Analysis of the interrelationship between houses, trees and damage in a cyclone affected city

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International Journal of Disaster Risk Reduction

journal homepage:www.elsevier.com/locate/ijdrr

Analysis of the interrelationship between houses, trees and damage in a cyclone a ﬀ ected city: Can landscape design and planning utilising trees minimise cyclone impact?

F.J. Van der Sommen

^a

, D.M. Pearson

^a,⁎,1

, G.S. Boggs

^a,2

aResearch Institute for Environment & Livelihoods, School of Environment, Faculty of Engineering, Health, Science and the Environment, Charles Darwin University, Darwin, Northern Territory 0909, Australia

A R T I C L E I N F O

Keywords:

Arboreal services Risk mitigation Disaster management Landscape ecology Climate change Green infrastructure

A B S T R A C T

Under scenarios of climate change the likelihood of more intensive extreme weather events like tropical cyclones is expected to increase and many tropical regions most at risk from cyclones are still developing economically.

With increased urbanisation predicted over the next 20–50 years to cope with population growth, it is important that planning for urban development in these regions considers amelioration of danger, especially the impacts associated with cyclone damage. Approaches to risk management can learn a lot from past experiences with cyclonic events. The knowledge that was accumulated after the devastation of Darwin, Australia by Cyclone Tracy in 1974 provides important evidence that can contribute towards risk mitigation and disaster management in the future. Applying a mixed methods approach, this study examines historical information collected at the time of Cyclone Tracy to help understand the role of the urban forest and positioning of housing in reducing cyclone damage. It includes a review of whether the pattern of tree cover, which is influenced by geophysical and socio-cultural factors, mitigates or exacerbates cyclone damage. The results of the study show that although the relationship is complex, trees appear to have a role to play in ameliorating cyclone damage under certain conditions. This potential gain, along with the other benefits trees offer to tropical urban areas, means that trees are an important consideration for future urban planning in developing regions.

1. Introduction

Climate change is thought to be one of the most signiﬁcant challenges facing the world and in particular urban centres, in the 21st Century[1]. Under scenarios of climate change the likelihood of more intense extreme weather events like tropical cyclones is expected to increase[21,27,69]. Cyclones are storm systems that are characterised by a low pressure centre, thunderstorms, very strong winds and tor- rential rain that can bring devastating consequences for urban centres.

Tropical cyclones form over tropical or sub-tropical seas between 5 and 30 degrees latitude[44], a region characterised by developing econo- mies, with some of the most vulnerable and disadvantaged people living in these areas. Steiner[60]. It is not possible to stop cyclones occurring but measures can be taken to lessen the severity of impacts, reducing possible loss of life and damage to infrastructure that can cause issues for socio-economic functioning after an extreme event (Hoque et al. 2016). It is important that population growth in the next

20–50 years and associated urban construction activity in developing tropical regions is carried out in the context of planning for urban development that considers amelioration of cyclone impacts, strategies to minimise the risks associated with cyclone damage and possible ways of reducing the exposure of people to these potentially deadly natural hazards.

When constructing urban areas certain physical features can help to reduce damage to buildings from natural hazards and protect lives.

Action can also be taken to reduce urban vulnerability and improve resilience to extreme events. This can include the construction of more wind resistant homes and buildings and the inclusion of features like shutters which can act as shock absorbers against debris and strong winds[12]. Proactive human behaviour can also increase the level of preparedness to face these extreme events which can have the knock on eﬀect of reducing personal vulnerability. Insight to risk management solutions can also be gained from past experiences with cyclonic events and learnings from the patterns of previous damage[60].

https://doi.org/10.1016/j.ijdrr.2018.01.031

Received 17 October 2017; Received in revised form 24 January 2018; Accepted 27 January 2018

⁎Corresponding author.

1Present address: Institute of Agriculture and Environment, College of Sciences, Massey University, Palmerston North, 4442, New Zealand.

2Present address: Western Australian Biodiversity Science Institute. University of Western Australia, Perth, 6401, Western Australia.

E-mail addresses:[email protected](D.M. Pearson),[email protected](G.S. Boggs).

Please cite this article as: Van der Sommen, F.J., International Journal of Disaster Risk Reduction (2018), https://doi.org/10.1016/j.ijdrr.2018.01.031

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Planning to reduce the impacts of cyclones is of particular concern to regions like monsoonal northern Australia that experience cyclones each year and fast developing cities like Darwin, in the Northern Territory, that are located in this zone and that have already borne witness to the extreme devastation that cyclones can cause. The knowledge that was accumulated after the devastation of Darwin by Cyclone Tracy in 1974 has the potential to provide important scientiﬁc and anecdotal evidence that can contribute towards risk mitigation and disaster management in the future. This paper presents theﬁndings of a study that examined historical information collected at the time of Cyclone Tracy, and ecological studies looking at the impact of sub- sequent severe cyclones that have hit northern Australia, to help understand the role of trees and positioning of housing, relative to landscape characteristics, in reducing cyclone damage.

