The classification of forests into different communities or types provides an impor- tant basis for basis for forest management and conservation planning. Numerous different classification systems have been developed and implemented in different parts of the world. Details of these different approaches are described by Mueller- Dombois and Ellenberg (1974) and Whittaker (1975). Recent examples of approaches to classifying forests are provided by McNab et al. (1999) and Carter et al. (1999). Kimmins (1997) presents a useful overview of the different approaches that have been used to classify forests in the past, which can be sum- marized as follows:
● Climatic classificationsare based on the fact that climate is one of the major factors influencing vegetation distribution. Climate types can be defined in terms of temperature and precipitation, which are associated with particular types of vegetation—for example, tropical rainy and warm temperate climates are associated with tropical rain forest and temperate rain forest, respectively.
It is important to note, however, that climatic classifications represent poten- tial, rather than actual, vegetation distributions.
● Landform or physiographic classificationsare based on soil characteristics and landform features such as topography and altitude. These are often employed when classifying vegetation using remote sensing data. Although they have a firm ecological basis, as with climatic classification, this approach is an indir- ect method of classifying vegetation as floristic composition is not taken into account.
● Biophysical or ecosystemic approachesare based on climate, soils, and landform together with information about vegetation composition. This approach is widely used to classify areas at the regional scale.
● Physiognomic classificationsare based on the growth form of dominant plants and the environments in which they grow; for example, vegetation dominated
by coniferous trees, deciduous trees, or shrubs might be differentiated as different vegetation types.
● Floristic compositioncan also be used to define vegetation types. This is the main method used by ecologists, but different approaches have been adopted in different regions. For example, the Braun–Blanquet approach has been used widely in Europe, and the method developed by Daubenmire has been widely used in the USA.
This diversity of approaches greatly complicates the integration or comparison of maps of forest types produced in different areas, and inhibits the development of generalizations about the patterns of distribution of different forest types and their relative conservation status. If the objective is some form of conservation assess- ment, it is very important to note the classification methods used in producing any that are drawn upon. For example, a map representing potential forest distribution (as produced by climatic or physiographic approaches) would be of little use for assessing the actual current extent of a particular forest type.
The problems of using applying available forest classifications to conservation issues are illustrated by Miles et al. (2006) in their global assessment of the con- servation status of tropical dry forests. A first glance at available maps of forest types, such as that presented by FAO (2001b), suggests that extensive areas of tropical dry forest remain. However, the FAO map of ecofloristic zones is based on a climatic classification of potential vegetation, which gives a very misleading impression of current vegetation cover. Miles et al. (2006) therefore used a global map of current forest cover derived from remote sensing imagery (MODIS) to examine the actual current extent of tropical dry forest. The next step was to over- lay this map with ecological classifications of forest types. Blasco et al. (2000) pro- vide a comparison of 10 regional classification schemes developed for tropical woody vegetation, and present a common framework based on ‘bioclimatic types’
to help make comparisons between them. However, this has proved to be of little use for mapping tropical dry forests, as forest types (or formations sensuBlasco et al., 2000) labelled as ‘dry forest’ in available regional classifications are grouped under no fewer than four of the six bioclimatic types considered. This suggests either that tropical dry forests have a very broad edaphic tolerance range, or that the concept of what constitutes a tropical dry forest is interpreted variously by different authors.
To produce their assessment, Miles et al. (2006) used the global biogeographic classification presented by Olson et al. (2001). This has the great merit that it was explicitly developed to support global conservation assessments. Importantly, the classification was based primarily on biogeographic information rather than climate, and included information regarding distributions of animal species as well as plants, unlike most alternative classifications that are available. This classifica- tion is applicable for use at global and regional scales, but has also been used by WWF for conservation assessments at subregional (ecoregion) scales (Olson et al.
2000).
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When choosing an appropriate classification scheme, it is necessary to decide whether compatibility with existing schemes is necessary or desirable. Such compatibility can be of great help in enabling communication between people working on a particular forest type. Selection of forest types used in a map will depend on the map’s purpose, and whether the classes can be accurately and effi- ciently delimited. When a classification system has been selected, it is important to document the details of each class and apply these definitions in an objective and consistent manner.
Many vegetation classification systems use a hierarchical approach in which classes are nested; major classes are divided into subclasses, which themselves can be divided into further subclasses. Such systems can be easily adapted to different scales. Non-hierarchical approaches tend to be used at a specific scale, when they might be preferred because they can be more readily customized to the particular needs of the investigation in question.