Forest Definition

Revision as of 15:08, 18 January 2011

General observations

In large area forest inventories, estimation of forest area is one of the most important results. Forest area is a concept that is easily understood and extremely important in the context of discussion sustainability of the forest development in a region or country. Of course, a forest definition is not necessary for an inventory of forest stands for which up-to-date and accurate maps are available. Of course, the definition of forest is crucial when deforestation is discussed. Deforestation refers to lands where forest has formerly been. Then it becomes extremely important to use exactly the same unambiguous definitions of forest at both points in time – otherwise, the stated deforestation can not be properly understood.

The definition of forest varies with the context; there are differences in forest definitions from a silviculture, ecology or legal point of view. However, for large area forest inventory, the definition of forest needs to comply with two – some times slightly contradicting – requirements:

1. it should be close to the definition used in the National Forest Act viz. in national forest policy processes; and
2. it must be operational so that the decision can be made without major complications whether at a defined point there is forest or not. This refers to both data sources: field observations and observations in remote sensing imagery.

The issue of forest definition has been discussed in the forestry, forest policy and forest inventory community for decades and the discussions are likely to continue. A very comprehensive compilation of forest definitions from all parts of the world has been published by Gyde Lund.

Components of a forest definition

When we look at forest definitions, we can clearly distinguish two different classes of criteria: quantitative and qualitative criteria.

Quantitative criteria include measurable thresholds such as minimum area of a “tree cluster” to be called “forest”, minimum width, minimum height of the tree vegetation, minimum crown coverage, and in some countries also minimum productivity (\(m3/ha\)).

Qualitative criteria include the definition of “tree”, statements how to deal with roads, buildings, timber storage sites and other infrastructure within a forested area, and how to deal with creeks or lakes, with temporary clear-cuts etc.. Usually, roads and creeks are included as forest land up to defined maximum width. In many regions it may also be necessary to explicitly exclude some land use systems ‑ such as agroforestry systems ‑ which fulfill many of the “biophysical” criteria of sufficient tree cover, but are obviously not a forest because used for agriculture.

Forest definition of FAO

The FAO definition of “forest” is widely adopted in many national forest inventories and it is the forest definition to which the international forest reporting of FAO refers to. The definition is published as part of the documentation of the Forest Resource Assessment Project within FAO which can be looked up at the FAO´s Forestry Department´s web site in Working Paper 33.

According to the FAO, forest is defined as

• “land with tree crown cover (or equivalent stocking level) of more than 10 percent and an area of more than 0.5 hectares (ha).
• The trees should be able to reach a minimum height of 5 meters (m) at maturity in situ.
• May consist either of closed forest formations where trees of various storey and undergrowth cover a high proportion of the ground or open forest formations with a continuous vegetation cover in which tree crown cover exceeds 10 percent.
• Young natural stands and all plantations established for forestry purposes which have yet to reach a crown density of 10 percent or tree height of 5 m are included under forest,
• as are areas normally forming part of the forest area which are temporarily unstocked as a result of human intervention or natural causes but which are expected to revert to forest.”

This definition includes “forest nurseries and seed orchards that constitute an integral part of the forest; forest roads, cleared tracts, firebreaks and other small open areas; forest in national parks, nature reserves and other protected areas such as those of specific scientific, historical, cultural or spiritual interest; windbreaks and shelterbelts of trees with an area of more than 0.5 ha and width of more than 20 m; plantations primarily used for forestry purposes, including rubberwood plantations and cork oak stands”, but it excludes “land predominantly used for agricultural practices.”

Summary

FAO´s forest definition puts emphasis on tree crown cover of more than 10% and area of more than 0.5ha. Besides that, FAO defines forest according to land use; therefore, any land use with agricultural purpose will not be defined as forest.

A forest “patch” can be defined for trees that are outside forest but in an agricultural land which has crown cover more than 10% and cover an area more than 0.5ha. When remote sensing is used for classification of forest and non-forest, local knowledge is essential because sometimes, it is hard to distinguish the land use system from images without knowing the area well: grassland with trees may be forest (if the criteria are met) – but if cattle is around it is agriculture (pasture) and can not be forest.

Forest Definition examples: Examples for Forest Definitions

Some discussion points referring to forest definitions

The definitions discussed here refer essentially to field measurements or to high resolution remote sensing imagery, where the criteria described above can actually be observed and measured. However, in many forest mapping studies, coarser resolution imagery is being used and the above specified definitions can not be applied, at least not directly. Then, if an interest is there to make the remote sensing based forest mapping compatible with the results of inventory studies that have forest definitions as described above – proxies need to be developed for image classification. Vegetation types are being distinguished in remote sensing imagery mainly from spectral reflectance and image texture. Of course, these criteria can not directly be converted into the above described ones. When comparing forest cover estimates based on field sampling with those based on remote sensing imagery, compatibility of definitions is an important point.