Duncan Slater

Interlocking wood grain patterns provide improved wood strength properties in forks of hazel (Corylus avellana L.)

Xylem found in the stems of woody plants has contrasting strength in different planes due to its anisotropy. At the apex of the junctions of woody plant branches, sufficient wood strength is needed to prevent the arising branches splitting apart. The main aim of this research was to address an issue so far overlooked, which is the contribution of wood grain orientation and patterns at this location (the apex of such junctions) to supplying the required strength. In this study, the wood grain patterns of xylem produced at the apex of junctions of hazel (Corylus avellana L.) were investigated. The mechanical properties were determined using compression and tensile tests of excised wood samples with an Instron® Universal Testing Machine. Sample strength was contrasted with the strength of xylem produced at the side of the junctions and in the adjacent lower stems. Wood formed at the top of junctions in the hazel exhibited more tortuous wood grain patterns and had twice the radial and tangential tensile strength when compared with adjacent wood formed in the stem. It also had significantly higher radial and tangential compression strength. Density of xylem at the apex of hazel junctions was 13.4% greater than in the adjacent stem. The authors conclude that tortuous wood grain patterns supply additional strength to junctions in hazel trees. This mechanical arrangement should inform anatomical studies of junctions and may inform the design of manufactured Y-shaped components made from fibrous composite materials.

An assessment of the movement behaviour of bifurcations in hazel (Corylus avellana L.) under dynamic wind loading using triaxial accelerometers

Accelerometers are potentially a valuable tool in analysing the movement behaviour of trees and their component parts. Most previous research using similar tools has assessed the stability of tree stems and roots under wind loading, but accelerometers can assess a wider range of component parts of trees, associated defects and growth forms. In this study, the movement of sixteen bifurcations in semi-mature hazel trees (Corylus avellana L.) was assessed using triaxial accelerometers during seven differing wind loading events. Seven of the bifurcations were normally formed, with the other nine bifurcations containing bark inclusions, of which a subset of five were restricted by being in direct contact with neighbouring branches. Analysis of the acceleration data from the three windiest days showed that synchronised movement of the pairs of branches arising from the normally formed bifurcations reduced the potential acceleration of one branch away from the other by an average of 57.72%, whereas this reduction was found to be only 40.38% for the bark-included bifurcations. The data collected from this study indicate that normally formed bifurcations are not necessarily structural flaws in trees as the synchronised movement of the two branches of a bifurcation means they typically avoid damaging stresses occurring at the join under dynamic wind loading: the presence of rubbing or touching branches may, however, give rise to abnormal movement behaviour and heightened stresses.

An argument against the axiom of uniform stress being applicable to trees

The axiom of uniform stress is based on the theory that trees carry out their secondary growth in such a way that bending stresses are averaged out over the outer surfaces of the tree and the tree’s resulting aerial form avoids stress concentrations and repairs “notch stresses”. However, this can be shown, by combining a series of logical propositions, to be an errant or incomplete concept of how tree form develops, even in the limited scope of local stress levels that component parts of trees experience due to gravitational or dynamic wind loading. This paper presents an argument against the veracity of this axiom and highlights the extent of contradictory scientific work on the adaptive responses of trees to strain, their dynamic movement in the wind and the habit of some species to fail in a graduated way. It is concluded that a better model of the biomechanical behaviour of trees can be achieved through a synthesis of existing scientific models.

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.

Natural bracing in trees: management recommendations

Abstract The assessment of risks from tree failure through surveying techniques involves making informed judgements on the structural integrity of major component parts of a tree. A tree fork or branch junction is often the focus of attention for risk assessment if that junction has bark included in the join, as such junctions are well known for being weak components in the crowns of trees. Further to the uncovering of a high level of association between the presence of naturally occurring bracing structures in trees and the formation of weak forks set below those structures, this second paper on natural bracing in trees defines the nine types of natural brace found in broadleaves, providing notes on their physical appearance and their effects on the trees’ mechanical performance. By combining data from a series of experiments, an assessment method for junction strength in trees is provided, which can then be modified to take into account the presence of natural braces. This method allows for the prioritisation of risk management actions for those bark-included junctions that are more likely to fail.

The association between natural braces and the development of bark-included junctions in trees

Abstract Bark-included junctions are a malformation that occur in a wide range of tree species. Although the formation of an acute internal angle at a branch junction has been suggested as the primary cause of bark-included junctions in trees, it is logical to assume that a weak structure of this kind may develop under a much lower loading regime than that of a normally formed junction. One factor that would affect the mechanical loading of a junction is the presence of crossing or touching limbs that are situated above such a junction and that act as a ‘natural brace’ that straddles the join, thus restricting the natural movement of the junction and its associated limbs. Survey and analysis of data from a cohort of broadleaved trees identified a very strong association (93.9%) between bark-included junctions that exhibited no discernible bulging (n = 99) and the presence of one or more natural brace above those junctions. Additionally, the bark-included junctions that exhibited a major bulge (n = 30) were significantly associated with the absence of natural braces (93.3%). This finding has several important ramifications for professional tree surgery operations and tree surveying techniques, which will be the subject of a companion paper.

