TRENDS IN ECOSYSTEM DIVERSITY
Biodiversity of Forest Ecosystems
What is a forest?
The FAO has defined natural or semi-natural forests as “ecological systems with a minimum of 10% crown cover of trees and/or bamboo, generally associated with wild flora and fauna and natural soil conditions and not subject to agricultural practices”. This is an extremely wide definition, and includes many open vegetation systems that would not normally be regarded as forests. A more rigorous definition, which accords much more closely with wider perceptions of what constitutes a forest, is that of closed-canopy forest. Thresholds for defining closed-canopy forest range from as low as 30% to as high as 75% crown cover.
Forests are ecosystems in which trees are the predominant life forms. A more precise definition than this remains surprisingly elusive, because trees occur in many different ecosystems, at different densities and in different forms. Most definitions of forest refer to canopy or crown cover, which is essentially the percentage of ground area shaded by the crowns of the trees when they are in full leaf.
Clearly, estimates of the area of forest both globally and in any given place must vary enormously depending on the definition of forest adopted. In this discussion a threshold of 30% canopy cover is generally used as defining a forest.
Where are forests
The factors determining the distribution of forests are largely climatic: tree establishment and growth requires a minimum number of days in the year with adequate moisture and warmth for active growth. Substrate characteristics are also important as trees require access to enough soil for supply of nutrients and water, and to provide anchorage. Trees are therefore absent from areas that are too dry, or too cold or have inadequate soil cover. Other natural factors that may limit the distribution of forests include fire, flooding, the presence of toxic minerals in the substrate, and the impact of large herbivores. Around one-half of the Earth's land surface is climatically suitable for forest, but as a result of human actions, around one-half of this area is not forested, so that current forest cover amounts to just under 40 million square kilometres (Map 11).
Major forest types
Classifying forests is in some ways an even more difficult task than defining forest. A number of global classification systems have been proposed, some extremely complex, but as yet none has gained universal acceptance. All such systems are more or less artificial as in nature forests rarely form homogenous units clearly separable from other such units; instead species composition and forest structure tend to change gradually or irregularly with, for example, changing altitude, exposure and latitude.
Nevertheless simplified global level classification systems can be a useful guide to the world's major forest types. Table 1.8 provides a breakdown of 22 forest types in five major categories: temperate needleleaf; temperate broadleaf and mixed; tropical moist; tropical dry; and sparse trees and parkland. Map 12 shows forest types broadly classified by ten types.
Table 1.8 Global areas of 22 main forest types
|Boreal and Temperate Needleleaf
||Fresh water swamp
||Semi-evergreen moist broadleaf
||Mixed needleleaf and broadleaf
|Temperate Broadleaf and Mixed
|Freshwater swamp forest
|Sclerophyllous dry forest
||Sparse Trees and Parkland
|Lowland evergreen broadleaf rainforest
|Lower montane forest
|Upper montane forest
||38 808 671
Source: Global Biodiversity
, table 7.4
Boreal and temperate needleleaf forests
cover a larger area of the world than any other forest type. They occur mostly in the higher latitudes of the northern hemisphere as well as high altitude zones and some warm temperate areas (particularly on soils poor in nutrients or otherwise unfavourable). Conifer canopies are efficient light absorbers, so limiting opportunity for extensive development of lower strata of vegetation, and the structure of these forests is often fairly simple. Species diversity is usually relatively low, and vast expanses of such forest in the northern hemisphere are dominated by a very small number of tree species. However, diversity amongst some groups of organisms, such as mosses and lichens, may be surprisingly high. These forests are of great importance in the carbon cycle, acting as major reservoirs of organic carbon both above and below ground.
Temperate broadleaf and mixed forests
are generally characteristic of the warmer temperate latitudes but extend to cool temperate ones, particularly in the southern hemisphere. These forests tend to be structurally more complex than needleleaf forests, and to have considerably higher species diversity. Above-ground biomass tends to be lower than in temperate needleaf or tropical moist forests, while that below ground tends to be intermediate.