Trees under cyclonic conditions have the potential to be both a source of, and a buffer for,flying debris impacting on houses. Tropical Cyclone Tracy, which struck Darwin city on Christmas Eve in 1974, can provide a valuable insight into the impact of cyclones on both trees and houses and the relationship between the two. From reviewing available historical information, it is evident that arboreal landscape design, planning and management in tropical landscapes has the potential to ameliorate the impact of cyclone damage providing that the trees planted can cope with the stresses imposed and do not contribute to the hazard. The science of integrating and managing trees for ecological or arboreal services, particularly, in urban landscapes, is a growingfield of research with particular interest associated with climate change adaptation and mitigation ([43]; Mellaney et al. 2015). However, to date, most cyclone research in urban landscapes has been reductionist in approach, focusing on the role of structural and material attributes of houses in relation to cyclone damage ([24,55]; Pielke and Landsea 1998), with the work that has focused on trees and cyclones mainly examining their response from a debris source perspective [64,67].

Little research to date has taken the more holistic approach needed to examine complex interacting factors and the possible positive role played by trees on urban landscapes during cyclones. In contrast, in non-urban environments, but in similar bioclimatic regions, there has been more in-depth research into the response of trees to cyclones and role of trees in the wider natural environment[14,57,68,69]. By taking a more holistic spatial landscape ecological approach to the analysis of tree response in urban areas under cyclonic situations it is possible to develop a greater understanding of the ecology of cyclone impacts and eﬀects and knowledge acquired that is more relevant to urban landscape planning for both houses and trees. This is particularly important given the growing value placed on incorporating urban green infrastructure into urban design and planning[49].

In this paper the authors examine the influence of geophysical and socio-cultural factors on tree and housing characteristics using a spatial landscape analysis. Presented are findings from the case study using Cyclone Tracy that demonstrate the complex nature of the relationships between these factors. This research highlights the need for greater understanding of the relationship between trees and urban environments and provides justification for further in depth analysis into the role of trees in mitigating or exacerbating cyclone caused housing damage. This paper provides a valuable contribution to the discussion about sustainable urban design for growing cities in tropical regions in the face of climate change.

1.1. Background

It is expected that by 2030 61% of the world's population will live in urban areas [71]. As development intensiﬁes and human use of land grows, there is inevitably greater competition for land and as a result there is more of a need for good landscape and urban design and planning[17]. Eﬀective planning and design needs to take into account biological, social and physical components of the landscape and make strong links to landscape architecture. Sustainable urban planning

needs to pay particular attention to the eﬀect urban development has on environmental processes and be aware of important feedback me- chanisms between the urban and more natural elements of the environment. A popular opinion of a sustainable city is one that has buildings and infrastructure that are built to conserve energy. Of particular concern to urban developments in areas aﬀected by cyclones and other extreme climatic events and natural hazards is how to make a city sustainable in the sense of being able to sustain and withstand the impact of these events, particularly if they become more frequent or intense.

It is known that urban development changes local climatology[71].

In fact, urbanisation has been noted as something that alters the local environment, by causing an increase in air temperature and altering water cycles, and by aﬀecting ecological processes and landscape functioning[2]. Urbanisation also changes the structure of landscapes so that patterning of vegetation as well as natural processes are altered.

This means that urbanisation causes the landscape to become‘compo- sitionally more heterogeneous’, ‘geometrically more complex’, and

‘ecologically more fragmented’ ([2]pg 1;[74]). The implications of what these changes mean under situations associated with extreme climatic events and natural disasters and how appropriate landscape design can ameliorate negative impacts is something that needs to be better understood.

Due to the potential catastrophic impact of cyclones there is a recognised need to consider the vulnerability of urban areas to these events especially given the severe impact they can have on the socio- economic systems that operate within urban environments[29]. Vul- nerability can be described as being‘exposure to an event’,‘sensitivity of a system to an event’, and the‘system's ability to adapt to an event’ [29]. The key to vulnerability is exposure, sensitivity, and adaptive capacity. In order to assess vulnerability one needs to understand the systems exposure to a hazard, as well as its sensitivity and resilience to the hazard. An important part of human vulnerability to global environmental change is associated with the danger of extreme climatic events [29]. Climate change adaptation strategies need to be in- corporated into urban and land use planning and actions taken to help empower the people that are the most at risk from extreme climatic events by getting them to participate in activities and strategies that will increase resilience and reduce vulnerability[21]. In areas likely to be aﬀected by extreme climatic events as a regular occurrence, urban design needs to consider the best way to minimise the impact of these events on infrastructure and human wellbeing[66]. This can be done by reducing exposure, decreasing the sensitivity of the landscape or improving the urban environments adaptive capacity and resilience to extreme climatic events.

The physical benefits of trees in urban areas is now well recognised [3,34,38,43,48,66,75]. In urban areas trees can play an important role in evaporative cooling, reducing heat island effects, cooling buildings, providing shade, reducing surface run offfollowing rainfall, reducing noise, reducing particulate pollution, sequesting carbon, reducing crime, and increasing property prices (Royet al.2012;[23]; Boland and Hunhammer 1999). There is also evidence to suggest that trees in urban areas make economic, environmental, social, cultural and spiritual contributions to the wellbeing of people[34]. Also, it has been noted that different types of trees have different impacts on the urban environment and the ecosystem services they provide is often species specific[34]. This means that trees can be seen to be economically, socially and culturally important components of urban areas [35].

Urban greenways have also been seen to have multiple functions, helping with urban sustainability and counteracting fragmentation of habitat [65]. Current priorities based on sustainable urban development are making urban planners pay more attention to the important role trees can play in urban environments [3,34,38,43,48,71,75].

