Investigation of tree stands of public spaces in Szeged JOURNAL OF ENVIRONMENTAL GEOGRAPHY Journal of Environmental Geography 8 (3–4), 33–39. DOI: 10.1515/jengeo-2015-0010 ISSN: 2060-467X INVESTIGATION OF TREE STANDS OF PUBLIC SPACES IN SZEGED Ágnes Takács*, Márton Kiss, Eszter Tanács, Levente Varga, Ágnes Gulyás Department of Climatology and Landscape Ecology, University of Szeged, Egyetem u. 2-6, H-6722 Szeged, Hungary *Corresponding author, e-mail: takacsagi@geo.u-szeged.hu Research article, received 10 September 2015, accepted 1 November 2015 Abstract In urban areas vegetation (especially woody vegetation) is of utmost importance, since it affects the ecological conditions of the city. Urban trees play an important role in improving urban climate both at the local (city, district) and the micro-level (e.g. in public squares). Establishing and maintaining advanced and detailed information systems necessary for the management of urban tree stands is an important task of environmental and climate-conscious city management. Despite that, few of the Hungarian municipalities have a regularly updated tree database. The city of Szeged started efficient green space management in autumn 2013, when we started the creation of a detailed and up-to-date tree register for the public areas, which has been continuously expanded ever since. The survey of the present study covers the period of the growing season, from late spring to early autumn of 2013. All the trees are included in the survey and quite a number of data are recorded for each individual (e.g. species, age, size parameters, exact location, health status, etc.). The recorded data are paper-based, however they are included in a GIS-based green space inventory software, Greenformatic, where coordinates are associated to each object, while information on the state of the tree, its location and handling can be found in the attribute table. The trees included are mostly concentrated in the inner city of Szeged, but the surveys will gradually cover ever larger areas of the city. The results highlight the fact that the structural attributes of the different species’ populations are formed by the integrated effect of the species’ urban tolerance and planting policies of the past decades. The current database already allows highly complex analysis, which contributes to the well-being of city residents. Keywords: urban tree, green space management, tree database, Greenformatic INTRODUCTION Tree stands have many positive effects on the ecological status of a city, its population's health and well -being, making the urban built environment more liveable. One of the most noticeable direct effects is changing the mi- croclimatic conditions (Andrade and Vieira, 2007; Bow- ler et al., 2010). During the active period of the growing season the daytime near-surface air temperature has been proved to be lower under the trees than above the free surface (Lin and Lin, 2010). This is primarily due to the canopy reducing the amount of radiation energy from the surface, as it reflects a part of it and absorbs another part - although the extent of this effect depends on the season and the time of day (Shashua-Bar et al., 2011; Konarska et al., 2013). This directly causes a decrease in temperature, while on the other hand it has an even more significant impact on human comfort, because it results in further physiolog- ical (heat) stress reduction, which is well detectable using different human comfort indices (Égerházi et al., 2013). In areas planted with trees a much larger amount of water leaves to the air through evapotranspiration than either in grasslands or built-up areas. This increases humidity, which indirectly contributes to the development of lower temperatures under the trees and has a generally beneficial effect on human comfort, especially during heat waves (Zhang et al., 2013). Surface roughness increased by the presence of woody vegetation decreases the speed of near- surface air movement, which can have both positive and negative implications. In winter, it can lead to significant heating-related energy savings (Loehrlein, 2014). An important element of improving air quality is the absorption of air pollutants (e.g. ozone, nitrogen, sulphur- dioxide, settling dust, etc.) - the actual quantity depending on the amount of leaf surface. During photosynthesis trees use a substantial amount of CO2 (extracting it from the air), one of the most important greenhouse gases (Nowak et al., 2006). Except for the latter, all those listed here have an indirect or direct impact on human health, either through human respiratory diseases or through otherwise affecting comfort. A very important ecosystem service of urban tree stands is the massive interception of precipitation on the leaf surface, a part of which evaporates directly, while the rest is slowly conveyed towards the ground, making infiltration easier, and significantly reducing the size of flash floods following extreme precipitation events (to an extent depending on the size and state of the stand). The water trapped in this way does not burden the sew- age system at the time, but seeps into the soil gradually and thus more efficiently. This in turn improves the quality of otherwise poor urban soils (Day and Dickin- son, 2008). 