Supplementary material:

Mod, Heidi K. & Miska Luoto. Arctic shrubification mediates the impacts of warming climate on

changes to tundra vegetation. Environmental Research Letters.

Table S1. The range and mean values of the predictors used to train the models. GDD = growing degree days, TCQ =

temperature of the coldest quarter, TWI = topographic wetness index, RAD = radiation, CALC = proportion of calcareous

bedrock, Shrub = shrub cover, Tree = tree cover.

GDD (summed °C)

TCQ (°C)

TWI

RAD (MJ/cm

2/a)

CALC (%)

Shrub (%)

Tree (%)

min. 126 -16 5 0.2 0 0 0

mean 486 -12 6 0.4 41 20 0

median 479 -13 6 0.4 46 28 5

max. 685 -9 10 0.8 99 126 68

Table S2. Parameters of the 20 shrub species and tree canopy cover in the dataset. Cover values of the 20 shrub species

reaching at least 15 cm height was used to form shrub predictor. For tree cover, cover value of canopy of a site was

used. For shrub species (n = 14) that had minimum of 10 observations in the dataset also individual species distribution

models (SDMs) were built.

Growth form

Number of observations

Total cover (summed %)

Mean cover (%)

Max. cover (%)

Min. height

Max. height

SDM

Empetrum hermaphroditum evergreen 1492 26803 11.7 95 10 30

x

Betula nana deciduous 932 10369 4.5 90 20 80 x

Vaccinium myrtillus decidious 904 6865 3 70 10 30 x

Vaccinium vitis-idaea evergreen 1725 6322 2.8 55 5 30 x

Juniperus communis evergreen 313 4764 2.1 85 20 100 x

Vaccinium uliginosum decidious 682 4053 1.8 70 15 50 x

Phyllodoce caerulea evergreen 619 2445 1.1 40 10 15 x

Cassiope tetragona evergreen 443 1956 0.6 65 10 30 x

Salix glauca decidious 128 385 0.2 75 50 200 x

Salix lapponum decidious 23 199 0.1 50 50 100 x

Salix hastata decidious 64 174 0.1 15 50 100 x

Salix lanata decidious 15 1.5 0.1 55 80 100 x

Salix caprea decidious 4 66 0.02 35 300 1000

Calluna vulgaris evergreen 1 35 0.02 35 10 40

Salix myrsinifolia decidious 9 35 0.02 25 200 1000

Salix phylicifolia decidious 27 26 0.01 5 200 300 x

Salix myrtilloides decidious 28 26 0.01 3 20 80 x

Salix myrsinites decidious 8 7 0.003 2 20 100

Ribes spicatum decidious 2 5 0.002 5 100 150

Rhododendron lapponicum evergreen 3 3 0.001 2 5 15

Tree cover trees 132 sites with canopy cover 3109 5.4 68

Figure S1. The test area showing the distribution and mean values of the 7 predictors.

Figure S2. Scheme of analyses. For further details, see Material and Methods section in the article.

Table S3. Vegetation change as predicted using abiotic and biotic SDMs for all species and different species groups. The

table shows absolute values, with percentage changes between current and future conditions counted for ‘mean

predicted species richness’ and ‘species with increase in distribution’. For ‘mean predicted species richness’, increase in

between current and future conditions are marked with green, while decreases are marked with purple. For ‘species

with increase in distribution’, red indicates that majority of species (> 50 %) increase in distribution between current and

future conditions, and blue indicate that majority of species decrease in distribution. Significance of changes in ‘mean

predicted species richness’ between abiotic and biotic models was tested with paired t.test: * p < 0.05; ** p < 0.01; *** p

< 0.001.

