Changes in oak wood buried in waterlogged peat: shrinkage as a complementary indicator of the wood degradation rate

A study was made on the degradation of contemporary heartwood of oak (Quercus sp.) buried in waterlogged peat on the archaeological site at Biskupin (Poland). The state of wood preservation was evaluated after 2, 4, 6, 8 and 10 years of deposition of samples in conditions similar to anaerobic. The changes that occurred in the wood were assessed on the basis of microscopic examinations, chemical composition, mass loss and selected physical properties of the wood. Despite a distinct mass loss of the samples (7.4% after 10 years of the experiment), only the initial phase of cell wall degradation and almost unchanged contents of major chemical components were observed in the examined wood. Greater changes were recorded in the content of water-soluble extractives, maximum water content, basic density and wood porosity, as well as in the total tangential and the total radial shrinkage. Wood mass loss resulted mainly from the loss of substances soluble in water (up to 70.8% after 10 years of the experiment) and less from the biological degradation of cell walls (loss of major chemical components of wood). A close relationship was observed between content of substances soluble in water, mass loss, and wood shrinkage. It was concluded that tangential and radial shrinkage may serve as a simple complementary indicator of the state of preservation of wood decomposing in the conditions of a monitored archaeological site.