Storm damage on urban trees in Guangzhou, China

LAN DSCAPE AN D U RBAN PLAN N I N G ELSEVIER zyxwvuts Landscape and Urban Planning 38 (1997) 45-59 Storm damage on urban trees in Guangzhou, China C.Y. Jim *, Howard H.T. Liu zyxwvutsrqponmlkjihgfedcbaZYXWVU Department of Geography and Geology, University of Hong Kong, Pokfulam Road, Hong Kong Received 26 August 1996; accepted 7 March 1997 zyxwvutsrqponmlkjihgfedcbaZYXWVUTS Abstract Guangzhou is the largest city in subtropical South China with a significant tree cover and a long history of urban greening. Its precious tree stock is subject to a wide range of natural and artificial stresses, one of which is strong wind associated with thunderstorms and typhoons. On 9 April 1995, a windstorm of extreme intensity struck the city and brought havoc to the tree population. A detailed survey was conducted immediately after the storm to collect quantitative information on 1782 trees encompassing 89.5% of the total spoiled-tree population. The study was aimed at understanding the nature and extent of wind damage on urban trees with reference to species, dimension (age), site characteristics, land use and urban history. A brief review of tree composition, habitat conditions and tree management responsibilities in the city, and a computer inventory on urban trees established earlier, provided a benchmark for data interpretation. Damage was generally independent of size despite the conspicuous loss of some large and champion-calibre specimens. Tree size, however, was related to the mode of damage. Ten species constituted the bulk of the destruction. Roadsides and green spaces had somewhat different assemblages of afflicted species. Recent widespread construction activities had weakened many street trees and predisposed them to injuries. Old districts with more old trees were more seriously affected. North districts which registered much stronger winds took up over 80% of the toll. Different phases of management responses to the natural disaster and the related administrative organization were assessed. Management implications of the findings were discussed with a view of rationalizing responses and reducing future wind damage. 0 1997 Elsevier Science B.V. Key words: Guangzhou; Urban tree; Urban forest; Storm damage; Tree management 1. Introduction Trees in cities constitute a major biomass component of urban ecosystems and provide many essential benefits and functions (Schmid, 1975; Grey and Deneke, 1986). Whether dwelling in natural or manmade habitats, urban trees grow and survive in a wide variety of conditions. They are often beset by a multitude of both human and natural impacts and * Corresponding author. Tel.: (852) 2859-7020, fax: (852) 2559-8994,e-mail: hragjcy@hkucc.hku.hk. 0169.2046/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PIf SO 169.2046(97)00018-2 suffer from a plethora of damage and constraints (Jim, 1987, 1993; Miller, 1988; Harris, 1992). Besides the common environmental stresses and constraints, occasional natural disasters such as storms and hail can bring havoc to urban trees (Gibbs and Greig, 1990; Sisinni et al., 1995). The impacts of strong winds on trees have been studied in detail for forested areas in the countryside (e.g., Grayson, 1989; Sellgren and Sellgren, 1990; Frangi and Lugo, 1991; Weaver, 1994). kesearch on tree failure in urban areas, until recently, has received less attention (e.g., Duryea et al., 1996). 46 C. Y. Jim, H.H.T. Liu /L.undscrrpe Chinese cities have a long urban planting trddition, some of which can be traced back to 200 BC (Government of Guangzhou City, 1924; Du et al., 1986). Urban greening has been a major objective of city planning and development in China’s recent history. In the last few decades, a series of laws have been enacted and corresponding management systems established in order to enhance and protect urban forests and green spaces (Ministry of Construction of China, 1992). Despite limitations due to inadequate funding and due to high density of population and economic activities, the average green cover in Chinese cities managed to increase from 2.9% in 1949 to 17.0% in 1988 (State Statistical Bureau of China, 1990). Urban trees in Chinese cities have provided their residents with various environmental benefits including, especially, the amelioration of microclimate and particulate air pollution (Yang, 1991). Guangzhou, a major subtropical city in South China and the provincial capital of Guangdong, has a high green cover of 23.7%, ranking third among the extra-large cities of China (State Statistical Bureau of China, 1990). Urban trees in Guangzhou have played a very important role in decorating the urban landscape and enhancing environmental quality (Yang, 1991; Huang et al., 1994; Zheng, 1995). The important green resource encounters various constraints and destruction by human and natural factors due to, in particular, massive infrastructure development, renewal of old districts, and occasional storms and tropical typhoons. A brief but severe windstorm battered the urban areas of Guangzhou on 9 April 1995. The half-hour onslaught resulted in the most serious destruction to urban trees since the 1970s; some 2000 trees were damaged (Guangzhoushi Yuanlin Guanliju; Landscaping Bureau of Guangzhou Municipality; hereinafter referred to as the Landscaping Bureau, 19951. A reconnaissance survey was conducted immediately after the episode to record damaged trees, and a detailed survey was subsequently carried out to census the affected trees with reference to location, species, habitat, structure and extent of damage. Combined with pre-storm tree-survey results, the post-storm survey data provide a good opportunity to assess tree-habitat-storm relationships in a subtropical Chinese city. and Urlmn Planning 38 (1997) 45-59 The main objective of this paper is to evaluate, through field investigation, the nature and extent of wreckage induced by the said storm on urban trees in Guangzhou, mainly in terms of species composition, structure and distribution of damage. Special attention is paid to the spatial spread of harmed trees and their relationship with land use. Management responses to the windstorm are also discussed to provide the basis for rational long-term planning and management of urban trees, vis-a-vis the occasional but deleterious natural hazard. 2. