Leslie R. Holdridge, after working for several years as a forester in the Caribbean region, found that, assigning biotemperature and rainfall parameters, it was possible to find out the limits between the higher units of vegetation.

Life zone could be defined as a natural climatic unit in which are grouped different associations corresponding to specific ranges of temperature, rainfall and humidity.

To measure heat, it is used the mean annual biotemperature which is the sum of the mean daily biotemperatures (calculated adding the hourly temperatures between 0º and 30º C of every day and dividing by 24). Biotemperature indicates the limits among which there is active vegetative life.

The second important climatic factor in determining life zones is rainfall. The value used for this factor is the mean annual total, expressed in millimeters, of water that falls from the atmosphere as rain, snow, hail or sleet. This value is always calculated averaging the totals of many years because rainfall tends to vary much from one year to the next. The water that is condensed in the vegetation or in the soil, as dew, is not included. The condensed water, as dew or as fog, when it is important, is considered as a factor of an atmospheric association.

The third and last important climatic factor is humidity which is determined by the relationship between temperature and rainfall, without taking in account other sources of humidity. The value that is useful to represent humidity is the named "relationship of potential evapotranspiration". The potential evapotranspiration is the theoretical quantity of water that the mature natural vegetation of an area would give back to the atmosphere. The value for the relationship of potential evapotranspiration is found out dividing the annual potential evapotranspiration, in mm, by the value of the total annual rainfall, also in mm.

The following descriptions are referred, mostly, to the Dominican Republic and they are based in the study made by the Pan-American Union in 1967.

The symbols used correspond to the Spanish names and common names for plants are those used in the Dominican Republic.

The more extensive areas of this life zone are found in the Southwest of the country. A small area is observed to the southwest of Baní, crossing Arroyo Hondo, and extending westward till Calderas Bay. The borderline between dry forest and thorn woodland is found again to the east of Azua, between Hatillo and Las Charcas. A more extensive area begins around Puerto Viejo, extending to the northwest, crossing part of the Azua valley, the Yaque del Sur river to around the Enriquillo lake.

To the southeast of Pedernales, and around Cabo Rojo, another area with this life zone is observed; it is not well defined because it is on limestone and it is easy to confuse it with the dry forest.

In the Northwest, there is another small area in the foothills of the Los Aguacates mountain, close to the Cruce de Guayacanes.

In this life zone, the climatic conditions are represented by bright days most of the year and a very low annual rainfall. Trade winds cross these regions; those winds, changing direction depending on the season, do not allow that the air masses filled with water could get saturated and so rains do not fall. Most rainfalls are convective ones, and fall mainly from august to october, matching the season with higher rainfall in the Southwest. In Puerto Viejo (Azua), the mean annual total of rainfall is 455 mm.

The mean annual biotemperature could be estimated in 22.0 °C. The potential evapotranspiration could be estimated, on average, to be 130% higher than the amount of mean annual rainfall. In this life zone, the rain water rarely gets to flow by the river beds.

Lands in this life zone are flat to slightly hilly. Altitude ranges from sea level to 300 m high.

Natural vegetation consists mostly of shrubs and thorny plants. Among the main species that prevail in this formation are cacti, such as guasábara (Cylindropuntia caribaea), common in the region between Azua and the Enriquillo lake; cagüey (Neoabottia paniculata), common from the Tábara river to around Enriquillo lake; bayahonda or cambrón (Prosopis juliflora), common to all regions within this zone. Besides cacti, there are isolated Capparis spp. plants.

The land in this life zone could be used in agriculture only with irrigation, mainly those of alluvial origin. The recommended crops are those with a short vegetative cycle.

The regions within this life zone are located in different places of the country. In the Southwest, it is found in along a belt that, starting in the Enriquillo town, extends westward by the southern foothills of Sierra de Bahoruco till the Haitian border, close to Pedernales. In the northern section of the Barahona peninsula, there is another belt that goes from the Caribbean Sea coast to Jimaní, between the limit of the thorn woodland and the northern foothills of Sierra de Bahoruco. The dividing limits between the dry forest and the Subtropical humid forest are confusing in some places in the southern and eastern foothills of Sierra de Neyba and this region continues till the neighborhoods of Pedro Corto and Las Matas de Farfán. From here, the limits extend to the Southeast along the southern foothills of Cordillera Central, from Juan de Herrera, Padre las Casas, Baní, till Hato Viejo to the South of San Cristóbal. In the Southeast of the country, it goes from Cabo Engaño to San Rafael del Yuma; in the Northwest, it extends itself from Santiago to Monte Cristi, among the northern foothills of Cordillera Central and the southwestern foothills of Cordillera Septentrional.

