Base-age invariance and inventory projections

One of the most important functions of forest inventory is to facilitate management decisions towards forest sustainability based on inventory projections into the future. Therefore, most forest inventories are used for predicting future states of the forests, in modern forestry the most common methods used in inventory projections are based on implicit functions describing time and site dependent relationships derived from panel data. The essence of the implicit functions used for inventory projections is that each function is defined by its own value at one point in time - usually at the inventory time - called the initial conditions or reference values. For this reason, these functions are also called self-referencing, and initial conditions are obtained from sampling, measurements, re-measurements, or other type of inventories. Classic examples of such functions, although not exclusive, are the site index models. They can have different algebraic forms using fixed or variable base ages and be base-age invariant or base-age variant. We explain the implications of different algebraic forms of the self-referencing models that can be used for inventory projections and discuss the forestry literature on base-age variant models under the base-age invariance agenda.