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5. How healthy are the world’s forests?

  • 5.1 What are the impacts of forest fires?
  • 5.2 How are forests affected by insect and disease outbreaks?
  • 5.3 What other disturbances can affect forests?

Forests need to be managed so that the risks and impacts of unwanted disturbances are minimized, including wildfires, airborne pollution, storm felling, invasive species, pests, diseases, and insects. Disturbances and their impacts vary from place to place and even the definition of what constitutes disturbance events varies among countries.

Forest disturbances can be devastating, but they have been severely underreported. Based on available data, this assessment focused on three kinds of disturbances:

  • Forest fires: Fire often gets out of control and destroys forest vegetation and biomass. Both uncontrolled agricultural expansion and recreational uses increase the risk of forest fires.
  • Insects and diseases: While insects and diseases are integral components of forests, outbreaks can have adverse effects on tree growth and survival, yield and quality of wood and non-wood forest products (NWFP), wildlife habitat, and the social value of forests.
  • Other disturbances (including wind, snow, ice, floods, tropical storms, drought, and damage by animals): Climatic events have always influenced forest ecosystems, but now global climate change primarily resulting from human activities appears to make forest ecosystems more prone to damage.

Disturbances interact with one another, for instance damage caused by fires or storms can facilitate insect pest outbreaks. Hence they need to be considered as a whole. Several disturbance factors were not taken into account in this assessment since quantitative information was lacking on illegal logging, encroachment, unsustainable management practices, pollution, and the impact of invasive plant species. More...

5.1 What are the impacts of forest fires?

Fire, from natural and human causes, has been a major factor in the development and management of forests. Though some forest ecosystems have adapted to frequent fires and benefit from them, others are damaged by them. Every year, millions of hectares of forests are consumed by fire, with loss of human and animal life and substantial economic damage along with loss of biodiversity and release of carbon to the atmosphere. Most fires today are caused by humans, when fire is misused in order to convert forests into agricultural lands or for other uses, and to maintain grazing lands, extract mineral resources, or settle ownership conflicts.

Around 2000, the area burned per year was at least about 277 000 km2 of forests, equivalent to roughly 1% of the forest area of the 91 countries that reported data on this subject. An additional 51 000 km2 of other wooded land were reported as significantly affected by fire. The largest shares of forest area affected by fire were reported from Africa and Asia, while Europe reported the smallest. Information was missing from many countries in Oceania and Africa.

Table on area of forest affected by fire

Between 1990 and 2000, the average area of forest affected by fires each year was reported to have increased in 35 countries, decreased in 31, and remained almost constant in 25 countries. From the data provided, it is difficult to discern any global trends. More...

5.2 How are forests affected by insect and disease outbreaks?

The types of problems caused by insects and diseases — often interlinked— have changed rapidly in recent years. Movement of insects and diseases has been facilitated by faster long-range air travel along with increased international trade of agricultural and forest products, and the exchange of plant material.

Identifying insects and diseases as the cause of damage to forests is difficult and the available data mainly focuses on the overall extent of forest affected, and not on the specific underlying causes.

Insect and disease problems are often either cyclical or chronic and determining when they begin or end can be a challenge. Because of such difficulties in assessment few countries provided data for insect infestations and diseases.

Globally, from 1998 to 2002, an average of 680 000 km2 of forest area were harmed by insects and/or diseases every year. The largest area of insect disturbance reported for a single country was 142 000 km2 (Canada), and that of disease disturbance, 174 000 km2 (United States) – both countries within the top five in terms of forest area and with good data-collection systems.

Table on area of forest affected by insects

Table on area of forest affected by diseases

Between 1990 and 2000 the level of disease reported increased while the level of insect damage decreased. More...

5.3 What other disturbances can affect forests?

Other forest disturbances include climatic factors such as wind, snow, ice, floods, tropical storms and drought, or impacts of animals such as camels, beavers, deer and rodents. Because “other disturbances” cover many different causes, information about them is highly erratic and not comparable.

The average area affected by other disturbances between 1998 and 2002 was about 84 000 km2 per year. Other disturbances included:

Table on area of forest affected by other disturbances

  • major catastrophic events such as hurricanes, which may not only cause widespread destruction but also make forests susceptible to infestations.
  • chronic pressures, such as constant feeding by animals which either cause direct damage to trees or have indirect effects such as increased soil compaction beneath the trees leading to dieback and decline.

However, very little detailed data is available on these other disturbances and countries have varying perceptions about what is included in ‘other disturbances’. In Europe the average area affected by other disturbances per year almost doubled between 1990 and 2000. This was primarily due to the effects of severe storms such as those that hit Western Europe in December 1999. More...

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