Windthrown Trees: Storms or Management? (#AREA 04)
Images of windthrown trees make for dramatic news coverage. The implied message in most coverage is that strong winds and heavy rain are the cause of the tree failure. However, is the storm the only cause of the tree falling? Many other, and often bigger, trees did not fall. This paper reviews some of the literature relating to windthrow of trees. The size and characteristics of tree canopies have a profound influence of the forces that winds exert on tree trunks and roots systems while the characteristics of tree root systems often determine whether trees fail during storms. The results of site inspections suggest that there may be other factors, such as the past history of the tree and the management practices to which it has been exposed, that may contribute to its failure during a storm.
Trees withstand physical loads from gravity and persistent winds throughout their lives, but for most trees the greatest loads that they will experience will come from occasional and sporadic wind gusts. Above the ground, forest trees tend to have a similar shape consisting of a straight central columnar trunk with little side branching until there is a tuft of foliage and branches at the apex. Trees growing on the edges of forests and plantations and in urban areas develop large numbers of often large side branches and tend to have greater trunk diameters, which makes them more stable than typical forest trees.
Analysis of windthrown trees has shown tree size to be a significant variable. Wind is not a static force nor is a trees response to gusts of wind but to simplify analysis wind loading was often considered to be a static force. The forces applied to the trees depended on factors, such as wind speed, upwind conditions, and tree characteristics such as size, shape and mass. Dynamic loads can be defined simply as time-varying and may vary with magnitude, direction and/or position with time. The responses of tree structures vary with time and their leafy canopies are flexible and the surfaces realign themselves in high winds by reconfiguring their shape and reducing the total canopy area. Mass damping is a dynamic parameter which estimates how much energy is absorbed or transferred. The mass damping capacity of foliage and branches during storm events is significant which raises questions about the validity of the view that mature and bigger trees are more likely to fail and suggests that the failure of senescent trees may have more to do with root systems than sail area (Coder 2010).
It is clear that all components of a tree’s root system contribute to its stability, but the two major components of the root system contributing to anchorage are the resistance of leeward roots to bending (25%) and the resistance of taproots and descending roots to uprooting (75%). Trees growing subject to directional winds tend to have windward roots that are smaller in diameter but longer and more branched at greater distances from the trunk than leeward roots. On the leeward side, the roots are shorter, thicker and tend to have more descending roots. The root plate is likely to be elliptical and skewed to the windward side of the trunk. During a windthrow event, the leeward lateral roots bend and eventually break often close to their base near the root crown, the windward lateral roots are pulled from the soil, often with their descending roots, if present, intact and the tap root or one, or more, of the larger descending roots closest to the centre of the tree trunk rotate
Site inspections of 80 windthrown trees from eight different genera were conducted over a period of 20 years. They revealed that damage to exposed lateral roots (87.5%), loss of descending roots (88.8%) and evidence of soil compaction at the base of the tree (65%) were often coincident with windthrow. Evidence of trenching near the trunk of the tree (58.8%) and waterlogging of the soil around the base of the tree (56.3%) were also common correlates. The literature surveyed and the results not only suggest where aspects of urban tree management might be improved, but may also prove helpful to arborists in assessing tree hazard related to possible windthrow. Inspection protocol criteria should include damaged or decayed lateral roots, the loss of descending roots, evidence of site or trenching works close to the trunk and whether trees are growing in compacted and waterlogged soil.
Management practices have a profound influence on the health of trees, growing under environmental stress. Trees that are growing in ideal locations often remain healthy and vigorous as they age and are capable of dealing with many of the pests, diseases and stresses that might otherwise affect them. Trees in urban areas that have had their root systems interfered with are more likely to be stressed and prematurely senescent. They are also more likely to suffer windthrow. Even if the canopies of tree appear to be healthy, root systems may be stressed and compromised. If roots are severed on the prevailing windward side of the tree then the risk of windthrow is heightened. In many instances, older urban trees will have been subjected to major root damage from construction, road and infrastructure works.