Table 1. The multi-level binomial model (Model 1) estimated for the mean percentage coverage of bilberry on the 2 m2 quadrates in the stands of the PSP3000 data. Sites I–V and VII–VIII refer to different site quality classes (see Suppl. file 1). Mineral soils, spruce mires and pine mires pertain to forest land, i.e. categories (a–e) of this study.
Variable Estimate Std error t-value Odds ratio p-value
Intercept –4.3356 0.0934 –46.44 0.013 < 0.001
Site (ref. III, mineral soils) a
   site I, mineral soils –2.4140 0.1708 –14.14 0.089 < 0.001
   site II, mineral soils –0.5627 0.0630 –8.93 0.570 < 0.001
   site IV, mineral soils –0.4702 0.0513 –9.17 0.625 < 0.001
   site V, mineral soils –1.6286 0.1085 –15.01 0.196 < 0.001
   site VII, mineral soils –1.3500 0.2149 –6.28 0.259 < 0.001
   sites ≤ II, spruce mires –1.1044 0.1173 –9.41 0.331 < 0.001
   sites ≤ III, pine mires –0.4081 0.1220 –3.35 0.665 0.001
   site IV, pine mires –0.2592 0.0812 –3.19 0.772 0.002
   site V, pine mires –1.0210 0.0922 –11.08 0.360 < 0.001
   site VIII, poorly productive land 1.1348 0.1893 6.00 3.111 < 0.001
ArtificialRegen b, mineral soils –0.2713 0.0500 –5.42 0.762 < 0.001
FormerAgrLand c, mineral soils –1.7342 0.1468 –11.81 0.177 < 0.001
Spruce d, mineral soils –0.1196 0.0522 –2.29 0.887 0.023
Deciduous trees d on sites I and II, mineral soils –0.7146 0.1123 –6.37 0.489 < 0.001
Deciduous trees d, spruce mires –0.7355 0.1246 –5.90 0.479 < 0.001
Altitude (m) 0.0040 0.0005 8.06 1.004 < 0.001
Altitude2/100 (m), poorly productive and waste land –0.0004 0.0001 –3.97 0.999 < 0.001
Stand age (a), mineral soils 0.0094 0.0011 8.43 1.009 < 0.001
Stand age2/100 (a), mineral soils –0.0027 0.0005 –5.32 0.997 < 0.001
Stand basal area (m2 ha–1), forest land e 0.1071 0.0055 19.35 1.113 < 0.001
Stand basal area2/100 (m2 ha–1), forest land e –0.2050 0.0151 –13.59 0.815 < 0.001
Variance components at f
   forestry centre region level 0.0187 (14)      
   municipality level 0.1279 (367)      
   cluster level 0.1328 (983)      
   sample plot level 0.1585 (2515)      
   stand level (‘pseudo’ level) 0.4913 (2801)      
a The parameter estimates of site variables “site III, spruce mires”, “site IV, spruce mires” and “site VIII, waste land” were not statistically significant.
b ArtificialRegen (artificial regeneration) is an indicator variable for the regeneration method (ref. natural regeneration).
c FormerAgrLand (former agricultural land) is an indicator variable for stand history (ref. former forest).
d An indicator variable for the dominant tree species (the reference is other tree species).
e In this context, forest land refers to categories (a–e) of this study.
f The number of observations at each level is given in parentheses. A random term at ‘pseudo’ level accounts for the overdispersion.
1

Fig. 1. Predicted coverage of bilberry in pine and spruce stands of different site fertilities (i.e. sites I–V; see the definitions in Suppl. file 1). The development of stands, representing mineral soils (A, B), pine mires (C) and spruce mires (D) in southern Finland, was simulated using the Motti simulator (arrows indicate thinnings).

2

Fig. 2. Bilberry yield indices calculated on the basis of Model 2 (Table 2) and mean annual bilberry yields (kg ha–1) presented by Salo (2015). The former index series covers years 2001–2014 and the latter one years 1997–2013. In both series, the mean value of the indices is 100.

Table 2. The multi-level Poisson model (Model 2) estimated for the mean number of bilberries on five 1 m2 quadrates in MASI stands, measured in 2001–2014.
Variable Estimate Std error t-value p-value
Intercept 4.4549 0.7005 6.36 <0.001
Year effect (ref. 2006)
   2001 0.1960 0.1719 1.14 0.256
   2002 –0.2007 0.1806 –1.11 0.268
   2003 0.4469 0.1708 2.62 0.009
   2004 –0.0602 0.1681 –0.36 0.721
   2005 0.5694 0.1622 3.51 0.001
   2007 0.3161 0.1708 1.85 0.066
   2008 0.0533 0.1647 0.32 0.747
   2009 0.0960 0.1598 0.60 0.549
   2010 –0.2072 0.1612 –1.29 0.200
   2011 0.2025 0.1569 1.29 0.198
   2012 0.5280 0.1632 3.24 0.001
   2013 –0.0334 0.1723 –0.19 0.846
   2014 0.0164 0.1788 0.09 0.927
Coverage of bilberry, % 0.0135 0.0057 2.38 0.028
1000/Temperature sum (dd) –1.0888 0.5101 –2.13 0.050
Variance components at a
   forestry centre region level <0.0001 (13)    
   municipality level 0.1235 (29)    
   stand level 0.1132 (50)    
   stand × year level (‘pseudo’ level) 0.2656 (306)    
a The number of observations at each level is given in parentheses. A random term at ‘pseudo’ level accounts for the overdispersion.