Table 1 Strengths and weaknesses of large-scale forest inventories such as the German NFI to assess surrogates for biodiversity based on forest structural diversity

Large number of inventory plots for different strata such as jurisdictions or biogeographical regions and broad forest types

Sampling based on angle count method; only a selection of trees are sampled, which leads to a loss of information at the plot-level (probability proportional to size)

Approach applicable to NFIs of other countries

The large-scale design (2 km × 2 km grid) does not capture effectively small areas like forest reserves

Adequate number of sampling plots per forest type available (for main forest types, see Additional file 5)

Biodiversity-relevant variables were originally not included in inventory-samplings; increasing integration of biodiversity-relevant variables only in recent inventories (NFI₂₀₀₂ and NFI₂₀₁₂)

Low costs for acquisition of data that are attached to or can be derived from classical inventory variables

No precise information about harvesting and other management activities at the plot-level

Dynamic changes over inventory periods can be considered (ongoing process)

Changes in sampled variables and sampling thresholds between NFIs (e.g. threshold-value for the minimal diameter for downed deadwood or the presence of hollows)

Same plots are re-sampled

→ Analysis on changes of structural elements and development of individual trees (over periods of 10 years)

While broad forest types can be analysed, local (regional) aspects may not be sufficiently well represented

A large number and variety of structural variables can be derived from inventory data

Owing to the sampling method and related small radius of sampling circles, plot measures are not representative of the stand in which they were collected; therefore extrapolation to hectare values is problematic

Some important variables of forest structure are not quantified directly. They can only be addresses through surrogates (e.g. the occurrence of large living trees as surrogate for habitat-tree characteristics)