Adapted VTA and SIA method in tree static assessment with use of resistography

Background and Purpose: This paper introduces a method for assessment of static safety by using the combination of adapted VTA and SIA methods and a resistograph. The aim is to assess the static as reliably as possible and without the need for expensive equipment. The purpose of this paper is to propose amethod for the static assessment which could be applied in practice in the future. Materials and Methods: Seven trees in the Botanical Garden of the Faculty of Science (University of Zagreb) were selected for the assessment of vitality and static safety. It has been detected that those trees were affected by possible indicators of decreased vitality, such as fungi fruiting bodies on the root collar or trunk, dead branches, growthmalformations, burrs, dead bark etc. The selected trees were: Fagus sylvatica L. »Purpurea Tricolor«, Fraxinus pennsylvanica Marshall, Acer pseudoplatanus L. »Leopoldii«, Fagus grandifolia Ehrh., Quercus petraea L., Quercus phellos L. and Salix alba L. The static stability of the said trees was assessed using a combination of the following three methods: i) adapted VTA (Visual Tree Assessment) method; ii) SIA (Static Integrated Assessment) method; iii) resistograph measurement using IML-Rezi F-300.The height of each treewasmeasured using the Vertex device, while their diameters were measured using a measuring band. The trees were examined for external symptoms indicating their vitality. The crowns were observed using the Pentax XCF (12x50) binoculars. Afterwards, based on the basic data on the trees, the minimum thickness of sound wood (SW) necessary to maintain the static safety was determined using a computer program. The obtained data were then used to determine the manner in which and the place where the diagnostic instruments shall be used. Results and Conclusions: Based on the knowledge of the subject trees and themeasurement of their diameters and heights, SW, as well as the basic safety of trees was determined using the SIA method which was 140% to 410%. For all selected trees, the required safety was 100%, so SWminimum requirement for intact wood ranged from4.4 cmfor themaple to 9.4 for the oak. The resistograph measurements of the 7 assessed trees established that four complied with the requirements, while the American ash, the white willow and the sessile oak were decided for cutting down. The decision was made based on the median measured values, which were below the threshold determined by the SIA method. The sessile oak demonstrated the highest non-compliance, since it scored only 37.0% of the value required for intact wood. The willow scored 48.7%, while the ash scored 62.5%. The results of the resistograph measurement were compared to the r/R ratio. Those results show that there was no difference between the two methods, although the weak point of r/R ratio could be clearl