However, trees in disaster prone urban areas, including high cyclone and bushﬁre risk areas, are often considered to exacerbate this risk which results in active removal campaigns undertaken post disaster

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[32]. Recent research by Roy [54] has highlighted the inﬂuence of personal opinions on perceived threats to vital infrastructure posed by street trees in municipal areas and cite an absence of research to bal- ance views and opinions. The subjective nature of current thinking and available information highlights the need to more scientiﬁcally understand and document the relationship between trees, tree cover and damage from cyclones to support evidence based decision making.

Cyclones, which can also go by the common name of typhoons and hurricanes depending on where they occur geographically, are categorized based on their strength and can range in intensity from what are classed as weak systems, with wind speeds of about 160 km per hour, to the extremely destructive classification which can have winds in excess of 350 km per hour[69]. Category 1 are the weakest cyclones and category 5 are the strongest. In this paper the authors hypothesize that certain species of trees growing in specific locations might be able to mitigate some of the extreme damage caused by cyclonic winds. In doing so the authors highlight the important connections between landscape pattern in the form of tree distribution in relation to house location, environmental functioning and urban design, and the need to consider these with regard to any development of tropical cities likely to be affected by cyclones. As such the authors suggest taking a landscape ecological approach to urban planning and design that considers pattern and function. A holistic approach such as this that considers complex interactions and interrelationships has the potential to better inform urban planning decisions in tropical areas affected by cyclonic events. This could be particularly useful in guiding planning and policy making decisions necessary to cope with the implications society will face from the growing need for more extensive urban development in areas prone to cyclone risk, whilst trying to minimise vulnerability of the urban areas to cyclones. A key consideration in this paper is the general view that tropical cyclone intensity will increase under scenarios of climate change meaning that newly developing urban areas such as the proposed expansion planned for urban Darwin[18]need to be designed to withstand the onslaught of what could be possibly stronger winds than previously experienced.

1.2. Study area

With a predominantly coastal based population and an already harsh climate and high incidence of natural disasters and extreme climatic events, climate change adaptation and disaster risk reduction are major considerations for Australia[21]. Tropical cyclones are a regular feature of northern Australia. Cyclones can occur in northern Australia any time between November and May. In this locality they have a mean rate of occurrence of 9.4 per year[57]. This study focuses on Darwin, which is the capital city of the Northern Territory and is located at a geographical location of 12°28'S 130°50′E (seeFig. 1a). Its climate is tropical with a distinct wet and dry season. The wet season falls from October to April and mean annual rainfall is in the order of 1730 mm/

year[8]. Climatically Darwin is more closely associated with its south Asian neighbouring capitals than Sydney. Its location makes it the most northerly of Australian capital cities. It is also currently the smallest but growth in population is one of the highest in Australia.

The city has been rebuilt twice due to the bombing of Darwin in February 1942 from Japanese air raids and Cyclone Tracy in 1974. The city has recently been going through an economic boom and development is high on the agenda. Post war suburban development of the Darwin landscape up to 1974 saw a progressive change of tree cover from naturally evolved eucalypt dominated savanna woodland, open forest and monsoon rainforest to fragmentation of trees in these communities and cultivation of introduced trees in parks and gardens.

These form a complex mosaic with buildings and associated infrastructure.

The geophysical environment of the land systems and land units on which Darwin is built, identiﬁable by their land forms, soils and natural vegetation, have been described elsewhere

Wood et al.,[20,72]and serve as an indicator of the spatial variability of environmental constraints imposed on land use for both construction and tree cultivation. Trees in these landscape have to deal with inherently infertile soil, highly seasonal rainfall and soil moisture, high temperature associated evapotranspiration rates, damage fromﬁre and more relevant to this study direct wind destruction. In urbanised landscapes additional cultural processes of soil scalping, vehicle com- paction, top soiling, fertilization, mulching and irrigation, construction with associated building microclimate has create a complex“anthropic”

landscape ecology.

Cyclone Tracy, an estimated category 4 cyclone, crossed directly over suburban Darwin, on 24th December 1974 (Fig. 1b). Much of the city was encompassed by the eye wall zone of the cyclone[40,6,7,70].

Cyclone Tracy is a prime example of the tenuous relationship between the scale of a hazard and its social and environmental consequences, as although comparatively small in size, with a variablealbeitsmall core size and shape, it had a devastating impact on Darwin suburban houses, trees, and people, causing 71 deaths and many injuries, mainly from ﬂying debris. It was also responsible for causing approximately $800 (AUD 1974) million in damage with more than 70% of Darwin's buildings (about 80% of houses) destroyed. Over 80% of the population of Darwin at the time became homeless and about 60% were evacuated.

The social, cultural and psychological impact of this cyclone is well documented [13,42,62,63]. Details of the eﬀects of the cyclone and associated debris on buildings has been described in an extensive report Fig. 1.a) Map showing the geographic location of Darwin, Northern Territory, Australia and b) the distribution of housing allotments (red) during the time of TC Tracy (1974) relative to Darwin in 1999 estimated cyclone centre line (CCL), eyewall boundary and coastline datum.

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[70]and recorded in a unpublished survey undertaken for the Darwin Reconstruction Commission (DRC) between 1975 and 1977 and held in the National Archives of Australia.

More recent severe cyclones (category 4 and above) to hit northern Australia include: Debbie –northern Queensland March 2017; Lam– Arnhem land Northern Territory February 2015; Marcia – northern Queensland February 2015; Yasi–northern Queensland February 2011;

Monica – Arnhem land, Northern Territory April 2006; and Larry– northern Queensland March 2006.

