Chapter 33. North and Central America: ecological zones

Figure 33-1. North and Central America: ecological zones

Figure 33-1 shows the distribution of ecological zones in North and Central America, as identifed and mapped by FRA 2000. Table 33-1 contains area statistics for the zones by subregion, and Table 33-2 indicates the proportion of forest in each zone by subregion.

BOREAL TUNDRA WOODLAND

The Hudson Plain occupies a major area of this zone, while the western portion consists of subdued broad lowlands and plateaus incised by major rivers. The climate, influenced by cold arctic air, is characterized by short, cool summers and long, cold winters. Mean annual temperature ranges from -10° to 0°C, with mean temperature in summer from 6° to 14°C and in winter from -26° to -16°C. Snow and ice persist for six to eight months of the year. The mean annual precipitation is low in the west, ranging from 200 to 500 mm, but reaches 500 to 800 mm in the east, with portions of Labrador reaching 1 000 mm.

Vegetation associations of the Hudson Bay lowlands consist of arctic tundra and some boreal forest transition types. The better-drained sites support open woodlands of black spruce (Picea mariana), tamarack (Larix laricina) and some white spruce (Picea glauca). Balsam poplar (Populus balsamifera), white spruce and white or paper birch (Betula papyrifera) are common along rivers.

East of these lowlands are large open stands of black spruce woodland as well as stunted black spruce and tamarack on the windswept plateaus. White spruce is also present. Alder (Alnus incana) thickets are common along riverbanks and other drainage areas. Other species include quaking aspen (Populus tremuloides) and balsam fir (Abies balsamea). Limited tree vegetation occurs along the exposed headlands of the Atlantic Coast and within the interiormost windswept barrens.

West of Hudson Bay, open stands of black and white spruce and tamarack dominate. Sometimes these open forests include jack pine (Pinus banksiana) as well. The western limits of the zone are characterized by open, generally slow-growing black spruce. Upland and foothill areas and southerly locales tend to be better drained and are somewhat warmer. Here, mixed-wood forests of white and black spruce, lodgepole pine (Pinus contorta), tamarack, white birch, trembling aspen and balsam poplar are common. Along nutrient-rich alluvial flats, white spruce and balsam poplar grow to sizes comparable to the largest in the boreal forest to the south.

Table 33-1. North and Central America: extent of ecological zones

Subregion	Total area of ecological zone (million ha)
	Tropical						Subtropical					Temperate					Boreal			Polar
	Rain forest	Moist	Dry	Shrub	Desert	Mountain	Humid	Dry	Steppe	Desert	Mountain	Oceanic	Continental	Steppe	Desert	Mountain	Coniferous	Tundra	Mountain
Central America	33	55	22			24			30	63	20
Caribbean	10	11	1			2
North America		2					106	9	86	45	39	4	225	211	75	197	225	266	118	324
Total North and Central America	43	68	23			26	106	9	116	108	59	4	225	211	75	197	225	266	118	358
TOTAL WORLD	1468	1117	755	839	1192	459	471	156	491	674	490	182	726	593	552	729	865	407	632	564

Note: Data derived from an overlay of FRA 2000 global maps of forest cover and ecological zones.

Table 33-2. North and Central America: proportion of forest by ecological zone

Subregion	Total area of ecological zone (million ha)
	Tropical						Subtropical					Temperate					Boreal			Polar
	Rain forest	Moist	Dry	Shrub	Desert	Mountain	Humid	Dry	Steppe	Desert	Mountain	Oceanic	Continental	Steppe	Desert	Mountain	Coniferous	Tundra	Mountain
Central America	69	59	44			65			11	6	74
Caribbean	46	28				55
North America							46	23	8	3	54	39	34	2	8	47	52	31	36	2
Total North and Central America
TOTAL WORLD	64	53	44			65	46	23	8	5	61	39	34	2	8	47	52	31	36	2

Note: Data derived from an overlay of FRA 2000 global maps of forest cover and ecological zones.