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJ The storm The windstorm struck urban Guangzhou in the evening of 9 April 1995, moving mainly from the north towards the south. Starting at 18:55 and ending at 19:25, the storm attacked for a period of 30 min. It reached its peak velocity from 19: 10 to 19: 15 at wind scale nine (22.6 m/s), while the average wind speed was between wind scale six and seven (12- 15.5 m/s). Wind velocity was higher in the northern parts of the city and it declined towards the south, with urban areas south of the Pearl River only slightly affected by the storm. The accompanying heavy downpour resulted in a record high rainfall of 36.4 mm. The windstorm was an extreme case of the thunderstorm weather phenomenon that often engenders in the afternoon and evening from March to June in the subtropical coastal city. Such thunderstorms are likely to develop under meteorological conditions that nurture atmospheric instability, such as strong convective air movement and high relative humidity. When a strong convection is induced and strengthened by the interaction of the high-altitude low-pressure trough and the South China stationary frontal surface, there is a higher probability of breeding exceptionally violent thunderstorms. Historical meteorological records indicate that the frequency of windstorms of extreme magnitude occurring in the central urban area of Guangzhou is about once every 10 years (Huang et al., 1994). The windstorm caused widespread destruction to Guangzhou City, with total losses valued at millions of RMB dollars (US$l .OO= RMB$8.30) (Guangzhou Daily, 11 April 1995). The major disruption included nine snapped main electricity cables and blockage of C. Y. Jim, H.H.T. Liu /Landscape more than 40 streets, mainly due to uprooted and broken trees. Other losses were related to buildings and vehicles damaged by fallen trees and the interference to routine air and land transportation (Yungcheng Euening Post, 10 April 1995). As the wind velocity was uneven and declined from the north towards the south, the northern districts, particularly those with open spaces, suffered heavier damage than the southern ones. 3. Study area The study area is focused on the main urban districts of the Guangzhou municipality, which had encountered different levels of damage. It comprises the three old urban districts (namely Liwan, Yuexiu, and Dongshan) and the built-up areas of newly developed urban districts (i.e., Haizhu and Tianhe). It covers an area of 56.9 km2 with approximately 2.13 million permanent inhabitants (Table 1). Guangzhou has a long urbanization history of over 2800 years since its founding in 900 BC (Government of Guangzhou City, 1924; Zeng, 1991). As one of the leading foci of China’s recent reform and open-door policy, the city has, since 1978, experienced fast economic development (Statistical Bureau of Guangzhou City, 1995). In the near future, Guangzhou is being planned to spread towards the east to be developed into a modem metropolis with a population of 35 million and 500 km* of built-up area (Urban Planning Bureau of Guangzhou City, 1994). Guangzhou City is situated in the southern part of Table 1 Basic statistics 41 and Urban Planning 38 (1997) 45- 59 the north subtropical humid climatic zones (Huang et al., 1994). The average annual precipitation is 1690 mm. The average number of annual rainy days (precipitation > 0.1 mm/day) is 150. The average annual radiation balance on the earth surface is 2676 X lo6 J/m*. The average annual air temperature is 2 1,8”C, with the hottest month in July and the coldest month in January. The average annual and diurnal temperature ranges are 15.1”C and 7.6”C, respectively. The average annual relative humidity is 79%. There are evident and high-frequency developments of heat-island and urban temperature inversions, both with strong impacts on urban environmental conditions. The frequent typhoons and occasional severe thunderstorms and tornadoes sometimes bring serious destruction (Huang et al., 1994). The topography of urban Guangzhou is composed of low hills, low terraces, lowlands, wetlands and water surfaces (Zeng, 1991). The bulk of the study area comprises alluvial plains formed by sediments of the Pearl River during the past several thousand years. The lowlands cover more than 70% of the study area, most of which are covered by buildings and transportation facilities. 4. Methods In order to evaluate wind damage on urban trees, a survey was conducted immediately after the storm. The selection of survey targets, streets and green spaces was based on our comprehensive pre-storm field survey database of the urban forests in and tree density in four districts of urban Guangzhou Tree density Basic statistics District Area (km’) Population (millions) Density (thousands/km’) Stree& (trees/m) Parks (trees/km’) Districts (trees/km’) Dongshan Haizhub Liwan Tianheb Yuexiu Total 17.2 14.3 9.4 7.1 8.9 56.9 0.55 0.47 0.52 0.15 0.44 2.13 32.0 32.9 55.3 21.1 49.4 37.4 0.19 0.21 0.19 0.37 0.13 0.19 895.1 172.0 399.9 296.3 2513.9 8 10.0 1706.2 861.7 1345.6 1241.0 3307.3 1626.8 “Streets without trees are excluded from the calculation. ‘Only built-up area has been included. Source: Statistics Bureau of Guangzhou City (19951, and the authors’ tree survey. 48 C.Y. Jim, H.H.T. L&/Landscape Guangzhou. Sampling targets were focused on the areas where the storm had induced serious damage, which is defined as a rate > 1% of the existing trees. Six urban parks and streets in the four northern urban districts were surveyed in detail. Moreover, large damaged trees with diameter at breast height (DBH) > 50 cm located outside the chosen sampling sites were also evaluated. A total of 1782 trees, which account for 89.5% of the population of damaged trees in the study area, were surveyed in the field. The post-storm field survey was carried out on 1 l-20 April 1995 to record the locations of damaged trees, and the nature and extent of damage. The major attributes of the damaged trees, such as species, DBH, crown spread and height, were extracted from pre-storm survey results collected in early 1995. A more elaborate assessment of the different modes of failure (Barry et al., 1982) could not be attempted due to the rather swift clearance of damaged trees. A comparison of the two sets of data, namely pre- and post-storm, provided much information on the destructive impacts. The working reports prepared by the Landscaping Bureau recorded all damaged trees located in the urban districts of the city. This data set, gathered immediately after the storm, was used as a reference to verify our survey results. Field data were recorded quantitatively