NORBERTO SANCHEZ MEJORADA, Chief of Forestry Department, Celulosa Michoacan, and
LOUIS HUGUET, FAO Technical Assistance Officer
The pines, firs and cypresses of Mexico and other genera of less importance have a potential value as exotics for planting programs comparable with that of eucalypts. They occur over a wide range of ecological conditions and present a diversity of characteristics and technological qualities.
The main causes of their diversity are the following:
1. They are found from 14° to 30° North latitude, i.e., on both sides of the Tropic of Cancer.2. The altitude varies from about 800 to 3,500 meters with correspondingly large variations in average temperatures.
3. Annual precipitation varies with exposure from 600 to 2,000 millimeters.
4. The geological origin of the soils includes granites, lavas, volcanic ash, calcareous rocks and alluvial material and, according to the site conditions, has resulted in the formation of lateritic soils, prairie soils and even, it appears, true tchernozems. Very often, because of the geologic youth of the country, the soils have been only incompletely developed.
5. Frequent hybridization occurs, and probably also mutations (9).*
* The numbers in parenthesis refer to titles given in the bibliography at the end of this paper.
Botanists (8, 9) claim to distinguish 38 species and 25 varieties or forms of pines, but there is good reason to think (9) there are even others. There are also eight species and five varieties of Abies; six species and two varieties of Cupressus; four species and one variety of Pseudotsuga; 12 species and six varieties of Juniperus; one species each of Taxodium, Picea and Libocedrus.
Until now, only three or four species of pine and several species of cypress have aroused interest for planting programs. Obviously planters have hardly begun to draw on the enormous reserve of genera, species, varieties, races and genetic characters represented in the entire array of Mexican conifers. But much more needs to be known about them before they are likely to be widely acceptable. This paper only summarizes the general information already available.
Distribution
Mexico extends on both sides of the Tropic of Cancer from latitude 15° to 32° North. It is estimated that only 5 to 7 percent of the land area, or about 100 to 150 thousand square kilometers are covered with true coniferous forest, but the actual area of distribution of conifers is much greater, extending over 500,000 square kilometers, or about a quarter of the country. This whole area was probably at one time covered by coniferous forests which were later cleared, burned or gradually destroyed by excessive cutting or grazing.
For the purpose of this paper, one may exclude the Peninsula of Yucatan, whose climate is tropical and where there are no conifers, and Lower California with a desert or dry Mediterranean climate. One can then divide Mexico into three natural zones, each extending from the extreme south to the extreme north of the country.
These are (see Figure 1):
1. The two coastal plains and the lower slopes of hill ranges up to about 800 meters in altitude, extending along the Pacific and the Gulf of Mexico. These two bands, one tropical and the other desert, bear no conifers.2. The central plateau narrowing toward the south and with individual valleys south of Mexico City. This plateau carries little or no coniferous forest and is of practically no interest for this paper.
3. Between the two preceding bands or, in the south, between the two coastal plains, are regions of distorted relief, nearly always of volcanic origin, which coincide with the area of distribution of conifers. Altitudes vary from 800 meters to more than 5,500 meters (Orizaba peak 5,747 meters).
FIGURE 1. - Mexico: the situation of its coniferous forests.
Courtesy, Ecology, 31 (4), 507-518, 1960.
FIGURE 2. - Mexico: its rainfall distribution.
Courtesy, Servicio Meteorologico Mexicano
There are three main mountain chains, partially covered by coniferous forests:
1. western Sierra Madre, parallel to the west coast continuing to the Sierra Madre of the south and the Sierra de Chiapas;2. eastern Sierra Madre which continues as the Sierra of Oaxaca;
3. the neo-volcanic Cordillera, which cuts the central plateau from coast to coast, and produces forested mountain islets rising from plateau.
Precipitation
The tropical zone is usually described as being crowned with deserts, with a transition zone merging from moist to dry vegetation conditions. Mexico cannot, in general, be considered as a desert country. In fact, using as criteria to define deserts or more simply arid zones, an annual precipitation of less than 400 millimeters, a dry season of more than 6 months, or even the fact that agriculture is impossible without irrigation, then only in the northern and central parts of the Central Plateau, in the northern part of the Pacific coast area, and in Lower California, can true arid conditions be said to exist (see Figure 2).
Forest vegetation appears in areas with a minimum precipitation from 400 millimeters and conifers are found from a minimum of 600 millimeters. Figures 1 and 2, although schematic, give an idea of the relation between forests and rainfall.
