How liverworts build hanging gardens from volcanic ash in Costa Rica

Nardia succulenta, a pioneer liverwort occurring on foot slopes of the 1968 cone of the volcano Arenal (Costa Rica), forms layered deposits with volcanic ash up to 15 cm thick. These adhere to vertical cliffs and bridge volcanic boulders to form a continuous carpet, thus creating a habitat that may favour the liverwort and facilitate establishment of vascular plants. The silt loam soil material of these "hanging gardens" is sorted from much coarser volcanic ash source materials provided by daily low-intensity eruptions. Sedimentation starts on pioneer Nardia growing in shallow grooves and voids on the vertical walls of andesitic boulders and erosion gullies. When covered by a thin layer of ash these form new shoots, so the ash is stabilised by stems and rhizoids, It contains up to 1.5% organic C, and has greater P retention than the source volcanic ash. It is also slightly enriched in oxalate and pyrophosphate-extractable amorphous Fe, Al and Si. A pH (H2O) of 5.3 probably results from acid deposition by fumarolic activity. The Nardia carpets are locally invaded by ants and other small insects and their larvae. (C) 2001 Elsevier Science B.V. All rights reserved.