J. JASSO M.J. JASSO M. is director of the National School of Forest Rangers at Uruapan, Michoacan, Mexico.
ALL ORGANISMS evolve but, in the struggle for existence and survival, the best adapted is not always the best in regard to characteristics of economic importance. This is a natural phenomenon, but it is also possible to exert great influence over organisms to alter their evolution. Man is responsible for this influence. He can be either a positive or negative influence in regard to the economic characteristics of the raw materials of forestry, depending on how he treats forests in his charge.
Since time immemorial people have had to use forest resources and forest land for agricultural and livestock purposes. Through technology and research, better utilization of forests and the land has been achieved. Unfortunately, education in forestry matters has not progressed at the same rate. Very few people are directly concerned with forestry problems. Only in countries like Sweden, where a large part of the economy is dependent upon forest industries, are people forest-conscious.
Humans beings have had, and continue to have, a great influence in altering the gene resources in forests. They have eliminated variability within genera by eliminating entire species. They have altered patterns of variability by changing gene frequency within species. Man has eliminated vast numbers of genotypes that he will never be able to recover. Natural evolutionary forces, and those brought on by man, tend in an ever increasing manner to alter genetic combinations, eugenically or dysgenically.
Therefore, any measures that can be taken to avoid the destruction of certain genetic components or specific genotypes are of great importance for the future.
Once the frequency of certain genotypes reaches equilibrium, these remain constant from generation to generation within a population until some evolutionary force causes a change. Furthermore, when this population is polymorphic genetic variation will be great, being proportional to the size of the population in question (Grant, 1963). Nevertheless, such suppositions are greatly affected by certain forces. In spite of their lengthy life cycles, forest tree populations suffer constant changes, either drastic or partial, in their genetic constitution.
It is impossible to recognize individual genes in forest tree species. This is in contrast to certain types of Pisum, which contain 500 known genes (Blixt, 1967) or to the well-understood genotypes of Zea mays and Drosophila species. Furthermore, the characteristics of importance for forestry are polygenic and controlled by a number of genes.
The importance of controlled or accidental events in changing certain genotypes can only be suggested. Foresters are completely in the dark as to the potential variability and as to the characteristics these genotypes may show under conditions imposed by their native environment or by an exotic environment. Certainly the elimination of genotypes, whether controlled or accidental, will have positive or negative repercussions on forest gene resources from an economic point of view.
A number of man-caused and natural processes can result in evolutionary changes. These are: mutation, migration, selection and genetic drift. Even though these are normally considered from the point of view of natural evolution, it should be remembered that man is one of the major influences on these forces.
MUTATION
Normal human activity in forests has probably not produced any unnatural mutations. It is possible that forestry operations which change environments drastically could influence mutation rates, but nothing is known about this. The increasing use of atomic radiation and chemicals in warfare and in meeting the needs of mankind could alter mutation rates
MIGRATION
This subject involves both emigration and immigration. In the case of emigration, man carries away a limited quantity of seeds representing only certain genetic combinations. He may perhaps be able to produce again the exported genotypes if the parent trees continue in existence.
The act of gathering seed eliminates certain genotypes from the area where seed is gathered, but the consequences (good or bad) can only be noted in the place of immigration, where the gathered seeds are planted. On the negative side, unsatisfactory results may be obtained because:
1. the trees may be of inferior form (e.g., misshapen, stunted, forked, etc.);2. the strain, ecotype, or species introduced may be an undesirably aggressive invader;
3. the trees may be easily attacked by pests and diseases, and even on occasions become the harbourers of some pest which destroys other forest gene resources;
4. the trees may show inferior timber characteristics, as occurred with the first Pinus sylvestris L. plantations in Europe (Schreiner, 1950).
On the positive side, certain beneficial cases of immigration of species, races, ecotypes, and so on occurred in New Zealand, Australia, certain Latin American countries, and Africa. Here immigration gave such good results that Australia is already undertaking inheritance studies for introduced species. Similar results have been obtained in certain African countries, which have seed orchards planted with Pinus patula Schl. and Cham. Furthermore, South Africa is obtaining very good results with Pinus pseudostrobus Lindl. A team in Mexico has recently been selecting plus trees of this species, as well as some others, for propagation on a large scale in South Africa. The export of these seeds from just a few widely scattered plus trees will have a negligible effect on Mexican gene resources, but a magnified positive effect in South Africa.