The incidence of precipitation related to altitude is especially determined by exposure. This is illustrated in Figure 3. Exposure should be understood to be orientation in relation to the general direction of the prevailing winds which bear the moisture from the sea, i.e., from the east on the Atlantic slope and from the west on the Pacific slope. On windward slopes there is produced the classic phenomenon of condensation, and on the leeward slopes the effect of foehn and of evaporation (Table 1).
TABLE 1. - VARIATION OF PRECIPITATION BETWEEN WINDWARD AND LEEWARD SLOPES
|
|
Windward |
Leeward |
||||
|
Station |
Altitude, in m. |
Precipitation, in mm. |
Station |
Altitude, |
Precipitation, in mm. |
|
|
Atlantic |
Jalapa |
1 338 |
1 588 |
Perote |
2 465 |
470 |
|
Pacific |
Uruapan |
1 610 |
1 674 |
Zamora |
1 664 |
826 |
This applies not only to the slopes which directly face the sea, but also to interior slopes, even though they are separated from the coast by other mountain chains. There are thus found "moist islets" covered with conifers in the middle of the central plateau and far from the two coastal Sierras.
The seasonal distribution of rainfall throughout the area is of the tropical type; one dry season of seven to eight months, except in the south where a second but short dry season occurs (curve III on Figure 4). This general distribution is illustrated in Figure 4. Total annual precipitation varies, of course (from about 600 to 2,000 millimeters in the coniferous forest), but 85 to 90 percent of this precipitation is concentrated in five rainy months from June to October and, in the north, in only four months.
The dry season is severe besides being long and this points up the great importance of the forest microclimate as in other countries with long dry seasons, both Mediterranean and tropical. The forest retains its own particular climate which is difficult to restore once the tree cover has been destroyed.
Temperatures
The mean annual temperature ranges between 10° and 24.5° C. over the area of distribution of conifers. Monthly mean temperatures vary only by 4° in the south to 8° in the north. However, the daily range is very great, varying from 10° to 20° C.
In Figure 5 the two heavy curves show recorded extremes of mean monthly temperatures. Between them are curves relating to other stations of the coniferous zones. Supplementary details in regard to the stations are given in Table 2.
Humidity
There is little recorded information for the forested areas because of the small number of meteorological stations. Using data from stations near the forests, as Durango in the north and Morelia in the central part of the country, it can be said that, during the six to seven dry months, humidity is low or very low. From 35 percent in April it goes up to 75 percent in July and August. Humidity increases as one approaches the slopes near the sea, particularly toward the windward side. On windward slopes in the zone of 2,400 to 3,000 meters, that is to say, where the firs occur, there is a zone of maximum humidity characterized by frequent mist.
FIGURE 4. - Rainfall variations.
KEY
I Patzcuaro
II El Salto
III San Cristobal de las Casas
FIGURE 5. - Temperature variations.
KEY
I Patzcuaro
II El Salto
III San Cristobal de las Casas
IV Desierto de los Leones v Tacambaro
Conclusion
The areas of the world where Mexican conifers can be suitably introduced must be mountainous and lie astride the Tropics of Cancer and Capricorn, receiving a minimum of 600 millimeters of rainfall annually, and perhaps high altitudes in the equatorial regions (Table 3).
This does not mean that other regions where the climate presents certain similarities with that of Mexico, for example, southwest France and parts of the Mediterranean Region, should not be interested in trying some carefully chosen Mexican conifers. Information on their adaptability is lacking, but other genera or species, for example, certain eucalypts, have been remarkably successful in climates quite different from that of their native habitat.
TABLE 2. - DETAILED INFORMATION CONCERNING FIVE FOREST STATIONS
|
No.* |
Station |
Latitude |
Altitude |
Total annual rainfall, in mm. |
Absolute minimum temp. °C |
Mean annual temp. °C |
No. of days of rain |
|
|
Name |
State |
|||||||
|
I |
Patzcuaro |
Michoacan |
19°31' |
2 211 |
1 109 |
-5 |
12.5 |
97 |
|
II |
El Salto |
Durango |
23°46' |
2 538 |
924 |
-15 |
11.4 |
74 |
|
III |
San Cristóbal de las Casas |
Chiapas |
16°44' |
2 128 |
1 172 |
-6 |
14.5 |
120 |
|
IV |
Desierto de los Leones |
Distrito Federal |
19°17' |
3 220 |
1 281 |
-7.4 |
133 |
|
|
V |
Tacambaro |
Michoacan |
19°12' |
1 577 |
1 240 |
+3 |
21.5 |
109 |
* These numbers refer to the curves in Figures 4 and 5.