The climatic characteristics of this life zone are bright and sunny days during the dry months and partially cloudy during the raining season and months January to March.

In the South of the country, rains occur in two well marked season; the first season corresponds to May and June, when the trade winds determine the country's more important convective rains, even if they do not correspond to the highest rainfalls in this region. The second rain season is September and October, with the highest rainfalls in the region. In the North, the situation is more variable; in some places, the main rains occur in April - June and in other places it is in September - October.

Rainfalls range from 545 mm, in Puerto Escondido, to 980 mm in Santiago de los Caballeros. On average, it rains in 51 days per year, some times as showers.

For this life zone, mean annual biotemperature is very close to 22.5 °C and it corresponds to a mean annual temperature of about 26 °C, mainly in those places close to great water masses. In places located in higher elevations, the mean annual temperature could be around 23 °C.

Potential evapotranspiration for this life zone could be estimated, on average, in 60% higher than the total annual amount of rain. Rain waters do not get to flow by rivers, except those coming from more humid life zones.

Topography in this life zone is variable, from flat close to the coast to hilly in the foothills of the mountain ranges. Altitude ranges from sea level to about 700 m.

The natural vegetation consists mainly of species of the family Cactaceae (cacti), shrubs and some trees. Among the main species of this life zone are baitoa (Phyllostylon brasiliense), bayahonda or cambrón (Prosopis juliflora), aroma or cambrón (Vachellia farnesiana), guayacán (Guaiacum officinale) and vera (Guaiacum sanctum). Species vary after soil quality; in some places, almácigo (Bursera simaruba) and frijolillo (Capparis spp.) prevail; meanwhile in other places the most common species are guano (Coccothrinax argentea) and some members of genus Croton (tremolina blanca - C. discolor). Uva de playa (Coccoloba uvifera) is common in soils close to the coast. In those places where the natural vegetation has been eliminated then cacti prevail, such as cayuco (Pilosocereus polygonus), alpargata (Consolea moniliformis), bombillito (Mammillaria prolifera), guasábara (Cylindropuntia caribaea), melón espinoso (Melocactus communis) and others.

Native species grow slowly in this life zone. Only where there is enough soil humidity, there could be formation of new tree stands by natural regeneration.

Agriculture without irrigation is restricted to crops with short vegetative cycles because the short rain season. However, pests and diseases are rare and the climatic conditions are appropriate for the development of agriculture.

The segregation of this transitional zone to the dry forest itself is based mainly in its climatic characteristics and its orographic location. In those places where this zone is close to the humid forest, annual rainfalls occur during 108 days. In some places, the month with the highest rainfall is May but in others is October. In transitional zones, located in higher altitudes, biotemperature could be 2 °C lower in some periods. This atmospheric phenomenon helps to keep longer the humidity, even if those places get the same amount of rain that in dry forest.

To identify this transitional zone, the palm cana (Sabal umbraculifera) is an indicative species that helps to separate this zone from the humid forest.

The most important agricultural regions of the country are found in this life zone.

In the North, the Subtropical humid forest is found in the valleys of those rivers flowing into the Atlantic Ocean; these areas go from sea level to 500 m of altitude, in the northern foothills of the Cordillera Septentrional. It goes along the southern foothills of the range, covering most of the Eastern Cibao Valley and the valleys in the low portion of Yuna river watershed. In the Samaná peninsula, it is found from sea level till 400 m. In the Northwest, if is found south of Dajabón, in the valleys of the Yaque del Norte river's tributaries, from 400 m to 800 m of altitude.

In the West, it is found in the valleys of the left bank tributaries of the Artibonito river and in the southern foothills of the Cordillera Central and the northern ones of Sierra de Neyba.

In the Southwest, this life zone is found in the southern foothills of Sierra de Neyba, at altitudes from 400 m to 800 m. In the foothills of Sierra de Bahoruco, it is restricted to narrow strips between the Subtropical very humid forest and the Subtropical dry forest or to its transitional zone to humid forest; altitudes go from sea level (Barahona) to 700 m.

In the southern foothills of Cordillera Central, it is found in the valleys of the tributaries of the Yaque del Sur river and those of the rivers Ocoa, Nizao and Haina.