SELECTION
Selection is perhaps one of the most important forces determining the frequency of certain genie components. It discriminates for or against certain characteristics. Selective forces are exerted by the demand for forest products, by silvicultural practice and experiments, by the implementation of forest improvement methods and, most dramatically, by a change in land use.
With only a few exceptions forest industries always seek wood of superior quality. They want trees easy to process that provide them with the least possible waste. The best trees are cut, and trees of poor quality are left standing to regenerate the area. Unfortunately, such selective pressure is dysgenic. The negative effects can be seen in many areas of the world.
A silviculturist who adopts good treatment methods for natural regeneration, to speed growth or to achieve healthy trees, always leaves trees standing that are phenotypically acceptable. He does this on the assumption that a good phenotype always represents a good genotype. In this case selection is eugenic. However, other silvicultural practices can be dysgenic. For example, thinning even-aged stands from above probably removes superior genotypes before the regeneration period. As another example, nurserymen who transplant inferior seedlings into transplant beds instead of culling them probably are perpetuating genetically inferior stock.
Genetic improvement programmes for forest trees concentrate on selecting individuals with superior phenotypical characteristics (Figures 18 and 19). Trees that are selected are suited to the type and class of industry that prevails at that time. In this sense, the practice is eugenic. However, if the needs of industry change and the trees selected do not represent a broad genetic base, then the practice could become dysgenic.
Clearing land to change the use from forestry to agriculture, mining, industry, or community development is the most dysgenic practice of all. Selected populations, races, varieties or species may be eliminated. Such clearing should not be done without realization of the genetic consequences. When it has to be done, appropriate precautions should be taken to preserve genes threatened with extinction.
GENETIC DRIFT
The evolutionary concept of genetic drift relates to the changes in frequency of genes in a population due to chance. Anyone can appreciate that the chance of a gene being eliminated from a population of trees increases as the number of trees in the population is reduced. Probably the distance over which effective pollination occurs is comparatively short for many forest trees, even though their pollen may be carried long distances by air currents. Thus, cutting systems that leave too few seed trees may cause the chance elimination of desirable genes from the population. Similarly, land clearing operations that leave only small, isolated populations of trees could result in an accidental loss of genes due to genetic drift.
Like all other living organisms, man tends to multiply and to disperse to areas where he can provide for his subsistence. In this way, he creates innumerable problems, mainly of a socioeconomic character. Together these socioeconomic factors can strongly influence genetic structure; they are:
1. Change in the use of the land due to deliberate government policy:(a) new settlements
(b) agriculture
(c) livestock
(d) forestry operations.2. Damage due to carelessness, ignorance or disrespect for law (to be remedied through forestry education):
(a) fires
(b) illicit felling
(c) breakdown of biotic equilibrium.
A change in the use of the land is most dangerous for gene resources. Generally such changes do not occur as a result of prior study. Thus the establishment of new settlements resulting from substantial demographic expansion requires locations easy of access, large in size and preferably level. Such settlements include a collection of buildings with communication, electrification and transport networks. This would not be so serious on its own, but settlement also brings with it the clearing of fields for agriculture and livestock and the starting of forestry operations. It could well be that forestry operations create the need for new settlements.
Settled agriculture usually requires relatively homogeneous and level ground. Given the topographical conditions of a mountainous country such as Mexico, agricultural expansion would eliminate those tree species that are restricted to valleys, deep soils, and areas on the lower slopes of mountains. While not requiring level land, stable livestock production does generally require large expanses of relatively clear land. Thus destruction would gradually occur at higher altitudes too.
Settlers generally lack an appreciation of forestry. This is dangerous for it can result in the wholesale destruction of certain species or in a destruction that occurs little by little over a large region. Intentional fires to produce feed for shifting livestock management or to prepare undulating ground for cultivation can be disastrous. Illicit felling also causes substantial damage because it eliminates young and well-formed trees that will then not be able to reproduce. Finally, settlement results in the elimination or flight of certain predatory forest animals, the result being a breakdown in the biotic balance. As a result, other fauna such as the squirrel can increase dramatically and eliminate up to 70 percent of a conifer seed harvest.
Certain examples may be given from Mexico:
1. The Perote plain, Vera Cruz, 2200 metres above. sea level, and the southwestern slopes of the Cofre de Perote are typical areas where forests have been. destroyed on a large scale. The vegetation varies from arid subtropical to alpine (Table 10).