and stored in an Excel version 5.0 spreadsheet. Then, each variable was subsequently coded into four ordinal classes to facilitate statistical analysis using SPSSPC release 6.1 for Windows. Tree damage was classified according to mode (uprooted, crown loss); location (offroad, roadside); DBH (O-20, 20-40, 40-60, > 60 cm); height (O-5, 5- 10, 10-15, > 15 m); and crown spread (O-4, 4-8, S- 12, > 12 m). Botanical nomenclature follows mainly Chen and Wu (1987, 1991) and Wu (19951, with verification using Mabberley (1990). and Urban Planning 38 (1997) 45-59 other green spaces (Landscaping Bureau, 1994). According to our 1995 pre-storm survey, there are 128 765 trees and 238 tree species in the study area (Table 2). Street trees, as the principal component, amount to 46743, accounting for 36.3% of the total tree population. Park trees rank second at 34.2% (43988). Trees in institutional, residential and other green spaces comprise 29.5% (38034). Species diversity, however, follows a reverse sequence. Some 117 species are found at streetsides, compared to 176 in urban parks. The institutional, residential and other spaces have the richest assortment with 214 Table 2 Main tree species in urban Guangzhou Land use Species Count %’ Street sides Ficus cirens Bauhinia blakeana Aleurites moluccana Ficus microcarpa Bauhinia uariegata Chukrasia tabularis Melaleuca leucadendra Bombax malabaricum Casuarina equisetifolia Acacia auriculifotmis Subtotal Eucalyptus tereticomis Acacia confusa Pinus massoniana Livistona chinensis Bridelia monoica Celtis sinensis Cinnamomum burmanii Melaleuca leucadendra Ficus hispida Michelin alba Subtotal Melaleuca leucadendra 8072 4244 3485 3286 3094 2880 2678 2253 2016 1177 33 185 2744 255 1 2148 1713 1591 1511 1409 1332 1187 1164 17350 4046 17.3 9.1 7.5 7.0 6.6 6.2 5.7 4.8 4.3 2.5 71.0 6.2 5.8 4.9 3.9 3.6 3.4 3.2 3.0 2.1 2.1 39.4 10.6 2988 2142 1580 1256 1190 1032 964 860 798 16 856 7.9 5.5 4.0 3.2 3.1 2.6 2.5 2.2 2.0 43.2 Urban parks Institutional, residential and other open spaces Roystonea regia Michelia alba Bauhinia blakeana Casuarina equisetiolia 5. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Tree composition and distribution in Aleurites moluccana Guangzhou Mangifera indica Catyota mitis Ficus L,irens In urban Guangzhou, there are some 195 streets Liuistona chinensis totalling 276.9 km in length that are planted with Subtotal trees, 3 1 urban parks covering a total of 3.16 km’, and a large number of institutional, residential and ‘Expressed as % of total count in each land use category. C.Y. Jim, H.H.T. Liu /Landscape species. Moreover, different land uses tend to have different species (Table 2). Trees in Guangzhou are unevenly distributed. The largest urban park, Yuexiu Park, accommodates 41.8% of total urban park trees (18 391), but occupies only 20.7% of the total urban park area. Similarly, the campus of Zhongshan University has 28.2% (11016) of the total tree population in institutional, residential and open spaces, but takes up only 10.6% of the collective area in the category. Tree density in the urban districts of Guangzhou also varies greatly (Table 11. and Urban Planning 38 (1997) 45- 59 7. Tree management 49 in Guangzhou Guangzhou is a Chinese city known for its urban greening. It owes the alias, the City of Blossoms, to its exceptionally rich flora of over 1400 species (Hou, 1956). Urban landscape planting has a long lineage that began in private gardens in ancient Guangzhou around 130 BC (Situ and Wei, 1994). The first public park, however, was not established until 1918. By 1949, there were only four urban parks in urban Guangzhou with 5200 street trees and a combined area of 32.6 ha. The planting of trees and other amenity vegetation has been accelerated since, and urban greening has been considered a major duty of the municipal government. 6. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Tree habitats in Guangzhou The greening works of Guangzhou is shouldered by a number of government agencies at different Trees in parks and green spaces exist in relatively levels. The Development Committee of the Municinatural habitats with little physical and chemical pal Government of Guangzhou (Guangzhou constraints both above and below ground. According Shizengfu Jianshe Weiyuanhui) is the overall coordito 1985- 1994 records, few trees in parks are damnator of urban greening for the city. The Urban aged by natural stresses and vandals, but over 90% Planning Bureau of Guangzhou Municipality of transplanting and felling are due to construction (Guangzhoushi Ch engshi Guihua Ju) plans the conactivities. In comparison, the storm had caused park tent and coverage of urban greenery in various urban trees the most serious damage, collectively more zones. severe than the total natural losses in the past 10 The Landscaping Bureau and its subdivisions bear years (Landscaping Bureau, 1995). the responsibilities of planting and managing all Roadside trees have endured more constraints than trees at streetsides, city-owned parks and other green park trees, especially in old districts with narrow spaces. It dispenses its varied tasks via three Departstreets and cramped distances between trees and ments. The Greening Administration Department adjacent buildings, often closer than half the diame(Luhua Guanlichu) manages trees along streets and ter of the mature crowns. In the old Liwan district, protects mature trees in urban areas. In addition, it most streets are narrower than 10 m and the pavehas the power to permit or prohibit transplanting or ments are less than 1.5 m. The major street species felling of trees exceeding 30 cm DBH regardless of in the established neighbourhoods, such as Ficus ownership. The Park Administration Department virens and Ficus microcarpa, have matured into (Gongyuan Guanlichu) is responsible for general large entities with crowns spreading beyond 15 m. management of city-owned parks, and for coordinatTree expansion is commonly limited by adjacent ing and supervising management of district-level buildings, and crown configuration is usually unnatuparks and other smaller public green areas. The ral and confined. The shallow and high-density unScience and Technology Department (Kejichu) is in derground services also hamper root spread and rencharge of promoting and applying new scientific der street trees predisposed to windstorm uprooting. knowledge and techniques concerning urban forestry Furthermore, the renewal of old buildings, widening for better tree management