In the Southeast region, it is found in almost all the Coastal Caribbean Plain, between San Cristóbal, the southern foothills of the Cordillera Oriental and San Rafael del Yuma. It is found also in parts of narrow valleys found in the northern and eastern foothills of the Cordillera Oriental.

The ecological conditions in this life zone result from a complex climatic system under the main influence of the subtropical anticyclones and the direction of the trade winds which prevail most of the year. The continental anticyclone has variable and temporal effects but the oceanic anticyclone has permanent effects.

The raining season is from April to December, with different intensities related the orographic location. In general, rainfalls begin after disappearing the influence of the continental anticyclone, determining so an atmospheric instability over the island. By the other hand, since April the eastern trade winds get loaded with humidity; those winds, when they pass over the island, determine both convective and orographic rainfalls.

Those places located in the western half portion have a rainfall pattern that diminishes from 1,500 mm to 1,000 mm as mean annual average but those located in the eastern half show an increase in the mean annual average from 1,500 mm to 2,000 mm.

Temperature in this life zone is variable, depending on location. In those open places close to the coast, the mean annual biotemperature is 23 ° - 24 °C but in higher places o in those next to the mountain ranges the mean annual biotemperature is 21 ° or less.

Potential evapotranspiration could be estimated to be, on average, 20% lesser than the mean total annual rainfall. In this life zone, a fourth of the rain water is not evapotranspirated and is lost by running off, above all in those months with the higher rainfalls.

The original natural vegetation of this formation was composed by forests which very few remain because most of them has been cut to be used in agriculture. Only secondary stands remain around the field crops or along riverbanks.

The main indicative species, useful to recognize this life zone, are roble (Catalpa longissima), mainly in well drained lands, and caoba (Swietenia mahagoni), a characteristic species of this life zone. In calcareous soils, the Hispaniolan royal palm ("palma real") (Roystonea hispaniolana) is commonly found.

The small secondary stands are composed by Juan Primero (Simaruba glauca), anón de majagua (Lonchocarpus pentaphyllus) and jagua (Genipa americana); the species more commonly found in the Caribbean Coastal Plain and the foothills of Cordillera Septentrional are guaraguao or grigrí (Bucida buceras), yaya (Oxandra lanceolata) and amacey (Tetragastris balsamifera). It is possible to find some isolated trees of fustete (Chlorophora tinctoria), common in Luperón, Imbert and Villa Isabel; campeche (Haematoxylon campechianum), found in Puerto Plata and surrounding Santiago; lirio (Hippeastrum puniceum), common in the calcareous soils of San Pedro de Macorís; caracolí (Pithecellobium glaucum), found in Enriquillo, Barahona and Azua; guácima (Guazuma ulmifolia) and palo de leche (Rauwolfia nitida), common in the Caribbean Coastal Plain and in the Barahona peninsula; penda (Citharexylum fruticosum) and córbano (Pithecellobium berterianum), common in places of San José de las Matas, Santiago, La Romana and Barahona.

The vegetation of savannas or shallow soils is characterized by the presence of the shrubs peralejo (Curatella americana), hojancha (Coccoloba pubescens), memiso (Trema micrantha) and roble (Tabebuia spp.). Cajuil (Anacardium occidentale) is found in very marginal places with rainfalls close to those in the Subtropical dry forest.

The vegetation of swamps and mangroves is formed mainly by drago (Pterocarpus officinalis), higo (Ficus spp.), mangle colorado (Rhizophora mangle), mangle prieto (Conocarpus erecta), mangle (Avicennia nitida) and mangle blanco (Laguncularia racemosa).

The natural regeneration of native species is easy because of the soil humidity. Species in this life zone have a moderate growth.

From a climatic viewpoint, these lands are the most appropriate for the development of agriculture due to the optimal combination of temperature and rain. In these places are located the populated centers with the higher demographic density. Most of inhabitants are dedicated to agriculture and cattle raising. Because of those favorable climatic conditions, the rural populations here are in better economic conditions that in any other life zone.

The regions with this life zone are found mainly in the foothills of the Septentrional, Central and Oriental mountain range. Many of the humid forest cultivated fields extend into this zone what makes sometimes very difficult to establish the limit between these two formations. Normally this life zone limits with both bh-S and bmh-MB life zones.

In the northern region it is found mainly in the hilly lands of the Cordillera Septentrional. In the Northeast, it occupies the Promotorio de Cabrera, getting to sea level. In Samaná Peninsula, it is found from 200 m to the highest elevations found in that peninsula.