TABLE 10. - DESCENDING ALTITUDINAL DISTRIBUTION OF VEGETATION ON THE COFRE DE PEROTE, VERA CRUZ
|
Location and species |
Altitude |
|
Metres |
|
|
Cofre de Perote Point |
4300 |
|
Upper level of timber vegetation |
3800 |
|
Pinus hartweggii |
3500-3800 |
|
Abies religiosa |
3100-3500 |
|
Pinus rudis. |
3000-3100 |
|
Pinus montezumae and Pinus rudis |
2800-3000 |
|
Pinus teocote |
2500-2800 |
|
Perote plain (hops, Pinus spp., fruit trees and semiarid vegetation) |
2200 |
In the decade 1930-40 a large part of the Perote plain was still covered with conifers (Hernandez, personal communication). At present almost all the area is cultivated to maize, broad beans and onions. In spite of this, some small areas of pines persist.
Although 40 years is too short a time to draw conclusions, the genetic component of Pinus pseudostrobus is apparently being eliminated on a large scale. It may at one time have been the dominant species in association with Pinus montezumae, Pinus rudis, Pinus teocote, and Pinus pseudostrobus var. oaxacana.
On the lower slopes of the Cofre de Perote pines are being cut for sawmilling and for the manufacture of posts. At the same time, poor people are taking advantage of the timber harvesting areas to sow potatoes. They speed up the destruction of the pines either by agriculture or by burning them. The weakened pines are subject to attacks by bark beetles, fungi, and mistletoe. Usually they die within a very brief period. When trees have died, they can be legally felled and sold. In this way more areas are opened up for cultivation of potatoes. However, only between 2800 and 3200 metres above sea level can this crop prosper. In this zone, the species in danger is Pinus rudis. It is found in small clusters in some parts of Mexico, and many of these are of very bad phenotypes.
2. To underscore what has been said about poor types, the situation at Texcoco, along the Mexico-Calpulalpan highway might be mentioned. There, Pinus rudis grows in mixtures with other pines, such as Pinus montezumae. In isolated patches of heavy growth, trees are misshapen and very short. The whole of this area was at one time covered with forests, but it has been improperly exploited. Today, it is being invaded by barley crops.
3. Another example is Pinus leiophylla spread over the Sierra Madre Occidental and over the Mexican neovolcanic axis, but only in small clusters. It is being rapidly eliminated because it grows at the lower levels of the mountains.
(a) At San Juan Tetla, Puebla, fruit trees and maize are taking its place.(b) At Villa del Carbon, Mexico, it is found in very degraded sites, in association with Pinus teocote. Both are from 10 to 12 metres in height and of very poor shape at ages 30 to 40 years. Both are being replaced by maize.
(c) At Bosencheve, Mexico, mixed with Pinus teocote, both are of good height. They tend to concentrate in humid places where one can still find very good phenotypes of Pinus leiophylla. Elimination of this species is due to the expansion of maize cultivation.
(d) At Meseta Tarasca, Michoacan, it is being eliminated for the cultivation of maize and barley. Also, it is being subjected to constant resin tapping. This species soon will be found only in ravines and certain other positions, but almost all of the remaining timber is of poor quality.
4. Some other conifers whose distribution is being restricted, although their phenotypes are not very degenerate, are Pinus strobus var. chiapensis, Pseudotsuga spp. and Picea spp. Pinus oocarpa, while spread all over Mexico, is not found in great quantities, and it has very poor timber.
5. Among the angiosperm trees, preservation of Swietenia macrophylla and Cedrela mexicana in Mexican tropical areas presents a serious problem. In this case, the peasants very often turn to felling, clearing, and burning in order to start cultivation.
It has been seen that natural forests tend to diminish in proximity to civilization. Artificial forests tend to expand, sometimes with poor genetic material.
Most geneticists consider that gene resources and variability are being alarmingly reduced at present. This reduction, aside from the individual phenotypical aspect, may bring serious consequences. Genotypes that can resist pests or disease may be eliminated, in addition to certain intrinsic characteristics in timber of importance for a specific industry. However, it must be remembered that variability may possibly remain latent. Certain types of maize native to South America, instead of reducing their variability, are increasing it (Hernandez, personal communication).