in Guangzhou. of streets, and construction of the mass-transit subThe Greening Company of Guangzhou way have caused massive tree losses. From 1990 to 1994 in urban Guangzhou, construction activities (Guangzhou Yuanlin Luhua Gongsi, hereinafter referred to as the Greening Company) is the major alone have eliminated as many as 4670 street trees implementation unit of the Landscaping Bureau. It (about 10% of the total street-tree population), including many outstanding and large specimens. harbours most of the city’s arboricultural staff and 50 C. Y. Jim, H.H. T. Liu / Landscqx equipment, and undertakes the bulk of the large-scale planting works in streetsides, parks and other public spaces. It is also in charge of emergency clearing, cutting or removing large trees broken by windstorms or other causes. At a more local hierarchy, the Green Guangzhou Committee (Guangzhou Luhua Weiyuanhui) coordinates and supervises various institutions to plant and maintain greeneries located in their compounds, and organizes voluntary planting programmes in urban Guangzhou. All districts of Guangzhou have established their respective Greening Offices (Luhua Bangongshi) which are local branches of the Landscaping Bureau and the Greening Company, to facilitate management of amenity vegetation in district-level open spaces and parks. The Greening Action Areas (Luhua Gongqu), as subdivisions of Greening Offices, conduct small-scale planting and maintenance of street trees. The Management Department (Guanlichu) of individual urban parks looks after smallscale planting and vegetation in their control areas. and Urban Pluming 38 CI9971 45-59 Bauhinia variegata 86 (2.27%); Ficus t:irens 83 (0.9 1%); Acacia confusa 59 (1.96%); Cleidiocarpon caualieri 55 (14.55%). These 10 species account for 62.8% of the total damaged population; another 18 species had a few damaged trees each and they collectively make up only 1% of damaged trees in the study area. In terms of botanical grouping, the 65 damaged species are spread out in 32 families. Caesalpiniaceae contribute 5 species and 514 trees (28.8% of the population of damaged trees). Equivalent values for other main families are: Myrtdceae 263 trees (14.8%), Mimosaceae 214 trees (12.0%), Moraceae 137 trees (7.7%), and Pinaceae 113 trees (6.3%). The above five families collectively contribute 35 species (53.8%) and 1241 trees (69.64%). Species differ significantly in terms of susceptibility to storm damage. For example, the two species planted along Huangshi Road Central, namely Bauhinia blakeana and Ficus uirens, have similar attributes in terms of age and dimensions, yet the former was more severely damaged. A summary of the post-storm survey results including 32 main af8. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Tree damage by species fected species, each with more than 10 damaged individuals, is given in Table 3. They can be divided Some 1992 trees in the city were damaged by the into four classes according to damaged rates, with windstorm, among which 798 were severely damthe notional average at 2.51%. Class A (damage rate aged due to uprooting or broken trunk (40% of the > 10%) includes some particularly vulnerable species damaged tree population). Six champion specimens such as Albizia chinensis (24.67%), Anthocephalus with DBH > 1 m were uprooted or had snapped chinensis (2 1.2 1%o), Cleidiocarpon cat,alieri trunks; the largest annihilated specimen, an old Eu(14.55%), Cassia suruttensis (12.97%), Sabina chicalyptus saligna, had 1.2 m DBH, 32 m height and nensis (12.47%) and Hibiscus tiliaceus (1 1.49%). 24 m crown spread. The damage was spatially disSome species are relatively less affected by the tributed as follows: 893 trees were located in urban storm with Class D (damage rate < lo%), including parks, 869 along streets, and the remainder scattered Aleurites moluccana, Chukrasia tabularis, Eucalypin off-road spaces. tus tereticomis, Ficus altissima, Ficus microcarpa, Of the 1992 total damaged trees, 1782 were evalFicus sirens, Michelia alba, Mangifera indica, and uated in detail in the present study, and they account Taxodium distichum. In addition, there are some for 1.38% of total tree population in the study area. species with less than 10 damaged individuals, such They are represented by 65 species, which account as Bombax malabaricum, Celtis sinensis and Cinfor 27.3% of the total species count in the study area. namomum burmanii, although the population of each Ten main affected species reckoned according to of these species reaches over 1000 in the study area. frequencies are (damage rate as per cent of trees of a The significant differences in windstorm susceptigiven species are given in parentheses): Bauhinia bility are related to species characteristics. The more blakeana 277 (5.56%); Cassia suruttensis 149 vulnerable species are closely related to relatively (12.97%); Melaleuca leucadendra 113 (1.87%); Pilower wood strength. Most Class A and Class B nus massoniana I 13 (4.94%); Casuarina equisetifospecies, such as Acacia auriculijiomis, Albizia chilia 99 (2.75%); Acacia auriculiformis 86 (6.70%); nensis, Anthocephalus chinensis, Bauhinia blakeana, C. Y. Jim, H.H. T. Liu /Landscape Cassia surattensis, Eucalyptus robusta, and Hibiscus tiliaceus, were planted as fast-growing trees with weak wood, Species more liable to storm damage also tend to have wide crown spread and dense foliage, which are, unfortunately, attributes essential for much-needed shading in tropical cities. In addition, Acacia auriculifomis and Eucalyptus robusta have shallow root system, and Bauhinia blakeana Table 3 Wind-destruction susceptibility Species Acacia auricul~formis Acacia confisa Albizia chinensis Albizia falcara Alewires moluccana Anrhocephalus chinensis Araucaria heterophylla Bauhinia blakeana Bauhinia uariegata Bridelia monoica Broussonetia papyrifera Caryta ochlandra Cassia surattensis Casuarina equisetifolia Chukrasia rabularis Cleidiocarpon catlalieri Etyhrina uariegata Eucalyprus robusta Eucalyptus rerericornis Ficus alrissima Ficus microcarpa Ficus [ireas Hibiscus riliaceus Khaya senegalensis Mangifera indica Melaleuca leucadendra Michelia alba Pinus massoniana Sabina chinensis Syzygium cumini Svzygium jambos Taxodium disrichum Total (average) and damage classification and Urban Planning 38 (1997) 45-59 51 have v-shaped crotches, both being characteristics that are predisposed to storm damage. A pertinent characteristic for species to resist storm damage, strongly manifested in the present