Tree Definition
In particular in large area forest inventories, it is necessary to define what a tree is; it is hardly necessary for example, for a plantation forest inventory with few planted species or in forest management inventories in temperate regions where there are maybe 10 species. There are always shrubs that can, in some cases, possibly reach larger dimensions and it must be clear how to deal with them when they occur on a forest inventory plot. It also needs to be decided how to deal with bamboo and palms: include them or not? It is a good idea to generate before the inventory field work starts a list of all species that are known to grow in the inventory regions. It is clear that in some cases, in particular in species-rich tropical forest environments, it will be difficult or impossible to make that list complete, but it is needed in any case. In this list, all species are then listed with their scientific names and the (known) local names; each species receives a code; in many countries there is an established coding system for tree species. According to the FAO definition (FAO 1998), a tree is defined “as a woody perennial plant with a single main stem or in the case of coppice, with several stems, and having more or less definite crown. This definition includes bamboos, palms and other woody plants that meet the above criteria.” Sometimes, a minimum height at maturity, e.g. 5m and 7m, is also given as a part of tree definition.
Definitions are crucial for any empirical study. Clear definitions generate transparency of concepts and add to the overall credibility which is eventually beneficial to both those who do studies and those who wish to understand the results and reports. Every step in an inventory project needs to be defined in terms of its technical components (what?), of the responsible persons (who?) and of when it should take place (when?). This is essential above all in larger forest inventory projects where various people are involved at various steps of the project. Without clearly defined concepts, it is hardly possible to maintain consistency in the entire process and the risk is big that, for example, different field crews take different decision for the same situation. All definitions are being written down in the inventory protocol (Inventory Manual or field manual) which needs to be available to all involved. Each variable (attribute) to be observed needs to be defined. And that refers also to the seemingly so clear attributes like “forest” or “dbh”. The definition should be unambiguous so that each member of the inventory team knows exactly how to measure it; this includes also the definition of the measurement devices to be used. For some hard-to-measure variables such as needle or leaf loss or percentage of crown cover where visual interpretation is done, a reference for comparison is needed. The reference can be a series of photographs. It should be stressed that, for some variables, there are well established definitions which are being widely used. It is recommendable to use these definitions if not convincing reasons speak against that. Also, you should be clear that there is no “right” or “wrong” definition (because, at the end, it is a matter of definition…) but there are only “well-formulated” and “not well formulated” definitions.