2. Methods

The study outlined in this paper reviews and evaluates aspects of the hypothesis that trees are not just responsible for damage under cyclonic conditions but if strategically placed can help to mitigate the damage caused by cyclones. To do this a mixed methods landscape ecosystem approach is taken which is interdisciplinary and draws on a range of available resources and multiple layers of historical biophysical and sociological data, as well as available ecological information associated with cyclone impact. The analysis examines the spatial patterns of house damage in relation to tree location and age and considers the response and role of trees by reviewing available literature covering previous ecological studies relating to tree damage and reactions during recent severe cyclones in northern Australia.

Biophysical data analysed in this study includes information relating to the cyclone, as well as topographic and tree cover data. To undertake the spatial analysis, the Cyclone centerline (CCL), as defined by the Bureau of Meteorology [7], was digitized and a 1 km buffer applied using GIS software. Contour, slope, and aspect layers were created from a DEM generated from 1:10 K spot height data. Tree cover data, derived from shadow proxies, was estimated for each house allotment in the study area using 1:7500 black and white aerial photo- graphy taken at about 15,000 m altitude at 10 am on the 7/6/74. These photos were scanned with a high-resolution scanner at 400dpi and rectified using image-to-cadastre georeferencing and then were converted to pseudo-colour to discriminate shades of black shadows from white roofed buildings. The outcome was two images which were di- gitally placed over the original photo. The house shadow was removed from other shadows by determining the shadow offset (in pixels) from the building at the centre of the image relative to coordinates of the corners. When processed, the compositefiles with house shadows largely eliminated were merged into a mosaic and the proportion of each cadastral block occupied by tree shadow was analysed. Zonal statistics were used to calculate the percentage cover of shadow /tree cover for each allotment in the cadastre dataset and converted to a 0–1 scale (the

‘Tree Cover Index (TCI)’). Validation of the shadow/tree cover mapping was undertaken by comparison with manual measurement of tree cover for 50 randomly selected producing a signiﬁcant liner regression (R²

= 0.68).

Sociological data analysed in this study consisted of house type and house damage data. This was extracted from an unpublished post Cyclone Tracy survey undertaken for the Darwin Reconstruction Commission (DRC) between 1975 and 1977 and held in National Archives of Australia. From this study information could be obtained on house type (elevated open, elevated built under and ground level), house material (brick, cement, ﬁbrocement, ﬁbrocement and brick, others), and house damage (House Damage Index (HDI) scaled from 0 to 1). As no record of how HDI was estimated, it was assumed that similar statistical criteria were used to calculate this damage index to those reported by Waker (1975),i.e.the Walker Building Damage Index (BDI) which is a function of maximum wind velocity impacting on the building and a measure of structural resistance of the building.

Comparison of HDI and BDI revealed that both data sets showed similar normal distribution patterns. The normalized basic damage distribution curve was rescaled to that of the DRC survey, called HDIw. A Kendall tua_b correlation analysis in SPSS 15 (SPSS Inc 2006) performed on

these two indices showed a highly signiﬁcant correlation value (0.94, p < 001) in all cases suggesting that similar criteria were used.

An estimate of house and tree age was determined using‘year of street naming’data provided by the Darwin Street Naming Committee within the Northern Territory Government Department of Lands and Planning. House ownership data was derived from the DRC survey.

Houses were categorized into‘Government’,‘Housing Commission’and

‘Private’ownership. Allotment area data was derived from the Northern Territory Cadastral data set on the assumption that this had remained largely unaltered since Cyclone Tracy.

To evaluate what variables appeared to contribute to the state of houses before and following Cyclone Tracy spatial and statistical ana- lyses were performed. The binning extension in SPSS was used to group tree cover into three and five classes, elevation and slope (%) for combined data into six classes, and elevation and Ln slope for north and south of the CCL into five classes. Statistical analysis involved determining and plotting means for total Darwin data together with standard deviation (95%) confidence level to assess trends and, differ- ences between, the various landscape determinants, TCI and HDI and their interactive responses. Pearson's Correlation coefficient was used to test for trends in such relationships with ANOVA together with the Dunnett's C test used to test for significance between responses as required.

For an evaluation of the impact of cyclones on trees in northern Australian landscapes ecological research undertaken after some of the most severe cyclones to hit the region was investigated. Published work on these events that was reviewed as part of this mixed methods approach included Turton[69], Turton,[68]Cook and Goydens[14]and Staben and Evans[57].

3. Results

3.1. Cyclone tree damage review

The results of the mixed methods approach show that of the research that has taken place on the impacts of recent cyclones to hit the Northern Territory coast, Staben and Evans[57](who looked at the impact of destructive winds on vegetation in the Magela Creek Catch- ment by evaluating tree canopy loss in natural areas, and at the Ranger mine near Kakadu National Park using remotely sensed data through an evaluation of thefirst ten days after Cyclone Monica and then the same area one year later), found that areas closer to watere.g. in riparian zones or inundated areas exhibited a bigger loss in terms of tree canopy cover than upland areas. Cook and Goydens [14]also looked at the impact of Cyclone Monica and found that from their investigation that different species of trees react differently to cyclones, with their observations indicating that the sclerophyllous leaves of eucalypts found in the savanna woodlands of the Northern Territory tend to resist de- foliation under cyclonic conditions for longer than the typical leaves of rainforest species found in northern Queensland. They also observed that the harder wood species appeared to resist breakage for longer.

