An assessment of tree susceptibility and resistance to cyclones A study based on Severe Tropical Cyclone Yasi 2nd February 2011 Proudly sponsored by Ergon Energy and Townsville City Council our vision A healthy, diverse and productive environment treasured by the whole community. our mission To engage the community in vegetation management to protect and restore the health, diversity and productivity of our unique Australian landscapes. Greening Australia Greg Calvert Senior Environmental Scientist T: 07 3902 4444 F: 07 3902 4422 Greening Australia (QLD) 333 Bennetts Road Norman Park, Qld 4170 W www.greeningaustralia.org.au W www.breatheeasynow.com.au Cyclone Yasi Tree Report - 2011 Document history and status Rev Issued to Author Initial and Date Description 1 GC 09/06/2011 First Draft David Follington DF 14/06/2011 Internal review David Wood DW 14/06/2011 Client review Greg Calvert GC 27/06/2011 Incorporate changes 2 GC 07/07/2011 Final Document details Author: Dr Greg Calvert Project Manager: Greg Calvert Name of Client : Townsville City Council and Ergon Energy Name of Project: Cyclone Yasi Tree Assessment An assessment of tree susceptibility and resistance to cyclones – with particular Title of Document: reference to Severe Tropical Cyclone Yasi in Townsville on 2nd February 2011 Document Version: 2 © COPYRIGHT: The concepts and information contained in this document are the property of Greening Australia Disclaimer/Commercial in Confidence This document has been prepared by and remains the property of Greening Australia (Queensland). It may only be used for the purposes for which it was commissioned and in accordance with the Terms of Engagement or contract. Unauthorised use of this document in any form is prohibited. The author assumes no responsibility for errors, omissions or contrary interpretation of the subject matter herein. The author as- sumes no responsibility or liability whatsoever on the behalf of any reader of these materials and expressly does not guarantee any outcomes you may or may not get as a result of the information provided. This document may also contain information which is considered commercial-in-confidence. You agree to hold any such information in confi- dence and will not disclose to any third party other than for the purposes directly related to the Terms of Engagement or contract under which Contents Executive Summary 6 Acknowledgements 9 Introduction 11 Aim of the Project 11 Tropical Cyclones 13 Defining tropical cyclone categories 13 Historical Impacts on Townsville by Tropical Cyclones 22 Severe Tropical Cyclone Yasi 26 Cyclones and Trees 31 Damage to Townsville and other tropical cities by Tree impact 31 How do trees protect life and property? 32 How do cyclones damage trees 38 Methodology 50 Limitations 50 Species Contribution to damage and green waste 50 Species contribution to power failure 51 Proportions of individual tree species 51 Tree species response to different cyclone categories 52 Results and Discussion 54 Species Contribution to damage and green waste 54 Species contribution to power failure 60 Proportions of individual tree species 64 Impacts on beach front communities 80 Characteristics of tree damage by different cyclone categories 87 Identifying traits of susceptible or resistant trees 89 Assessing Cyclone Resistance 95 Recommendations for Risk Mitigation 98 Potential for use of tree cyclone ratings in planning schemes 98 References 103 Appendix A: Raw data for observed impacts of Cyclone Yasi on trees in Townsville 108 Appendix B: Tree species considered likely to be resistant to impacts by tropical cyclones 114 Appendix C: Tree species considered likely to be sensitive to impacts by tropical cyclones 120 Appendix D: Cyclone damage behaviour of some commonly grown trees 124 4 Cyclone Yasi Tree Report - 2011 this document was commissioned. executive summary 5 Executive Summary benefits for buildings. Flying debris is one of the most significant causes of death and property damage during cyclones and the role of trees in Townsville has a continuous history of cyclonic immobilising this material is widely recognised. events, varying considerably in frequency and the The removal of trees from around buildings interval between events. On average, the 100km can leave them exposed to cyclone damage, long Townsville coastline can expect to get 8-10 and increase the likelihood of debris from that cyclone crossings per decade. Since Townsville’s structure damaging other structures further first recorded cyclone in 1867, the city itself downwind. Careful species selection is necessary has experienced at least 18 cyclonic events. to ensure that vegetation captures rather than The most severe cyclone in living memory contributes to flying debris. was Category 3 