Table 1. Duration of thermal growing season (d), effective temperature sum of growing season (°Cd), annual mean temperature (°C) and annual mean precipitation (mm) in the study area in 2012–2015.
Year Duration of thermal
growing season (d) ab		 Effective temperature sum of growing season (°Cd) b Annual mean
temperature (°C) c		 Annual mean
precipitation (mm) c
2012 156 921 0.7 692
2013 157 1246 2.2 481
2014 136 1141 2.8 515
2015 151 903 2.7 783
a Number of days when the mean daily temperature exceeds 5 °C, b Based on data collected by the nearest weather station at Rovaniemi airport (Finnish Meteorological Institute), c Based on grid data (Finnish Meteorological Institute).
Table 2. Numbers of all study plots, control and picking treatment plots at four sites during the study. Measurements of the bilberry and aboveground vegetation in the control and picking treatment plots during the study are denoted by plus and minus -signs for each year separately.
Number of study plots and the measurements in control (C) and picking treatment plots (PT) 2012 2013 2014 2015
Number of
  all study plots, sum (at sites 1,2,3,4) 60 (20,10,10,20) 59 (20,9*,10,20) 59 (20,9,10,20) 40 (10,0,10,20)
  control plots, sum (at sites 1,2,3,4) 30 (10,5,5,10) 29 (10,4*,5,10) 29 (10,4,5,10) 20 (5,0,5,10)
  treatment plots, sum (at sites 1,2,3,4) 30 (10,5,5,10) 30 (10,5,5,10) 30 (10,5,5,10) 20 (5,0,5,10)
Number of berries, C/PT +/+ +/+ +/+ +/+
Weight of berries, C/PT -/+ -/+ -/+ +/+
Biomass removed by raking, C/PT -/+ -/+ -/+ -/-
Aboveground undamaged biomass** + - - -
Number of flowers, C/PT -/- +/+ +/+ -/-
Abundances of vascular plant, moss, litter and soil C/PT -/- +/+ +/+ +/+
Aboveground biomass*** -/- -/- -/- +/+
* One control plot was lost due to the tree felling during the summer, time between flowering and berry maturation, at the site 2. C/PT = Control/Picking treatment, + = measured, - = not measured, ** aboveground biomass of bilberry collected outside the study plots, *** aboveground biomass of all plant species collected from the study plots at the end of the experiment.
Table 3. Undamaged aboveground biomass of bilberry (Vaccinium myrtillus) (dry weight g m–2, mean ± SE) in 2012 and the amount of biomass, which was removed by rake together with berries in the berry picking treatment plots at four study sites during 2012–2014. The proportion of removed biomass from undamaged bilberry biomass is shown for 2012.
Site 2012 2013 2014
Undamaged
biomass (g m–2)* 		Removed
biomass (g m–2) 		Proportion of removed biomass from total biomass (%) Removed
biomass (g m–2) 		Removed
biomass (g m–2)
1 145.28 ± 16.21 0.59 ± 0.15 0.40 0.31 ± 0.08 2.63 ± 1.10
2 127.90 ± 26.66 0.19 ± 0.10 0.15 0.88 ± 0.39 0.59 ± 0.19
3 196.57 ± 21.89 0.28 ± 0.10 0.14 0.74 ± 0.22 0.43 ± 0.13
4 153.55 ± 16.10 0.63 ± 0.14 0.41 0.35 ± 0.10 0.69 ± 0.25
* Biomass of bilberry collected at randomly selected 50 × 50 cm plots outside the study plots at each site (N = 6).
Table 4. Results of linear mixed model analysis of the number of Vaccinium myrtillus berries, flowers and fruit set.
Source Number of berries Number of flowers Fruit set
F p F p F p
Picking treatment (PT) 7.92953,314 0.007 0.66528 0.422 6.86027,857 0.014
Year (Y) 0.22494,384 0.799 0.79728 0.380 0.36127,338 0.553
PT × Y 0.00894,384 0.992 0.23528 0.632 1.10527,388 0.302
Numerator degrees of freedom Ndf = 1, 2, 2 for PT, Y and PT × Y in the number of berries, respectively. Ndf = 1, 1, 1 for PT, Y and PT × Y in the number of flowers and fruit set. Subscripts under F ratios indicate denominator degrees of freedom (Ddf).
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Fig. 1. Number of Vaccinium myrtillus berries (mean ± se) in the control and picking treatment plots before the experiment in 2012 and during the study period. The difference in the number of berries between the control and picking treatment plots in 2012 was not statistically significant (ns: not significant) as revealed by a one-factor ANOVA. The capital letter (A) above the columns denotes non-significant between-year variation in the number of berries during 2013–2015 (as revealed by a linear mixed model analysis). Small letters indicate the difference between the control and picking treatment at the level p < 0.05. In 2012: control N = 30, picking treatment N = 30. In 2013 and 2014: control N = 29, picking treatment N = 30. In 2015: control N = 20, picking treatment N = 20.

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Fig. 2. Fruit set (%) of Vaccinium myrtillus (mean ± se) in control and picking treatment plots in 2013 and 2014. The fruit set was estimated as the value of number of mature berries/number of flowers × 100. The capital letter (A) above columns denotes non-significant between-year variation in the fruit set of Vaccinium myrtillus. Small letters indicate the significant difference between the control and picking treatment plots at the level p < 0.05. Results are obtained by a linear mixed model analysis. In 2013: control: N = 15, picking treatment: N = 15, and in 2014: control: N = 14, picking treatment: N = 15.

Table 5. Results of linear mixed models testing the effect of picking treatment on the total vascular plant abundance and the relative abundance of plant functional types over years 2013‒2015. View in new window/tab.
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Fig. 3. Relative abundance (percentage values from the total vascular plant abundance) of plant functional types in the control and picking treatment plots in 2013–2015. Prefix “C” refers to the control and “PT” to the picking treatment on the X-axis. The picking treatment had no effect on the relative abundance of plant functional types. Small letters indicate the significant difference in the relative abundance of deciduous and graminoid species between years as revealed by a linear mixed models and LSD post hoc-test. A significant interaction between treatment and year was found for deciduous dwarf shrubs, which increased when combined with picking in 2015. A relative abundance of forbs is not shown in the figure due to their low abundance. In 2013 and 2014: control: N = 29, picking treatment: N = 30. In 2015: control: N = 20, picking treatment: N = 20.