Temperate needleleaf forests cover
a larger area of the world than any other forest types. They mostly
occupy the higher latitude regions of the northern hemisphere, as well
as high altitude zones and some warm temperate areas, especially on
nutrient-poor or otherwise unfavourable soils. These forests are composed
entirely, or nearly so, of coniferous species (Coniferophyta). In the
Northern Hemisphere pines Pinus, spruces Picea, larches
Larix, silver firs Abies, Douglas firs Pseudotsuga
and hemlocks Tsuga, make up the canopy, but other taxa are also
important. In the southern hemisphere most coniferous trees, members
of the Araucariaceae and Podocarpaceae, occur in mixtures with broadleaf
species that are classed as broadleaf and mixed forests.
Temperate broadleaf and mixed forests,
which include a substantial component of trees in the Anthophyta, cover
over 6.5 million km2 of the Earth's surface. They are generally
characteristic of the warmer temperate latitudes, but extend to cool
temperate ones, particularly in the southern hemisphere. They include
such forest types as the mixed deciduous forests of the USA and their
counterparts in China and Japan, the broadleaf evergreen rain forests
of Japan, Chile and Tasmania, the sclerophyllous forests of Australia,
the Mediterranean and California, and the southern beech Nothofagus
forests of Chile and New Zealand.
Tropical moist forests cover more than
11 million km2 of the humid tropics and include many different
forest types. The best known and most extensive are the lowland evergreen
broadleaf rainforests, which make up over half this area and include,
for example: the seasonally inundated varzea and igapó
forests and the terra firme forests of the Amazon Basin; the
peat forests and moist dipterocarp forests of Southeast Asia; and the
high forests of the Congo Basin. The forests of tropical mountains are
also included in this broad category, generally divided into upper and
lower montane formations on the basis of their physiognomy, which varies
with altitude. The montane forests include cloud forest, those forests
at middle to high altitude, which derive a significant part of their
water budget from cloud, and support a rich abundance of vascular and
nonvascular epiphytes. Mangrove forests also fall within this broad
category, as do most of the tropical coniferous forests of Central America.
Tropical dry forests are characteristic
of areas in the tropics affected by seasonal drought. Such seasonal
climates characterise much of the tropics, but less than 4 million km2
of tropical dry forests remain. The seasonality of rainfall is usually
reflected in the deciduousness of the forest canopy, with most trees
being leafless for several months of the year. However, under some
conditions, e.g. less fertile soils or less predictable drought regimes,
the proportion of evergreen species increases and the forests are characterised
as
"sclerophyllous". Thorn forest, a dense forest of low stature with a
high frequency of thorny or spiny species, is found where drought is
prolonged, and especially where grazing animals are plentiful. On very
poor soils, and especially where fire is a recurrent phenomenon, woody
savannas develop (see 'sparse trees and parkland').
Sparse trees and parkland are forests
with open canopies of 10-30% crown cover. They occur principally in
areas of transition from forested to non-forested landscapes. The two
major zones in which these ecosystems occur are in the boreal region
and in the seasonally dry tropics. At high latitudes, north of the main
zone of boreal forest or taiga, growing conditions are not adequate
to maintain a continuous closed forest cover, so tree cover is both
sparse and discontinuous. This vegetation is variously called open taiga,
open lichen woodland, and forest tundra. It is species-poor, has high
bryophyte cover, and is frequently affected by fire.
Forest plantations, generally intended
for the production of timber and pulpwood increase the total area of
forest worldwide. In 1999 FAO has estimated that total plantation area
in developed countries is about 600,000 km2 and in developing
countries it is about 550,000 km2. Commonly mono-specific
and/or composed of introduced tree species, these ecosystems are not
generally important as habitat for native biodiversity. However, they
can be managed in ways that enhance their biodiversity protection functions
and they are important providers of ecosystem services such as maintaining
nutrient capital, protecting watersheds and soil structure as well as
storing carbon. They may also play an important role in alleviating
pressure on natural forests for timber and fuelwood production.
UNEP-WCMC's harmonised general forest
classification consists of 15 different tropical forest types and 11 temperate
and boreal forest types. This classification reflects characteristics
of forests that can be of conservation importance. For example, whether
the forest is tropical, temperate or boreal, whether it is a plantation
of exotic species, or whether it is a degraded natural forest type. Not
wishing to exclude much of the important areas that support trees which
are sparsely distributed, a category for sparse trees and parkland was
included. This agrees with the definition of "forest" used by FAO in their
Forest Resources Assessments. Apart from this criterion for the limits
of tree cover for "forest" (canopy cover between 10-30%) there was no
particular height limit used, just that the trees had to be mainly phanerophytes
(single-trunked individuals, as most trees) and not chamaephytes (multi-stemmed
individuals, as most shrubs).
