Wounding of trees induces various reactions at the tissue and cell level. At the wound surface cells mostly die rapidly by desiccation, whereas the tissue beneath is stimulated to develop a protection zone against the penetration of air and microorganisms. In hardwoods, the major reaction to wounding in the differentiated xylem is the occlusion of water conducting vessels by tyloses and/or fibrillar/granular material. Very intense wound reactions occur in a narrow zone beneath from the wound surface. This zone is called the boundary layer and shows distinct discoloration.
Tylosis formation has been investigated in detail by transmission electron microscopy (TEM) for oak and black locust, and vessel occlusions by fibrillar/granular material have been described for birch, beech and lime trees. Vessel-adjacent parenchyma cells are responsible both for the production of tyloses and synthesis of fibrillar/granular materials, which are then secreted into lumens of vessels and fibres. Such a secretion process requires the modification of pit membranes between parenchyma cells and vessels or fibres. Cellular UV-microspectrophotometry allowed a more detailed chemical characterisation of the wound reaction compounds. Their absorbance pattern, with a distinct maximum beyond 300 nm, identified them as flavonoid compounds. Precursor substances with an additional maximum at 278nm were found in parenchyma cells. In addition to the occlusion of water conducting elements, parenchyma cells especially within the boundary layer develop a suberin-like layer, which is attached to the secondary wall. Modified wall structures were also observed in fibres and vessels of the boundary layer.
Wounds later become closed by callus tissue formed at the wound edges. In some cases, however, wounding of trees, e.g. by debarking during the growth period, results in the formation of callus tissue which develops over the entire wound surface or parts of it. The development of such a surface callus is divided into three stages. During the first stage, numerous cell divisions take place in regions where differentiating xylem remains at the wound surface. During the second stage, cells in the callus undergo differentiation by forming a wound periderm. In the third stage, a new cambial zone develops between wound periderm and xylem laid down prior to wounding, which is then producing wound xylem and wound phloem. These various wound-associated processes lead to compartmentalization around a wound, which restricts the spread of damage and thus maintains the functionality of inner xylem. The principles of this compartmentalization strategy were originally formulated by Alex L. Shigo in his CODIT model, which was later variously modified, e.g. by Walter Liese and co-workers.
Uwe Schmitt is Research Director at the Institute of Wood Technology and Wood Biology of the Johann Heinrich von Thünen-Institute (vTI) / Federal Research Institute for Rural Areas, Forestry and Fisheries in Hamburg/Germany. He studied Biology at the University of Heidelberg, where he graduated in 1980 with his diploma in the field of Cell Biology. Also in Heidelberg, Uwe Schmitt finished his PhD in 1983 in the field of fine structure of plant cells and moved then to Berlin as Research Assistant at the Federal Biological Research Centre for Agriculture and Forestry. After nearly three years of research on the involvement of biotic agents in forest dieback
in Europe, in 1985 he took over a position as senior research scientist at the former Federal Research Centre for Forestry and Forest Products in Hamburg (BFH). His main research activities are ultrastructure of wood, bark and wood composites, growth periodicity of trees with special regard to climate response as well as wound reactions and discolourations in the xylem of trees. From 2004-2008, Uwe Schmitt was interim head of the Institute of Wood Biology and Wood Protection of the BFH and since 2009, Team Leader Biology of the newly founded Institute of Wood Technology and Wood Biology of the vTI. He is German coordinator of the bilateral research activities between Germany and New Zealand in the frame of plant cell wall research and variously involved as national delegate in the European research network COST. He published 160 original papers, about 100 of them in internationally reviewed journals. Uwe Schmitt was elected Fellow of the IAWS in 2000 and since 2002 he has been Secretary of our Academy.