The drastic reduction in genetic variability caused by destruction of forests results in:
1. Elimination of desirable genotypes. People who need forest products or who eliminate large forest areas in order to turn them over to agriculture may not leave trees standing that are phenotypically good. It is impossible to determine which trees possess gene complexes that will be resistant to pests and diseases. Reduction in variability can thus be considered as dysgenic selection from an economic viewpoint.2. Invasion by inferior species. The oak is a tree that characteristically is a great invader of Mexican pine forests (Figure 20). Once pines are eliminated, the areas become covered with Quercus spp.
3. Increase in erosion. This is one of the most serious problems in Mexico and possibly in many other places in the world as well (Figure 21). In this case, reduction in genetic variability is only in directly responsible, as most deforested areas might well be regenerated artificially, even with some other species.
4. Creation of new species. This is very difficult to prove in practice. Nevertheless, as an example, one might imagine a possible hybridizing between two species, such as Pinus leiophylla and Pinus lawsoni, thereafter continuing their natural evolutionary process. This is very easy to understand as, at this time, many varieties and forms of certain species occur in Mexico. Maybe they are the result of the whole complex collection of species that exist in areas with such uneven topography and a mosaic of different habitats.
Selection of superior types of trees to achieve rapid growth, increased yield, improved timber characteristics for specific industries, and so on is highly justifiable, even though it may be accompanied by some loss in genetic variability. To date, great advances have been made in this respect. In Mexico, at this time, there are already various seed production areas spread over the country (Jasso, 1968; Jasso and Villarreal, 1968). Plus trees are being selected and attempts made at vegetative propagation, although with only limited success so far.
With every day that passes forests are ever more exposed to change. Changes may be in a negative or positive direction from a genetic point of view. For esters are always trying to increase production and improve the quality of raw materials. Nevertheless, the other side of the balance sheet unfortunately involves many people who lack education and understanding of forestry matters. Their negative effect is very much stronger than the positive influence of the forester. Thus, on balance, genetic deterioration is substantial.
Dysgenic alterations will favour the elimination of desirable genotypes, their replacement by inferior invading species, and erosion of the soil. On the positive side, genetic improvement in trees and the possible creation of new natural and artificial genie combinations can be of great advantage to humanity.
There is no doubt that genetic alteration is important, whether in agriculture, livestock, or forestry. In the first two cases, experiments to determine the variation that may be induced in certain genotypes take little time and space. In forestry, the research problem is more difficult, because of the number of years required and the substantial areas that are necessary for experiments.
For this reason making short-term recommendations is futile. Any suggestions should rather relate to solving the problems of negative genetic alteration and achieving positive genetic alteration. In order to overcome much of what has been described as negative, the forest geneticist is recommended to take the following steps:
1. Explore and evaluate genera and species of worldwide economic importance.2. Evaluate variability of species that have gone unnoticed and which may have economic importance.
3. Insist to governments that changes in land use should be based on adequate studies.
4. Increase research on vegetative propagation of forest species, with emphasis on those threatened with extinction or with loss of irrecoverable genotypes.
5. Collect and preserve pollen and seed of species that are in danger of extinction, under international coordination and control.
6. Establish cooperative experiments aimed at genetic improvement of indigenous species in each country for the benefit of all.
7. Establish forest banks in the country where each species is native in order to preserve trees of superior qualities on the one hand and an adequate range of variability on the other.
8. Establish arboreta in various countries with imported germ plasm.
9. Coordinate activities of all research institutions devoted to the genetic improvement of forest trees to combine their efforts and avoid duplication of work.
10. Promote and organize intensive studies of forestry species on the basis of the physical and technological properties of the timber.
11. Disseminate information on suitable methods to speed the forestry education of people and of news media in order to. encourage the rational use of forests and to avoid limitless destruction and illicit felling.
BLIXT, S. 1967. Edible legumes. FAO Technical Conference on the Exploration, Utilization and Conservation of Plant Gene Resources, p. 35-37.
GRANT, V. 1963. The origin of adaptations. New York, Columbia University Press. 606 p.
HERNANDEZ X., E. 1969. Personal communication.
JASSO, M., J. 1966. Tree improvement program for conifers in Mexico. Raleigh, N. C., North Carolina State University (Thesis).
JASSO, M., J. & VILLARREAL, C. R. 1968. Genética forestal aplicada y propagación de árboles. México, D. F., Centro Nacional de Productividad. Ref. Agr. III-31.
SCHREINER, E. J. 1950. Genetics in relation to forestry, J. For., 48: 33-38.