In the East, this formation occupies the higher places of Sierra de Yamasá and Cordillera Oriental. In the eastern end of Cordillera Oriental, it extends over slightly hilly lands and to sea level.

In the southern and eastern Cordillera Central, it is found in the tributaries of the rivers Yuna, Nizao, Haina and Ozama. In the northern and western parts, it is found in the tributaries of the rivers Yaque del Norte, Dajabón and Artibonito.

The climatic conditions corresponding to this life zone are variable because the influences of the anticyclones and the trade winds that pass over the country. Rainfall is similar to those of the Subtropical humid forest, except that the orographic rains are more intense. These rains, because they last longer, have influence on the flora composition and in the physiognomy of the vegetation. Rainfalls in this life zone range, on average, from 2,200 mm to 4,400 mm.

Temperature varies in relation to the localization. Those areas close to the coast has an annual mean biotemperature of 24 °C. However, those areas that are in the foothills of the mountain rages have mean biotemperatures as low as 18 °C.

The potential evapotranspiration could be estimated to be, on average, 60% lower than the mean total average annual rainfalls. In this life zone, 3/5 of the rain waters are lost by running off, arising rivers with water the whole year.

In this life zone, most of the lands are in the mountains. Altitude ranges from sea level to 850 m.

The main species that help to identify this life zone are guaraguao (Buchenavia capitata), mainly in the Septentrional and Oriental mountain ranges; and, in a general way, sablito (Didymopanax morototoni), peralejo o madroño (Byrsonima spicata) and aguacatillo (Alchornea latifolia).

The natural vegetation in this life zone is formed mainly by the following species: membrillo or almendrito (Prunus myrtifolia), in the forests of Los Haitises and of the northern foothills of Cordillera Central; mara or baría (Calophyllum brasiliense), cocuyo (Hirtella triandra) and cabirma (Guarea guidonia), in those forested areas close to Jarabacoa, around the Isabel de Torres mountain, Samaná peninsula and the foothills of Cordillera Oriental; palo de yagua (Casearia arborea), found in areas close to Barahona and Nagua; algarrobo (Hymenea courbaril), found in the proximity of Bonao, La Vega and Miches; balatá (Manilkara domingensis), common in the areas of the Samaná peninsula and in the foothills of the Septentrional and Oriental mountain ranges; and the palm manacla (Prestoea montana), that occurs in forests with a specific soil. Another common species is pino (Pinus occidentalis), mostly in lateritic soils.

The climatic conditions in this life zone are favorable to an easy natural regeneration and the fast growing of the plants.

In general terms, only very fertile alluvial soils could be used for intensive agriculture, and it would be profitable only with very remunerative crops. Those lands with shallow soils and slight slopes are appropriate for perennial crops such as cacao, rubber, some subtropical fruits and, in lesser proportion, coffee.

The location of this zone is related to the physiographic position within the bmh-S. The largest area is found between the sources of Masipedro and Jima rivers, in the Casabito mountain. Other small areas are found in the Oriental and Septentrional mountain ranges.

The surplus of rainfall in this area is due, without any doubts, to orographic rains. Potential evapotranspiration could be estimated, on average, to be 75% lesser than the annual mean rainfall. In this life zone, 3/4 or more of the rain waters are lost through running off, and so the rivers have water along the whole year

This are mountainous areas and their altitudes range fro 400 m to 850 m.

The natural vegetation is composed by trees, markedly covered with parasitic and epiphytic plants. Among the main indicative species are the tree fern (Cyathea spp.) and lirio (Linociera spp.).

In this life zone, the native species grow faster than those of the bmh-S and their natural regeneration is abundant.

From the forestry and agriculture viewpoint, this lands of the Subtropical pluvial forest are worthless due to the excessive humidity. This lands will have to remain with their natural cover as a means to control the running off and soil erosion.

Most of this life zone is found in the Cordillera Central, between the bh-S and the bmh-MB, in hilly lands and soils with low productivity. In the Neyba and Bahoruco mountain ranges, this life zone is found above 800 m.

Climatic conditions are the most appropriate for human and animal life, even if in the country this areas are underpopulated because of poor soils, lack of roads and also because farmers have not developed the appropriate techniques for this land. Temperatures are moderate, similar to those in a temperate climate, and rains, even if they are not regular, keep a certain degree of soil humidity throughout the whole year.