TABLE 3. - AREA SUITABLE FOR THE INTRODUCTION OF MEXICAN PINES
|
Hemisphere |
America |
Africa |
Asia |
Oceania |
|
Northern |
None |
Cameroons |
India |
|
|
Fouta Djalon, Guinea |
Burma |
|
||
|
Uganda |
Thailand |
|
||
|
Ethiopia |
Viet-Nam |
|
||
|
Southern |
Southern Peru |
Northeastern parts of Union of South Africa |
|
Australian Alps (Tropical zones) |
Pines occur on soil over a substratum of all kind of geological formations including volcanic rocks such as the andesites and basalts, schists, and sedimentary rocks such as limestone and sandstone. Rather than the parent rock, it is the differences in evolution of the soils resulting from the effects of weathering, fire or destruction of forest cover, that is important. Pine is found on lateritic soils with iron concretions, as well as on soils derived from limestone and from basalt. There exist also many almost virgin soils of recent volcanic origin where pines have become successfully established. This is especially true on volcanic ash and lava.
Pines are found at altitudes of 800 to about 2,200 meters on yellow or red lateritic soil. The evolution of this soil depends on its exposure, and degree and date of the clearing of the forest. At higher elevations there occur black prairie soils and typical podzols and even tchernozems at elevations of 2,300 to 3,000 meters. In the zone of podzols where the coniferous forest has not been too much affected, and where there is a mixture with hardwoods, there are found brown forest soils without well-differentiated horizons under the humus layer. These are the soils which grow the best forests, and it seems that the climax is a mixture of hardwoods and conifers.
Additional information on the distribution of pines and other conifers in relation to the soil is given later in this paper.
Shifting cultivation with the burning of the forest has been practiced throughout Mexico since antiquity. Fire was the instrument of the hunter, and after the Spaniards had introduced cattle, was used as a primitive method to favor the growth of green grass at the end of the dry season. Fire generally passes over all the pine forest at intervals of less than five years. It affects the fir forest less except on their edges because of the higher humidity.
The burning of the forests under the practice of shifting cultivation on long rotation of 10 to 15 years and on parcels of only 2 to 3 hectares permits the reestablishment of even-aged pine forest from the seed of surrounding trees. But in the zones of great density of population, as in the States of Michoacan, Jalisco, Mexico, Puebla and Hidalgo, nomadic agriculture is no longer practiced and burning ends in permanent clearing which may extend to hundreds of hectares.
The remaining areas accidentally burned or fired for grazing have a park-like appearance with no young growth. It is in the mountainous zones of low population density, as in the States of Oaxaca, Guerrero, Durango, Chihuahua and certain regions of Michoacan or where by some miracle fires are less frequent, where there are found relatively dense forests with a rather well-formed understory of deciduous species and with a good distribution of age and diameter classes.
Fortunately in the coniferous forests, fire does not destroy whole trees as is the case in the Mediterranean type forests which contain a highly inflammable undergrowth. Grass fires appear to affect only young pines below 5 or 6 meters in height. Nevertheless, fire always has other harmful consequences such as soil degradation, weakening of the trees left standing and destruction of the moist humid microclimate. However, it seems that five years of complete protection against fire are usually sufficient for the establishment of a lower story of young pines and various species of shrubs and plants.
Commercial exploitation is often blamed in Mexico for forest destruction. It appears rather that this has been due either to clearing for cultivation, or to a combination of heavy cutting and uncontrolled fires afterwards which destroy all young seedlings. Grazing, except by goats, does not in itself seem to affect the coniferous forests seriously, but it is indirectly dangerous as the reason for fires started by shepherds.
Of the 10 to 15 million hectares of conifer forests which now exist in Mexico, it is estimated that three million hectares in the States of Chihuahua, Durango, Michoacan, Guerrero and Oaxaca, have not yet been systematically exploited, but they have been subject to clearing, fire and grazing. The fir forests, considered less valuable for lumber, are extensive only in the central region at the higher elevations and have only been affected in the vicinity of villages and settlement centers; in the states of the north, they cover only small areas, in mixture with Pseudotsuga and with the very rare Picea chihuahuana on northern exposures or in cool moist ravines.
The pine is practically the only conifer which occurs below about 2,400 meters elevation. Above that the fir begins and there is found a transition forest type composed of pine, fir and sometimes cypress in varying proportions. Then, up to about 3,100 meters, fir and sometimes cypress are the only conifers. From 3,100 to 3,500 meters elevation there are again found pines succeeded by alpine meadows. Permanent snows occur at the top of three or four very high peaks. (Figure 6).
