PINE-OAK FOREST: CHARACTERISTICS, CLIMATE, FLORA AND FAUNA - ENVIRONMENT - 2024

The pine-oak forest is an ecoregion of temperate zones in which there is codominance of pine (Pinus) and oak (Quercus) species. They are characterized by presenting three strata.

The upper stratum is generally dominated by pine trees, while oaks are located in the second. It is common to see a greater number of oaks, but pines tend to have a greater trunk area.

Pine-oak forest. Source: Zereshk, from Wikimedia Commons

The forests develop in temperate subhumid climates. They are located between 1200-3000 meters above sea level. The average annual temperature ranges from 12 to 18 ° C and frosts are frequent. The rains can go from 600 to 1000 mm per year.

They are distributed from the southeast of the United States to the north of Nicaragua and in Mexico they represent the largest extension of temperate forests. The most important ones are found in the mountainous areas of the Eastern and Western Sierras Madre. They also occur in the Transversal Volcanic Axis and in the Sierra de Chiapas.

Its flora is quite diverse. The presence of more than 40 species of pines and more than 150 of oaks have been reported. Strawberry trees, poplars and cypresses are also common.

The fauna is abundant. We can find pumas, lynxes, white-tailed deer, raccoons, and armadillos. There are also a large number of birds and insects. Among the latter, the monarch butterfly stands out, which fulfills its hibernation period in these forests.

General characteristics

Pine-oak forests are considered an ecoregion, since they occupy a fairly large area and share species and ecological dynamics. The vegetation is interpreted as a mixed forest, since there is a codominance between two groups of plants.

They are generally distributed between 1200-3200 meters above sea level. However, some pine-oak forests have been observed at altitudes of up to 600 meters above sea level.

In many temperate and sub-temperate mountainous regions of North America, pine and oak forests are common. Some authors consider that pine-oak forests are transitional between pine and oak forests, but others argue that they have their own identity and dynamics.

The trees present in these forests are predominantly of boreal origin. However, there are neotropical species mainly in the shrub and herbaceous groups.

Codominance

In pine-oak forests, the species of both groups share the dominance of the vegetation. Due to the great diversity of environments in which this type of forest can occur, the associations can be highly variable.

The composition and proportion of the species will depend on the environmental factors present. Pines tend to predominate in higher humidity conditions. When the environment is a little drier, the proportion changes and the oaks tend to be more abundant.

Likewise, in the structure of the forest it has been observed that both groups can dominate in some aspect. For example, there may be a higher density of oak individuals, but the basal area may be higher in pines.

Vertical structure

Pines and oaks are considerably different in their physiognomy. With regard to phenology, pines are evergreen, while oak trees have deciduous species. Therefore, the proportions of coverage between both genders in a given place will define the structure of the forest.

In general, these forests are characterized by presenting three strata. The arboreal layer can reach up to 40 m in height. This stratum is generally dominated by pine trees.

Later there is a second layer that can reach up to 20 in height. In this there are mainly oak species, although species from other arboreal groups may be present.

Then we have a shrub layer that can reach up to 10 m. Here juvenile individuals of the pines and oaks are presented, as well as other associated species.

In relation to the herbaceous stratum (1- 0.20 m) it may or may not be present. This is going to be related to how closed the tree stratum is. In very closed forests, it will only be present in the clearings that form. While in those forests with the most open tree stratum, there is a greater diversity of herbaceous species.

A great diversity of epiphytes and climbing plants can also be found that grow associated with oaks. The highest frequency of these life forms is related to humidity and temperature conditions. Thus, some groups of epiphytes such as orchids do not appear when the temperature is very low.

Ecological relationships between pines and oaks

The relationship between these two groups of plants in the same type of vegetation can be beneficial for both. It has been found that there is an effect that can be considered almost symbiotic between pines and oaks when they grow together.

In the first successional stages of the forest, the pines are the first to establish themselves due to their light requirements. Later, the oaks develop, which due to their physiognomy do not intercept large amounts of light.

In already established forests, the pines frequently regenerate under the oaks, because there are better conditions of soil fertility in these areas, favoring the germination and establishment of the pines.

In addition, pine seeds reach the ground more easily under oak trees. The mantle of leaves that forms under the pines makes it more difficult for the seed to have favorable conditions for its germination.