Rainfalls are heavier from April to November. The amount of rain varies depending on the place, and it ranges from 900 to 1,800 mm as annual total mean.

Temperature varies very slightly along the day and in season. Between December and February, it could be as low as -1 °C, arising the possibility of occasional frost. However, the annual mean temperature ranges from 18° and 21° C. The annual mean biotemperature for this life zone has values close to the annual mean temperature because this last one never gets above 30 °C, and very few times is lower than -1 °C. The thermal conditions of this zone is limiting factor for the development of most cold sensible crops.

The percentage of the mean potential evapotranspiration is the same that the total annual rainfall and, because of that, many ravines arise here that carry water only in those months of highest rainfall.

Lands in this life zone are located in the mountains and the areas with slight slopes are few and dispersed, forming narrow valleys, without communications. Altitude ranges from 800 m to 2,200 m.

The primary natural vegetation consists mainly of pine trees. Other conifers are found here, mainly in the southern side of Cordillera Central, and these are sabina (Juniperus gracilior) and Podocarpus buchii. Some of the main broadleaf species are Garrya fadyenii and Vaccinium cubense, found in areas close to Constanza and in the Sierra de Bahoruco; Rapanea ferruginea, found in the northern side of Cordillera Central, close to Jarabacoa and San José de las Matas; and Buddleia domingensis, that is a species found in areas close to the bmh-MB in Cordillera Central.

Native species have an easy natural regeneration due to soil humidity, and they are of moderate growth.

Lands, from an ecological point of view, have optimal characteristics for agriculture and forestry. Pests and diseases are sporadic. However, agriculture are restricted because of the soils and, in fact, very few lands are appropriate for agriculture.

It is found mainly in the mountains of the Cordillera Central, where arises the most important rivers of the country. Other areas where this life zone is found are the highest mountains of Neyba and Bahoruco ranges.

Climate in this life zone is characterized by occasional frost and because it gets more rainfall than the bh-MB.

It can be estimated that rainfalls are over 2,000 mm per year. The rainfall pattern is similar to that of the bh-MB, although orographic rainfalls are more intense.

The potential evapotranspiration could be estimated to be, on average, 55% lower than the total annual mean rainfall. In this life zone, close to 50% of the rain water is not evapotranspirated and so rivers have water most of the year.

This life zone is found in mountains. Altitude ranges from 850 to 2,100 m.

Tree species are the main constituents of the natural vegetation. Among the main indicative species that could help to identify this life zone are Garrya fadyenii, Weinmannia pinnata, Oreopanax capitatum, Brunellia comocladifolia and Didymopanax tremulum.

The most valuable species of the natural forest are pine and some broad-leaved such as ébano (Diospyros ebenaster) and almendro (Prunus occidentalis).

The native species in this life zone have an easy natural regeneration and a moderate growth.

From an ecological viewpoint, lands in this life zone offer few possibilities for agriculture; they are essentially for forestry and its natural vegetation, in some places, should remain unexploited to allow the control of the rain water running off and to avoid soil erosion in the watersheds.

This zone is found in the eastern part of Cordillera Central, between the tributaries of Nizao and Yaque del Norte tributaries, and in the western part, between the tributaries of the river San Juan.

This life zone is characterized, as well as the other formations of the Lower Montane belt, by the presence of frost in certain times of the year. Rainfall could be estimated to be over 4,000 mm per year. Potential evapotranspiration could be estimated, on average, to be 76% lower than the mean annual rainfall. Here about 3/4 of the rain waters are not evapotranspirated, and they are lost by running off.

This life zone is located high in the mountains and the altitude ranges from 1,600 m to 1,800 m.

Its vegetation is characterized by the predominance of tree ferns and the presence of parasitic and epiphytic plants covering the tree trunks.

From an ecological point of view, this life zone has neither agricultural nor forestal value. Its natural vegetation should remain without modification.

It is found mostly in the higher elevations of Cordillera Central, where the highest peaks of Hispaniola island and of the Caribbean region are found. The altitudinal belt Montane begins, in this country, at 2,100 meters above sea level.

In this life zone, the climatic conditions differs from those of the bmh-MB because frosts are more frequent. The rainfall in this zone could be estimated in 1,500 per year; and the potential evapotranspiration could be estimated, on average, 60% lower than the annual mean rainfall.

The main tree species is pino (Pinus occidentalis) which, in this life zone, has a very irregular shape and is not very tall.

These lands should remain with their natural vegetation to protect the watersheds.