Tropical moist forests
include many different forest types. They are without doubt overall the most diverse ecosystems on earth. Some estimates suggest that at least 60% of all species (and possibly near 90%) occur in them, despite the fact that they cover little more than 7% of the world's land surface, and around 2% of the surface of the globe. Diversity is usually extremely high at all spatial scales; in some parts of the western Amazon and of the Atlantic coastal forest of Brazil (`Mata Atlântica
') there may be as many as 300 tree species per hectare. Notable exceptions are mangrove ecosystems, which have low tree species diversity, and forests on very nutrient-poor soils. Diversity also tends to decrease with increasing altitude and with increasing seasonality of climate. Lowland tropical moist forests can have very high above-ground biomass, although not as high as some needleleaf forests. Below-ground biomass, however, with the exception of some peat-swamp forests, tends to be relatively low. Tropical moist forests are estimated to account for nearly one-third of global terrestrial annual net primary production.
Tropical dry forests
are characteristic of areas in the tropics affected by seasonal drought. Seasonality of rainfall often results in a largely deciduous forest canopy; however, under some conditions, such a low fertility soils or unpredictable rainfall regimes, the canopy may become dominated by evergreen sclerophyllous species (i.e. those with typically small, thick-skinned leaves). On very poor soils and especially where fire is a recurrent phenomenon, woody savannas may develop. In general dry tropical forests have lower species diversity than tropical moist forests; however they appear to be characterised by high levels of local endemism.
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 where these formations occur are the boreal region (i.e. at very high latitudes) and the seasonally dry tropics.
, generally intended for the production of timber and pulpwood, are believed to cover well over one million square kilometres worldwide. Commonly composed of only one species, often non-native, plantations are not generally important as habitat for native species. However, they can be managed in ways that enhance the role they play in maintaining biodiversity. They may also be important providers of ecosystem services, such as maintaining nutrient capital and soil structure as well as protecting watersheds. They may also alleviate pressure on natural forests for timber and fuelwood production.
Changes in forest cover
The Earth's climate has been extremely unstable for the past two million years and for most of this time much colder than today's. The extent and distribution of forests during the cold periods were very different from today, not least because much of the northern hemisphere was covered in thick ice, but also because the climate in much of the tropics was apparently drier.
The beginning of the Holocene, which followed the last major glacial period between 11,000 and 12,000 years ago, would undoubtedly have been a period of major forest expansion as the ice retreated and wetter conditions began to prevail in much of the tropics. However, it also evidently marked the emergence of agriculture and the spread of humans through almost all the habitable world, including the Americas. Human impact on forest cover undoubtedly dates back at least to this period, but because these impacts started at a time when forest extent and distribution would have been rapidly changing also for climatic reasons, it is extremely difficult to determine just how much land has been permanently deforested by human hands. In other words, it is very difficult, and perhaps of little value, to seek a precise global baseline of `original forest cover' against which to measure human impact. Such a baseline could theoretically be established before the start of the Pleistocene ice ages, over two million years ago, but the climate at that time is not sufficiently well known to make this feasible.
Despite these problems, it is possible to make some general observations on the history of forest loss, which differs markedly between different regions and different forest types. In Western Europe it seems that forest clearance has an extremely long history, with, for example, much of the British Isles deforested in the Neolithic, between 7000 and 5000 years ago. In Europe as a whole, forest cover continued to decline until the 19th century when it stabilised. Since the early 20th century, forest cover in Europe has expanded, often through the establishment of conifer plantations although latterly also through re-establishment of mixed and broadleaf forests. Similarly, in eastern North America, forest cover reached a minimum around 1860, but has since expanded. Forests west of the Appalachians in North America suffered the most severe impacts in the late 19th and early 20th centuries, but are still under pressure from demand for timber and pulp.
It has been widely assumed that large-scale forest clearance outside of Europe had only taken place following European colonisation. However, there is increasing evidence that indigenous peoples in some areas (for example the Yucatan region of central America, parts of the Andes in south America and present day Cambodia in Southeast Asia) may have had major impacts on forest cover before the arrival of Europeans. Nevertheless, in most areas European colonisation undoubtedly precipitated a major phase of deforestation, which appears to have reached unprecedentedly high levels in the last half-century or so. Table 1.9 shows that rates of deforestation continue to be high in the developing countries of the tropics, in both absolute and proportional terms. In contrast, temperate (mostly developed) countries are losing forests at lower rates, or indeed showing an increase in forest area.