Fracture properties of green wood formed within the forks of hazel (Corylus avellana L.)

Key messageCentral apex of bifurcations has higher specific fracture energy in TR fracture system than that of four sampling locations. This could be due to higher density and interlocked grain formation.AbstractForks are one of the important biomechanical structures in trees because of their potential vulnerability to splitting. Many researchers have investigated the strength and stiffness properties of tree forks, but very little is known about the toughening mechanism within tree forks. In this study, the specific fracture energy (Gf, Jm−2) of forks of hazel (Corylus avellana L.) was investigated in the RT (Radial-Tangential) and TR (Tangential-Radial) fracture systems using double-edge-notched tensile tests. Sample Gf values were measured at between the central apex of bifurcations, at the side apices of bifurcations, in the parent stems and in the two branches of forks. The fracture surfaces were analysed by Scanning Electron Microscopy (SEM), and the wood density was determined. Gf was found to be considerably greater at the central apex of a bifurcation than in other sampling locations. Surprisingly, Gf of TR was greater than Gf of RT at the central apex, while the other four locations showed greater Gf values in their RT fracture systems. The density of the central apex of bifurcations was found to be around 22% greater than elsewhere. In addition, it was shown that there was a more tortuous and interlocked wood grain formation at the central apex of bifurcations. The combination of higher density and tortuous grain structure provides reinforcement at the central apex.

Supporting failure? Damage inflicted to establishing trees in London by a range of tree support and protection systems

ABSTRACT Urban foresters employ a diverse range of different tree support and protection systems (TSPS) to help trees establish in the urban environment; however, in some cases, TSPS can cause damage to their host trees. A survey was undertaken of 762 establishing trees across 14 London boroughs to determine which types of TSPS are in current use, to assess how different systems perform and to record the frequency and severity of damage to trees associated with these different types and configurations of TSPS. We found that TSPS-associated damage was highly prevalent within the sampled population, affecting 34.8% of the trees surveyed. Failure to remove TSPS components after the establishment phase was responsible for more than 80% of the cases of severe damage to trees found by this survey. A comparison between the performance of several TSPS configurations in terms of their effectiveness and association with different TSPS damage types identified that the “two posts and ties” system performed consistently well in comparison with all other TSPS assessed. Other damage related to the use of protective structures and rubber spacers. We conclude that simple changes to current tree planting practice would minimise the impact of TSPS-associated damage in our urban forests.

Attitudes and approaches in London boroughs to the use of photographic records in the justification, management and enforcement of tree preservation orders

Abstract English law allows a local planning authority (LPA) to impose legal protection for special trees through the making of a tree preservation order (TPO). Against the penalty of criminal prosecution, these orders preserve the appearance of trees for the benefit of public amenity. Current literature and guidance notes disagree on the significance of photographs in the management of protected trees. This study investigated whether there was any variation in approach to the management of photographic records of protected trees by LPAs responsible for the administration of TPOs in 33 London boroughs. It was found that no responding boroughs adopt a formal procedure for the management of photographic records. Analysis of responses (n = 21) indicated that 10 boroughs were photophobic, with their attitude to digital recording technology adversely influencing their working practices. This contrasts with the finding that most officers saw considerable benefit in developing and using a photographic archive of protected trees. It is concluded that current pressure on local authority resources is an obstacle to the capture of such a photographic archive of protected trees, and this, in turn, highlights an opportunity for the creation of a best practice model through tree officer group and governing body cooperation.

The impact of three periods of housing development upon the urban forest of Shrewsbury, England

Abstract A significant body of recent research has identified that trees in the urban environment provide considerable benefits to society, both providing desirable places to live and helping to reduce the impact of the urban population on the wider environment. If we are to continue to benefit from the urban forest in the future, a knowledge of the existing resource, its condition and future viability is a fundamental requirement. The design and creation of newly built development present one of the biggest threats to and greatest opportunities for urban forests but have not been analysed in depth. An assessment using i-tree ECO recorded the extent, nature and quality of the urban forest resource in residential areas of the town of Shrewsbury. Comparisons were then made between three separate study areas within the town based upon the period in which they were primarily constructed. The pre-1950 residential housing areas had significantly greater canopy cover (17.8%) when compared with 1951–1985 housing (13.1%) and post-1985 housing areas (9.9%). Analysis of tree age, size and ownership distributions identified that larger growing garden trees in the earlier housing schemes were the major contributor to this difference in canopy cover between areas. For this case study, we can conclude there is a negative trend in urban canopy cover that relates to the size of garden space allocated to more modern residential properties, which is not sufficiently compensated for by tree planting in adjacent public areas.