All species (n = 130) NOW 2050 2070 % 2050 % 2070

Mean predicted species richness abiotic 26.5 28.8 27.7 8.7 % 4.5 %

Species with increase in distribution abiotic 76 75 58.5 % 57.7 %

Mean gains/losses abiotic 2.34 1.22

Mean turnover abiotic 0.44 0.53

Mean extinctions (%) abiotic 37.1 47.6

Shrub species (n = 14) NOW 2050 2070 % 2050 % 2070

Mean predicted species richness abiotic 1.9 3.7 4.2 94.7 % 121.1 %

Species with increase in distribution abiotic 12 12 85.7 % 85.7 %

Mean gains/losses abiotic 1.24 1.56

Mean turnover abiotic 0.51 0.62

Mean extinctions (%) abiotic 31.8 43.3

All species (n = 116) NOW 2050 2070 % 2050 % 2070

Mean predicted species richness abiotic 24.6 25.2 23.5 2.4 % -4.5%

Mean predicted species richness biotic 21.1*** 20.9*** 19.1*** -0.9 % -9.5 %

Species with increase in distribution abiotic 64 63 55.2 % 54.3 %

Species with increase in distribution biotic 53 48 45.7 % 41.4 %

Mean gains/losses abiotic 0.60 -1.03

Mean gains/losses biotic -0.23 -2.07

Mean turnover abiotic 0.43 0.52

Mean turnover biotic 0.47 0.57

Mean extinctions (%) abiotic 37.8 48.3

Mean extinctions (%) biotic 42.5 54.3

Arctic species (n = 51) NOW 2050 2070 % 2050 % 2070

Mean predicted species richness abiotic 15.8 13.4 11.7 -15.2 % -25.9 %

Mean predicted species richness biotic 12.9*** 10.3*** 8.8*** -20.2 % -31.8 %

Species with increase in distribution abiotic 19 16 37.3 % 31.4 %

Species with increase in distribution biotic 17 13 33.3 % 25.5 %

Mean gains/losses abiotic -2.49 -4.17

Mean gains/losses biotic -2.62 -4.15

Mean turnover abiotic 0.45 0.55

Mean turnover biotic 0.48 0.59

Mean extinctions (%) abiotic 44.4 55.6

Mean extinctions (%) biotic 49.1 61.3

Boreal species (n = 65) NOW 2050 2070 % 2050 % 2070

Mean predicted species richness abiotic 8.7 11.8 11.9 35.6 % 36.8 %

Mean predicted species richness biotic 8.2*** 10.6*** 10.3*** 29.3 % 25.6 %

Species with increase in distribution abiotic 45 47 69.2 % 72.3 %

Species with increase in distribution biotic 36 35 55.4 % 53.8 %

Mean gains/losses abiotic 3.09 3.12

Mean gains/losses biotic 2.39 2.08

Mean turnover abiotic 0.45 0.52

Mean turnover biotic 0.49 0.58

Mean extinctions (%) abiotic 24.4 33

Mean extinctions (%) biotic 30.1 41.3

Graminoids (n = 30) NOW 2050 2070 % 2050 % 2070

Mean predicted species richness abiotic 7.9 8.9 8.4 12.7 % 6.3 %

Mean predicted species richness biotic 5*** 5.7*** 5.1*** 14.0 % 2.0 %

Species with increase in distribution abiotic 18 17 60.0 % 56.7 %

Species with increase in distribution biotic 16 13 53.3 % 43.3 %

Mean gains/losses abiotic 1.00 0.45

Mean gains/losses biotic 0.64 0.10

Mean turnover abiotic 0.39 0.47

Mean turnover biotic 0.47 0.59

Mean extinctions (%) abiotic 26.4 36

Mean extinctions (%) biotic 31.3 43.8

Forbs (n = 63) NOW 2050 2070 % 2050 % 2070

Mean predicted species richness abiotic 11.9 11.3 10.3 -5.0 % -13.4 %

Mean predicted species richness biotic 10.3*** 9.4*** 8.4*** -8.7 % -18.4 %

Species with increase in distribution abiotic 32 33 50.8 % 52.4 %

Species with increase in distribution biotic 23 22 36.5 % 34.9 %

Mean gains/losses abiotic -0.65 -1.60

Mean gains/losses biotic -0.86 -1.90

Mean turnover abiotic 0.47 0.57

Mean turnover biotic 0.55 0.66

Mean extinctions (%) abiotic 44.9 55.9

Mean extinctions (%) biotic 52.8 64.3

Table S4. Minimum, median, mean and maximum values of predicted shrub and tree covers in the test area.