study, is to have strong wood. For instance, despite exceptionally wide crown spread as well as highdensity foliage, Ficus microcarpa, Ficus virens and Michelia alba are less susceptible to damage (Table of 32 main affected species Number of trees” Damage rate (%Ib Damage class‘ Total Damaged Uprooted 1285 3004 150 318 4665 66 1387 4985 3783 1591 675 484 1150 3602 3198 378 978 879 2928 876 4552 9122 174 978 2023 603 1 3312 2289 393 241 1512 1396 68414 86 59 37 27 27 14 40 277 86 25 50 17 149 99 17 55 17 46 32 11 40 83 20 17 16 113 30 113 49 23 28 12 1715d 6.70 75 B 31 C 1.96 5 A 24.67 3 B 8.49 15 D 0.58 0 A 21.21 2.88 31 C B 5.56 76 2.27 C 26 12 2.33 C B 24 7.41 17 3.51 C A 0 12.97 80 2.75 C D 0 0.53 A 0 14.55 1.74 3 C B 33 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 5.23 D I .09 0 1.25 D D 27 0.88 D 22 0.9 I A 11.49 0 1.74 10 C D 3 0.79 73 1.87 C D 0.92 20 B 4.94 108 A 12.47 B 9.54 0 1.85 C D 0.86 0 (2.51) zyxwvutsrqponmlkjihgfedcbaZYXWVUTSR 650 ‘Total tree count refers to trees in the study area. bExpressed as % of the total tree count. ‘Damage class A indicates that damage rate of the species is higher than 10%; class B indicates that of between 5% and 10%; class C indicates that of between 1.5%and 5%; class D indicates that of lower than 1.5%. dIncludes 32 main species out of the total of 65 affected by the windstorm; hence, the total damaged tree count is less than the 1782 evaluated in the study. 52 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA C. Y. Jim, H. H. T. Liu /Landscape and Urban Planning 38 (1997) 45- 59 Table 4 Tree size in relation to damage mode Damage Height (ml Mode <5 5-10 IO-15 >15 crown <4 (ml 4-8 8-12 > 12 DBH (cmlil < 20 20-40 40-60 > 60 Uprooted Crown Total Cramer’s V b 10.4 89.6 100 0.287 41.9 58.1 100 53.0 47.0 100 51.6 48.4 100 12.2 87.8 100 0.340 55.6 44.4 100 37.3 62.7 100 50.4 49.6 100 12.0 88.0 100 0.310 49.1 50.9 100 48.6 51.4 100 67.9 32.1 100 Total count 865 911 1776 “Values in %. bSignificance level at p < 0.001 for the three tests. 3). Species with tall but narrow crown spread and low-density foliage are also more able to endure wind onslaught, such as the main Classes C and D species Aleurites moluccana, Casuarina equisetifolia, Chukrasia tabularis, Eucal.yptus tereticornis, Melaleuca leucadendra, Michelia alba, and Taxodium distichum. Due to a relatively tall stature in relation to tree size, they are more likely to suffer uprooting damage than other species (Table 3). indicate that these size classes also contribute over 70% of the total tree population. The overall impacts of the storm are, in general, independent of tree size. Tree size, however, has some control over the specific mode of damage (Table 4). There are statistically significant differences between damage mode on the one hand, and tree size attributes on the other. The smaller trees are more susceptible to crown damage, i.e., the partial loss of foliage and branches. The larger trees are more susceptible to uprooting damage (Table 5) i.e., more related to weaknesses at 9. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Tree damage by dimension and age the soil-root interface and anchorage. Tree damage in relation to DBH shows some In terms of height and crown spread (Table 4) the aberrant trend in comparison with height and crown frequency distributions of damaged trees are very attributes. As much as 63.8% of damaged trees fall similar to those of the total tree population. Trees into the 20-40-cm class, vis-a-vis only 48.2% of the with median size at height 5- 15 m and crown diametotal tree population in the study area. Thus the ter 4-12 m account for most (over 70%) of the mainly semi-mature trees with 20-40 cm DBH are damaged population. Results of the pre-storm survey more susceptible to storm damage. Furthermore, the Table 5 Trees DBH and age distribution in relation to damage mode (a) By DBH Damage mode Uprooted Crown Damaged trees 742 (41.7%) 1039 (58.3%) DBH I (O-20 cm) II (20-40 5.9% 31.1% 75.2% 55.7% cm) III (40-60 16.3% 12.3% cm) 2.6% 0.9% (b) By age Total Damage mode Age < 10 years IO-20 years 20-30 Uprooted Crown 79 (10.6%) 289 (27.8%) 454 (61.2%) 502 (48.3%) 137 (18.5%) 169 (16.3%) years > 30 years 72 (9.7%) 89 (8.6%) Total IV(>60cml 742 (100%) 1039 (100%) 100% 100% C. Y. Jim, H. H. T. Liu /Landscape Table 6 Champion trees totally damaged 53 zyxwvut and Urban Planning 38 (I 997) 45- 59 by the storm Species Location count DBH (cm) Estimated age (years) Cause of loss Albizia chinensis Dongshan district Caonuan park Yuexiu park Yuexiu district Dongshan district Dongshan district Yuexiu district Dongshan district Liwan district Yuexiu park 2 80- 90 60- 65 3 80-100 60- 70 1 1 1 1 1 3 90 120 Broken trunk Broken trunk Broken trunk Uprooted Uprooted Broken trunk Uprooted Uprooted Uprooted Uprooted Albizia falcata Bombax malabaricum Eucaly ptus saligna Ficus microcarpa Ficus microcarpa Ficus microcarpa Ficus virens Ficus virens Ficus kens 1 1 same group of trees are more vulnerable to uprooting than the other DBH classes, taking up 75.2% of trees affected by such damage mode (Table 5). The age distribution of damaged trees has been estimated by the Landscaping Bureau staff and verified by the authors. In the post-storm survey, DBH and other dimensional characteristics of the damaged trees (or their remnants) were compared with those of the relatively unaffected trees of the same species in the vicinity to confirm their age class. The results register 61.2% of uprooted trees as lo-20 years old (Table 5), suggesting that this particular age group had suffered more damage than the others. However, 72 mature trees > 30 years old were uprooted and another 89 had broken branches. The young trees (less than 10 years) are largely spared of the worst damage due mainly to small biomass, hence limited hindrance to air movement. Age and the associated maladies due to a stressful tenure in the city environment have weakened many old trees and destined them for a premature demise. Among the damaged trees, 14 champion specimens with DBH > 80 cm and age > 60 years had been uprooted or had snapped trunks (Table 6). They are mainly Ficus microcalpa and F. uirens (57.1%) being the principal species both in the streets and green spaces symbolic of the subtropical city, and two related species Albizia falcata and A. chinensis (35.7%), which constitute the most numerous and largest trees in the study area. The largest existing old trees (> 60 years) in the study area belong to the genus Ficus (77.6%) with wide crown spread and decaying trunk and root (post-mature symptoms of urban trees in stress). 