Failure of forks in clonal varieties of Platanus x acerifolia

Established plantings of clonal London Planes (Platanus x acerifolia (Aiton) Willd.) in Bristol city centre have suffered such a high proportion of failures at their forks and branch junctions that many semi-mature trees have been removed on the grounds of safety. This issue of "Problem Planes" has been noted in arboricultural literature (Tubby & Rose, 2008), but the phenomenon has up until now not been subject to rigorous scientific investigation. Young plane forks harvested from a modern problematic clonal type and from traditional non-problematic clonal stock were compared in relation to the size of their growth increments, wood density, load-bearing capacity and load-bearing capacity of the smaller branch arising from the bifurcation. Based on two-dimensional (2D) images taken of each fork, finite element analysis (FEA) software was used to estimate the relative stress concentration levels of the harvested forks if the two branches arising from the bifurcation were bent apart. The stems of the modern problematic clones situated just below the junctions tested were found to be growing 79% faster than the traditional non-problematic clones by analysis of transverse growth increments of the test samples collected. However, wood density was not found to be significantly different between these clonal types. Forks of modern problematic clones which were each subjected to an in-plane static tensile test had only 50.5% of the bending strength of their smaller arising branches which were subjected to a three-point bending test, whereas the forks of traditional non-problematic clones were 68.6% as strong as their smaller branches. Finite element analysis predicted that the shapes of the forks formed on the problematic clones would lead to around 16% higher stress concentrations at their fork apices when compared with the shapes of the junctions found on the non-problematic clone samples. From this evidence, the authors find that at least part of the explanation as to why the junctions of these modern problematic clones of Platanus x acerifolia are failing is that they develop more "V-shaped" junctions that lead to greater stress concentrations on the inside of the fork when the branches arising from the bifurcation sway apart in windy conditions. Selection of planting stock of London Plane by arboriculturists should include an assessment of the shape of their branch junctions and forks, to avoid perpetuation of this problem. © 2012 Taylor & Francis and Aboricultural Association.