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Fig. 1. Locations of MASI stands used in this study. Black circle means that there was one stand per municipality. The cases with two stands and five stands per municipality are marked with an unfilled circle and a star, respectively. Numbers 0–13 refer to Forestry Centres of Finland.

Table 1. Main characteristics of cowberry coverage and stands in the PSP3000 data used in this study, by categories (a)–(g). Categories (a)–(e) pertain to forest land. For each category, number of stands (N), mean values and ranges of variation (min–max) of the characteristics are presented. Sites I-V and VII-VIII refer to different site quality classes (I = herb-rich forests, eutrophic mires, II = herb-rich heath forests, mesotrophic mires, III = mesic heath forests, meso-oligotrophic mires, IV = sub-xeric heath forests, oligotrophic mires, V = xeric heath forests, poor ombro-oligotrophic bogs, VII = rocky and sandy soils, VIII = summit and fell forests). View in new window/tab.
Table 2. Main characteristics of cowberry and stands in the MASI data (193 annual observations in 34 stands). Sites III-V refer to different site quality classes of mineral soil forests (III = mesic heath forest, IV = sub-xeric heath forest, V = xeric heath forest).
Characteristic N Mean Min Max
Number of berries (m–2) 193 130.4 0 2338
on site III 15 233.8 11 1173
on site IV 154 122.4 0 2338
on site V 24 117.1 8 332
Coverage of cowberry (%) 34 48.2 9 86
on site III 3 63.0 52 77
on site IV 26 46.2 9 86
on site V 5 49.4 22 76
Altitude (m) 34 149.2 19 314
Temperature sum (dd) 34 1015.5 682 1337
Stand age (a) 34 75.7 5 300
Stand basal area (m2ha–1) 34 12.0 0 35
Table 3. The multi-level binomial model (Model 1) estimated for the mean percentage coverage of cowberry 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 Table 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 –4.7902 0.4571 –10.48 0.008 <0.001
Site (ref. IV, mineral soils) a)
site I, mineral soils –5.1730 0.2410 –21.47 0.006 <0.001
site II, mineral soils –2.5690 0.1396 –18.40 0.077 <0.001
site III, mineral soils –0.4216 0.0687 –6.13 0.656 <0.001
site V, mineral soils –0.4185 0.1403 –2.98 0.658 0.003
sites I-II, spruce mires –2.0679 0.1567 –13.20 0.126 <0.001
site III, spruce mires –0.7984 0.1179 –6.77 0.450 <0.001
sites I-III, pine mires –1.8198 0.1543 –11.79 0.162 <0.001
site IV, pine mires –0.5644 0.0959 –5.88 0.569 <0.001
site V, pine mires –1.7620 0.1121 –15.72 0.172 <0.001
site VIII, poorly productive land –1.4831 0.2776 –5.34 0.227 <0.001
site VIII, waste land –2.9819 0.333 –8.95 0.051 <0.001
FormerAgrLand b), mineral soils –0.9438 0.1993 –4.73 0.389 <0.001
Spruce c) on sites I-III, mineral soils and spruce mires –0.4327 0.0663 –6.52 0.649 <0.001
Deciduous trees c) on sites I-III, mineral soils and spruce mires –0.7528 0.1009 –7.46 0.471 <0.001
1000/Temperature sum (dd) 2.5592 0.5561 4.60 12.925 <0.001
Altitude (m) –0.0039 0.0008 –4.62 0.996 <0.001
Stand age (a) on sites I-II, mineral soils 0.0106 0.0019 5.65 1.011 <0.001
Stand basal area (m2ha–1), forest land d) 0.0157 0.0025 6.19 1.016 <0.001
Variance components at e)
forestry centre region level 0.1211 (14)      
municipality level 0.1839 (367)      
cluster level 0.1819 (983)      
sample plot level 0.2148 (2515)      
stand level (“pseudo” level) 0.9463 (2801)      
a) The parameter estimates of site variables “site VII, mineral soils” and “site IV, spruce mires” were not statistically significant.
b) FormerAgrLand (former agricultural land) is an indicator variable for stand history (ref. forest).
c) An indicator variable for the dominant tree species (the reference is other tree species).
d) In this context, forest land refers to categories (a)–(e) of this study.
e) The number of observations at each level is given in parentheses. A random term at “pseudo” level accounts for the overdispersion.
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Fig. 2. Predicted coverage of cowberry in pine stands of different site fertilities (i.e. sites III-V; see the definitions in Table 1). The development of stands, representing mineral soils (A, B) and pine mires (C, D) in southern (A, C) and northern (B, D) Finland, was simulated using the Motti simulator (arrows indicate thinnings). Predictions were calculated using Model 1 (Table 3).

Table 4. The multi-level Poisson model (Model 2) estimated for the mean number of cowberries on five 1-m2 quadrates in pine-dominated stands of the MASI data, measured in 2001–2012. Sites III-V refer to different site quality classes of mineral soil forests (see Table 2).
Variable Estimate Std error t-value p-value
Intercept 6.5404 1.0099 6.48 <0.001
Year effect (ref. 2012)
2001 0.6276 0.3083 2.04 0.044
2002 0.1742 0.3011 0.58 0.564
2003 0.3927 0.3118 1.26 0.210
2004 0.1290 0.2941 0.44 0.662
2005 1.0525 0.2960 3.56 0.001
2006 0.4146 0.2919 1.42 0.158
2007 0.1529 0.2700 0.57 0.572
2008 –0.2921 0.2682 –1.09 0.278
2009 –0.3474 0.2710 –1.28 0.202
2010 –0.2773 0.2698 –1.03 0.306
2011 0.1925 0.2714 0.71 0.479
Coverage of cowberry (%) 0.0966 0.0208 4.64 0.010
Coverage of cowberry2/100 (%) –0.0837 0.0217 –3.86 0.018
Ln (Stand basal area + 1) (m2ha–1) –0.4716 0.0993 –4.75 0.009
Altitude (m) 0.0071 0.0023 3.06 0.009
1000/Temperature sum (dd) –4.6264 1.0758 –4.30 0.001
Variance components at a)
forestry centre region level 0.2270 (12)    
municipality level 0.0904 (27)    
stand level <0.0001 (34)    
stand x year level (“pseudo” level) 0.5024 (193)    
a) The number of observations at each level is given in parentheses. A random term at “pseudo” level accounts for the overdispersion.
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Fig. 3. Predicted average annual yields of cowberry and their 95% confidence intervals in pine stands on site IV (i.e. sub-xeric heath forest) in southern and northern Finland. The development of stands was simulated using the Motti simulator (arrows indicate thinnings). Predictions were calculated using Models 1 and 2 (Tables 3 and 4), and were also compared with the estimates computed using the models of Turtiainen et al. (2005).