Risk assessment of Celtis australis affected by Inonotus rickii in Lisbon town

Abstract

In Lisbon, 20% of urban trees are Celtis australis (European hackberry), but since 2000 Inonotus rickii, a basidiomycete fungus, is causing heart white rot and the failure of these street trees. In the present work, 374 C. australis located in a residential area of Lisbon were surveyed in 2007 and in 2015 to evaluate the incidence of the disease and the risk of failure associated to each tree, following the Visual Tree Assessment method (VTA). Trees' total height, height to the live crown base and diameter at 1.30 meters height (DBH) were measured. Trees were assessed for the overall condition of trunks, branches and leaves as well and rated with a risk rating value. Major defects were registered and rated according to their failure potential. Following ground level visual inspection, trees showing severe defects on the trunk were further assessed using a IML-Resistograph F-400S. Out of the 374 trees assessed, 279 individuals have symptoms caused by I. rickii. Fruit bodies were seen in 15% of the trees, mainly on individuals with DBH greater than 40 cm. The incidence of I. rickii increased in 10% between 2007 and 2015 and the diseased trees exhibited unbalanced crowns, dead branches and cavities on branches and on the trunk. The evaluation of the extension of such defects led to their classification as critical with a probable or imminent likelihood of failure. In Lisbon I. rickii threatens the future of a significant part of street trees affecting their longevity and mechanical stability.