphasized the increasing importance that this type of research will acquire in the future, owing to the dangerous consequences stemming from an ever larger use of insecticides.
Appropriate controls should be envisaged in normal planting programmes when improved stock is utilized on a large scale. In Italy, for instance, special control measures have been studied for the poplar plantations of the Po valley. These have been attacked recently by Marssonina brunnea, causing early defoliation. In the States of Texas and Mississippi, United States, selected poplar clones can be used only when the trees are preventively treated with systemic pesticides to prevent attacks by defoliators. This represents further but indispensable expenditure, if the fruit of long-term improvement work is not to be destroyed.
Intensive culture can have both positive and negative effects on damage caused by pests. On the positive side, treatments that accelerate growth generally improve health of trees and reduce their susceptibility to attack by pests. On the negative side, intensive culture can lead to epidemics. The latter is particularly true where only one or a few susceptible varieties are planted over large, contiguous areas. Biological controls can be effected by breaking up the mass of susceptible material, for instance, by planting multi-clonal hybrid varieties (Schreiner, 1966).
Evaluating the interaction between breeding and intensive culture proves extremely hard. However, it appears obvious that one must provide all the cultural care necessary, in order not to cancel or reduce the advantages deriving from the use of improved stock.
Most of the cultural treatments mentioned should also be practiced in plantations established with unimproved planting material. It can be expected that breeding aimed at improving effects of single cultural practices will lead in the future to the highest biological and economic productivity.
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