A review on promising approaches for liquid permeability improvement in softwoods

The low liquid permeability of refractory wood species such as Norway spruce [Picea abies (L.) Karst.] and white Fir (Abies alba) is related mainly to the aspiration of bordered pits during wood drying. The resulting low permeability complicates treatments with liquid preservatives or wood modification substances. This article provides a literature review on various mechanical and biotechnological approaches that were developed for improving liquid permeability. In this context, we focus on the incubation of Norway spruce wood with a white rot fungus, Physisporinus vitreus (Pers.) P. Karst. The process is termed "bioincising" and results in a significant increase in wood permeability. This is most probably caused by the selective degradation of bordered pit membranes and simple pits of xylem ray parenchyma during the initial period of wood colonization. Subsequently, we discuss how bioincising could be a potential pretreatment method for wood preservation and selected wood modification substances. Considering that these wood modification systems require specific penetration depths for optimal performance, we discuss the capability of bioincising to enhance permeability at the required penetration depths. In this regard, we propose a terminology for better differentiation of penetration depths by liquid substances into the wood.