However, theirfindings suggested that retention of leaves could result in an increased proportion of trees snapped and uprooted in cyclones due to greater wind load[14]. The damage that cyclones cause has been observed to be affected by–wind velocity, variations in site exposure or effects of topography, and response of systems to wind disturbance as a function of species composition and vegetation structure [69]. Tur- ton's[69]research looked at trying to secure resilience to cyclones in the wet tropical forests of northern Queensland using information from Cyclone Larry and Yasi. He found that at the local scale within forest structures the impact of the cyclone is controlled by composition and structure and small scale topography. This provides evidence that the relationship between tree behaviour and cyclones is complex. In urban environments, this is further complicated by modified physical landscapes, varied species selection, active tree management and the built environment, patterns of which are heavily influenced by socio,

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cultural and economic drivers. Within the scope of this study which used historical data and literature it was not possible to evaluate the direct impact relevant to species composition but the eﬀects of some of the socio-cultural characteristics are illustrated in the results below.

3.2. Cyclone Tracy characterisation

An evaluation of historical information relevant to Cyclone Tracy showed that extent of house damage was inﬂuenced by their structural attributes such as house type (i.e.elevated or ground level), cladding material, as well as the construction process associated with ownership, and landscape topography relative to the cyclone path, although the relative contribution of these factors is poorly documented.

The damage to trees, both in the remnant native forest and in the suburban landscapes of relic endemic and cultivated parts of Darwin has also been well documented[10,22]. Following Cyclone Tracy, Ca- meron, et al.[10]observed an inverse relationship between tree cover and housing damage suggesting that trees provided cyclone protection, although there was no statistical evidence to support this. However, reports that document the fact that during periods of strong winds there is alteration to the predicted uplift forces onflat roofed, low buildings which can be seen to be associated with surrounding trees of similar dimension[58,73], lend support to this hypothesis. Possibly more significantly in favour of this argument is the fact that much house damage noted at the time of Cyclone Tracy appears to result fromflying debris, which has much greater force than the transporting wind [26]. It is argued from this interpretation of the information available that urban tree cover may act as“debrisfilters”absorbing both debris energy and matter thereby playing some role in reducing damage, providing that trees themselves do not become a hazard.

Evidence from general cyclone models and from cyclone damage pattern data analysed in this study, as well as from the post Cyclone Tracy survey[70], indicates that the winds of the approaching cyclone had a stronger on-shore (westerly) component, associated with reduced boundary layer friction, on the northern (left) side than southern (right) side of the cyclone path where they were weaker and more oﬀshore (easterly). The mean wind velocity in such a tract has also been shown to increase radially out from the cyclone centre to the eye-wall and then decrease beyond from the centre (see[15]).

3.3. Cyclone Tracy housing damage and tree cover analysis

The landscape analysis of housing damage and tree cover patterns provides insights into potential relationships between trees, cyclones and damage to the built environment. The radial HDI profile in this study followed the wind profile closely with damage peaking close to the eye-wall (5–6 km) estimated by the Bureau of Meteorology[6]. The cyclone tract intercepted and divided the south to north spatio-temporal gradient of developing allotments, such that the decrease in modelled radial windfield intensity was accompanied by newer houses with lower tree cover northwards of the CCL (Fig. 2a). This was in contrast to an increase in house and tree age (and tree cover) south- wards of the CCL. Here the modelled wind velocity increased at least up to the eye-wall, contributing to a direct correlation between the TCI and HDI.

The anticipated influence of decreasing cyclone intensity inland from the coast was not evident in HDI patterns (Fig. 2b). What was observed in the data was an increase in HDI with distance inland, particularly north of the CCL. Whilst the TCI, consistent with development patterns, was seen to decrease with distance inland. This pattern was less evident, if not reversed south of the CCL. This relationship is further explored in this study through the analysis of a range of social and biophysical parameters that may underlie the relationship between tree cover and housing damage, and can affect the role of trees in protecting or inflicting damage.

The Walker report[70]had previously identiﬁed that terrain factors

north and south of the cyclone centreline may have played a role in damage, although how this might have occurred was not fully ex- plained. The role of landforms in regulating windflow and determining the degree of cyclone exposure is generally recognised [26]. Bluff bodies such as hills can establish pressure gradients which are positive on the windward side and negative on the lee side. According to Reardon[53], structures located on the crest of hills, can be expected to experience wind speed increases in the order of 20–50% depending upon the extent and slope of the hill. For small hills and rises or de- pressions, typifying the Darwin landscape, topographic barriers of 3–6 m may also have modified wind impact[70].

This study suggests that north of the CCL HDI increased with elevation, while TCI decreased (Fig. 3a to d). This contributes to the observed inverse relationship between tree cover and damage. HDI did however increase with tree cover on higher elevation sites suggesting that increased exposure increased tree related hazards. However, the inﬂuence of elevation on HDI is not apparent from data south of the CCL. Here damage appears to increase with tree cover but only at low elevation sites. Low elevation sites are located closer to the coast where trees are older, and where the winds from both the approaching cyclone would have been stronger. At the more sheltered mid elevation sites (both north and south of the CCL) there appears to be both higher TCI and lower HDI suggesting that in these locations topography and tree cover combine to enhance house protection. HDI is also observed to increase with slope angle north of the CCL, particularly where there is increased tree cover although this relationship is not observed south of the CCL.