Weather

They generally thrive in sub-humid temperate climates. However, some are distributed in colder climates (subhumid semi-cold) or warmer.

The temperate subhumid climate is characterized by an average annual temperature of 12-18 ° C. The coldest months of the year can show temperatures below 0 ° C, so they tend to be subject to frost every year.

Average annual rainfall ranges from 600 to 1000 mm, although it can reach 1800 mm. The rainiest months are generally July and August. The first months of the year are the driest. Humidity ranges from 43-55% annually.

Distribution

Pine-oak forests are distributed from the southwestern United States to Nicaragua. They also occur in some areas of Cuba.

In Mexico they are located in the Sierra Madre Oriental and Occidental, which are the mountain ranges to the east and west of the Mexican state. They are also found in the Transversal Volcanic Axis between both mountain ranges, which is located in the center of the country.

These plant formations are also found in the Sierra Madre Sur that extend along the Pacific coasts in the states of Guerrero and Oaxaca. Also to the southeast in the Sierra Madre and the Chiapas Plateau.

Main pine-oak forests in Mexico

In Mexico, pine-oak forests occupy approximately 16 million hectares. It is considered that almost 90% of the surface can be usable from the forestry point of view.

Sierra Madre Occidental

This area has the largest extension of pine-oak forests in Mexico. On the other hand, it is considered that there is the largest association of pines and oaks worldwide.

It goes from the states of Sonora, Sinaloa and Durango to Jalisco. Pine-oak forests occupy approximately 30% of the surface of the Sierra Madre Occidental.

This region is transitional between the Holartic floristic kingdoms (with species whose center of origin are the temperate zones) and the Neotropical (with species originating from the American tropics). In this sense, it is appreciated that the woody elements have a boreal affinity. The mostly herbaceous flora tends to be neotropical and endemic species are common.

Depending on the region, the altitude and the climate there is a predominance of different species of Pinus and Quercus. Thus, in the north there are mainly P. arizonica and P. engelmanii, and the white oaks Q. rugosa and Q. gambelli.

Interesting to highlight are the forests in Chihuahua and north of Durango, where there is an oak of very restricted distribution (Q. tarahumara). This species grows in shallow soils.

Likewise, in areas with high environmental humidity, it is very high, the pine-oak forests are intermediate with the mesophilic forest.

Sierra Madre Oriental

They occupy a large area, being considered the third largest in the Mexican territory, representing 4.5% of the country's pine-oak forests. They extend from the center of Nuevo León and South of Coahuila and continue south to the center of Puebla. It reaches Hidalgo, Querétaro and Veracruz, where it connects with the Transversal Volcanic Axis.

There is a great diversity of species of both genera. The Sierra Madre Oriental is considered a center of diversity for both Pinus and Quercus.

In the case of Pinus, 17 species have been recorded, two of which are endemic to this region. For Quercus, more than 30 species have been identified.

The climate tends to be slightly more humid than in other temperate zones, due to the favorable exposure to the trade winds of the Gulf of Mexico. For this reason, oak species may slightly predominate in some areas.

The Sierra de San Carlos north of Tamaulipas is an isolated area, where these forests predominate. The species that dominate are mainly the oaks (Q. rysophylla, Q sartorii and Q sideroxyla) accompanied by Pinus oocarpa.

Transverse Volcanic Axis

This forms a mountain range that marks the limit between North America and what is currently the Isthmus of Tehuantepec to Central America. 77% of its surface is made up of mountain ranges, so temperate forests predominate.

Pine-oak forests are the second largest in Mexico. They are located from Jalisco, north of Michoacán, south of Querétaro, south of Guanajuato, Mexico City to the center-west of Veracruz.

The diversity of species of pines and oaks is considered superior to those found in the Sierra Madre Oriental and Occidental. In the case of oaks, it has been found that it has a high genetic variability in these forests.

The pine-oak forests in this area are considered among the most threatened in Mexican territory. In this region are the largest populated centers of the country such as Mexico City, Puebla and Guadalajara. Therefore, the forested areas have been deforested for urban development and other uses.

Sierra Madre of Chiapas

In Central America there is a region with the presence of pine-oak forests. It occupies an approximate area of ​​more than 110,000 km2. It extends from the central part of Chiapas, southern Guatemala, Honduras, El Salvador, to small areas of Nicaragua.