A broadly rolling mosaic of uplands and associated wetlands dominates this zone. The climate is generally continental with long, cold winters and short, warm summers, modified in the east by the Atlantic Ocean. The mean annual temperature ranges from -4°C in central Canada to 5.5°C in the boreal regions of Newfoundland. Mean summer temperature varies between 11° and 15°C, with mean winter temperature from -20.5° C in the west to -1°C in the east. Mean annual precipitation varies between 100 and 625 mm with the exception of boreal Newfoundland, where average precipitation is higher, from 900 to 1 600 mm.

Much of the zone is distinguished by closed stands of conifers, largely white spruce (Picea glauca), black spruce (P. mariana), balsam fir (Abies balsamea) and tamarack (Larix laricina). Common deciduous species include white birch (Betula papyrifera), trembling aspen (P. tremuloides) and balsam poplar (P. balsamifera). In the south, conifers such as eastern white pine (Pinus strobus), red pine (P. resinosa) and jack pine (P. banksiana) are evident. At the transition with forests to the south, species such as sugar maple (Acer saccharum), black ash (Fraxinus nigra) and eastern white cedar (Thuja occidentalis) are found.

Towards the western boundary of the zone the vegetation is medium to tall closed stands of trembling aspen, balsam poplar and jack pine with white and black spruce occurring in late successional stages. Lodgepole pine (Pinus contorta) may dominate in some of the upland areas along with white spruce and balsam fir. Black spruce tends to be concentrated in the poorly drained valleys. Trembling aspen and balsam poplar characterize the transition to the south. White spruce and balsam fir are the climax species but are not widespread because of the frequent occurrence of fire.

Both open and closed black spruce and balsam fir forests are characteristic in the east. White birch and trembling aspen are typical of disturbed sites. White spruce is generally more tolerant of ocean spray and is more prevalent near the ocean. Wetlands are extensive, with a cover of stunted black spruce, tamarack and shrubs.

The northern part of the zone is transitional to the boreal tundra. Pure stands of jack pine or mixed stands of jack pine, white birch and trembling aspen are typical of the drier sites, while black spruce and balsam fir dominate wet sites.

BOREAL MOUNTAIN SYSTEMS

Mountain ranges with numerous high peaks and extensive plateaus separated by wide valleys and lowlands characterize this zone. The climate ranges from cold, subhumid to semi-arid with long, cold winters and short, warm summers. Mean annual temperatures range from -10°C in the north to 5°C in the south. Mean summer ranges are 6.5° to 11.5°C and mean winter temperatures range between -13° and -25°C. Annual precipitation is lowest in valleys in the rain shadow of the Coast Range (less than 300 mm) and increases up to 1 500 mm at higher elevations of the interior mountains.

Vegetation at higher elevations ranges from arctic to alpine tundra. At lower elevations in the north, open woodlands of white spruce (Picea glauca) and white birch (Betula papyrifera) are mixed with dwarf birches and willows. The unglaciated Old Crow Basin has stunted stands of black spruce and tamarack with some white spruce. To the south, vegetative cover ranges from closed to open forest of white and black spruce, subalpine fir (Abies lasiocarpa), lodgepole pine (Pinus contorta), trembling aspen (Populus tremuloides), balsam poplar (P. balsamifera) and white birch. Lodgepole pine and subalpine fir tend to disappear rapidly towards the north.

TEMPERATE OCEANIC FOREST

This relatively small ecological zone occupies a north-south depression between the Pacific Coast Range and the Cascade Mountains. The nearness of the ocean profoundly moderates the climate, and annual temperatures average 9° to 13°C. Average rainfall ranges from around 400 to 1 500 mm, but more typically is from 750 to 1 150 mm. Fog partially compensates for the summer drought.

These forests are composed of mixtures of western red cedar (Thuya plicata), western hemlock (Tsuga heterophylla) and Douglas fir (Pseudotsuga menziesii). In the interior valleys, the forest is less dense than along the coast and often contains such deciduous trees as big-leaf maple (Acer macrophyllum), black cottonwood (Populus trichocarpa) and, to the south, Oregon ash (Fraxinus latifolia). There are woodlands that support open stands of oaks or are broken by groves of Douglas fir and other trees such as Oregon white oak (Quercus garryana) and Pacific madrone (Arbutus menziesii). Clearing for cultivation has greatly reduced the area of these forests.

