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[Usage Notes] Piston_et_al_2018_Data.zip All data used to assess whether two contrasted canopy phenotypes (tight and loose) of the shrub Cytisus galianoi differed in their effects on the microhabitat and on subordinate plant community composition in a dry subalpine system. We also experimentally distinguished the relative contribution of above- (canopy) and below-ground (soil) effects of C. galianoi on the most frequent subordinate species, Festuca indigesta, as well as the reciprocal effects of F. indigesta on C. galianoi. Pistón_et_al_2018_Data.zip, 1. The impact of plant-plant interactions on species diversity patterns has been broadly addressed in stressful environments, such as alpine ecosystems, where foundation species promote species richness by creating habitat for other species. However, foundation species with contrasting phenotypes might modify the microhabitat differently, which would alter the subordinate community composition, and coincide with distinct feedback effects of those subordinate species on the foundation species. However, the precise interaction mechanisms that facilitate species are not fully understood, especially the relative contribution of above- and below-ground compartments of foundation species to subordinate species and the potential feedbacks they receive. 2. We explored whether two contrasted canopy phenotypes (tight and loose) of the shrub Cytisus galianoi differed in their effects on the microhabitat and on subordinate plant community composition in a dry subalpine system. We also experimentally distinguished the relative contribution of above- (canopy) and below-ground (soil) effects of C. galianoi on the most frequent subordinate species, Festuca indigesta, as well as the reciprocal effects of F. indigesta on C. galianoi. 3. We performed observational and manipulative experiments to assess the influence of phenotypic differences of the shrub on understory microhabitat and subordinate plant community composition. Reciprocal effects were assessed by removing either F. indigesta from the understory of the two shrub phenotypes or the C. galianoi canopy from the immediate vicinity of F. indigesta. 4. The two C. galianoi phenotypes differed in mean values of functional traits (like stem density or plant height), modified their understory microhabitats differently, and hosted distinct subordinate communities. Loose phenotypes had more positive effects on community composition and diversity than tight phenotypes. Additionally, tight phenotypes simultaneously showed both more positive aboveground and more negative belowground effects on F. indigesta than loose phenotypes. There were no significant feedback effects of F. indigesta on C. galianoi. 5. The two phenotypes of the foundation species C. galianoi showed contrasting effects on the subordinate plant community: compared to the tight phenotype, the loose phenotype had higher associated species diversity and reduced reciprocal interaction intensities above- and below-ground with the subordinate species F. indigesta. This highlights the impact of phenotypic variation for plant interactions and community-level diversity., Peer reviewed

This study was supported by the Office of Secretary of Public Education of Mexico through the program Further Education Programme for Teachers (UNICACH-110). Further support was provided by the Spanish Research Agency (grant CGL2017-84515-R to FIP and grant CGL2015-66192-R to JML). We thank the Cabo de Gata-Níjar Nature Park for support during field work (permit: 2016-101-657)., UV radiation UV radiation exposition with two levels: UV+ and UV- Biocides Exposition of biocides with two levels: biocides+ and biocides- Species Identity Leaf litter species identity with three levels: S. tenacissima, C. humilis and R. sphaerocarpa Mixtures Plant diversity treatments with four levels (three monospecific plus the mixture) Isopods Isopod presence with two levels: Isopods+ and Isopods- Mortality Total number of dead isopods after 80 days of experiment w_inic Litter initial dry weight (g) w_uv Litter mass after 200 days of UV exposition during the first stage of the experiment; dry weight (g) w_iso Litter mass after 80 days of exposition to isopods during second stage of the experiment; dry weight (g), Peer reviewed

Litter decomposition in field experiment. A total of 280 mesocosms were settled under and away of 20 individuals of Z. lotus. The litter boxes thus contained either one single species of litter (20 litter boxes for each species and position treatment), 2-sp mixtures (20 litter boxes for each 2-species combination and position treatment) or 3-sp mixtures (20 litter boxes with the 3-species mixture and position treatment)., This study was supported by the Office of Secretary of Public Education of Mexico though the program Further Education Programme for Teachers (UNICACH-110). Further support was provided by the Spanish Research Agency (grant CGL2017-84515-R to FIP and grant CGL2015-66192-R to JML). We thank the Cabo de Gata-Níjar Nature Park for support during field work (permit: 2016-101-657)., Date Date of collection (1 - 07/13/2017, 2 - 01/13/2018, 3 -08/30/2018) Plant Plant (Ziziphus lotus fertility island) ID utmx Geographic Longitude for each plant utmy Geographic Latitude for each plant Box Mesocosm ID Island Location of the mesocosm (Away or Under shrub) Mixtures Litter plant diversity (species richness 1, 2, 3) Species Identity of the litter plant species with three levels: S. tenacissima, C. humilis and R. sphaerocarpa w_inic Litter initial dry mass (g) w_end Litter dry mass (g) after 184, 367 and 597 days in the field, Peer reviewed

Litter decomposition in field experiment. A total of 280 mesocosms were settled under and away of 20 individuals of Z. lotus. The litter boxes thus contained either one single species of litter (20 litter boxes for each species and position treatment), 2-sp mixtures (20 litter boxes for each 2-species combination and position treatment) or 3-sp mixtures (20 litter boxes with the 3-species mixture and position treatment)., This study was supported by the Office of Secretary of Public Education of Mexico though the program Further Education Programme for Teachers (UNICACH-110). Further support was provided by the Spanish Research Agency (grant CGL2017-84515-R to FIP and grant CGL2015-66192-R to JML). We thank the Cabo de Gata-Níjar Nature Park for support during field work (permit: 2016-101-657)., Date Date of collection (1 - 07/13/2017, 2 - 01/13/2018, 3 -08/30/2018) Plant Plant (Ziziphus lotus fertility island) ID utmx Geographic Longitude for each plant utmy Geographic Latitude for each plant Box Mesocosm ID Island Location of the mesocosm (Away or Under shrub) Mixtures Litter plant diversity (species richness 1, 2, 3) Species Identity of the litter plant species with three levels: S. tenacissima, C. humilis and R. sphaerocarpa Predators Abundance of predators Decomposers Abundance of detritivores, Peer reviewed