Results of one-way ANOVA and Dunnett's C post hoc test indicates both north and south of the CCL NE aspect (0–90) had marginally lower HDI than NW (90–180) (p < 0.05) but significantly lower HDI than SE (180–360) (p < 0.001). More broadly, locations where aspect had a westerly component, significantly higher HDI (p < 0.001) values were observed than those locations with an easterly aspect, while TCI values appeared lower. This inverse relationship, although indicative of protection benefits, also indicates the influence of exposure to the eastward moving cyclone and associated westerly winds. North of the CCL only the more sheltered south easterly aspects indicated a significantly lower HDI with increased tree cover both benefitting from the microclimate of this location. South of the CCL most aspects showed the reverse trend suggesting that higher tree cover, particularly significant for south westerly aspects, was associated with higher HDI. Therefore, it is possible that the older trees found here were contributing to higher HDI values.

The spatial and topographic landscape factors described above, that inﬂuence both the pattern of house construction and associated tree cover management, are under pinned by a number of cultural factors.

For example, certain house attributes were shown to also influence the damage caused by Cyclone Tracy. Such attributes include the material used to clad the house and the style of house. The resistance to wind offered by a house has been noted to be a function of the nature of cladding material, internal structural members, windows and other modular units by the eminent civil engineer Joseph Minor[45,46]who is recognised in wind engineering. The results of one way ANOVA and Dunnett's Ç test for difference in HDI scores for house structures and materials indicates a significant difference (P < 0.05) between all house types and all materials except betweenfibrocement, cement and others which were not significantly different. Overall, this study shows that brick houses demonstrated significantly less damage than fi- brocement and HDI was significantly greater for elevated open houses than those at ground level. As emphasised by Holmes[26], debris pe- netration of cladding is the primary mechanism by which internal pressure is rapidly increased. Trees may have played a role in both being a source of and intercepting debris that impacted on elevated homes constructed largely offibrocement.

Although there was a signiﬁcantly negative correlation with TCI and HDI, particularly north of the CCL, this relationship was weaker and

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reversed south of the cyclone centreline particularly for elevated houses, with no relationship evident for elevated “built under” and

“ground level”houses (Fig. 4). This suggests that younger, smaller trees to the north may have provided more protection against stronger winds for these more vulnerable houses, whereas the older, taller trees to the south of the CCL may have exacerbated such damage even under the weaker wind regime.

Reference has been made to the important role played by age in

explaining temporal patterns of TCI inﬂuencing relationships to HDI.

Both TCI generally increased and HDI decreased with age, the latter showing greater temporal variability with similar cyclic changes evident north and south of the CCL, suggesting a progressive improvement followed by deterioration in house vulnerability.

The influence of the‘human factor’on house construction, as well as tree planting and maintenance influencing cyclone vulnerability, is reflected in primary house ownership. Of the‘Government’houses, the Fig. 2.Mean pooled HDI and TCI with distance to the a) CCL b) and Coastline).

Fig. 3.Least distance means square plot of HDI as a function of landscape elevation (contour) and slope (log percent) for a) south the CCL and b) north of the CCL and Least distance means square plot of tree cover (Arcsine cover %) in Darwin, 1974, as a function of landscape elevation (contour) and slope (log %) for c) south of the CCL and d) north of the CCL.

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post cyclone damage assessment in Walker[70]noted that‘Housing Commission’houses appeared from general inspection to have fared somewhat better than departmental ones, although the greater number of hipped roofs amongst‘Housing Commission’houses also increased their vulnerability as noted in this study. The analysis of data also showed that the pooled‘Government’and‘Housing Commission’houses were found to have sustained signiﬁcantly higher HDI values than

‘Private’houses, with an inverse relationship to TCI (Fig. 5). North of the CCL the more vulnerable‘Government’houses showed an inverse relationship with TCI, not evident in‘Private’houses. South of the CCL higher tree cover, linked to larger older trees in‘Private’house allotments, generally located along the western coastal margin, was associated with increased HDI.

The space around and between houses in cyclone affected landscapes has had different, often conflicting, affects. Walker[70]suggested that brick houses, mixed with elevated houses may have provided some protection. Building height /spacing ratio or house density has also been considered a contributing factor in shielding[25,30,39].

In this study increased allotment area, up to 2800 m², serving as a proxy for house spacing, showed a small change in HDI for‘Private’houses, but did not for‘Government’houses. TCI both north and south of the CCL also increased with allotment area suggesting that the combination of area and TCI reduced housing damage for the more sensitive houses.

4. Discussion

Relationships identiﬁed in the initial analysis of pre-Cyclone Tracy allotment tree cover and post-Cyclone Tracy house damage data held in archives lends strong support to the hypothesis put forward by Cameron

et al.[10]that there is an inverse relation between these two variables.

That is, trees can offer some form of protection to houses during cyclones. However, further analysis of data revealed that the relationship between the cyclone windfield, houses and trees is a complex one, as it appears to be influenced by temporal, spatial, topographic and cultural factors. These, individually or collectively, have been shown to dom- inate, and, in some circumstances, reverse the relationship. The complexity of the relationship is illustrated by the fact that the tract of Cyclone Tracy and the associated cyclone wind field, particularly within the eye-wall zone that covered most of Darwin, inflicted damage on both houses and trees that intersected a north south spatio-temporal gradient of changing house construction and associated tree age and cover.