TEMPERATE CONTINENTAL FOREST

Warm summers and cool winters are typical of this zone. The weather is highly changeable. Mean annual temperatures range from 2° to 10°C. The mean summer temperature ranges from 16° to 18°C, with the winter mean ranging from -2.5° to -7°C. Annual precipitation over much of the zone ranges from 720 to 1 000 mm, reaching 1 500 mm near the Atlantic Coast. The proximity of the Atlantic Ocean moderates the climate of the eastern portion of the zone.

At one time the entire zone was heavily forested, but most of the forests around the Great Lakes and in the northeastern United States have succumbed to urbanization and conversion to agriculture. Forest cover varies from mixed coniferous/deciduous stands of white and red pine (Pinus strobus and P. resinosa), eastern hemlock (Tsuga canadensis), red oak (Quercus rubra), sugar maple (Acer saccharum) and white birch (Betula papyrifera) in the northern portions to the rich diversity of the deciduous Carolinian forest in the southwest.

The mixed mesophytic association, the deciduous forest with the greatest diversity, occupies well-drained sites. Widespread dominants include sugar maple, American beech (Fagus grandifolia), white elm (Ulmus americana), basswood (Tilia americana), red and white oak (Quercus rubra, Q. alba), walnut (Juglans nigra, J. cinerea), hickory (Carya ovata, C. cordiformis), buckeye (Aesculus spp.) and eastern hemlock (Tsuga canadensis) in addition to 20 to 25 other species. An oak association, with white oak and northern red oak as dominant species, occurs east of the Appalachian Mountains.

Further inland, where precipitation is lower, the drought-resistant oak-hickory association is dominant, with white oak, red oak, black oak (Quercus velutina), bitternut hickory (Carya cordiformis) and shagbark hickory (Carya ovata). Wetter sites typically feature American or white elm, tulip-tree (Liriodendron tulipifera) and sweetgum (Liquidambar styraciflua). Northern reaches of this association contain maple, beech and basswood (Tilia americana).

Forests in the northeastern portion of the zone are generally mixed stands of conifers and deciduous species characterized by red spruce (Picea rubens), balsam fir (Abies balsamea), yellow birch (Betula alleghaniensis) and sugar maple. Red and white pine and eastern hemlock occur to a lesser but significant degree. Some boreal species are present, including black spruce (Picea mariana), white spruce (P. glauca), balsam poplar (Populus balsamifera) and white birch. Jack pine (Pinus banksiana) is prominent on sandy soils. Pine-oak forest occupies dry sandy soils along the northern coastal plain of the United States and is frequently exposed to naturally occurring fires. Eastern white cedar (Thuja occidentalis) occurs on mesic sites.

TEMPERATE STEPPE

The climate of this zone is greatly influenced by its location in the heart of the continent. The zone has a continental climate that is subhumid to semi-arid with short, hot summers and long, cold winters. Generally, precipitation is low and evaporation is high. Mean annual temperature ranges from 1.5° to 3.5°C. Mean winter temperature ranges from -12.5° to -8° C, with summer means from 14° to 16°C. Annual precipitation is variable, from 250 mm in the arid grasslands to near 700 mm in the higher-elevation wooded portions.

Park-like stands of trembling aspen (Populus tremuloides) and balsam poplar (P. balsamifera) lie at the northern edge of this zone, a transition to the boreal forest to the north. The aspen parkland has expanded considerably southwards since prairie fires were effectively eliminated. Patches of scrubby aspen and cottonwood (Populus spp.), willow (Salix spp.) and box-elder (Acer negundo) occur on shaded slopes of valleys and river terraces. To the east, the zone consists of a mosaic of trembling aspen, bur oak (Quercus macrocarpa) and grasslands. Further south, oak and hickory become the dominant tree species in the transition zone with the eastern broadleaf forests.

TEMPERATE DESERT

This zone covers the Great Basin, the northern Colorado Plateau in Utah and the plains and tablelands of the Columbia-Snake River Plateaus and the Wyoming Basin. The aridity of this zone is the result of the rain shadow of the Sierra Nevada and Cascade Mountains. Summers are hot and winters are cold, with stronger seasonal temperature extremes on the higher plateaus. The average annual temperature ranges from 4° to 13°C. Annual precipitation averages about 130 to 400 mm. Almost no rain falls during the summer months. Part of the winter precipitation falls as snow.