150 130 120 80 105 90- 120 110 85 100 60 80 70- 90 80 65 Thus, they are highly susceptible to storm damage. In absolute terms, only a small number of Ficus microcarpa and F. virens were damaged (Table 3). However, some 68.3% of the damaged trees of the two species are large specimens with over 12 m crown spread; thus, the storm has preferentially destroyed the larger trees. Although Albizia falcata and A. chinensis have only 421 trees in the study area, their weak wood, huge final form and a high proportion (27.7%) of post-mature trees ( > 50 years old) have rendered them easy prey to strong winds. The 120-year-old Bombax malabaricum with DBH Table 7 Tree damage in main sampling Sampling areas Area/length areas Total tree count Damaged Count Parks (ha) Caonuan Huanghuagang Lanpu Liesilinyuan Yuexiu Zoological Subtotal Districtsa (km) Baiyun Dongshan Tianhe Yuexiu Subtotal Other Areas Total “Includes roadside trees % 1.4 12.9 4.0 18.0 65.5 43.3 145.1 246 3304 543 3659 18391 7229 33372 13 43 10 182 339 169 915 5.28 1.30 1.84 4.97 1.84 2.34 2.74 6.5 74.1 18.0 52.6 151.2 1968 13951 6707 706 1 29687 5284 68 343 44 234 451 230 959 111 1985 2.24 1.68 6.72 3.25 3.23 2.10 2.90 trees. C. Y. Jim, H. H. T. 54 Liu / Lmdxuppe Planning38 i 1 Y97145-59 and Urhm The roadside versus offroad tree locations have no obvious effects on overall damage frequency (Table District” By topography By location 9). However, the effects are significant in relation to tree size (height, crown spread and DBH), and also Lowland Hill Low Northern Southern Total part terrace part to some major species. Most of the 31 main damaged Dongshan 82 39 507 509 119 62X species have significantly different damage rates for Tianhe 340 0 III 399 52 451 the two locations (Table 10). Among these species, Yuexiu 43 339 210 526 66 592 Alhizia chinensis, A. ,falcatu, Aleurites moluccana, Others 48 47 16 93 18 III Anthocephalus chinensis, Araucaria heterophyllu, Total 513 425 844 1557 255 1782 Bauhiniu rariegutu, Broussonetia pupyiferu, Gary % 28.8 23.8 47.4 87.4 12.6 100 ota ochlandra, Casuarina equisetifolia, Chukrasia “Includes all damaged trees located in the district tabularis, Erythrina cariegutu, Ficus microcarpa, F. t:irens, Hibiscus tiliuceus, Pinus massoniana, Sabinu chinensis, Syzygium jambos and Taxodium distichum, collectively accounting for 43.2% of the > 90 cm was also destroyed under similar circumdamaged tree population, suffered damaged rates in stances. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA offroad spaces twice those at roadsides. However, other main damaged species, including Acacia auricuhfiwmis, A, confusa, Bauhinia blakeana, Cassia 10. Tree damage by location and land use surattensis, Cleidiocurpon cacalirri, Eucalyptus robusta, E. tereticornis, Ficus altissimu, Khuya seneThe spatial pattern of tree failure varies with galensis, Melaleuca leucudendra, and Syzygium location (Table 7). Of the 1782 damaged trees, 84.4% cumini, which make up 50.8% of the total damaged were distributed in the northern half of the study area population, suffered damage rates in streetsides twice (Table 8). Moreover, 89.7% of the afflicted large those in offroad spaces (Table IO). trees with DBH > 40 cm, and all damaged chamHuman activities can explain some of the wind pion specimens, were found in the same half. This damage. For example, the development history of pattern proves that winds in the northern districts urban districts in Guangzhou influenced the strucwere far stronger than in the southern districts. In tural and age attributes of damaged trees, especially terms of topographical position, 71.2% of the damthose in streetsides. Street tree damage varies signifiaged trees were situated in the low hills and low cantly in three districts with different development platform areas (Table 8) in the northern and northtenure, namely Dongshan, Tianhe and Yuexiu (Table eastern sections, which together take up only 22% of I I). The damaged trees in older districts tend to be the study area. Differences in tree habitats in terms older and larger. Thus, the larger trees in the older districts do not bestow strength and protection against of elevation and topography, therefore, play an imwind destruction; rather they suffer from an accumuportant role on storm damage. Table 8 Spatial distribution Table 9 Relationship Location of damaged trees among tree location, age and tree damage Height (m) Crown (m) Total count DBH(m) Class <5 5-10 IO-15 >I5 <4 4-8 8-12 > 12 < 20 20-40 40-60 > 60 Roadside Offroad Cramer’s V h 151 118 0.797 340 398 312 265 95 97 219 143 0.130 393 425 257 223 42 79 217 150 0.096 554 583 131 118 9 19 “Values in %. bSignificance level at p < 0.01 for all three tests 870 912 C.Y. Jim, H.H.T. Liu/L.andscape Table 10 Location and damage frequency Species Offroad Location Acacia auriculformis Acacia confisa Albizia chine&s Albizia falcata Aleurites moluccana Anthocephalus chinensis Araucaria heterophylla Bauhinia blakeana Bauhinia rariegata Broussonetia papyrifera Cayota ochlandru Cassia surattensis Casuarina eyuisetifolia Chukrasia tabularis Cleidiocarpon cacalieri Erythrina variegata Eucalyptus robusta Eucalyptus tereticornis Ficus altissima Ficus microcarpa Ficus sirens Hibiscus tiliaceus Khaya senegalensis Mangifera indica Melaleuca leucadendra Michelin alba Pinus massoniana Sabina chinensis Syzygium cumini Syzygium jambos Taxodium distichum 55 of the main tree species Species subtotal “Expressed ‘Expressed and Urban Planning 38 (1997) 45-59 Roadside location Location subtotal (no.1 Species Subtotal (%‘c), 2 13 35 21 15 14 40 28 86 48 17 1 56 5 55 17 33 30 11 25 41 20 17 4 20 11 113 49 23 28 12 2.33 22.03 94.59 100 55.56 100 100 10.11 100 96.00 100 0.67 56.57 29.41 100 100 11.74 93.15 100 62.50 44.09 100 100 25.00 17.70 36.61 100 100 100 100 100 1.85 0.47 24.65 Il.16 1.27 43.75 2.30 3.77 11.10 23.65 6.07 0.16 3.53 1.57 14.55 1.99 3.99 I .05 1.85 1.97 3.90 23.47 2.08 0.52 0.60 0.49 4.95 33.33 2.47 (BP Species subtotal (nos.) Location subtotal Species subtotal (7c’c) (%jb 84 46 2 97.67 17.97 5.41 7.14 18.62 2.33 12 44.44 0.34 249 89.89 5.87 4.00 0.74 148 43 12 99.33 43.43 70.59 29.6 2.13 0.42 13 2 28.26 6.25 15.85 2.47 15 42 37.50 45.91 0.46 0.52 12 93 19 75.00 82.30 63.33 0.95 3.47 1.76 1.85 0.86 as o/c of total species. as % of all trees of the species in the location in question. lation of maladies and lack of proper care over the years. Widespread urban