1
Supporting Information
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Appendix S1. Main characteristics of the study sites along the abiotic stress gradient evaluated.
site
soil type
easting
northing
altitude
average annual
average
max. temp.
(°C)
average annual
precip. (mm)
average
min. temp.
(°C)
(m.a.s.l.)
temp. (°C)
Las
Colominas
Calcareous
692494 4273202
630
391
9
21
15
Huelves
Calcareous
508716 4436139
911
497
7
20
13
Ontígola
Calcareous
447271 4427264
593
431
7
22
14
Sierra Espuña
Calcareous
616839 4187005
663
364
10
22
16
Villarobledo
Calcareous
542064 4340264
753
446
8
21
14
Zorita
Calcareous
510441 4467336
602
433
7
22
15
Almeria
Calcareous
584367 4087691
201
334
13
21
18
Jaén
Calcareous
493939 4165914
804
517
9
21
15
El Plano
Gypsiferous 687597 4268937
571
370
9
21
15
Ontígola II
Gypsiferous 446915 4427092
598
427
7
22
14
Titulcia
Gypsiferous 456397 4446093
598
440
8
21
14
1
Carabaña
Gypsiferous 478288 4455557
592
473
8
21
14
Chinchon
Gypsiferous 456433 4458275
609
448
8
21
14
Fuentidueña
Gypsiferous 488787 4443880
601
445
8
21
14
Jaén II
Gypsiferous 495283 4175781
493
632
10
22
16
Los
Monegros
Gypsiferous 708985 4620387
507
428
7
20
14
Los Yesos
Gypsiferous 563409 4103742
537
364
11
22
16
Villamanrique Gypsiferous 482031 4432562
600
437
8
21
15
3
4
5
6
7
8
9
10
11
12
13
14
15
16
2
17
18
19
Appendix S2. Biological soil crust functional traits.
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strongly in chemistry and physical morphology, impacting their function in the ecosystem. This
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is well illustrated by the morphological differences between larger mosses (a), which are likely
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to be a large water sink and under certain conditions may enhance infiltration, and crustose or
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foliose lichens (b) which are likely to shed runoff at microscales. Large photo b: Rebecca Mau.
Figure A1-1. Despite apparent niche overlap, species within biological soil crusts may differ
a.
b.
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3
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Appendix S3. Effect of soil type upon species co-occurrence.
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Methods
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We tested the hypothesis that soil type determines intensity of species interactions using one way
28
ANOVA. To determine if relationships between abiotic stress and co-occurrence differed among
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soil types (calcareous and gypsum), we separated the data from the two soil types and conducted
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linear regression analyses. We used JMP IN 4.0 (2000 SAS Inst.) for these analyses.
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32
Results
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Soil type, gypsiferous or calcareous, had essentially no effect on species interactions, regardless
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of which index or algorithm was used (C-scorefixed-fixed t = 0.62, P = 0.55; C-scorefixed-equiprobable t
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= 0.43, P = 0.67; Spp. pairsfixed-fixed t = -0.42, P = 0.68; Spp. pairsfixed-equiprobable t = - 0.99, P =
36
0.34). To examine the possibility that these responses of species interactions to abiotic stress
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were dependent upon the soil type considered, we conducted a second set of analyses using the
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data from one soil type at a time. This set of analyses indicates that in both soil types the
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prevalence of competition increases at higher abiotic stress (Table A3-1), although the
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relationship tended to be stronger in calcareous soils compared to gypsiferous soils.
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42
43
44
4
45
Table A3-1. Relationships of co-occurrence indicators to abiotic stress (Axis 1 of PCA
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conducted with climatic variables) in two contrasting soil types.
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Calcareous soils
Gypsiferous soils
R2
P
R2
P
C-scorefixed-fixed
0.56
0.03
0.40
0.05
C-scorefixed-equi.
0.02
0.75
0.02
0.68
Spp. pairsfixed-fixed
0.70
0.01
0.11
0.36
Spp. pairsfixed-equi.
0.47
0.06
0.08
0.43
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
5
63
64
65
66
67
68
69
70
6