34 Takács et al. (2015) Creating and maintaining a green surface property cadastre is a statutory obligation for municipalities in Hungary; woody vegetation represents a substantial part of this. However, the property value in this case is much more than just the value of wood. The pollutant and car- bon sequestration, the reduction in runoff, the energy sav- ings resulting from the shading of buildings are relatively easily expressible in monetary terms. Defining the mone- tary value of much more abstract concepts such as the re- duction of thermal comfort, the aesthetic and / or cultural value, the mental and physical regeneration effect - the further benefits of well-maintained green areas and tree stands - is much more difficult, however without doubt these also should be given some consideration. It makes monetary evaluation particularly difficult that the idea of a property gaining value over time instead of losing it is quite foreign to traditional economic thinking (McPher- son, 2003). STUDY AREA Szeged is situated in the south-eastern part of Hungary (46°N, 20°E; 78-85 m above sea level), in the Lower-Tisza- valley, at the confluence of the rivers Tisza and Maros. Ac- cording to Péczely’s climatic classification (Péczely, 1976) the Great Hungarian Plain is characterised with a dry-warm continental climate, therefore in summer heat is typical and drought susceptibility is high in the Szeged area. The an- nual sunshine duration is high and air humidity is typically low. Winter snow cover is rare and the amount of snow is also low. Szeged is the biggest city of the Southern Great Plain region with an area of 281 km2 and a population of about 170 000 (Fig. 1). Due to its size, the city of Szeged has easily detecta- ble climatic effects on the local scale. The most apparent phenomenon is the formation of a so-called urban heat is- land as a result of artificial surfaces, which is most pro- nounced a few hours after sunset. In Szeged the added heat from the heat island is on average 2-3°C, but in calm, anticyclonic periods it may reach up to 6-8°C (Balázs et al., 2009). This (along with many other climatic effects) significantly affects the life chances of urban vegetation. It may therefore be useful if vegetation studies are supple- mented with a climatological perspective, and vice versa, the climate-modifying effects of the vegetation are inves- tigated. METHODS In order for a city to have an efficient green space man- agement, which is also sustainable in the long term, a de- tailed, up-to-date database is necessary. To this end, in 2012 the Department of Climatology and Landscape Ecology of the University of Szeged (in collaboration with the Environmental Management Office of Szeged) started to set up such a detailed tree register which helps the performance of operational tasks and maintenance while also provides an opportunity for the scientific ex- amination of the urban ecological role of trees (e.g. com- plex ecosystem services evaluation). The data-recording Fig. 1 Tree alleys and squares recorded by the end of November 2013 Investigation of tree stands of public spaces in Szeged 35 during the field surveys was paper-based, with a high de- mand on time and human resources; university students were heavily involved in the work. The ideal period for the survey is from late spring to early autumn, namely the growing season, when the foliage has fully developed and autumn defoliation has not yet started. Some parameters, such as the exact extent of the canopy or the health status of individuals can only be estab- lished with reasonable accuracy in this period. All the trees or shrubs with a dbh (diameter at breast height) over 5 cm are included in the survey and quite a number of data are recorded for each individual (e.g. species, age, size param- eters, exact location, health status, etc.). Photos are also taken of each tree and added to the database. Additional data are related to the surroundings of the tree such as the size of the planting space, the nature of a protection measure or nearby damaging factors. Health data contain information on injury and lesions detected on the root system, the trunk or the canopy, as well as other anomalies included as comments. Based on the data rec- orded separately for the tree parts, an assessment of the whole tree’s health status is provided on a 5-point scale. The person recording the experienced damage can also make management proposals and include them in the da- tabase. In order to get more accurate scientific analysis some extra parameters are also recorded for each individual, which so far have not featured in such registers. These in- clude for example the proportion of dried-out crown parts, the proportion of missing or truncated parts (as a percent- age) and the degree of light availability. These enable more realistic calculations of tree volume and leaf area serving as input for pollutant absorption and carbon-se- questration calculations. Thus regulatory ecosystem ser- vices can be evaluated more precisely (Takács et al., 2014). In order to record and store spatial data a GIS is nec- essary, since visualization and spatial analysis are part of the complex requirements of users (both managers and scientific experts). The Greenformatic - Geospatial Infor- mation Software, is a targeted GIS-based green space in- ventory software developed in Hungary. Coordinates are associated to each object, while information on the state of the tree, its location and handling can be found in the attribute table. This primarily serves to directly make us- ers' everyday work easier. The results shown in the present work were derived from the data of over 5000 tree individuals recorded until November 2013 (see the extent of the area on Fig. 1.). As shown on the map in Figure 1, the