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This study examines the effects of altitudinal, temperature and aspect gradients on vascular plant species richness on mountain tops in Sierra Nevada (Spain) at different spatial scales (1 m2 quadrats, plot clusters of 4 m2, upper summit area down to the 5-m contour line, entire summit down to the 10-m contour line). The methodology follows the Global Observation Research Initiative in Alpine Environments (GLORIA) programme. Floristic and soil temperature data of eight summits sites in two neighbouring regions of the high part of Sierra Nevada (from 2668 m to 3327 m a.s.l.) were used in this study. In total, 102 taxa were recorded (84 genera; 29 families). The species richness decreased, whereas the proportion of endemic taxa increased with elevation. There were significant linear relationships between species richness and altitude and average soil temperature at each spatial scale. However, there was no significant relationship between species richness and aspect variables. Facing continued climate change, the high-altitude flora of Sierra Nevada is expected to be particularly vulnerable and prone to warming-induced biodiversity losses due to the high proportion of endemic taxa, ranging from 23% at lower elevations up to 67% at higher ones. [ABSTRACT FROM PUBLISHER]