The pine forest up to about 2,400 meters is certainly never the climax type. As in the southern United States, complete protection against fire would probably bring about a return toward a climax of hardwoods. In those forests which are relatively well protected against fire, one finds a great abundance of hardwood species (Figures 7 and 8) comprising:
1. at all altitudes, a large number of species of oaks which are not yet well known;2. at the lower altitudes in the subtropical zones, spiny species such as Prosopis juliflora, Acacia farnesia and other acacias: these occur on the dry leeward slopes already mentioned, and are joined here by Arbutus sp.;
3. at higher elevations, alders are found in the moister zones.
In cool ravines, naturally protected against fires and where pine is almost totally absent, there is found a flora rich in such deciduous species as hornbeam, oaks and basswood (lime). These ravines are moist, receive little sunlight, and their vegetation is completely different from that of the pine forest.
In addition to the trees or shrubs already mentioned, cactus occurs at low altitudes on leeward slopes and, at higher altitudes, Solanum madrense, Senecio salignus, Verbesina montanoifolia, Baccharis conferta under which the fir seedlings establish themselves, and a legume Lupinus hartwegii important for covering denuded land and for the formation of a forest microclimate.
As has been said, the greater the frequency of fire, the more the pine forest resembles parkland, with several ferns such as Pteridium aquilinum (indicating soil acidity), grasses, and scattered oaks and a much reduced organic soil cover.
The better the soil, the less open is the forest. Other things being equal, such as climate, exposure and slope of the ground, the better preserved forests occur on brown soils, on brown lateritic or even yellow soils, or on black soil at appropriate altitudes. The forest most adversely affected by fire are those growing on red lateritic, sometimes very eroded, soils.
|
Soil types: |
Species |
|
Red soils of the lateritic or iron bearing soil type: |
Pinus oocarpa, Pinus michoacana |
|
Black prairie soils or tchernozem: |
Abies sp., P. rudis, P. hartwegii, Cupressus sp. |
|
Sandy or rocky soils: |
P. lumholtzii, P. duranguensis (in Central Mexico), P. teacote |
|
Brown forest soils: |
P. montezumae, P. pseudostrobus, P. tenuifolia, P. patula, P. duranguensis (in the north), P. cooperi |
|
On clearings: |
P. leiophylla, P. pseudostrobus. |
FIGURE 7. - Old second-growth forest, partially protected against fire, in the State of Michoacan.
The current classification of the various genera, species, and principal varieties of Mexican conifers, has been developed by Professor Maximino Martínez of the Institute of Biology of the University of Mexico. For the pines, this classification is based on the character of the needles and the cones, on the presence or absence of wings on the seeds, and on the persistence of the bracts which surround the needle clusters. The reader is referred to two basic books by this eminent botanist (8).
N. T. Mirov (9), who has studied the composition of resin of the various Mexican pines, considers that there are certainly other species than those indicated by Martínez. It has been noted that the pines of the central and southern parts of the country present a greater variability in their botanical characteristics than those of the pines in the northern states such as Durango and Chihuahua. The reason for this is not clearly understood.
The Mexican conifers and especially the pines are of considerable interest to geneticists. There is already growing at Placerville, California, an interesting hybrid of Pinus ponderosa and P. montezumae (9). In the natural forests, the following hybrids have been noted: Pinus montezumae x P. michoacana, P. montezumae x P. pseudostrobus, and there certainly exist others in the transition zones of the central states at elevations between 1,500 and 2,000 meters where at least six or seven different species of pine occur on the same hectare.
The silvical characteristics of the Mexican conifers are similar to those of all pines and firs and, for this reason, only a few general remarks are given here.
As in the case of other pines, the Mexican pines are pioneer species, strong light demanders which invade bare land, mineral soils, and openings in the forests. Nevertheless, in forests having an abundant undergrowth with adequate protection against fire, sufficient young plants get away to assure adequate natural regeneration. This is fortunate, because it does not appear to be advisable to use clear-cutting practices in the pine forests as it leaves the soil too exposed to the tropical climate.
The firs are tolerant to shade, and in fact require shade during their early years. However, they appear better adapted to light than the firs of Europe, and resemble in this regard the Mediterranean firs. They come in on bare soil under brushy cover such as Baccharis conferta, or in natural or artificial openings in the forest.
Cypress appears to have characteristics similar to those of the fir.
The coniferous forests of Mexico are in general irregular as regards distribution of diameter classes, if not from the standpoint of age. This is normal in old growth forests, but is also the case in second growth forests as a result of the official regulations controlling forest exploitation which forbid clear-cutting and restrict fellings to not more than 35 to 40 percent of the standing volume. The forests coming in on clearings are certainly even-aged but, because of the wide differences in rates of growth of species and of individuals, they already take on the aspect of uneven-aged forests after a few years.