The main vegetation, sometimes called sagebrush steppe, is made up of sagebrush and other shrub species mixed with short grasses. Above the sagebrush belt lies a woodland zone dominated by pinyon pine (Pinus edulis) and juniper (Juniperus spp.).

TEMPERATE MOUNTAIN SYSTEMS

This zone includes the Coast Range, the Rocky Mountains and the Appalachian Mountains. The climate is extremely varied, from a relatively humid maritime climate at low elevations along the Pacific Coast to cold, arctic conditions above the tree line in the Rocky Mountains. Along the coast the mean annual temperature ranges from 4.5°C in the north to 9°C in the south. Average annual precipitation is extremely variable, from 600 mm in the Gulf Islands to 4 000 mm to the north. The interior portion of the zone is similarly variable. The climate of the Appalachian Highlands is more temperate, with a distinct summer and winter. Average annual temperatures range from below 10°C in the north to about 18°C at the southern end. Average annual precipitation varies from 900 mm in the valleys to 2 000 mm on the highest peaks.

The temperate rain forests of the Pacific Coast Mountains are among the most productive in North America and contain some of the world's largest and longest-lived trees. This vegetation association is dominated by western hemlock (Tsuga heterophylla) and amabilis or Pacific silver fir (Abies amabilis) as climax species, although several other species are common. Big-leaf maple (Acer macrophyllum) reaches its northern extension in the southern portion of this zone. It is generally found along creek beds and in other alluvial areas along with red alder (Alnus rubra) and black cottonwood (Populus trichocarpa). At high elevations, up to 2 000 m, mountain hemlock (Tsuga mertensiana), subalpine fir (Abies lasiocarpa) and amabilis fir assume prominence along with yellow or Pacific cedar (Chamaecyparis nootkatensis), becoming open and stunted at higher elevations.

Amabilis fir, lodgepole pine (Pinus contorta) and Sitka spruce (Picea sitchensis) are common in the north. At lower elevations in the north, western hemlock and western red cedar (Thuya plicata) dominate with red alder pioneering on disturbed sites. The coastal Douglas fir association is found in the lee of the coastal mountains. Douglas fir (Pseudotsuga menziesii) dominates. Western red cedar (Thuja plicata) is typical on wetter sites, and Garry oak (Quercus garryana) and arbutus (Arbutus menziesii) are abundant on drier sites.

Interior Douglas fir associations dominate in the rain shadow of the Coast Range and other mountain ranges. Fires have resulted in even-aged lodgepole pine stands at higher elevations, while ponderosa pine (Pinus ponderosa) is the common seral tree at the warmer and drier lower elevations.

At mid elevations of the interior plateau regions, closed stands of Englemann spruce (Picea engelmannii) and subalpine fir are common. Lodgepole pine, western white pine (Pinus monticola), Douglas fir and trembling aspen reflect past fire history. At higher elevations the Englemann spruce-subalpine fir association begins to dominate. The forest often has an open parkland appearance. Under drier conditions, extensive stands of lodgepole pine and whitebark pine (Pinus albicaulis) are common. Wetter areas may be dominated by mountain hemlock (Tsuga mertensiana).

A western red cedar-western hemlock forest with a wide variety of conifer trees is characteristic of the interior wet belt of this zone. In addition to the two dominant species, other common trees include white spruce (Picea glauca), Englemann spruce and subalpine fir. Douglas fir and lodgepole pine occur in drier areas. Englemann spruce, white spruce and subalpine fir are the dominant trees in subboreal plateau areas. Even-aged lodgepole pine and trembling aspen (Populus tremuloides) cover extensive areas of previously burned sites.

