Table 1. Results from the beta regression models explaining average ground cover of lichens, bryophytes, eudicots, grasses, total vascular plants (TVP) and no vegetation in stands of native P. sylvestris and P. abies when contrasted to non-native P. contorta. Significant p-values are given in bold. The presented models represent the models with lowest AIC (see Supplementary file 3). The pseudo-R2 value (the squared correlation of the linear predictor and link-transformed response) gives a measure of model fit.
  Estimate SE p-value Rp2
Bryophytes       0.25
Intercept 1.932 0.216 <0.001  
Picea abies –0.514 0.275 0.062  
Pinus sylvestris –0.976 0.266 <0.001  
Age –0.024 0.007 0.001  
P. abies × age 0.030 0.008 <0.001  
P. sylvestris × age 0.035 0.008 <0.001  
Lichens       0.37
Intercept –2.946 0.905 0.001  
Picea abies –1.198 0.992 0.227  
Pinus sylvestris 2.149 0.985 0.029  
Age –0.021 0.014 0.132  
Canopy cover –0.021 0.013 0.118  
P. abies × age 0.022 0.016 0.181  
P. sylvestris × age 0.002 0.015 0.881  
P. abies × canopy cover 0.007 0.017 0.665  
P. sylvestris × canopy cover –0.043 0.017 0.013  
Eudicots     0.36
Intercept –0.611 0.112 <0.001  
Picea abies –0.520 0.137 <0.001  
Pinus sylvestris –0.343 0.135 0.011  
Age 0.015 0.002 <0.001  
Grasses       0.51
Intercept –2.810 0.740 <0.001  
Picea abies 0.386 0.741 0.603  
Pinus sylvestris 0.527 0.770 0.493  
Age 0.022 0.015 0.137  
Canopy cover 0.026 0.012 0.035  
P. abies × age 0.004 0.014 0.756  
P. sylvestris × age –0.035 0.010 <0.001  
P. abies × canopy cover –0.032 0.013 0.017  
P. sylvestris × canopy cover 0.012 0.013 0.366  
Age × canopy cover 0.000 0.000 0.137  
Total vascular plant cover (TVP)       0.49
Intercept –0.804 0.521 0.123  
Picea abies 0.238 0.565 0.673  
Pinus sylvestris 0.019 0.601 0.975  
Age 0.031 0.009 <0.001  
Canopy cover 0.011 0.008 0.145  
P. abies × age –0.024 0.010 0.014  
P. sylvestris × age –0.025 0.009 0.006  
P. abies × canopy cover –0.012 0.009 0.209  
P. sylvestris × canopy cover 0.009 0.010 0.369  
No vegetation       0.68
Intercept –3.788 0.498 <0.001  
Picea abies 1.928 0.442 <0.001  
Pinus sylvestris 0.736 0.465 0.114  
Age –0.012 0.014 0.392  
Canopy cover 0.008 0.005 0.103  
P. abies × age 0.003 0.015 0.859  
P. sylvestris × age –0.014 0.015 0.343  
1

Fig. 1. Mean cover (%), ±SE, for the six different types of ground vegetation inventoried in stands of P. contorta, P. sylvestris and P. abies of different stand age classes. Note the different scales on the y-axes in (a) Lichens. Significant differences within age classes are indicated by different letters (Tukey’s test; p < 0.05), n.s. = not significant. The star (*) denotes the single 85 year old P. contorta stand which was not included in the significance test.

2

Fig. 2. Relationship between grass cover (%) and canopy cover (%). All stands included. Regression line is shown for Picea abies, the only tree species, under which significant correlation between grass and canopy cover was observed in the beta regression analysis.

Table 2. Mean values (± SE) for canopy cover in different stand types. Significant differences within age classes are represented by different letters (Tukey’s test; p < 0.05). The star (*) denotes the single 85 year old P. contorta stand which was not included in the significance test.
Tree species Age class Canopy cover (%)
P. contorta 15 57 (4.2)a
P. abies 15 26 (2.1)b
P. sylvestris 15 36 (3.4)b
P. contorta 30 62 (2.2)a
P. abies 30 58 (4.0)b
P. sylvestris 30 40 (3.8)a
P. contorta 85 27 *
P. abies 85 66 (1.9)a
P. sylvestris 85 38 (1.9)b