Table 3 Core variables used in the Forest Structure Index and their recognized importance for biodiversity of forests

DBHq (quadratic mean diameter of trees ≥ 7 cm at breast height)

GS growing stock

Spies and Franklin (1991); Uuttera et al. (1997); Acker et al. (1998); Ferreira and Prance (1999); Ziegler (2000); Tanabe et al. (2001)

Common variable to describe stand structure; higher DBHq implies older and taller stands with high biomass, typical forest microclimate, and more presence of habitat attributes of mature forests

DBH sd (standard deviation of diameter at breast height of trees ≥ 7 cm)

UA uneven-agedness

Acker et al. (1998); Neumann and Starlinger (2001); McElhinny et al. (2006)

High standard deviation of DBH implies a diverse stand structure with patches of different densities and tree dimensions; many niches are provided for different taxa; relates to canopy layering

Height sd (standard deviation of mean height of trees ≥ 7 cm DBH)

VH vertical-heterogeneity

MacArthur and MacArthur (1961); Sabatini et al. (2015); McElhinny et al. (2006)

Standard deviation of stand height describes the vertical heterogeneity of stands directly; relates to canopy layering

Bark (index to describe diversity of bark types)

BD bark diversity

Gilmore (1985); Dickman (1991); Pearce (1996); Eyre and Smith (1997); McElhinny et al. (2006); Bhadra et al. (2008)

Diversity of bark types (smooth, fissured, peeling, scaly, cracked, etc.) in forest stands implies a variety of habitats for many species to be found there (insects, fungi, yeasts, spiders, epiphytes). Tree diameter and bark-development phases are considered

Flower-diversity (diversity of fruiting and flowering trees)

FD Flower diversity

Kavanagh (1987); Andrews et al. (1994); Smith et al. (1994); Soderquist and MacNally (2000); Herrera et al. (2001); Singh and Kushwaha (2005)

Food source for nectarivorous and frugivorous species (mainly insects, bats and birds)

VolTrees40 (volume per hectare of trees with a DBH ≥ 40 cm)

LLT Large living trees

Spies and Franklin (1991); Tyrrell and Crow (1994); Koop et al. (1995); Acker et al. (1998); Van Den Meersschaut and Vandekerkhove (2000); Ziegler (2000); Larrieu and Cabanettes (2012)

Large trees have a special function as habitat or source of food for many taxa; they have a greater probability to provide microhabitat structures such as hollows, crown dead wood, etc.

N DC (number of decay classes)

DW DC deadwood decay classes

Lassauce et al. (2011); Blaser et al. (2013); Lachat et al. (2013); Dittrich et al. (2014)

Important for many taxonomic groups; many decay classes indicate a continuous recruitment of deadwood; indicator for natural forest conditions

Deadwood st mean DBH (mean DBH of standing deadwood)

DW s standing deadwood

Drapeau et al. (2009); Rondeux and Sanchez (2010); Lassauce et al. (2011); Verkerk et al. (2011); Lachat et al. (2013)

Important structural element for many taxa of xylobiotic species (habitat and food source); more suitable than volume/ha because of strong extrapolation effects when sampled on small plots; stumps are excluded from the calculation

Deadwood d average mean diameter (average mean diameter of downed deadwood)

DW d downed deadwood

Drapeau et al. (2009); Rondeux and Sanchez (2010); Brin et al. (2011); Lassauce et al. (2011); Verkerk et al. (2011); Lachat et al. (2013); Kappes and Topp (2004)

Important structural element for many taxa of xylobiotic species (habitat, food source, regeneration niche); surrogate for deadwood types and N/ha of dead wood pieces, justified by level of correlation and better distribution

SR (richness of tree species with DBH ≥ 7 cm)

CH compositional heterogeneity

Lähde et al. (1994); Maltamo (1997); Uuttera et al. (1997); Tilman (1999); Van Den Meersschaut and Vandekerkhove (2000); Uuttera et al. (2000); Sullivan et al. (2001); Pretzsch (2003); Pretzsch (2005)

Species richness of trees with DBH ≥ 7 cm is important for diversity of dependent species, in particular host-specific herbivores, detritivores, symbionts and pathogens

SR Reg (species richness of regeneration (DBH < 7 cm))

REG regeneration

Mosimann et al. (1987); Du Bus de Warnaffe and Deconchat (2008); Müller et al. (2008)

Important for many taxa like insects, mammals and birds; high SR Reg leads to more diverse future stand conditions