renewal and infrastructure deTable I I Damaged street trees in relation to history of urban development District Dongshan Tianhe Yuexiu History (year) Modal tree age (year) Maximum age (year) Modal DBH (cm) Maximum DBH (cm) 50-150 30-50 100 30-50 120 IO-15 8-20 40 15-30 45 >2000 40-70 120 35-70 150 velopment in urban Guangzhou in the last 15 years have contributed to reduced resistance of trees to storm attack. The installation of high-density underground service networks, situated mainly along streets and Inavements. has extensivelv . destroved ex* isting root systems and posed a severe constraint to future root growth (Landscaping Bureau, 1994). Such inimical physical and physiological limitations have predisposed the affected trees to wind damage. In the present storm, at least three large trees in with DBH > 1 m were uprooted because their main roots had been severed to give way to underground electricity 56 C. Y. Jim, zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA H. H. T. Liu / Landscnpe and Urban Planning 38 C1997) 45- 59 Table 12 cables. Many other smaller trees suffered a similar Treatment of damaged trees after the 9 April 1995 windstorm fate. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Site 11. Management responses Under the directions of Natural Hazards Control Centre of the Guangzhou Municipal Government (Guangzhoushi J’mzai Zhihuibu), the Landscaping Bureau coordinates treatment and salvaging of storm-damaged trees. The Greening Company and the Greening Action Areas of urban districts are responsible for removing and treating street trees damaged by the storms, and the park management offices, for damaged trees in park areas. The Greening Company, with specialized arboricultural equipment and trained staff to deal with large trees, is given the onus of emergency clearance of trees blocking streets or electricity lines, regardless of location or district. The Greening Action Areas assume the subsequent tasks on smaller trees in their respective districts after the Greening Company’s urgent works have been accomplished. The management strategies are divided into longterm prevention and post-storm treatments. An important preventive measure is to select species that are resistant to windstorm attack. Others include regular pruning to thin and reduce crowns, to reduce wind resistance. Regular inspection of large and old trees is another long-term approach to identify and remove, in normal time, hazardous trees which are decayed, infected, infested and dying. Special attention is paid to the management of the trees with a high probability of inflicting negative impacts on urban functions, properties or lives. In this regard, trees (in streetsides and in residential and institutional areas) that could potentially affect urban inhabitants, account for more than 65% of total tree population in urban Guangzhou. Furthermore, most electricity cables and telephone lines in the study area are suspended on poles along streets, making tree-induced damage particularly hazardous. Thus, management inputs, both preventive and curative, are focused on street sides and residential areas. In terms of immediate responses, clearing and removal of damaged trees which ruin houses, block streets and other transportation facilities, or break electric lines, are given top priority. The second step Parks Caonuan Huanghuagang Lanpu Liesilinyuan Liuhuahu Luhu Yuexiu Zhongshan Memorial Hall Zoological Districts Baiyun Dongshan Fongcun Haizhu Liwan Tianhe Yuexiu Greening Company” Total Manhours Felled trees 50 102 68 394 20 40 615 36 381 13 II 34 127 99 604 24 45 40 698 441 561 4218 2 173 0 4 15 89 93 112 921 Treated trees Total 0 13 32 43 4 38 61 189 7 8 0 21 21 198 189 387 4 14 10 166 184 18 zyxwvutsrqponmlkjihgfedcba 42 84 11 17 1 373 80 5 1071 “Large trees and emergency tree works are shouldered Greening Company, with separate statistics records. 44 275 11 21 16 462 173 117 1992 by the is cutting and clearing uprooted trees and damaged branches to eliminate their potential danger. The third is treating trees weakened by windstorm and planting trees to fill up street tree gaps resulting from the storm damage. The follow-up steps are executed by Landscaping Bureau’s different subdivisions. In this study, urgent action was conducted 40 min after the end of the storm. Over 60 workers and 10 vehicles of the Greening Company participated in the immediate works associated with damaged street trees. The works were carried out from 08:20 on 9 April to 18:30 on 10 April, dealing with 71 broken or uprooted large trees which blocked 41 streets. Among these trees, 66 trees were cut and cleared, and only five were lifted upright after pruning. The second step of treatment was undertaken by the Greening Action Areas in each district, from IO- 13 April, to treat some 192 1 damaged trees all over the city. Among them, 855 trees were cut down and cleared, and 1066 trees were trimmed and lifted. A total of 4218 manhours were used to treat the 1992 damaged trees during the two response steps (Table 12). The subsequent step, planting new trees C. Y. Jim, H. H. T. Liu /Landscape to substitute for the damaged ones, is initiated after the damage was cleared. 12. Management soon implications Trees in cities are exposed to many natural and cultural stresses, some of which are chronic while others are episodic. In southern Chinese coastal cities, windstorms and typhoons are occasional forces of destruction that can usher extensive havoc to urban greenery. They constitute an important selection force that, from time to time, serves to purge the tree population of its weak trees. The interplay between the frequency of recurrence and the magnitude of the wind force on the one hand, and the condition of the trees on the other, will largely determine the extent of the ‘culling’ process (Bell et al., 1991). As the continued addition of trees to the league of weaklings is a fact of life in the urban tree population, it is to be expected that every storm will extract a certain toll. The extent of damage, however, can be somewhat subdued by suitable management measures. Although the storm has resulted in massive damage to trees in different quarters, it is significant that street trees are more seriously affected and have induced more serious disruption than their counterparts in other green spaces. Street trees in urban Guangzhou have suffered extensive constraints and destruction due to various activities, especially since the economic take-off starting from