In the Appalachian Highlands, a vertical zonation prevails, with the lower limits of each forest belt rising towards the south. The valleys of the southern parts support a mixed oak-pine forest. Above this zone lies the Appalachian oak forest, dominated by a dozen species of white and black oaks. At higher elevations is hardwood forest composed of birch, American beech (Fagus americana), maple (Acer spp.), elm (Ulmus spp.), red oak (Quercus rubra) and basswood (Tilia americana), with an admixture of eastern hemlock (Tsuga canadensis) and white pine. Spruce-fir forest and meadows are found on the highest peaks. Mixed mesophytic forest extends into narrow valleys of the southern Appalachians, where oak vegetation predominates. The northern reaches are located in the transition zone between the boreal spruce-fir forest to the north and the deciduous forest to the south. Growth form and species are very similar to those found to the north, but red spruce (Picea rubens) tends to replace white spruce. Here the valleys contain a hardwood forest dominated by sugar maple, yellow birch (Betula alleghaniensis) and beech. Low mountain slopes are covered with a mixed forest of spruce, fir, maple, beech and birch. Above the mixed-forest zone lie pure stands of balsam fir (Abies balsamea) and red spruce.

SUBTROPICAL HUMID FOREST

This zone comprises the Atlantic and Gulf coastal plains and piedmont. Winters are mild and summers hot and humid. Average annual temperature is 15° to 21°C. Annual precipitation ranges from around 1 000 to 1 500 mm, relatively evenly distributed throughout the year.

On the coastal plains temperate evergreen rain forest is the dominant natural vegetation. Subtropical rain forest has fewer tree species than its tropical counterpart; trees are not as tall, leaves are usually smaller and more leathery and the leaf canopy is less dense. Common species include evergreen oaks (Quercus myrtifolia, Q. virginiana, Q. laurifolia) and species of laurel (Cordia alliodora, C. bicolor) and magnolia (Magnolia grandiflora, M. virginiana). Further inland, the climax vegetation is medium-tall to tall forests of broadleaf deciduous and needleleaf evergreen trees. Loblolly pine (Pinus taeda), shortleaf pine (Pinus echinata) and other southern yellow pine species dominate the stands, singly or in combination. Common associates include oak (Quercus spp.), hickory (Carya spp.), sweetgum (Liquidambar styraciflua), blackgum (Nyssa sylvatica), red maple (Acer rubrum) and winged elm (Ulmus alata). Gum and cypress dominate the extensive coastal marshes and interior swamps along the Atlantic and Gulf Coasts.

Along the Mississippi River, small patches of riverine deciduous forests still occur, with an abundance of green ash (Fraxinus pennsylvanica), Carolina poplar (Populus deltoides), elm, cottonwood, sugarberry (Celtis laevigata), sweetgum and water tupelo (Nyssa aquatica), as well as oak and baldcypress (Taxodium distichum). Pecan (Carya illinoiensis) is also present, associated with American sycamore (Platanus occidentalis), American elm (Ulmus americana) and roughleaf dogwood (Cornus drummondii).

Today, extensive forests of loblolly and slash pine (Pinus taeda, P. elliottii) are widespread in this zone, predominantly as plantations or second-growth forest following fire.

SUBTROPICAL DRY FOREST

This ecological zone is situated on the Pacific Coast between approximately 30° and 45°N latitude. The climate is typically Mediterranean, characterized by hot, dry summers and mild winters, with precipitation associated with winter storms. Annual temperatures average about 10° to 18°C, with average summer temperature above 18°C and average winter temperatures above 0°C. Annual rainfall ranges from 200 to 1 000 mm depending on latitude and altitude, always with a pronounced summer drought. Extreme droughts are not uncommon. Coastal fog is typical, particularly from May through July.

Redwood (Sequoia sempervirens) is characteristic of the fog belt on seaward slopes in coastal northern California. Associated with it are Douglas fir (Pseudotsuga menziesii) and other conifers such as western hemlock (Tsuga heterophylla) and western red cedar (Thuya plicata). Along the coast in a narrow, patchy belt lies pine-cypress forest. Inland, the south-facing mountain slopes are covered by mixed forest, including tanoak (Lithocarpus densiflorus), live oak (Quercus spp.), madrone (Arbutus menziesii) and Douglas fir.

The central and southern coastal areas are covered by chaparral, a mostly evergreen shrub vegetation. Several tree species are endemic to this region, including Monterey cypress (Cupressus macrocarpa), Torrey pine (Pinus torreyana), Monterey pine (P. radiata) and Bishop pine (P. muricata). Patches of live oak (Quercus spp., mostly Q. agrifolia) or valley oak (Quercus lobata) woodland are found on the hills and lower mountains.