These 26 forest categories (defined below) are used
to enable the translation of forest types from national and regional
classification systems to a harmonised global one. (Please
note that categories 12 and 13 have been created as a result of data
holdings which do not specify the forest type, hence 26 categories are
quoted, not 28).
Temperate and boreal forest types
Natural
forest with > 30% canopy cover, in which the canopy is predominantly
(> 75%) needleleaf and evergreen.
Natural
forests with > 30% canopy cover, in which the canopy is predominantly
(> 75%) needleleaf and deciduous.
Natural
forest with > 30% canopy cover, in which the canopy is composed of a
more or less even mixture of needleleaf and broadleaf crowns (between
50:50% and 25:75%).
Natural
forests with > 30% canopy cover, the canopy being > 75% evergreen and
broadleaf.
Natural
forests with > 30% canopy cover, in which > 75% of the canopy is deciduous
and broadleaves predominate (> 75% of canopy cover).
Natural
forests with > 30% canopy cover, composed of trees with any mixture
of leaf type and seasonality, but in which the predominant environmental
characteristic is a waterlogged soil.
Natural
forest with > 30% canopy cover, in which the canopy is mainly composed
of sclerophyllous broadleaves and is > 75% evergreen.
Any
forest type above that has in its interior significant areas of disturbance
by people, including clearing, felling for wood extraction, anthropogenic
fires, road construction, etc.
Natural
forests in which the tree canopy cover is between 10-30%, such as in
the steppe regions of the world. Trees of any type (e.g., needleleaf,
broadleaf, palms).
Intensively
managed forests with > 30% canopy cover, which have been planted by
people with species not naturally occurring in that country.
Intensively
managed forests with > 30% canopy cover, which have been planted by
people with species that occur naturally in that country.
Forest
plantations showing extent only with no further information about their
type, This data currently only refers to the Ukraine.
Forest
data showing forest extent only with no further information about their
type.
Tropical
forest types
Natural forests with
> 30% canopy cover, below 1200m altitude that display little or no seasonality,
the canopy being >75% evergreen broadleaf.
Natural forests with
> 30% canopy cover, between 1200-1800m altitude, with any seasonality
regime and leaf type mixture.
Natural forests with
> 30% canopy cover, above 1800m altitude, with any seasonality regime
and leaf type mixture.
Natural forests with
> 30% canopy cover, below 1200m altitude, composed of trees with any
mixture of leaf type and seasonality, but in which the predominant environmental
characteristic is a waterlogged soil.
Natural forests with
> 30% canopy cover, below 1200m altitude in which between 50-75% of
the canopy is evergreen, > 75% are broadleaves, and the trees display
seasonality of flowering and fruiting.
Natural forests with
> 30% canopy cover, below 1200m altitude, in which the canopy is composed
of a more or less even mixture of needleleaf and broadleaf crowns (between
50:50% and 25:75%).
Natural forest with
> 30% canopy cover, below 1200m altitude, in which the canopy is predominantly
(> 75%) needleleaf.
Natural forests with
> 30% canopy cover, composed of species of mangrove tree, generally
along coasts in or near brackish or salt water.
Any forest type above
that has in its interior significant areas of disturbance by people,
including clearing, felling for wood extraction, anthropogenic fires,
road construction, etc.
Natural forests with
> 30% canopy cover, below 1200m altitude in which between 50-100% of
the canopy is deciduous and broadleaves predominate (> 75% of canopy
cover).
Natural forests with
> 30% canopy cover, below 1200m altitude, in which the canopy is mainly
composed of sclerophyllous broadleaves and is > 75% evergreen.
Natural forests with
> 30% canopy cover, below 1200m altitude, in which the canopy is mainly
composed of deciduous trees with thorns and succulent phanerophytes
with thorns may be frequent.
Natural forests in which
the tree canopy cover is between 10-30%, such as in the savannah regions
of the world. Trees of any type (e.g., needleleaf, broadleaf, palms).
Intensively managed
forests with > 30% canopy cover, which have been planted by people with
species not naturally occurring in that country.
Intensively managed forests with > 30% canopy cover,
which have been planted by people with species that occur naturally
in that country.
UNEP-WCMC
has been gathering and compiling spatial data on the extent and conservation
status of forests since 1987. Until 1995, WCMC's work focused on tropical
moist forests because of their high species diversity. GIS data were first
assembled for closed moist tropical forests and used to publish the three
volumes of the Conservation Atlas of Tropical Forests, covering Asia (1991),
Africa (1992) and the Americas (1996). Because digital data were rare
at this time, the process of assembling the forest cover data sets involved
digitising manually many paper maps.
UNEP-WCMC's
forest category classification system is a simplification of other more
complex systems (e.g. UNESCO's forest and woodland 'subformations'),
which still retains a certain amount of basic information on forest
physiognomy and phenology. This system divides the world's forest into
26 major types, which reflect climatic zones as well as the principal
types of trees. Of course each of these major types comprises a great
range of forests. These 26 major types can be reclassified into 6 broader
categories.