Abstract
Tree barks are among the less studied forest products notwithstanding their relevant physiological and protective role in tree functioning. The large diversity in structure and chemical composition of barks makes them a particularly interesting potential source of chemicals and bio-products, at present valued in the context of biorefineries. One of the valuable components of barks is cork (phellem in anatomy) due to a rather unique set of properties and composition. Cork from the cork oak (Quercus suber) has been extensively studied, mostly because of its economic importance and worldwide utilization of cork products. However, several other species have barks with substantial cork amounts that may constitute additional resources for cork-based bioproducts. This paper makes a review of the tree species that have barks with significant proportion of cork and on the available information regarding their bark structural and chemical characterization. A general integrative appraisal of the formation and types of barks and of cork development is also given. The knowledge gaps and the potential interesting research lines are identified and discussed, as well as the utilization perspectives.
Highlights
Trees are externally covered on their stems and branches by the bark that represents 9 to 15% of the stem volume (Harkin and Rowe, 1971)
The present review presents a general overview of barks and of cork in particular and gathers the information available for some of the species with barks containing a substantial amount of cork, concerning the development and structural and chemical characteristics of the cork component, as well as their potential usage for cork-based bioproducts
Birch bark is described as having a rhytidome composed by periderms with successive cork layers and phloem layers with several sclerenchymatic tissues and showing conspicuous lenticels that contain suberized and non-suberized cells disposed in stratified layers (Bhat, 1982; Trockenbrodt, 1991; Ferreira et al, 2016b)
Summary
Trees are externally covered on their stems and branches by the bark that represents 9 to 15% of the stem volume (Harkin and Rowe, 1971). A synthesis of the existing knowledge on the cork of the different barks is made, organized by species, including, when available, details on periderm or rhytidome, cork cellular and chemical features, and use potential.
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