Nurse logs: an ecological strategy in a late Paleozoic forest from the southern Andean region

Decaying logs on the forest floor can act as 'nurse logs' for new seedlings, helping with the regeneration of the vegetation. Fossil evidence of this ecological strategy is exceptionally well preserved in the Argentinean Andes, where an ~300 m.y. old permineralized forest was found at 3000 m elevation in San Juan Province. The fossil trunks, some of them in life position, are intercalated between volcanic rocks and sediments deposited in flooded environments (probably coastal lagoons). More than 100 specimens studied allow us to suggest the dominance of only one tree species in the forest. The first issue of the research was to determine the means used by the vegetation to survive in such adverse environmental conditions. Fossil evidence supports the hypothesis of regeneration via nurse logs. Little rootlets preserved inside the wood of several specimens indicate that seedlings developed on these logs. Important additional information provided by the fossils is the presence of aerenchymatic tissue in the rootlets. Aerenchyma tissue is a common feature developed in plants living in flooded environments; therefore its recognition in the fossil forest helps in the ecological interpretation. doi: 10.1130/G30557.1