A blue oak (Quercus douglasii)-foothill pine (Pinus sabiniana) woodland community forms a ring around the Central Valley of California. Most of the coastal plains and interior valleys have been converted to urban use or irrigated agriculture.

SUBTROPICAL STEPPE

This zone is dominated by flat to rolling plains and plateaus. The climate is semi-arid subtropical. Summers are long and hot and winters are generally short and mild. Annual temperatures average 14° to 21°C. Annual precipitation varies considerably, from about 250 mm in the drier (mostly western) regions, to about 1 000 mm in the northeastern Prairie Parkland region. The zone is also subject to periodic intense droughts and frosts.

A variety of natural vegetation is found in this zone. Grasslands in which shrubs and trees grow singly or in bunches are predominant. Locally, oak and juniper are mixed with grasses and mesquite (Prosopis spp.). Because of the low rainfall they rarely grow higher than 5 to 7 m. The most characteristic tree is Ashe juniper (Juniperus ashei). Live oak (Quercus virginiana) forest is found along the Gulf Coast. In the northeastern part of the zone, oak savannah, dominated by post oak (Quercus stellata) and blackjack oak (Quercus marilandica), forms a transition with the more humid subtropical forest zone.

The generally higher Colorado Plateau has distinct vegetation. Woodland is the most extensive vegetation type, dominated by open stands of pinyon pine and several species of juniper (Juniperus spp.). Cottonwoods and other trees grow along some of the permanent streams.

SUBTROPICAL MOUNTAIN SYSTEMS

This zone comprises the southernmost portion of the Cascade Mountains and the Rocky Mountains, the Sierra Nevada, the Coast Range and the Western Sierra Madre. The climate is extremely diverse, with variation related to latitude, altitude and exposure. The prevailing west winds influence climatic conditions; the eastern slopes are much drier than the western slopes. Winter and annual precipitation increases with elevation; at high altitude precipitation is mostly snow.

Vegetation zones are well differentiated, generally in altitudinal belts. In the Sierra Nevada, southern Cascades and northern Coast Range, the slopes, from about 500 to 1 200 m, are covered by coniferous and shrub associations. On higher slopes, foothill pine (Pinus sabiniana) and blue oak (Quercus douglasii) dominate, forming typical open or woodland stands. Above this belt, between 600 and 1 800 m in the Cascades and between 1 500 and 2 400 m or higher in the south, the most important trees are ponderosa pine (Pinus ponderosa), Jeffrey pine (P. jeffreyi), Douglas fir (Pseudotsuga menziesii), sugar pine (P. lambertiana), white fir (Abies concolor), California red fir (Abies magnifica) and incense cedar (Calocedrus decurrens), but several other conifers are also present. The spectacular giant sequoia (Sequoiadendron giganteum) grows in a few groves on the western slopes. On the dry eastern slopes, Jeffrey pine replaces ponderosa pine. The subalpine zone begins at 1 800 to 2 500 m and extends upslope for about 300 m. Mountain hemlock (Tsuga mertensiana), California red fir, lodgepole pine (Pinus contorta), western white pine (P. monticola) and whitebark pine (P. albicaulis) are important. The timberline ranges from about 2 100 m in the north to 3000 m in the south.

Further south in the drier California Coastal Range, the vegetation consists of sclerophyll forest and chaparral. Chaparral is found on south-facing slopes and drier sites, while forest appears on northfacing slopes and wetter sites. The most important evergreen trees are California live oak (Quercus agrifolia), canyon live oak (Q. chrysolepis), interior live oak (Q. wislizeni), tanoak (Lithocarpus densiflorus), California laurel (Umbellularia californica), Pacific madrone (Arbutus menziesii), golden chinkapin (Castanopsis chrysophylla) and Pacific bayberry (Myrica californica). At higher elevations and near the ocean, chaparral is often interspersed with coniferous forest.