database assembled by that date mostly represents the densely built-up areas of the inner city, within the Great Boulevard (Nagykörút). RESULTS The tree population in the register recorded by November 2013 contains 5197 objects, the tree individuals belong to 110 species and 4 categories: stumps, empty planting spaces, former planting spaces and dried-out trees. The city is quite species-rich, however approximately 60% of the individuals belong to the 10 dominant species (Table 1). There are 48 species with less than 5 individuals in the database. Almost half of the recorded individuals belong to spe- cies non-native in Hungary (Fig. 2.). Of these, Sophora ja- ponica, Celtis occidentalis and Koelreuteria paniculata are present in highest numbers (over 200 individuals each). Of native species, different species of linden trees (Tilia sp.) are most frequent (1321 individuals). Fig. 2 The proportion of native and non-native species (miscel- laneous: e.g. dried-out tree, removed planting space, unidenti- fied species) Table 1 The most common tree species in Szeged within the surveyed areas Tree species Number of individuals % Littleleaf linden Tilia cordata 634 12.2% Pagoda tree Sophora japonica 542 10.4% Common hackberry Celtis occidentalis 472 9.1% Silver linden Tilia tomentosa 458 8.8% Large-leaved Lime Tilia platyphyllos 229 4.4% Goldenrain Koelreuteria paniculata 224 4.3% Hore-chestnut Aesculus hippocastanum 168 3.2 % Manna or flowering ash Fraxinus ornus 121 2.3% Plane Platanus hispanica 121 2.3% Norway Maple Acer platanoides 117 2.3% dominant species 3086 59.38% other species 1655 31.85% 36 Takács et al. (2015) It is noteworthy that empty planting spaces, tree stumps and other miscellaneous categories make up 8.6% of the whole database, which means 490 trees were wait- ing to be replanted. That number continued to increase in the winter of 2013, because in addition to the winter cuts a number of individuals had to be removed during the re- construction of Kossuth Lajos Avenue. However, some trees were also planted in the autumn of 2013 and 2014. These changes are not included in the present analysis. This also draws attention to the vital importance of an up-to-date database, which in addition to containing the existing trees, also includes the interventions, rec- orded in the shortest possible time. Only thus can the da- tabase facilitate effective green space management. In addition to allowing the approximate estimation of tree volume value, the size parameters of the individu- als serve as input to a number of other analyses. Although dbh depends on a number of parameters besides age, most importantly on species, light availability and other site conditions, the current distribution of stem diameter clas- ses may be used to refer indirectly to the age of the stands. There is little information available in the literature con- cerning urban trees; there are more examples of such esti- mations from forest stands. Sophora japonica and Platanus hispanica have the largest average dbh in the Szeged database, for both spe- cies it’s close to 50 cm. True, these species also have the largest standard deviation, therefore their dbh range (and probably the age as well) is higher than that of the others. Similarly, there is a large standard deviation in the case of Celtis occidentalis, but the average dbh is lower. Tilia to- mentosa, Koelreuteria paniculata and Fraxinus ornus have the lowest average dbh. In case of the latter two spe- cies standard deviation is also the lowest therefore they show the most homogeneous age distribution of all the species in the database (Fig. 3) and are probably the youngest stands as well. Looking at the age distribution of the total recorded tree population, the average estimated age is 36 years, while the software estimates the age of the oldest speci- mens as 103 years. The age group 15-45 makes up 66% of the total population (Fig. 4). The age distribution sug- gests that the last great tree planting campaign in this area was at the end of the 1980’s and the early 1990’s, but sig- nificant planting actions took place between the two world wars and in the 1970’s as well. (It should be noted that these results still only refer to areas of the city within Nagykörút. Since these events are intimately linked with the city's structural development, the extension of the da- tabase to the outer areas might significantly modify this picture). The number of the old trees (older than 90 years) is only 22. In the case of urban trees - with regard to the unfavourable ecological conditions and a fear of collapse damage – such high ages are very rare, especially in the light of Szeged having been destroyed by the great flood of 1879. As a result of the destruction, even the oldest trees can only be dated back to that time. Among the old- est trees are some of the planes in Széchenyi square, and a few old oak trees in Korányi Alley. Fig. 4 Age distribution of the whole recorded tree population Examining in detail the age distribution of the 10 most common species, the planting practices of the last hundred years are neatly outlined. It can be seen that Tilia plathyphyllos, Sophora japonica and Platanus his- panica have the most diverse age structure, which sug- gests that these species enjoyed an almost unbroken pop- ularity over the last hundred years; they were favoured throughout the last century in plantations (Fig. 5). Fig. 3 The average dbh of the 10 most common tree species along with the standard deviation 0 10 20 30 40 50 60 70 80 tr u n k d ia m e