Table 1.9 Estimated annual change in forest cover 1990-1995
||Annual change (km2)
||Annual change rate (%)
|Africa - tropical
|Africa - non-tropical
|Asia - tropical
|Asia - non-tropical
|Oceania - tropical
|Oceania - temperate
|Central America, Mexico and Caribbean
|South America - tropical
|South America - temperate
Source: FAO 1999. Note - forest cover change data are not available for the former USSR
Major values and uses of forests
- Timber and other wood products
- Fuelwood and charcoal
- Non-wood forest products (bushmeat, rattans and fibres, honey, edible plants, medicinals, aromatics, dyes)
- Climate amelioration
- Regulation of local and regional hydrological cycles
- Mediators in the carbon cycle, chiefly as carbon sinks
- Soil stabilisation and watershed protection
- Cultural values (aesthetic, spiritual, recreational)
Major impacts on forests
- Conversion, chiefly to cropland (including plantation forestry) but also for a range of other purposes, including road building, mining, urban and industrial development,
- Changing fire regimes,
- Invasive alien species,
- Extraction of non-timber forest products,
- Fuel wood extraction,
- Unsustainable shifting cultivation.
- Climate change,
- Pollutants, including acid rain,
Status and trends of species in forests
Because forests occur over such a wide area and encompass such a wide range of ecological conditions, it is difficult to generalise about the status of forest species as a whole. Most temperate and boreal forest species in the northern hemisphere are widespread and generally not highly threatened with extinction. Forests of warmer temperate and subtropical areas have a higher diversity of species, many of which are localised and therefore in general more vulnerable to extinction. A higher proportion of these is threatened. Similarly, southern hemisphere temperate forests are generally isolated from each other and have a high proportion of localised and threatened species. For those groups, namely mammals and birds, that have been analysed in any detail, far more tropical forest species are considered threatened than species from any other habitat. It is unclear, however, whether this is merely a reflection of the fact that tropical forests contain far more species in these groups than any other habitat, or whether a higher proportion of these species is threatened than of temperate and boreal forest species. In tropical and subtropical regions, there are clear differences between insular and continental biotas. The former appear far more extinction-prone and have a far higher proportion of threatened species than the latter.
Notes on a selection of the species that occur in forest, mainly in the tropics, and that are categorised as Critically Endangered, i.e. at highest risk of extinction, are provided in Table 1.10.
Table 1.10 Some Critically Endangered forest species
|Black-faced Lion Tamarin
|Discovered early 1990 on Superagüi island, south Brazil, where restricted to around 300 km2 of forest, with a total population of about 300 individuals.
|Golden Bamboo Lemur
|Discovered in 1987. Patchily distributed in a few areas of rainforest in southeast Madagascar. The small population of a few hundred animals is threatened by deforestation.
|Formerly widespread in upland forests of southeast Asia. Reduced by deforestation and hunting to a few hundred animals, mostly in Indonesia and Malaysia.
|Restricted to forests of the Goda and Mabla Mountains, Djibouti. Fewer than 1,000 birds, and declining because of habitat degradation and hunting.
|A large eagle endemic to the Philippines. Remains mostly on Luzon and Mindanao, where threatened by forest loss, hunting and trapping.
|Believed extinct until rediscovered in 1990. Restricted to the Hellshire Hills where threatened by predation from mongooses, dogs and cats and habitat loss through deforestation.
|Peppered Tree Frog
|Recorded from a few localities in the eucalypt forest of the New England Tableland, NSW, Australia. Reasons for decline not clear.
||Endemic to the mountain forests of Puerto Rico. Not recorded for a number of years and possibly now extinct.
|Tree Hole Crab
|Restricted to closed canopy rainforest in the Upper Guinea region of West Africa. Estimated to total less than 250 mature individuals in several fragmented populations.
|A small isolated population exists in riparian oak woodland in the Chisos Mountains, Texas, USA. The locality is threatened by the activities of tourists and occasional drought.
|Known from a few scattered individuals in cloud forest of the Canary Islands, Spain. With a very poor regenerative capacity, the few remaining populations are threatened by fire, grazing and exploitation of the medicinal bark.
|Scattered in the lowland rainforests near Darwin, Australia. The entire population consists of approximately 500 mature individuals and is believed to have stabilised since feral pigs and water buffaloes were excluded.