late 1970s. The widespread root injuries, in particular, have increased susceptibility to storm attack. More stringent measures to protect this valuable green endowment of the city in the relentless rush towards urban development should be adopted. The careful alignment of excavation work to avoid large and outstanding specimens, and a more earnest use of minimum digging and micro-tunnelling techniques (National Joint Utilities Group, 1995), should help to defend trees from unnecessary harm. The landscape professional should be given the chance to assess construction plans to gauge the potential impacts on trees (Leach, 1994). For new plantings, a properly designed soil environment with adequate rooting room, and a good growing medium in physical and chemical terms, should be provided to ensure well-devel- and Urban Planning 38 ( 1997) 45- 59 57 oped root systems and good anchorage (Craul, 1992; Quine et al., 1995). In terms of species selection, there are some conflicts between shading function and storm resistance. For instance, Bauhinia blakeana and B. uariegata have been planted extensively for fast growth, attractive blossom and shading. However, they are highly susceptible to storm damage, and it is doubtful if their popular use should be continued. A drastic rethinking on species selection is needed to link it more closely and rationally to tree-management liability. The present study can furnish the initial objective data to compile a list of wind-sensitive species, whose planting is to be avoided in problematic sites, and of wind-tolerant species for tree managers to choose. For some wind-vulnerable species that are well suited to the cramped and stressful city milieu with desirable tree-form and ornamental qualities, such as Cassia surattensis, Casuarina equisetifolia, Ficus uirens, and Melaleuca leucadendra, it may be desirable to continue planting them in less exposed sites. The use of their cultivars with wind-resistant features can be pursued. For newly-adopted species, trial planting with a limited number in different habitats should be closely evaluated for ‘ windfirmness’ and structural stability. A policy of maintaining diversity in terms of species, size and age should guard against an overzealous focus on wind-resistant trees. Symbolic of subtropical human settlements, Ficus microcarpa, F. sirens and other related species occupy a special landscape role in urban Guangzhou. With centuries of continuous planting history and with many exceptionally large specimens, they have achieved a dominant status in terms of biomass, landscape impacts and environmental amelioration. The huge final size, high-density foliage, longevity, and tolerance of air pollutants (Green Guangzhou Committee, 19901, are earnestly-sought characteristics for urban greening. However, many Ficus trees in the study area in the 100-200 year-old age bracket have prematurely passed their prime due to prolonged abuse and lack of proper care. Many commonly suffer from extensive wood decay and a host of other maladies. The storm badly battered Ficus trees, which account for 57% of the affected large trees with DBH > 1 m, resulting in significant landscape deprecia- 58 C. Y. Jim. H.H.T. Liu /Lmd.ccnpe tion and loss of amenity. Although some championquality specimens have been listed as Protected Trees (Green Guangzhou Committee, 1989) in an attempt to augment preservation, no effective measures have been adopted to minimize storm damage. There is an urgent need to move beyond a simple statutory listing of outstanding specimens (Mitchell et al., 1990; American Forestry Association, 1992) towards providing specialist care to enhance their continued survival (Jim, 1994). Much of the sylvan character of Guangzhou is contributed by the Ficus trees, especially their sizeable members. To sustain this unique image, it is necessary to protect the existing stock while continually adding replacements for the losses. 13. Conclusion In order to reduce storm-related damage of urban trees, the Landscaping Bureau can shift some efforts from its preoccupation with tree planting towards maintenance. This attitude shift is all the more important for developed districts, where many trees have reached the mature period and beyond. Preventive pruning in a several-year rotation associated with scheduled inspection are key measures to reduce storm havoc. Trees near streets, buildings, electric and telephone cable networks, in particular, should be trimmed regularly. The untenable, but unfortunately still common, practice of topping should be stopped forthwith. A thorough evaluation of hazardous trees with decayed stems or branches (Innes, 1990; Matheny and Clark, 1994) should identify candidates for prompt treatment or timely removal. A programmed felling can reduce the hazardous and more costly crisis clearance needed after a storm. Publicity on pre-storm maintenance and post-storm remedial works should help to muster community understanding and support. and dispel the misconception that trees are dangerous. As a short-cycle recurring natural disaster, it will be desirable to have a contingency plan to efficiently coordinate the package of mitigating measures. The ability to minimize damage is an essential yardstick of management capability. A strategy to cope with the preparation, response and recovery aspects, and an administrative setup to effect implementation, will help contain storm damage (Burban and Andresen, and Urban Pluming 38 (I9971 45-50 1994; Grayson, 1989). To facilitate the work, a comprehensive computer database of information on urban trees can provide the raw material and a synoptic perspective to schedule preventive maintenance, identify priorities in the annual tree-care cycle, rationalize the deployment of arboricultural equipment and staff, select appropriate species to match habitat conditions, and prepare planting plans (Crossen, 1989; Chacalo et al., 1994). Acknowledgements Special thanks are conveyed to Ms. F. Zhong and other staff of the Administrative Bureau of Parks and Forests of Guangzhou City; Mr. CF. Qin and Dr. D.S. Guan of the Department of Environmental Science, Zhongshan University; Mr. J.W. Ding of the Urban Planning Bureau of Guangzhou City; and to the research grants provided by the Dr. Stephen S.F. Hui Trust Fund administered by the University of Hong Kong. References Barry, P.J., Anderson, R.L., Swain, K.M. Sr., 1982. How to Evaluate and Manage Storm-damaged Forest Areas. 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