te r ( c m ) Investigation of tree stands of public spaces in Szeged 37 Aesculus hippocastanum was clearly a favourite of the 1970’s, the vast majority of today's white-flowered population belongs to the age group 30-45. The species is in many ways an ideal park tree; it has a very significant microclimate-improving effect since it allows only a small percentage of the direct radiation through the can- opy, and it is also very decorative through almost the whole year. The species was very popular until the late 1980’s (until the massive invasion of horse chestnut leaf miner - Cameraria ohridella) but by now the population is in a critical condition. The proportion of older individuals is the highest in the case of Sophora japonica and Platanus hispan- ica. However, it seems that these species have lost some of their popularity over the last 20 years, since there are very few young individuals in the database. Celtis occidentalis (with an average age of 42) shows the most uniform age distribution in the observed popula- tion. This is due to the fact it is one of most urban-tolerant species; much more tolerant to the unfavourable urban con- ditions (air and soil pollution, drought) than other species, so it was commonly used in the past as well as in today’s urban tree planting. In the last 20-30 years, however, the focus has clearly shifted towards Tilia cordata and tomentosa, Fraxinus ornus and Koelreuteria paniculata. The age distribution of lindens shows that from 1965 to the pre- sent day they are the fashion trees of the city of Szeged Fig. 5 Age distribution of the 10 most common species Fig. 6 Health status of the 10 most common species (1: Optimal 2: Well-cared for, 3: Deficiencies, 4: Serious deficiencies 5: Neglected) 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% over 90 75-90 60-75 45-60 30-45 15-30 0-15 Tilia tomentosa Tilia cordata Tilia platyphyllos Sophora japonica Celtis occidentalis Koelreuteria paniculata Aesculus hippocastan um Fraxinus ornus Platanus hispanica Acer platanoides 1 163 39 19 27 40 10 5 35 5 4 2 122 199 60 78 142 49 48 13 30 21 3 142 372 146 414 285 165 108 32 85 79 4 4 13 0 20 4 0 7 1 0 3 5 2 10 2 2 1 0 0 0 0 1 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 38 Takács et al. (2015) (Gaskó 2008), and in recent decades a rejuvenation process can be observed. However, Tilia tomentosa plays a more important role among younger individuals (about 50%). This is due to the fact that in recent years many perished Tilia platyphyllos trees were replaced with Tilia tomentosa individuals, more tolerant of the harsh urban environment. In the inner, more built-up areas of the city, it is no wonder that in the last 20 years species with rela- tively smaller proportions have been favoured, which would not outgrow the confined space very fast – such as Fraxinus ornus. The assessment of the health status of the popula- tion was carried out according to the standards pre- sented in the methods section. In terms of the whole observed population it is pleasing that 40% of the trees are in a relatively good state, so only minor changes of the trunk and canopy are observed. The other end of the spectrum represents individuals with severe deficien- cies, i.e. where serious trunk and/or foliage damage was observed, such as deep-penetrating trunk decay, rot of the root collar or the crown - which represents an imminent risk of an accident - or withering of the treetop, which warns of major root damage. These symptoms require immediate and significant interven- tion, and in some cases make it impossible to save the tree. The database contains 60 such individuals, which makes up only 3% of the total observed population, but since the most densely populated inner city areas are affected, they require increased attention. The health status data of the 10 most common spe- cies draw attention to a number of interesting fa cts. The highest proportion of significant deficiencies (i.e. more than slight deterioration of health status), can be observed in the case of Sophora japonica (77%), Plat- anus hispanica (71%), Koelreuteria paniculata (74%) and Acer platanoides (73 %) (Fig. 6). The first two spe- cies can be characterized with higher average ages, in- cluding a relatively high proportion of older trees. Of course, this may explain the health status being worse than average. The second place of Koelreuteria paniculata in these rankings is an interesting phenomenon since the age distribution suggests these trees to be relatively young, so poor health is not expected. It may be a re- minder of the fact that the environmental circum- stances in Szeged are probably not well tolerated by this species. The same applies to the case of Acer plat- anoides. The "serious deficiencies" or "neglected" category contain individuals with severe crown -base or root collar rot, or a strong deep -reaching parent branch decay. These two categories appear in major proportions in the case of four species, Tilia cordata, Sophora ja- ponica, Celtis occidentalis and Aesculus hippocasta- num, which seem to be the most threatened of the ob- served tree population of Szeged. In Hungary the horse chestnut leaf miner (Cameraria ohridella ) spread at a very fast rate in the early 1990’s and that infection did not spare the trees in Szeged either. Even today, there are serious problems with the chestnut trees. In many cases, the individuals lose most of their foliage by t he end of July, or the beginning of August and start flow- ering again in September – which in turn greatly weak- ens