The spatial analysis of HDI and TCI across a range of landscape situations in the Darwin study area revealed a consistent inverse correlation between these two variables, particularly in landscapes involving stronger winds, more vulnerable houses and smaller trees. This therefore provides sufficient indication that here tree cover may minimise house damage from cyclone debris. In landscapes where trees are larger and where both house and trees have greater topographic exposure to winds, even though houses are less inherently vulnerable, tree cover as a source of debris may contribute to cyclone hazards. Within the moving cyclone windfield there are sites where combined and alter- nating winds may amplify the tree cover house interaction effects, providing both benefits and dis-benefits depending on the type of cover and the vulnerability of houses and the effects of topography on both.

The landscape analysis approach adopted in this study demonstrates the complex relationship between tree cover and cyclone damage within the urban environment, providing an important base to develop Fig. 4.Mean and standard deviation of HDI and their relationship toﬁve binned TCI classes for three house structures a) north (N) and b) south (S) of the CCL.

a) b)

Fig. 5.Relationship between house ownership a) north and b) south of CCL and the HDI for three tree cover classes.

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more detailed investigation from which to unravel the real complexities in this relationship. In particular, the historical analysis undertaken in this study was unable, due to limited data, to reveal spatial patterns in tree species distribution across the 1974 Darwin landscape. Examining the relationship of tree species to cyclone damage is important and should be explored in future studies, but needs to consider both the species based and edaphic factors aﬀecting tree physiology and cyclone response. In particular, their context within the built environment, modiﬁed physical landscape and management.

Recognising that trees can also be a source of cyclone debris means that models that try to understand tree debris generated from cyclonic events (e.g. [67]) might be able to assist in getting a better understanding of how much debris can result from a cyclone and what its impact might have. These ﬁnding suggest that debris modelling relevant to species composition and impact should be explored in more detail in future studies. Debris estimation methods are complex but models based on tree density, tree height and wind data have tried to understand the potential amount of tree debris caused by cyclones[67].

Thompson et al.[67]pointed out that most estimates of debris were however based on the assumption that all trees behave uniformly and that trees greater than 30 m will generate debris. The result being that models generated for hurricanes in Virginia and North Carolina have overestimated debris quite considerably by 90% and 42% respectively.

Interesting for this study is that an evaluation of the results of the modelling and analysis conducted by Thompson et al.[67]shows that non-storm variables were found to be more important predictors of debris than storm variables. Theseﬁndings also supported the results of work carried out by Staudhammer et al.[59], Kupfer et al.[37]and Duryea et al.[19]that suggested tree damage caused by hurricanes was more than likely attributed to trees being improperly located, poorly maintained or a species that has a low tolerance to hurricane force winds. The conclusion being that tree, or site speciﬁc variables, and land cover, might have a stronger causal relationship with debris than meteorological variables. This means that as the Cyclone Tracy study hypothesises it might be possible for urban planners and tree managers to proactively undertake action by careful planting of trees i.e. considering structure, composition and location, that can reduce the pro- duction of tree debris from cyclones and contribute towards trees playing a more protective role against the damage by cyclones. To add more evidence to this argument and to provide more accurate information for urban tree management it is suggested that these factors be explored more fully through further experimental and modelling exercises.

As we move through the 21st century planning to reduce the risks associated with cyclones will be vital to deal with the impact of climate change on urban areas. Some of the measures taken will be major in terms of modiﬁcations to buildings and the structure of urban areas but we know from past experience that small actions taken prior to the cyclone can also help to reduce the damage[31]. Being prepared from knowledge of what has been experienced before and taking pre- ventative measures can reduce the impact of cyclones and incorporating a spatial understanding of patterns of damage can reduce risk. This means that spatial and temporal analysis of damage combined with predictive modelling from known behaviour can be very important for risk reduction and decision making about natural hazards (Hoque et al., 2016).

The development of fast growing urban areas like Darwin and their recovery from catastrophic events such as Cyclone Tracy is common to the evolution of natural complex systems involving adaptive adjust- ment. In anthropic ecosystems such adaptation involves selection of ideas that, from experience, work towards contributing to responses in design, planning and management. However, many of the lessons from Cyclone Tracy are in jeopardy of having slipped from community memory risking a repeat of some past mistakes. Much of the vulnerability was, and still is, a result of complex human processes within the urban community ranging from tree species selection and cultivation, to

house design, construction and management based on policies with little evidence or understanding of the complex processes involved. This means that optimum strategies for dealing with phenomena such as cyclones are not adequately being implemented at the current time. It is important to use the results of this study and further recommended analysis to inform future urban planning and decision making. There is great power in using historical information and knowledge to reveal important information on exposure, sensitivity and adaptive capacity of houses and urban centres, and in association with tree cover information, this information needs to form the basis of directing further research and guidelines for development in cyclone prone urban areas moving forward.

The urgency of developing informed policies and guidelines around minimising the impact of cyclones on urban area, based on a re- assessment of the eﬀects of Cyclone Tracy and other cyclonic events, combined with further targeted research on other urban cyclone phenomena, has increased as a result of the pressures associated with climate change. Evidence of increases in atmospheric and ocean tem- peratures, rainfall, wind and frequency of thunderstorms, are some indicators of possible anthropogenic, environmental feedback eﬀects, impinging on tropical landscapes. Projections from albeit limited cyclone data coupled with thermodynamic understanding of cyclone genesis and its link to the raising of sea surface temperature suggest that the intensity of cyclones may be increasing and could be maintained longer. The relatively recent occurrences of high intensity Cyclones Ingrid and Monica following on from Tracey that struck within the Darwin region and the devastation that resulted across Queensland and northern New South Wales from the very recent destructive Cyclone Debbie appear to support this with implications for both houses and trees

Urban planning solutions for the future lie in implementing adaptive approaches that contribute towards building urban resilience and sustainability to the changing climate[1]. Factors such as vulnerability to cyclones, taking into account exposure, sensitivity and adaptive capacity, are vital considerations for future urban developments. This study provides weight to the argument that informed urban landscape planning and design incorporating trees can potentially contribute to the amelioration of the impact of such changes. However, when considering the role of trees, as Kirkpatrick et al.[33]point out, the level of engagement people have with them needs to be considered as this tends to vary. The result of this being that some conflict exists between those who see trees as cost effective for achieving urban goals and love them, and those that are described as being scared of trees. This conflict may serve to restrict up-take of tree planting to cushion against cyclone events. However, it has been noted that some of the negative feelings towards tree use in urban areas can be reduced by better tree management which includes more appropriate choice of species planted.

