Forest inventory is the systematic collection of data on the forestry resources within a given area. It allows assessment of the current status and lays the ground for analysis and planning, constituting the basis for sustainable forest management.

Due to cost and time constraints inventories are generally carried out using sampling techniques. The general principle of sampling is to select a subset from a population and draw inferences from the sample to the entire population. The selection of the most appropriate sampling design is subject to several considerations (more details can be found in the Tools section of this module). Two basic considerations involve whether the objective is to set up a monitoring system (repeated measurements over time) and whether auxiliary information (i.e. aerial or satellite imageries) is available or not.

The main factors determining the overall methodology are the purpose and the scale of the inventory.

The purpose (objective/goal) and the targeted audience of a forest inventory must be clearly defined, and the focus of the data collection outlined accordingly with the information needs of the users. While in the past forest inventories were primarily aimed at assessing timber availability, in recent years the forest is recognized as a complex ecosystem with several elements (including humans) interacting. A forest inventory is now commonly conceived as a multipurpose forest inventory with the contribution of expertise from different fields such as sampling theory, surveying, information technology, remote sensing, social science, mensuration and modelling to assess the multiple functions of forests and trees.

In terms of scale, a wide range of needs, and therefore approaches, are possible. Global forest inventories are aimed at determining the extent and status of forest resources at the global level (i.e. FAO’s Forest Resources Assessment being carried out since 1946 which also serves as mechanism for facilitating the harmonization of terminologies and definitions).

Smaller areas inventories are usually driven by specific goals, often for forest planning and operations. They include regional inventories (portions of the country area); reconnaissance inventories (rough insights of forest resources in a limited area); diagnostic sampling to orient silviculture and forest management; exploitation surveys (focused at assessing harvestable timber availability and planning of harvest and logging operations); post-harvest inventories (to assess regrowth and damage caused by logging operations); forest health monitoring (often linked to salvage cuts operations).