The high tropical Andes host one of the richest alpine floras of the world, with exceptionally high levels of endemism and turnover rates. Yet, little is known about the patterns and processes that structure altitudinal and latitudinal variation in plant community diversity. Herein we present the first continental‐scale comparative study of plant community diversity on summits of the tropical Andes. Data were obtained from 792 permanent vegetation plots (1 m2) within 50 summits, distributed along a 4200 km transect; summit elevations ranged between 3220 and 5498 m a.s.l. We analyzed the plant community data to assess: 1) differences in species abundance patterns in summits across the region, 2) the role of geographic distance in explaining floristic similarity and 3) the importance of altitudinal and latitudinal environmental gradients in explaining plant community composition and richness. On the basis of species abundance patterns, our summit communities were separated into two major groups: Puna and Páramo. Floristic similarity declined with increasing geographic distance between study‐sites, the correlation being stronger in the more insular Páramo than in the Puna (corresponding to higher species turnover rates within the Páramo). Ordination analysis (CCA) showed that precipitation, maximum temperature and rock cover were the strongest predictors of community similarity across all summits. Generalized linear model (GLM) quasi‐Poisson regression indicated that across all summits species richness increased with maximum air temperature and above‐ground necromass and decreased on summits where scree was the dominant substrate. Our results point to different environmental variables as key factors for explaining vertical and latitudinal species turnover and species richness patterns on high Andean summits, offering a powerful tool to detect contrasting latitudinal and altitudinal effects of climate change across the tropical Andes.

Question: Species-specific interactions can connect particular species to others, which has important implications for species interdependence and co-existence. However, species-specific effects of ecosystem engineers remain little explored, particularly in the alpine tropics. We investigated the effects of two cushion plants with subtle differences in morphological traits in a tropical Andean ecosystem. We asked whether these foundation species differed in their effects on local abiotic factors, species richness and the structure of plant assemblages growing within them.Location: La Culata National Park, northern tropical Andes, Venezuela. Methods:We determined shallow soil temperature, soil organic matter and water content within two co-existing cushion species, the larger and more compact Azorella julianii and the smaller and denser Arenaria musciformis, and in adjacent open areas. We also recorded the total number of plant species growing within both cushion species, and in paired open areas at three sites. We then compared local species richness, total plant abundance and the abundance of each species within cushions and outside using the relative interaction index (RII), and community dissimilarity patterns using NMDS ordination. We also compared species accumulation curves across sites between the cushions and open areas.Results: Both cushions had similar positive effects on soil organic matter, but soil water content was increased only with Azorella. Both cushions had a positive effect on local species richness and total plant abundance. In addition, both species significantly modified species abundance patterns, showing a positive association with several species, many of them grasses. However, facilitative effects on other species were generally stronger for Azorella than Arenaria. Moreover, community structure of plants growing within the two cushion plants differed. The exotic invader Rumex acetosella increased in abundance within Azorella but not Arenaria. Conclusions:We found evidence for species-specific effects of ecosystem engineers in the alpine tropics; two cushion species with subtly contrasting traits had different effects on microhabitat conditions and plant assemblages. This could in turn increase abiotic and biotic heterogeneity and promote species co-existence and emphasizes interdependence among species in alpine plant communities.

9Studies on alpine and semi-arid environments indicate that plants that act as ecosystem engineers improve 10 microhabitat conditions and modify local plant abundance and diversity. However, few studies have linked these 16show that H. laricifolium dampened temperature oscillations and increased soil water and organic matter contents, 17 the effect being more pronounced at the drier, lower elevation site. While positively associated forbs showed an 18 improvement in their water status when growing under the crown, the species with the lowest water potentials and 19 higher water-stress tolerance were grasses with a negative spatial relation with the shrub; this was consistent at both 20 elevations. Moreover, the effect of the shrub on the water status and abundance of the exotic herb Rumex acetosella 21 changed from positive in the drier site to negative in the more mesic site. Our results provide mechanistic evidence 22 for interpreting spatial association patterns between nurse plants and other species in the alpine tropics. We propose 23 that stress resistance strategies (tolerance vs. avoidance) and origin (native vs. exotic) influence interactions with 24 ecosystem engineers. 25

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