| 1. |
- Klingberg, Jenny, 1978, et al.
(författare)
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Leaf area measurements of urban woodlands, parks and trees in Gothenburg, Sweden
- 2015
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Ingår i: ICUC9 – 9 th International Conference on Urban Climate jointly with 12th Symposium on the Urban Environment. 20-24 July 2015, Toulouse, France.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Leaf area of urban vegetation is an important characteristic since it influences e.g. the urban climate through transpiratory cooling, air quality through air pollutant deposition and water management through rainfall interception. Measurements of leaf area are fundamental to accurately model these processes. Few studies have however presented leaf area measurements in the urban environment. The aim of this study was to i) describe the urban greenery based on measurements of leaf area index (LAI) of trees in different types of urban environments and ii) compare two different methods to measure LAI of urban trees. During the summer of 2014, LAI was measured in a central urban deciduous woodland, a suburban mixed forest, a central old park, a grove adjacent to a traffic route and in allotment gardens in Gothenburg, Sweden. In addition, single urban trees of seven common urban tree species in Gothenburg were measured (Acer platanoides, Aesculus hippocastanum, Betula pendula, Fagus sylvatica, Prunus serrulata, Quercus robur and Tilia europaea). Two different indirect methods were used; the LAI-2200 plant canopy analyzer (Li-cor Inc.) and hemispherical photography. The digital images were analyzed with Hemisfer (Schleppi, WSL). The canopy measurements were performed in a grid or cross with 8-32 points with fixed intervals. For single urban trees, 3-6 specimen of each species was measured. Average LAI of the measured urban parks and forests ranged between 2.6 and 4.8. A better way to characterize the different sites was to use cumulative density functions, which visualized the degree of heterogeneity. The old urban park had the largest LAI range (0 - 8.3). As a comparison, LAI ranged from 2.4 to 5.9 in the more homogenous urban woodland. LAI based on hemispherical photos was similar to the values received by the LAI-2200 plant canopy analyzer. Both methods had advantages and disadvantages. The urban environment offers challenges not present in forest canopies, such as interference of buildings. A combination of methods might be necessary for an environment as heterogeneous as the urban, with both single trees and forest canopies.
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| 2. |
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| 3. |
- Konarska, Janina, 1986, et al.
(författare)
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319: Tree Health, Growth and Cooling Effects at Contrasting Urban Sites in Gothenburg, Sweden
- 2018
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Ingår i: 10th International Conference on Urban Climate/14th Symposium on the Urban Environment, New York, US, August 2018.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- One of the most important ecosystem services provided by urban trees is the mitigation of urban heat. However, this cooling effect may be compromised due to vulnerability of urban trees to harsh urban environment and ongoing climate change. In this project we aim to analyse the growth and health of urban tree species in Gothenburg, Sweden in response to different urban growing conditions, as well as the impact of heat and drought stress on tree cooling benefits through changes to shading effect and transpiration. Three species with different adaptation mechanisms to drought stress were selected: Aesculus hippocastanum, Tilia europaea and Quercus palustris. For each species, trees of the same age planted at adjacent paved and unpaved sites were chosen for the study. In 2017, measured traits included stomatal conductance and transpiration rates, pre-dawn and midday water potentials, chlorophyll content, and leaf area density. In 2018, additional microclimate and human thermal comfort measurements were added to assess the cooling benefits provided by trees. The summertime measurements in 2017 showed significant differences in multiple plant traits between contrasting sites for all species studied, with trees at paved sites showing reduced gas exchange, chlorophyll content and leaf area density compared to sites with a higher fraction of permeable surfaces. These differences were observed both on a hot summer day following a dry period, as well as on a cooler day following a wet period. A clear difference in water use dynamics between species with ”tolerant” (e.g. Q. palustris) and ”avoiding” (e.g. A. hippocastanum) mechanisms against drought stress were also observed. The results highlight the importance of proper planning and species selection in supporting the health and resilience of the urban forest as well as maximising its cooling benefits. The study will be continued in Melbourne, Australia with the focus on urban tree performance alongside water sensitive urban design (WSUD).
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| 4. |
- Konarska, Janina, 1986, et al.
(författare)
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Applications of dual-wavelength hemispherical photography in urban climatology and urban forestry
- 2021
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Ingår i: Urban Forestry & Urban Greening. - : Elsevier BV. - 1618-8667 .- 1610-8167. ; 58
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Tidskriftsartikel (refereegranskat)abstract
- Digital hemispherical photography (DHP) is widely used to measure the radiative environment and estimate sky view factors (SVF) in urban areas and leaf area index (LAI) in forests. However, a limitation is the difficulty to distinguish trees from buildings, or leaves from stems and branches. In this study, we collected and processed dual-wavelength photographs recording visible and near-infrared (NIR) light in order to classify pixels into sky, green and woody plant elements, and buildings. Three applications of the method are presented: calculation of partial SVFs accounting for the obstruction of sky by buildings and vegetation separately, the modelling of mean radiant temperature (Tmrt), and the correction of LAI estimates for light intercepted by woody elements and buildings. The obtained partial SVFs were in good agreement with values modelled based on digital surface models. Distinguishing between buildings and vegetation in the modelling of long-wave radiation fluxes in the SOLWEIG model resulted in differences in modelled Tmrt by up to 3 °C. The bias of LAI estimates in urban parks caused by the light interception by woody elements and buildings was found to be relatively small (3–4 %). However, the presented method shows a high potential for estimates of LAI of urban vegetation in densely built-up areas.