Cyclone Althea in December 1971, and there are no records available to Trees have been shown to vary significantly in suggest a stronger cyclone has hit Townsville their response to cyclones, and this study aims since European settlement. Considerable to evaluate the relative responses of different property damage and loss of life has occurred in species to such events. While loss of foliage and Townsville during past cyclones, primarily due to branches can be nearly 100% during severe the effects of storm surge and flooding. category cyclones, the failure of the trunk and roots is of greatest significance. Susceptibility to The landfall of Category 4 Cyclone Yasi on trunk snapping varies between species due to Mission Beach in February 2011 had significant differences in wind resistance, trunk flexibility, impacts on Townsville, as the exceptionally wood density, crown symmetry and the presence large diameter of the cyclone resulted in of hollows, often caused by termites. The Townsville experiencing winds equivalent to a damage itself can arise from tension (pulling), Category 2 event and an accompanying storm compression or torsion (twisting). An ‘adaptive surge. Unconfirmed reports suggested that growth hypothesis’ has been developed to approximately 65,000 trees were blown over in explain how trees adapt to the wind loading in Townsville, causing widespread power failure their environment, so planting advanced trees and blockage of roads. Following the cyclone, a or staking trees may deny them the opportunity total of 450,000 cubic metres of green waste was to develop sufficient strengthening. Most tree collected, in addition to the 30,000 loads of green species sit on a gradient or spectrum between waste deposited by residents. ‘resistance’ (ability to withstand disturbance) and ‘resilience’ (ability to recover from disturbance), Despite the damage caused by cyclone-affected and this is strongly correlated with wood density trees, trees can have significant benefits during and growth rates. Natural ecosystems tend cyclones. The benefits may include reducing to have a combination of the two strategies wind loading on buildings, intercepting potentially to maximise the options for recovery from lethal flying debris, offering protection to other disturbance. plants, reducing erosion along flooded rivers or storm-damaged beach fronts, and potentially Uprooting (wind throw) is a result of failure of the even preventing the loss of rooves from buildings. root system. The root / shoot ratio for some trees Windbreaks are particularly valuable in absorbing decreases with maturity, so taller trees become the energy from wind gusts as the force of wind top heavy and more prone to wind throw. Roots increases as a cube of its velocity, so even minor may have reduced depth due to either intrinsic or reductions in wind speed can have significant 6 Cyclone Yasi Tree Report - 2011 environmental factors and the resulting shallow . weeping fig (Ficus benjamina); and roots will make them susceptible to waterlogging. pink trumpet tree (Tabebuia impetiginosa * The physical extent of the root system is of great (syn. T. palmeri)) importance in preventing windthrow. In urban areas, trees may be increasingly susceptible to All size classes of trees suffered damage, though windthrow. Trees may have compromised root the greatest numbers of damaged individuals systems at the time of planting, be planted into an were large trees but the relationship between impenetrable substrate or develop shallow roots tree size and cyclone damage within a particular in response to shallow watering regimes. Planting species cannot be established from this data. trees close together provides them advantages While exotic species (not native to Queensland) from mutual support, but increases their chances contributed more than twice as much green waste of being damaged from adjacent falling trees and as non native trees, there are both resistant and debris. susceptible species amongst both native and exotic trees. Following Cyclone Yasi, Greening Australia collected tree damage data between Ayr and Despite the ‘Plant Smart’ Ergon Energy Mission Beach to capture the effects of different partnership with Greening Australia to assist in wind speeds, though the majority of data was vegetation management around powerlines, a collected in Townsville. The primary questions survey of power failures in Mundingburra and examined were to identify which species Aitkenvale showed 95.45% were caused by contributed the most to vegetation damage in vegetation, with 42.85% caused by yellow flame Townsville, which species contributed the most trees. While the Plant Smart program has aimed to cases of power failure, which trees suffered to reduce the height of plants grown