Giant stem rosettes have strong facilitation effects on alpine plant communities in the tropical Andes

<b>Background</b>: Giant rosettes constitute one of the most distinctive growth-forms in tropical alpine ecosystems. However, their interactions with other plant species remain unexplored in high Andean páramos. <b>Aims</b>: Quantify the effect of a dominant rosette (<i>Coespeletia timotensis</i>) on soil and microclimatic conditions and relate the impacts to plant community structure. <b>Methods</b>: We analysed topsoil temperatures, soil organic matter (SOM) and plant species cover in areas adjacent to <i>C. timotensis</i> and paired areas outside, at three sites (4250–4360 m a.s.l.) in La Culata National Park, Venezuela. Species richness, total vegetation cover and percent cover of each species near and away from the rosettes were compared. <b>Results</b>: Topsoil temperature amplitudes were lower and SOM greater near <i>C. timotensis</i> stems, compared to areas away from them. <i>C. timotensis</i> had a consistent positive effect on species richness, vegetation cover and the cover of many abundant species (including cushions and herbs). <b>Conclusions</b>: The facilitation effects of <i>C. timotensis</i> on abiotic conditions and community structure indicate they play a key role as nurse plants, pointing to giant rosettes as foundation species for the maintenance of plant diversity in the alpine tropics.