TABLE 4. - PRELIMINARY CLASSIFICATION THE PRINCIPAL MEXICAN CONIFERS ACCORDING; TO THEIR REQUIREMENTS IN ALTITUDE AND TEMPERATURE
|
Climate |
Mean annual temperature, in °C |
Approximate altitude, in meters |
Species |
|
Tropical |
Over 24° |
Below 900 (exceptionally) |
Pinus strobus var. chiapensis P. oocarpa * |
|
Subtropical |
19-24 |
900-1,650 m |
P. douglasiana (s), P. herrerai, P. lawsoni, P. leiophylla *, P. michoacana, P. montezumae, and var. P. oocarpa,* P. pringlei, P. pseudostrobus, P. strobus var. chiapensis, P. tenuifolia |
|
Warm temperate |
17-19 |
1,650-2,000 m |
P. douglasiana, P. herrerai, P. lawsoni, P. ayacahuite, P. ayacahuite var. veitchii, P. leiophylla, P. lumholtzii,* P. michoacana, and var., P. montezumae var. lindleyi, P. oocarpa, P. patula, P. pringlei, P. pseudostrobus, P. tenuifolia, P. teocote, Juniperus sp. * Taxodium mucronatum (Swampy soils and riverain soils) |
|
Temperate |
10-17 |
2,000-3,100 |
P. arizonica,* P. ayacahuite var. brachyptera, P. cambroides,* P. chihuahuana,* P. duranguensis,* P. engelmanni,* P. flexilis, P. greggii,* P. hartwegii, P. leiophylla,* P. lumholtzii,* P. cooperi, P. michoacana and var. P. montezumae, P. nelsoni*, P. oocarpa var. trifoliata,* P. patula, P. pinceana,* P. pseudostrobus, P. rudis, T. teocote, Taxodium mucronatum, Juniperus sp. * from 2400 - 3100 m or on cold wet north exposures: Abies sp., Pseudotsuga sp., and Cupressus sp. |
|
Cold temperate |
less than 10° |
over 3,100 |
P. hartwegii, P. rudis |
* Indicates species which are able to adapt themselves to dryness, i.e., less than 1,000 mm. precipitation per year.
Fir forests and the cypress forests are all uneven-aged.
The structure of almost all types of forests is represented by the diagram in Figure 9.
The pines and firs do attain diameter of 1.2 meters including bark, and heights of 50 meters. However, in the old growth forests, the average diameter of the larger trees is 80 centimeters and they may grow to 300 years of age. In second growth forests, the greater part of the growing stock falls into the 40 to 50 centimeters diameter class.
Standing volumes of growing stock vary considerably. They may attain 600 cubic meters per hectare in old growth forests, 300 cubic meters in second growth forests already exploited in the central zones, down to as low as 50 cubic meters. For all of Mexico, an average of 80 cubic meters per hectare applied to 10 million hectares of conifers, gives an estimate of total standing volume of 800 million cubic meters, of which 95 percent is pine.
Almost all commercial exploitation is still carried out in old growth forests. Only rarely is there cutting in second growth forests, apart, of course, from the very frequent unauthorized cutting by the rural population to meet their domestic needs.
Virgin forests or old second growth forests
In the virgin forests, although the standing volume is high, the rate of growth is low, 1 to 2 percent according to climatic and soil conditions, and there is an equilibrium between growth and natural mortality. In old second growth forests, where the older trees have been once removed, the growth rate varies from 3 to 5 percent according to site conditions and exceeds natural losses.
FIGURE 9. - Relationship between the volume and number of trees in the various diameter classes.
KEY
I Volume
II Number of trees.
Recently exploited
There is very little information for the relatively dry forests in the northern states which grow more slowly than the forests of the southern and central zones. For these latter zones, the following growth rates have been recorded from sample areas:
|
Sanding volume |
Growth rate |
Observations |
|
m³ per ha. |
percent |
|
|
107 |
6.40 |
Black or brown soil |
|
76 |
4.95 |
Black or brown soil |
|
168 |
3.34 |
Black or brown soil |
|
122 |
6.30 |
Black or brown soil |
|
137 |
2.92 |
Climate-leeward |
|
149 |
3.90 |
Climate-windward |
|
123 |
6.40 |
Climate-windward |
|
137 |
2.90 |
Red lateritic soils |
|
96 |
4.25 |
Red lateritic soils |
|
89 |
2.40 |
Red lateritic soils |
The data shown above relate to forests dominated by Pinus montezumae, P. tenuifolia, P. pseudostrobus, P. leiophylla, P. michoacana.