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| 5. |
- Konarska, Janina, 1986
(författare)
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Climate regulation provided by urban greening - examples from a high latitude city
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Cities exert a strong influence on urban climate, and consequently on human health and wellbeing. This increases the importance of considering climate issues in urban planning, particularly in the context of global climate change. One of the key adaptation strategies in climate-sensitive planning is urban greenery. The purpose of this thesis is to increase understanding of how urban greenery influences the air temperature and outdoor thermal comfort in a high latitude city. The thesis consists of three main parts. In the first part the aim is to describe the urban greenery at various scales in terms of the amount of foliage. In the second part different aspects of the cooling effect of urban vegetation and the resulting intra-urban thermal variations are discussed. Finally, the last part deals with the modelling of mean radiant temperature (Tmrt), an important parameter governing human thermal comfort, in vegetated urban areas. The thesis is based on extensive meteorological and plant physiological measurements conducted in Gothenburg, Sweden. Study sites ranged from single street trees to parks and woodlands. Moreover, a LiDAR dataset and high resolution digital surface models (DSMs) of ground, buildings and vegetation were used to analyse spatial characteristics of the study sites, including effective leaf area index (Le) describing tree foliage, and sky view factor (SVF), a measure of obstruction of sky commonly used in urban climate studies. The results show substantial variations in Le between different types of urban greenery, with the highest Le observed in an urban woodland and the lowest in residential green yards. These variations were accurately modelled using LiDAR data. However, when averaged over large areas only partly covered by trees, variations in Le were found to result mostly from tree fraction rather than structural characteristics of tree canopies. Single urban trees of five common species were shown to provide a strong shading effect throughout the year, with a potentially positive effect on thermal comfort in summer and negative in winter in high latitude cities. Parameterisation of transmissivity of solar radiation through tree crowns significantly improved the modelling of Tmrt in SOLWEIG, a model simulating radiation fluxes in complex urban environments. While tree transpiration in temperate climates is often assumed negligible in darkness, night-time transpiration was observed in all of seven common tree species, and data analyses indicated its contribution to the evening cooling on clear, calm nights of the warm season. The cooling effect of trees due to both shading and transpiration was found to be influenced by tree growing conditions and access to sunlight. Trees growing on wide grass lawns had denser crowns and higher stomatal conductance than those surrounded by impervious surfaces. When provided with good growing conditions, sun-exposed trees can strongly influence microclimate by providing additional shade and by intensive transpiration. Parks exhibited a cooler microclimate than built-up sites throughout the day and year, and in different weather conditions, with the strongest cooling effect on clear, calm days of the warm season. While the evening cooling in a high latitude city is best correlated with SVF, spatial characteristics describing buildings and vegetation proved useful in the analysis of intra-urban thermal variations. When high resolution DSMs are not available, near-infrared hemispherical photography can be used to calculate SVFs accounting for the obstruction of sky by buildings and trees separately. The findings presented in this thesis can be used in climate-sensitive planning, in urban climate modelling as well as in valuation of ecosystem services provided by urban greenery.
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| 6. |
- Konarska, Janina, 1986, et al.
(författare)
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Influence of vegetation and building geometry on the spatial variations of air temperature and cooling rates in a high latitude city
- 2016
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Ingår i: International Journal of Climatology. - : Wiley. - 0899-8418 .- 1097-0088. ; 36:5, s. 2379-2395
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Tidskriftsartikel (refereegranskat)abstract
- The aim of this study was to provide a better understanding of how vegetation and building geometry influence the spatial distribution of air temperature and nocturnal cooling rates (CR) in a high-latitude city. Intra-urban thermal variations were analysed in two seasons (May–September and November–March) and in different weather conditions (clear, calm and cloudy, windy) in Gothenburg, Sweden. Simultaneous air temperature measurements were conducted for 2years (2012–2013) at ten fixed park and street sites characterized by varying type and amount of vegetation, building geometry, openness and surface cover. Several spatial characteristics, including sky view factor (SVF) as well as the cover and volume of buildings and trees, were calculated within circular areas of radii ranging from 10 to 150m. Spatial characteristics were found to explain air temperature distribution in the studied area to a large extent throughout the day and year, in both clear, calm as well as cloudy, windy conditions. The highest correlations were found for weighted calculation areas accounting for the influence of both nearest (10m) and wider (25–150m) surroundings. Park sites remained cooler than built-up areas, with the most pronounced cooling effect (0.8°C) on clear, calm days of the warm season. The most important factor governing CR around sunset was SVF. However, on clear, calm nights of the warm season, they were also enhanced by vegetation, indicating the influence of evapotranspiration. Minimum night-time air temperature was governed mostly by the presence of buildings. Within the street canyon, a daytime cooling and night-time warming effect of a street tree was observed, particularly in the warm season. The study shows the importance of various spatial characteristics describing openness, amount of vegetation and building geometry in analysing intra-urban variations in daytime and night-time air temperature.