The Sierra Madre de Chiapas constitutes the limit of the Boreal floristic kingdom and has a great influence from the Neotropical kingdom. Here the pine-oak forests present their lowest altitudinal distribution (600-1800 masl).

The presence of 11 species of pine and approximately 21 species of oak have been reported. The most common species in these forests are P. strobus, P. ayacuahauite and Q. acatenangensis.

Flora

The most important floristic elements in these plant formations are pines and oaks. The species present vary in each region where these forests occur. The groups that make up the shrub and herbaceous strata are very different depending on the region.

Species of

In Mexico there are about 47 species of the genus, with a percentage of endemism of 55%. Most of these are important elements of pine-oak forests.

Some species such as the Chinese ocotes (P. leophylla and P. oocarpa) can occur in almost all regions where forests are distributed. Others do not reach the south, such as P. durangensis.

In other cases, pine-oak forests are made up of elements with a very restricted distribution. Such is the case of P. maximartinezii, which only occurs in two communities, one in Durango and the other in Zacatecas.

Species of

The presence of 161 species of oaks has been reported in Mexico, of which 109 (67.7%) are endemic to the country. Among the most common in pine-oak forests are Q. crassifolia (oak) and Q. rugosa (quebracho oak).

Most of the species have regional endemism, so their distribution is moderately restricted. Q. hirtifolia occurs only in the Sierra Madre Oriental, while Q. coahulensis occurs in Coahuila and Chihuahua.

Other groups of plants

Other common species in these plant formations are strawberry trees (Arbutus) and táscate (Juniperus deppeana). Also noteworthy are poplars (Populus), cypresses (Cupressus spp.) And zapotillo (Garrya sp), among others. Likewise, diverse shrub genera such as Baccharis (chamizo) and Vaccinum (chaparrera) are frequent.

The herbaceous strata are not very diverse, ferns being frequent. Species of Asteraceae are also presented. Epiphytes are rare and only some species of orchids and bromeliads occur in forests with higher humidity.

Fauna

The fauna of the pine-oak forests is quite varied. Among the mammals, felines such as the lynx (Lynx rufus) and the puma (Puma concolor) stand out.

The white-tailed deer (Odocoileus virginianus), armadillos (Dasypus novemcinctus), raccoons (Procyon lotor) and the northern coati (Nasua narica) are also very frequent.

Birds are among the most diverse groups. In some areas, more than 100 different species have been found. Woodpeckers, such as the greater woodpecker (Picoides villosus), and the acorn (Sialia mexicana), can be mentioned. Raptors are abundant, highlighting the golden eagle (Aquila chrysaetos), the American kestrel (Falco sparverius) and the red-breasted hawk (Accipiter striatus).

Among the snakes there are several of the genus Crotalus. The transvolcanic rattlesnake (Crotalus triseriatus) stands out, which is distributed in the Transversal Volcanic Axis.

There is an abundance of insects of various groups. Of special ecological and conservation interest is the monarch butterfly (Danaus plexippus). This species fulfills its hibernation period in forests of the Transversal Volcanic Axis between the states of Mexico and Michoacán.

References

1. Almazán C, F Puebla and A Almazán (2009) Bird diversity in pine-oak forests of central Guerrero, Mexico Acta Zoológica Mexicana 25: 123-142.
2. Gernandt D and J Pérez (2014) Biodiversity of Pinophyta (conifers) in Mexico. Mexican Journal of Biodiversity Supl. 85: 126-133.
3. González M, M González, JA Tena, L Ruacho and L López (2012) Vegetation of the Sierra Madre Occidental, Mexico: a synthesis. Acta Botánica Mexicana 100: 351-403.
4. Luna, I, J Morrone and D Espinosa (2004) Biodiversity of the Sierra Madre Oriental. Conabio, Autonomous University of Mexico. Mexico DF. 527 pp.
5. Quintana P and M González (1993) Phytogeographic affinity and successional role of the woody flora of the pine-oak forests of the Highlands of Chiapas, Mexico. Acta Botánica Mexicana 21: 43-57.
6. Rzedowski J (1978) Vegetation of Mexico. Limusa. Mexico, D F. 432 pp.
7. Valencia S (2004) Diversity of the genus Quercus (Fagaceae) in Mexico. Soc.Bot.Méx. 75 : 33-53.