The effect of cutting on growth is illustrated by the following data relating to a pine forest of 127 hectares protected against fire in the State of Jalisco, at latitude 19° N., on yellow lateritic soils. The four dominant species are Pinus douglasiana, P. oocarpa, P. michoacana, P. tenuifolia, growing in a relatively moist climate with about 1,500 millimeters of precipitation.
TABLE 5. - EFFECT OF CUTTING ON GROWTH
|
|
Before |
After cutting |
10 years cutting |
|
|
in cubic meters | ||
|
Standing volume |
127 |
86 |
176 |
|
|
percent | ||
|
Average rate of growth |
4.4 |
|
7.6 |
Thus, cutting out the trees of more than 45 centimeters in diameter, clearly had a beneficial effect on the remaining growing stock. This has been confirmed in other forests by many borings made by the authors using a Pressler borer. In other words, the pines of Mexico growing below the latitude 19°, under relatively favorable conditions, respond well to even elementary silvicultural practices. Information is lacking about the pines of the cooler and drier northern states of Mexico, but they are of much less interest for use as introduced species than are the pines from the central and southern part of the country.
Mexico
There has been little artificial planting of indigenous pines or other conifers in Mexico itself, because there is plenty of natural forest, natural regeneration is easy and establishing new plantations in areas with long dry seasons is difficult. The total area of plantations in the entire country is probably less than 500 hectares.
The most interesting plantation is north of Mexico City in the Desierto de los Leones. The elevation and climate of this station are indicated in a previous Table and correspond to curve IV in Figure 5. The conditions are favorable, the soils are deep and of good quality, approaching that of brown forest soil. About 150 hectares have been planted on land formerly occupied by forest and since denuded. The plantations were begun in 1920 with Pinus patula and P. montezumae. At the time of planting, there existed the remnants of the natural fir forest. Now, after 38 years and numerous thinnings, there is an average per hectare of 170 cubic meters of fir, and 260 cubic meters of pine with an annual growth rate of 6 percent (10.5 cubic meters per hectare) and 9 percent (23.5 cubic meters per hectare) respectively. The plantations were established with 1-year-old bare-rooted nursery stock.
Other countries
Mexican pines and cypress have been planted on a small scale in many parts of the world (see Unasylva, Volume 12, Number 3, page 121). In the Union of South Africa, large-scale plantings of these species have been made. The climate and altitudes are very similar to those of Mexico.
In South Africa, there are extensive plantations (1, 6, 7, 12, 14) of Pinus patula and certain areas of P. pseudostrobus, P. leiophylla and Cupressus lindleyi. The South African foresters would prefer to develop P. pseudostrobus because of its better wood quality, but due to the difficulty of obtaining seed, they have developed P. patula from seed obtained from existing plantations.
The seedlings are raised from seed sown in furrows and transplanted in the nursery. They are planted with bare roots at square spacing varying from 2 to 2.75 meters on land which has been previously cleared of brush and plowed. Pruning and thinnings are practiced. The yields reported are excellent, an annual average of 10 to 22 cubic meters per hectare for 30-year-old plantations. Average annual diameter growth exceeds 1 centimeter and height growth, 1 meter. Under favorable conditions there is an average increment of 32 cubic meters per hectare annually.
In Madagascar (16), the yields from P. patula are excellent. The methods of planting are similar to those used in South Africa but before planting roots are coated with clay and fertilizer.
These examples show that, at least for two of the Mexican pine species, remarkable results can be obtained. That other species can grow equally well is indicated by the results of natural forests in Mexico itself where no special silviculture treatment has been accorded other than protection from fire. The conifers thus seem to hold considerable promise for introduction to areas of comparable conditions.
TABLE 6. - CLIMATIC REQUIREMENTS OF PINUS PATULA
|
Area |
Latitude |
Annual average temp., in °C. |
Annual precipitation, in mm. |
|
1. Zone of distribution of P. patula in Mexico |
19°N. |
12-20 |
1 000-2 000 |
|
2. Plantations of P. patula in South Africa |
23-29°N. |
13-22.5 |
700-1 900 |
|
3. Madagascar |
21°S. |
22.5 |
1 739 |
It is surprising that the Mexican pines, which are so frequently subject to fire and excessive tapping for resin, do not suffer more severely from pests and diseases. There are many potentially noxious insects or fungi but it appears that only one is dangerous. This is a species of Dendroctonus, the larvae of which excavate galleries in the sapwood which leads to the death of the tree. It attacks especially Pinus leiophylla and sometimes P. montezumae but rarely other species. It is sometimes followed by Ips (10) which is only a secondary agent. Generally the damage does not affect more than a few hectares and the incidence of severe attack can be ascribed to drought, intense insolation and exposure, and to fires, or in a word to all of the external causes which weaken trees and affect their growing conditions (15). It is known that a tree which has been attacked can eliminate the larvae if it is capable of secreting sufficient resin.