Vegetation zones in the southern Rocky Mountains resemble those further north but occur at higher elevations. The foothill zone, reaching as high as 2 000 m, is characterized by mixed grasses, chaparral brush, oak-juniper woodland and pinyon-juniper woodland. At about 2 000 m, open forests of ponderosa pine are found, although pinyon and juniper occupy south-facing slopes. In Arizona, the pine forests are strongly infused with Chihuahuan pine (Pinus leiophylla var. chihuahuana) and Apache pine (Pinus engelmannii). Pine forest is replaced at about 2 400 m by Douglas fir. Aspen (Populus tremuloides) is common in this zone and limber pine (Pinus flexilis) grows in places that are rockier and drier. At about 2 700 m the Douglas fir zone merges into a belt of Englemann spruce (Picea engelmannii) and corkbark fir (Abies lasiocarpa var. arizonica). Limber pine and bristlecone pine (Pinus longaeva) grow in the rockier places. The alpine zone starts around 3 400 m.

The vegetation of the western Sierra Madre in Mexico includes both evergreen and deciduous forest, primarily composed of conifers and oaks. These grow usually from 10 to 30 m, sometimes reaching 50 m. Mountain cloud forest also occurs. Mexico has about 40 species of pine and more than 150 species of oak.

TROPICAL RAIN FOREST

This zone encompasses parts of the Gulf coastal plain and the lowlands of the Chiapas Sierra Madre in Mexico as well as lowlands along the Caribbean Coast and small areas along the Pacific Coast in Central America. Parts of the Caribbean islands are also included. Year-round temperatures average between 20° and 26°C with little seasonal variation. The average annual precipitation range is 1 500 to 3 000 mm and in some areas may total more than 4 000 mm. The dry season lasts less than three months, occurring in winter. North of about 12°S latitude, hurricanes (tropical cyclones) bring very heavy regional rains from August to October.

The evergreen to semi-evergreen forest along the Atlantic Coast is tall and dense. The forest has a complex and diverse flora with approximately 5 000 vascular plant species. Canopy trees reach 30 to 40 m high, with emergent trees up to 50 m. The subcanopy layer is dense, with trees from 5 to 25 m tall. The understorey layers present a great variety of palms and tree ferns. Common tree species include paque or paleto (Dialium guianense), allspice tree (Pimenta dioica), breadnut (Brosimum alicastrum), manteco (Ampelocera hottlei), masica (Brosimum alicastrum), masaquilla (Pseudolmedia cf. spurea), laurel (Cordia alliodora, C. bicolor), maria (Calophyllum brasiliense), hule (Castilla elastica, C. tunu), cuajada (Dendropanax arboreus), caobina (Mauria sessiliflora), seliyon (Pouteria izabalensis), sangre de pozo (Pterocarpus officinalis), varillo (Symphonia globulifera), caoba (Swietenia macrophylla), cumbillo or sombrerete (Terminalia amazonia), sangre real (Virola koschnyi) and San Juan or copai-yé wood (Vochysia hondurensis). There are also well-developed rain forests in specific places on the Pacific side of Central America. Pine grows in infertile locations, alone or in association with oak.

An evergreen forest, intermediate in height, with two or three strata, grows between 400 and 1 300 m altitude on the wetter (Atlantic) side of the Central American ranges. Canopy trees are mostly 30 to 40 m tall. The subcanopy is very dense with trees 15 to 25 m tall.

TROPICAL MOIST DECIDUOUS FOREST

This zone consists of the lower Pacific part of the central mountain ranges in Central America, the plains and hills of the Yucatan Peninsula, humid parts of the Gulf of Mexico plains and the Everglades in the United States. The climate is drier than in the rain forest zone and the dry season is more pronounced (three to five months). Average annual precipitation is around 1 300 mm in El Salvador. It falls to less than 1 000 mm in Honduras and increases again from Nicaragua to Costa Rica. Most of the Yucatan Peninsula in Mexico receives 1 000 to 1 500 mm.

The predominant vegetation is deciduous high forest with three or four storeys and approximately 100 tree species in association on fertile soils. The most typical tree species are Cordia alliodora, Carapa guianensis, Guarea spp., Vitex spp., Virola spp., Calophyllum brasiliense, Terminalia chiriquensis, Dialium guianense, Tabebuia pentaphylla, Ochroma lagopus and Manilkara spp. From Nicaragua southwards the associations are enriched by many South American species such as Anacardium excelsum, Dipteryx panamensis, Eschweilera calyculata, Lecythis spp. and Prioria copaifera. Certain distinct associations include pure stands of cativo (Prioria copaifera) on riparian flood lands, palm swamps and mangrove swamps on tidal estuaries.