Table S5. Variable importance values of the predictors in biotic models averaged across species of different species

groups.

GDD TCQ TWI RAD CALC SHRUB TREE

all species (n=116) 0.44 0.22 0.16 0.27 0.28 0.31 0.08

arctic species (n = 51) 0.38 0.23 0.14 0.29 0.27 0.34 0.07

boreal species (n = 65) 0.49 0.22 0.18 0.25 0.29 0.28 0.09

graminoids (n = 30) 0.40 0.21 0.18 0.25 0.25 0.34 0.07

forbs (n = 63) 0.46 0.21 0.16 0.30 0.28 0.33 0.08

Shrub cover (%) Tree cover (%)

NOW 2050 2070 NOW 2050 2070

min. 2.2 2.7 3.7 0.2 0.2 0.2

median 6.6 13.6 17.8 0.2 0.2 0.2

mean 7.2 16.7 20.3 0.2 0.3 0.7

max. 25.3 63.3 67.7 0.2 8.2 37.2

Figure S3. Gains and losses of arctic (a-d) and boreal species (g-j) as predicted for 2050 (a, c, g, i) and 2070 (b, d, h, j)

without (a-b, g-h) and with the effect of shrubification (c-d, i-j). Panels on the bottom row (e-f, k-l) show the effect of

shrubification on species gains and losses.

Figure S4. Species gains and losses of graminoids (a-d) and forbs (g-j) as predicted for 2050 (a, c, g, i) and 2070 (b, d, h,

j) without (a-b, g-h) and with the effect of shrubification (c-d, i-j). Panels on the bottom row (e-f, k-l) show the effect of

shrubification on species gains and losses.

Figure S5. Turnover of arctic (a-d) and boreal species (g-j) as predicted under RCP 4.5 scenario for 2050 (a, c, g, i) and

2070 (b, d, h, j) without (a-b, g-h) and with the effect of shrubification (c-d, i-j). Panels on the bottom row (e-f, k-l) show

the effect of shrubification: negative values (blue) indicate diminishing influence, whereas positive values (red) indicate

amplifying impact.

Figure S6. Species turnover of graminoids (a-d) and forbs (g-j) as predicted under RCP 4.5 scenario for 2050 (a, c, g, i)

and 2070 (b, d, h, j) without (a-b, g-h) and with the effect of shrubification (c-d, i-j). Panels on the bottom row (e-f, k-l)

show the effect of shrubification: negative values (blue) indicate diminishing influence, whereas positive values (red)

indicate amplifying impact.

Figure S7. Proportion of arctic (a-d) and boreal species (g-j) going extinct as predicted under RCP 4.5 scenario for 2050

(a, c, g, i) and 2070 (b, d, h, j) without (a-b, g-h) and with the effect of shrubification (c-d, i-j). Panels on the bottom row

(e-f, k-l) show the effect of shrubification: negative values (blue) indicate diminishing influence, whereas positive values

(red) indicate amplifying impact.

Figure S8. Proportion of graminoid (a-d) and forb species (g-j) going extinct as predicted under RCP 4.5 scenario for

2050 (a, c, g, i) and 2070 (b, d, h, j) without (a-b, g-h) and with the effect of shrubification (c-d, i-j). Panels on the

bottom row (e-f, k-l) show the effect of shrubification: negative values (blue) indicate diminishing influence, whereas

positive values (red) indicate amplifying impact.