There are preventive methods but once an attack has started, the only cure is to fell the attacked trees and carefully burn all of the bark, branches and other debris.
Pine defoliators of the family Lasiocampides do not seem serious: these include Coloradia pandora in the north and Neodiprion sp. in the central region (11).
No systematic study has yet apparently been made of the mechanical properties of the wood of Mexican conifers. They can be summarily divided into two classes:
1. The lightest, such as Pinus engelmanni, P. duranguensis, P. montezumae, P. pseudostrobus, P. tenuifolia, P. patula and Cupressus spp., have a specific gravity of less than 0.45. They resemble the American P. ponderosa and are suitable for manufacture of panels and moldings, light construction, containers, and in general for all uses where strength is not a primary factor.2. The heavier species, whose specific gravity exceeds 0.45 and sometimes even 0.50 (some are not floatable), include Pinus oocarpa, P. michoacana, P. lawsoni. They are of a subtropical type and resemble the "pitch-pines" of the southern United States. These are suitable for heavy and light construction, dock works, flooring, and in general for those uses where strength is an important factor.
The above is apparently confirmed by technological studies made in South Africa on plantation-grown Mexican pines (12).
With regard to wood quality it can be said that, in general, except for certain species like Pinus oocarpa and P. leiophylla, trees growing close together are self pruning, and from old forests it is common to obtain 20 percent clear lumber from logs over 50 centimeters in diameter. Many species are used for veneer: Pinus engelmanni, P. duranguensis, P. pseudostrobus, Pinus tenuifolia.
It seems then that the whole range of Mexican pines present a gamut of technological qualities, which make them suitable for very many uses, but more research is still needed on properties. Then those species could be selected, for introduction as exotics, best suited for the required end-use.
There is fortunately more information available on the utilization of Mexican pines and firs for the manufacture of chemical and mechanical pulp, and paper.
Four mills scattered through the coniferous zones manufacture sulphate pulp and kraft paper from a mixture of pine and small proportion of cypress. The more common pines are used and their products have qualities at least equal to those of imported products.
An unpublished study (13) shows that mechanical pulp and newsprint prepared from pines, with 15 to 20 percent of semibleached pine pulp, has the same characteristics as the equivalent products made from pines of the southern United States, where four large paper mills are manufacturing newsprint. In Mexico itself, a newsprint mill is being constructed and another is planned. These will use more than 15 different pines which have already been successfully tested in the laboratory and in the mill.
Another important use of Mexican pines is for the production of resin in the States of Michoacan, Jalisco and Mexico. The simplified French method of tapping is used and the annual production is over 33,000 tons of gum. The species which are most productive include Pinus leiophylla, P. lawsoni, P. pringlei, P. teocote, P. michoacana, P. oocarpa, all of subtropical character. Each tree produces 1 to 4 kilograms of gum per year and the yield per tree is greater as the climate is warmer and the forest more open.
As in the southern United States the stumps of certain pines of the subtropical type are distilled to produce pine oil. Such a distillation plant has just been constructed in the central region.
At the end of the last century, Mexican pines were introduced into South Africa. Since the beginning of this century, different countries have tried different Mexican conifers from seed sent from Mexico. In general, however, the supply made available was inadequate and not satisfactory in quality. Often the seeds received were of species different from what the labels indicated.
For this reason, the Forestry Department of the Union of South Africa sent Mr. E. E. M. Loock, author of The Pines of Mexico and British Honduras (7), to Mexico in 1947/48 to collect seed of known origin and authenticity. Due to many difficulties Loock was only able to collect in one year 158 kilograms of seed, principally of Pinus pseudostrobus.
Since that time Mexican organizations, and since 1951 officers of FAO who have worked in Mexico, have only been able to fill requests for seed in a most inadequate manner. There is no commercial organization for seed collection: several forestry enterprises collect seed of pine or firs for use in their own restricted planting work, but usually not of the species requested by other countries.
The authors, during their inventory of 300,000 hectares of coniferous forests in the State of Michoacan (see Figure 10 and Unasylva, Volume 12, Number 2), began in 1953 to organize on a modest scale the collection and shipping abroad of certified seeds. During the past four years about 40 to 50 kilograms of seed have been sent each year to a number of countries including the Rhodesias, Tanganyika, Cameroons, Madagascar, Nyasaland, Colombia, Paraguay, New Zealand, Australia, and the Belgian Congo. The species most often sent were not the most often demanded, but had to be limited to those which could be found in the area under reconnaissance. These include Pinus pseudostrobus, P. tenuifolia, P. oocarpa, P. michoacana and P. montezumae.