A two-layer semideciduous, seasonal forest of medium height grows in the drier parts of the zone, from 600 to about 1 600 m. The canopy is are mostly dry-season deciduous trees about 25 m tall. Understorey trees are 10 to 20 m tall.

TROPICAL DRY FOREST

This zone comprises flat narrow lowlands or low hilly areas up to 1 000 m altitude, located mainly along the Pacific Coast but also including interior depressions of the Sierra Madre and the northwestern plain of the Yucatan Peninsula in Mexico. The tropical climate of the zone is characterized by short intense episodes of rainfall, especially during the summer. Overall, average annual precipitation is between 600 and 1 600 mm. The dry season varies from five to eight months.

The dominant vegetation formation is dry deciduous forest. A diverse flora is present and low deciduous and semideciduous forests predominate. The forests are from 4 to 15 m tall and have three distinct strata. Southern floristic elements are prominent along with numerous endemic genera on the Pacific side. Legumes dominate the tree flora. On very infertile soils, Curatella americana and Byrsonima crassifolia form a distinctive association. Since these two species are extremely fire resistant, they are often found on soils seriously degraded by excessive cropping and burning. In northwestern Costa Rica, in the Guanacaste region, a similar association occurs on pumice soils. This association differs in that Quercus oleoides accompanies the other two species.

The two vegetation associations covering the major part of the zone on the Pacific Coast differ little in tree species but are quite distinct in terms of dominant species. Characteristic species include Cedrela mexicana, Swietenia humilis, Enterolobium cyclocarpum, Pithecellobium saman, Hymenaea courbaril, Andira inermis, Platymiscium spp., Chlorophora tinctoria, Astronium graveolens, Dalbergia spp., Sweetia panamensis, Achras zapota and Tabebuia chrysantha. From Mexico to Honduras Cybistax donnell-smithii is another important species, while from Nicaragua southwards Bombacopsis quinata is a conspicuous tree. In Mexico, the low deciduous forests contain about 6 000 vascular plant species, of which 40 percent are endemic.

Where the water table is high in fertile soils, as on river flats, a taller and more luxuriant forest occurs; Brosimum spp. and Anacardium excelsum are common species.

TROPICAL MOUNTAIN SYSTEMS

The climate in the mountain areas varies enormously. Wind-exposed areas are normally wet, while interior valleys are usually moist or dry. Monthly mean temperature shows little seasonal variation but ranges from 12°C at about 1 500 m to less than 6°C at 3 800 m on mountain summits.

Broadleaf forests prevail in highland areas of Mexico, Guatemala, Honduras and Nicaragua, but pine forests are also very common. In the mountain areas of Guatemala where annual rainfall is less than 1 000 mm, the most notable trees are Pinus pseudostrobus and several species of Quercus. Other genera from the temperate zone such as Salix, Sambucus, Ostrya and Acer are also represented. On sites where annual precipitation exceeds 1 000 mm, the climax forest consists of mixed broadleaf forest, including species of Prunus and Cornus, members of the Lauraceae and Ericaceae families and several other species. The forest here is tall and very dense, with canopy trees generally reaching 30 m in height, and it has a dense shrub layer.

The high area of Costa Rica and Panama includes several altitudinal belts. The so-called coffee belt, between 600 and 1 600 m, is an important zone in Central America since most of the population lives there. This belt is part of the previously described tropical lowland zone. From 1 600 m to approximately 2 800 m, the vegetation can is either very tall oak forest or mixed Lauraceae-rich forest. The tall oak forest is a high, comparatively open stand, characterized by emergent, large-crowned oaks, Quercus copeyensis and Q. seemanni, reaching up to 50 m, and a lower stratum of relatively small to medium-sized trees. The Lauraceae-rich forest is not as tall as the oak forest but still reaches 30 m in height. The forest is very dense, with multiple strata. Genera represented from the Lauraceae family include Ocotea, Phoebe, Nectandra and Persea. From 2 800 to 3 500 m there are many shrubs and a bamboo species. In the primary forest, evergreen oaks, including Quercus costaricensis, dominate the tree canopy, which reaches a height of some 25 to 30 m.

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