Forests are not just trees, but part of ecosystems that underpin life, economies and societies. Where these are privately owned, they are often managed mainly for production. Yet, in addition to directly supporting such industries as timber, pulp and biotechnology, they provide a wide range of ecosystem services. These services include prevention of soil erosion, maintenance of soil fertility, and fixing carbon from the atmosphere as biomass and soil organic carbon. They host a large proportion of terrestrial biodiversity, protect water catchments and moderate climate change. They also support local livelihoods, provide fuel, traditional medicines and foods to local communities, and underpin many cultures. The harvesting of forest products is putting severe stress on the world’s forests.

Where Are They?
Forests can be found in all regions capable of sustaining tree growth, at altitudes up to the tree line, except where natural fire frequency is too high, or where the environment has been impaired by natural processes or by human activities. As a general rule, they are dominated by angiosperms (broadleaf) are more species-rich than those dominated by gymnosperms (conifer, montane, or needleleaf), although exceptions exist (for example, species-poor aspen and birch stands in northern latitudes).

Sometimes they contain many tree species within a small area (as in tropical rain and temperate deciduous areas), or relatively few species over large areas (e.g., taiga and arid montane conifers). They are are often home to many animal and plant species, and biomass per unit area is high compared to other vegetation communities. Much of this biomass occurs below-ground in the root systems and as partially decomposed plant detritus. The woody component contains lignin, which is relatively slow to decompose compared with other organic materials such as cellulose or carbohydrate.

Forests are differentiated from woodlands by the extent of canopy coverage: the branches and foliage of separate trees often meet or interlock, although there can be gaps of varying sizes within an area. A woodland has a more continuously open canopy, with trees spaced further apart, which allows more sunlight to penetrate to the ground between them.

Forests Will Change.

The models used by the IPCC project that even with an increase of 1°C in global mean temperature, this is sufficient to cause changes in regional climates that will affect the growth and regeneration capacity in many regions. In several instances this will alter the function and composition of these areas significantly.

As a consequence of possible changes in temperature and water availability under doubled equivalent carbon dioxide equilibrium conditions, a substantial fraction (a global average of one third, varying by region from one seventh to two thirds) of the existing forested area of the world will undergo major changes in broad vegetation types. The greatest changes are likely to occur in high latitudes and the least changes in the tropics.

Climate change is expected to occur at a rapid rate relative to the speed at which tree species grow, reproduce and re-establish themselves. For mid latitude regions, a global average warming of 1-3.5°C over the next 100 years would be equivalent to a pole ward shift of the present isotherms by approximately 150-550 km or an altitude shift of about 150-550 m; in low latitudes, temperatures would generally be increased to higher levels than now exist. This compares to past tree species migration rates that are believed to be on the order of 4-200 km per century. Therefore, the species composition of is likely to change; entire types may disappear, while new assemblages of species, hence new ecosystems, may be established.

Although net primary productivity could increase, the standing biomass of of tree areas may not because of more frequent outbreaks and extended ranges of pests (like the Mountain Pine Beetle ) and pathogens, and increasing frequency and intensity of wildfires. Large amounts of carbon could be released into the atmosphere during transitions from one area type to another because the rate at which carbon can be lost during times of high boreal mortality is greater than the rate at which it can be gained through growth to maturity.

Also see deforestation.