For the moment it is difficult to extend this activity for lack of time, capital and means. In order to satisfy all requests and to guarantee the origin and quality of seeds, a special organization with certain resources at its disposal would be indispensable for the following reasons:
1. The large number of species, varieties or races which would require close supervision during collection.2. The distance between the zones of collection and their relative inaccessibility. These zones are scattered for more than 2,000 kilometers from north to south and 600 kilometers from east to west, requiring much rough travel.
3. A great variation in ecological conditions within short distances.
4. The irregularity of seeding. It is estimated that for each species there is one good seed year each five years. This involves the necessity of having available an establishment not only for extraction and cleaning of seeds, but also for proper storage in a cold chamber.
It is hoped that over the next few years it will be possible to find the resources to establish such an organization.
CHANDLER, N. G. Pulpwood plantations in South Africa. Proc. Aust. Pulp and Paper Indus. Tech. Ass.
CHAMPION, SIR HARRY, and BRASNETT, N. V. Choice of Tree Species for Planting, FAO, Rome (in preparation).
AUNG DIN, U. "Pines for tropical areas," Unasylva, Vol. 12. No. 3. 1958.
FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS. Pine bark beetles in Central Mexico. (See Perry, J. P. Jnr.) Unasylva, Vol. 5, No. 4. 159-165, Rome, 1951.
FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS. Composition of turpentines of Mexican pines. (See Mirov, N.T.) Unasylva, Vol. 8, No. 4, 167-173. Rome, 1954.
FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS. Pines for tropical areas (See Aung Din, U.) Unasylva, Vol. 12, No. 3, 121-133, Rome, 1951.
FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS. Tropical Silviculture I. (See Haing, I.T., Huberman, M. and Aung Din, U.) Rome, 1958.
FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS. Choice of Tree Species for Planting. (See Champion, Sir Harry, and Brasnett, N.V.) In preparation.
HAIG, I. T., HUBERMAN, M. A. and AUNG DIN, U, Tropical Silviculture, FAO, Rome, 1958.
KING, N. L. La experiencia Surafricana en la plantación de especies exóticas, Documentos de la Junta latinoamericana de expertos en la industria del papel y de la celulosa, Buenos Aires, 1954.
LOOCK, E. E. M. The Pines of Mexico and British Honduras Bull. 35, Union of South Africa, Department of Forestry, 1950.
MARTÍNEZ, PROFESOR MAXIMINO Los Pinos Mexicanos, Edición Botas, Mexico, 1948.
MARTÍNEZ, PROFESOR MAXIMINO Las Pinaceas Mexicanas, Secretaría de Agricultura y Ganadería, Mexico, 1953.
MIROV, N. T. Personal letter de 25 November de 1957.
MIROV, N. T. Composition of turpentines of Mexican pines, Unasylva, Vol. 8, No. 4, 1954, also unpublished manuscript on Mexican pines.
PERRY, J. P. Jnr. Pine bark beetles in central Mexico, Unasylva, Vol. 5, No. 4.
SCHWERDTFEGER, F., and G. BECKER, La entomología forestal de Guatemala, Informe FAO/ETAP No. 366, Rome, April 1955.
SCOTT, M. H. The quality and uses of exotic soft woods in South Africa, The Empire Forestry Review, Vol. 30, No. 3, Sept., 1951.
UNITED STATES FOREST PRODUCTS LABORATORY, MADISON. Ground wood pulping and newsprint Paper-making experiments on Pinus from Michoacan, Mexico.
VAN HOUTTE, JUAN. Detalles sobresalientes de las reforestaciones en Sudáfrica, Revista forestal argentina, Año I, No. 2, Buenos Aires, 1957.
VERDUZCO GUTIÉRREZ, JOSÉ. Algunos aspectos del problema de sanidad forestal en México, Tesis profesional, Mexico, 1952.
VIGNAL, P. Les roboisements en Pinus patula de la Haute Matsiatra, Bois et Forêts des Tropiques, No. 45, janvier - février 1956.
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FORTHCOMING MEETINGS 1960 Fifth World Forestry Congress, Seattle, Washington, U.S.A.: 25 August - 16 September (Executive Secretary: Dr I.T. Haig, Executive Secretary, Organizing Committee, Fifth World Forestry Congress, Department of State, Washington 25, D.C.) Far East Pulp and Paper Conference, Tokyo, Japan: October or November 1961 Second World Eucalyptus Conference, Brazil |