the tree's immune system. Tilia tomentosa and Fraxinus ornus are in the best state of health among the recorded species. The propor- tion of individuals in the Optimal and Well-cared-for categories is way above 50%. A likely reason is that these species have the youngest stands, many of them having been planted relatively freshly. DISCUSSION AND CONCLUSION Urban woody vegetation plays an important ecological role in settlements, however they are not yet always ap- preciated accordingly in Hungary. There are very few mu- nicipalities who have an up-to-date usable tree register. In the current budget cycle (2014-2020) of the European Un- ion there is particular emphasis on the development of green infrastructure. It is no coincidence, since the opti- mally chosen vegetation can locally mitigate the extremes of global change to a large extent and it can significantly improve the unfavourable living conditions in urban ar- eas. However that requires an up-to-date green space da- tabase, which shows health status, location of the individ- ual trees, and is also informative of the performed and to do tasks. Progress in Hungary in this respect has first ap- peared in some of the bigger cities. Szeged is at the fore- front in this, since the city’s tree register is constantly ex- tended and updated. Such data are essential to the effec- tive management of green areas, but keeping the data up- to-date is also the most labour-intensive part. During the everyday work trees are continuously replaced, there are rejuvenation actions, cuttings, so changes often occur, which need to be continuously monitored. Most of the trees are not older than 45 years; and only 10 individuals in the current database belong to the age group of more than 100 years. This can be explained by the fact that tree planting affecting today’s Szeged effectively began after the catastrophe of 1879, since trees planted before that time were destroyed by the flood. Wartime cuttings should also be taken into ac- count, when the citizens of Szeged needed to acquire fuel for heating. The oaks of Korányi Alley are among the oldest in- dividuals that were demonstrably planted first, right fol- lowing the flood. Therefore when the reconstruction of the river protection wall was carried out this was pre- cisely the reason for the need to exercise caution during the construction works, as these old, healthy individuals represent a huge unique value. The age distribution of the tree population raises interesting questions. The question of how long it is "worth it" to keep an old tree is important for decision-makers and managers; i.e. what is the age when maintenance would become significantly more expensive (due to an increasing need for mainte- nance works, risk of accident, curled concrete, etc.) than the positive effects of the individual. Such aging popu- lations usually have higher care needs, however due to their large canopy and consequently larger leaf surface can have a very significant air quality improving role; Investigation of tree stands of public spaces in Szeged 39 furthermore such important aesthetic, historical, cultural values are connected to them, which are difficult to ex- press in monetary terms. The periods of increased tree planting are clearly vis- ible in the age distribution data for each species, as is the fact that the constantly rejuvenated or freshly introduced species are in the best health. In some cases, however, contradictions arise, since Koelreuteria paniculata indi- viduals despite the young population are in a poor state so their "profitable" maintenance is more difficult. Concerning the health status of the Szeged tree pop- ulation, in the case of certain species the proportion of the “deficiencies” category is relatively high. White-flowered Aesculus hippocastanum trees are especially in poor health. When the funding becomes available most proba- bly the red-flowered variant of chestnut (Aesculus × car- nea) will be used to replace these, which seems to be re- sistant to the pest destroying the other type. A noteworthy initiative to improve the health of the remaining popula- tion was the opening of the formerly asphalt-surrounded planting spaces in a substantial part of Szentháromság street. The open soil surface was covered with mulch and shrubs, which is supposed to greatly improve the state of the stand through allowing a better infiltration of rainwa- ter. The poor health of the trees is probably related to the sometimes extremely parched urban soils. Unfortunately it is possible that the stand is already so heavily degraded that even this measure will not help. The health status of species represented by older stands (Tilia platyphyllos, Platanus hispanica and Sophora japonica) is also worse than the average, the cat- egory of major deficiencies appears in their case. At pre- sent, Tilia tomentosa has the best health status, which may be the result of partly the young age of the stands, and partly of the fact that this sub-Mediterranean species tol- erates urban conditions better than others. Therefore, it is an alternative to be considered when replacing other lin- den species. Although the data presented in this study involve only a part of the total of Szeged’s street tree population, even the current database allows a highly complex analy- sis, of which here only species composition, age-, size- and health status distribution were examined. The establishment of the appropriate species compo- sition is very difficult. 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