Decay stages of Jurassic wood debris from Scotland: evidence for the coevolution of fungal rot, arthropods and the nurse log strategy.

A key feature of extant conifer forests is the high percentage of seeds that germinate and establish on dead wood; in some forests, this can exceed 90%. This deadwood can act as an ideal nursery for young tree species, leading to this type of seedbed being termed 'nurse logs'. It is unclear how common this ecological strategy has been throughout the evolutionary history of conifers. Here, we describe a c. 150-million-year-old wood fragment from the Jurassic of Scotland using classic palaeobotanical techniques and microscopy. We interpret our new finding within the evolution of gymnosperms with a literature review of fossil nurse logs, and fungal and arthropod evolution. We interpret the timeline of decay processes on this log as consisting of three main stages: breakdown of the wood by white pocket rot; two types of arthropod boring, likely by beetles and mites; and colonisation by small roots. These findings demonstrate the complexity of decay processes occurring on a Jurassic forest floor, and the key role that white rot and arthropods play in the nurse log ecological strategy. Conifer germination on nurse logs facilitated by white rot fungi is a conserved ecological strategy of relevance in shaping forest ecosystems over geological timescales.