PLANT DEPOSITION IN MODERN VOLCANIC ENVIRONMENTS

Volcanic eruptions and their deposits provide paleobiologists with an array of depositional environments in which to investigate the conditi ons in which exceptionally preserved flora and fauna are preserved. St udies of vegetation patterns before and after eruptions have shown tha t tropical vegetation makes a very rapid recovery at the vegetational level (proportion of devastated land covered). The recolonisation of t he rare elements of a diversified flora, however, is slow, and so is t he rebound of floristic richness, assembled over centuries from surrou nding patches of vegetation and from in situ speciation. Two major vol canic events that occurred in the past 15 years were studied in an att empt to understand the processes and complex patterns of plant deposit ion in volcanic landscapes. Both volcanoes gave rise to terrestrial, a ndesitic, explosive eruptions that gave little warning of their absolu te magnitude or duration. In both eruptions, sediment-dammed lakes wer e formed in which a large quantity of plant material was trapped. In b oth instances, forests were killed and both standing and fallen trunks record the composition of the pre-eruption flora. The vegetation pres ent in the vicinity of the volcanoes was, in the case of Mount St Hele ns (Washington, U.S.A.), dense coniferous forest living in a cool temp erate climate and, in the case of El Chichon (Chiapas, Mexico), remnan ts of paratropical rainforest alternating with patches of agricultural land. Litter layers are present under the tephra at both volcanic sit es, yet the pattern of deposition and quality of the plant material di ffers dramatically between the two, in part because of differences in the types of eruptions and in part because of the nature of the plants available for burial. One of the most significant styles of burial, u nexpected in the air-fall ash deposits, is the presence of more than o ne eruptive layer generated by eruptions only hours apart. These separ ate eruption layers have different lithological characteristics and th e plant deposits buried in the different layers are different in taxon omic composition. Significantly, leaves in the upland volcanic-ash dep osits are preserved thus far for 10 years, even in the tropical settin gs where root growth might be expected to have obliterated all signs o f depositional stratigraphy. These impressions and compressions have a high likelihood of entering the fossil record and provide an excellen t example of upland deposition and preservation.