Also increasing evidence does show that the potential for tree cover to ameliorate the impact of climate change, can complement the arboreal services currently recognised as being offered to urban environments in terms of effluent treatment[47,61], amelioration of urban air quality and carbon dioxide assimilation[36,4], changing urban microclimate [50], reduction of urban heat islands [41,56] and shade cooling of houses [28], and offer a contribution to greenhouse gas reduction which can only be positive for urban development moving forward.

This being the case the increasing use of what has become known as

‘green infrastructure’to help build resilient urban systems has potential to minimise impacts associated with natural disasters like cyclones, as well as contributing to the health and economic wellbeing of tropical communities, However, as Matthews et al.[43]point out there is still some confusion over what exactly green infrastructure is. However, whatever way planners want to look at green infrastructure it appears that growing trees and considering plant life in urban development can have multiple beneﬁts for the urban environment. This study contributes to the mounting evidence that points to the value of trees in urban areas, the need for further research into the ecosystem services

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that trees can provide, the impact of natural disasters on urban areas and the role of green infrastructure in helping to mitigate the consequences of climate change.

The research presented in this paper illustrates that there are complex relationships that need more in depth and targeted analysis to understand completely. Significantly, this study demonstrates that trees can play both a protective and destructive role in cyclone affected urban landscapes, and the mixed methods analysis undertaken in this paper has highlighted the complex interplay of species, age, physical environment and management of trees and the layout and structural attributes of houses that contributes to the role trees and the urban forest play during cyclones. Thesefindings add considerable strength to the justification that further investigation is not only warranted but timely. The many questions that arise from this study such as around a deeper understanding of particular tree species that might act as potential debris or debris barriers, need to be more fully explored. Future research should be focused on addressing fundamental questions that will be capable of informing strategic planting and positioning of appropriate urban trees. These questions centre round determining defi- nitive answers to which species of trees to use, what density and distribution, what distance should they be from buildings and infrastructure and what tree management practices should be applied for greater protection benefits. Manipulating key factors to optimise the ability of the urban forest to mitigate cyclone damage in urban landscapes should be a priority and understanding the best way to do this is a key requirement to inform evidence based decision making by urban planners and policy makers

5. Conclusion

Since it is likely that society will want to build larger cities in cyclone prone areas in the future, then it is particularly important to learn from history and take on board the lessons learned from events like Cyclone Tracy so as to inform future planning. Even though this study highlights the complexity involved in the possible relationship between trees, cyclones and house damage in urban areas, thefindings indicate that holistic approaches to urban planning are required for sustainable urban development in the wake of climate change. The results of this study indicate that urban planning needs to cross disciplines and bring planners, builders, architects, meteorologists and ecologists to the table to discuss best practice urban design to ameliorate the impact of extreme events. Adopting such an approach offers an exciting possibility for the design of future cities. One in which humans and nature could truly work together to minimise the social-economic impacts of cyclone damage and potentially reduce the risk of high financial and human loss. The impact of urban landscape change on trees, the role trees can play in ameliorating the severity of damage associated with cyclonic activity in monsoonal landscapes, and how tree cover needs to be managed for sustainable landscapes are matters that this study has touched on but still require further research. This study of Darwin and what we can learn from its destruction by Cyclone Tracy serves to demonstrate the potential important and complex relationships that need to be explored further and at a more global scale if effective policy guidelines and adaptive strategies are to be developed for urban sustainability in areas affected by extreme climatic events.

Acknowledgments

The authors would like to acknowledge Charles Darwin University for supporting the research presented in this paper. The authors would also like to acknowledge the many people with whom Frank had end- less conversations about this research and in particular Cyclone Tracy, as he was formalizing his theories that formed a substantial component of this paper. You are too many to thank individually but you know who you are. As with any historical study much information is hard toﬁnd and many contributed towardsﬁnding its location. The authors would

also like to thank the many people who read and commented on earlier versions of this work, in particular Professors Bob Wasson, Andrew Campbell and Julian Gorman

Dedication

This paper is dedicated to the memory and work of Dr Frans-Josef (Frank) Van der Sommen who sadly lost hisfight with cancer on 10th January 2013. Frank was a passionate scholar and landscape ecologist, with an eager quest to gain a better understanding of landscape functioning right up to the end. His inquisitive mind, his holistic thinking, and his enthusiasm for the natural environment and its interactions with people, and the academic conversations that his conceptualizing generated are sorely missed by all who had the pleasure of working with him. It is hoped thatfinally through this paper aspects of the research conducted towards the fulfillment of his PhD, will inspire others to further pursue the knowledge required to address the yet to be an- swered complex questions on associations between trees, cyclones and urban environments.

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