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| 7. |
- Konarska, Janina, 1986, et al.
(författare)
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Surface paving more important than species in determining the physiology, growth and cooling effects of urban trees
- 2023
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Ingår i: Landscape and Urban Planning. - 0169-2046. ; 240
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Tidskriftsartikel (refereegranskat)abstract
- Urban trees provide numerous ecosystem services including the mitigation of urban heat. However, this cooling effect is often restricted due to poor tree growth and health caused by harsh growing conditions. The aim of this study is to analyse the influence of surface paving on the physiology, growth and cooling benefits of street trees of three common species with contrasting drought tolerance (Aesculus hippocastanum, Tilia × europaea and Quercus palustris) in Gothenburg, Sweden. Tree structural parameters, leaf water potentials, gas exchange, leaf area density and chlorophyll content were measured in three summertime periods in 2017 and 2018. Moreover, based on the measured data, the cooling effects of studied trees due to shading and transpiration were modelled. For all species, trees at highly paved sites were found to have significantly reduced growth, physiological performance and cooling effects compared to sites with a higher degree of permeable surfaces. This negative effect of surface paving was stronger than the effect of species, despite differences in drought tolerance and water use strategy between studied species. Based on the sensitivity of species to surface paving, we recommend Q. palustris and T. europaea for sites with good growing conditions due to their potential to maintain high water use and growth rates, and A. hippocastanum for highly paved sites where shading is not the primary ecosystem service sought. Our findings emphasise the importance of tree planting design in supporting urban tree growth and the continuous provision of ecosystem services, particularly in the context of the changing climate.
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| 8. |
- Konarska, Janina, 1986, et al.
(författare)
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Transmissivity of solar radiation through crowns of single urban trees—application for outdoor thermal comfort modelling
- 2014
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Ingår i: Theoretical and Applied Climatology. - : Springer Science and Business Media LLC. - 0177-798X .- 1434-4483. ; 117:3-4, s. 363-376
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Tidskriftsartikel (refereegranskat)abstract
- Trees play an important role in mitigating heat stress on hot summer days, mainly due to their ability to provide shade. However, an important issue is also the reduction of solar radiation caused by trees in winter, in particular at high latitudes. In this study, we examine the transmissivity of total and direct solar radiation through crowns of single street trees in Göteborg, Sweden. One coniferous and four deciduous trees of species common in northern European cities were selected for case study. Radiation measurements were conducted on nine clear days in 2011–2012 in foliated and leafless tree conditions using two sunshine pyranometers— one located in shade of a tree and the other one on the roof of an adjacent building. The measurements showed a significant reduction of total and direct shortwave radiation in the shade of the studied trees, both foliated and leafless. Average transmissivity of direct solar radiation through the foliated and defoliated tree crowns ranged from 1.3 to 5.3 % and from 40.2 to 51.9 %, respectively. The results confirm the potential of a single urban tree to reduce heat stress in urban environment. However, the relatively low transmissivity through defoliated trees should be considered while planning street trees in high latitude cities, where the solar access in winter is limited. The results were used for parameterisation of SOLWEIG model for a better estimation of the mean radiant temperature (Tmrt). Measured values of transmissivity of solar radiation through both foliated and leafless trees were found to improve the model performance.
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| 9. |
- Konarska, Janina, 1986, et al.
(författare)
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Transpiration of urban trees and its cooling effect in a high latitude city
- 2016
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Ingår i: International journal of biometeorology. - : Springer Science and Business Media LLC. - 0020-7128 .- 1432-1254. ; 60:1, s. 159-172
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Tidskriftsartikel (refereegranskat)abstract
- An important ecosystem service provided by urban trees is the cooling effect caused by their transpiration. The aim of this study was to quantify the magnitude of daytime and night-time transpiration of common urban tree species in a high latitude city (Gothenburg, Sweden), to analyse the influence of weather conditions and surface permeability on the tree transpiration, and to find out whether tree transpiration contributed to daytime or nocturnal cooling. Stomatal conductance and leaf transpiration at day and night were measured on mature street and park trees of seven common tree species in Gothenburg: Tilia europaea, Quercus robur, Betula pendula, Acer platanoides, Aesculus hippocastanum, Fagus sylvatica and Prunus serrulata. Transpiration increased with vapour pressure deficit and photosynthetically active radiation. Midday rates of sunlit leaves ranged from less than 1 mmol m−2 s−1 (B. pendula) to over 3 mmol m−2 s−1 (Q. robur). Daytime stomatal conductance was positively related to the fraction of permeable surfaces within the vertically projected crown area. A simple estimate of available rainwater, comprising of precipitation sum and fractional surface permeability within the crown area, was found to explain 68 % of variation in midday stomatal conductance. Night-time transpiration was observed in all studied species and amounted to 7 and 20 % of midday transpiration of sunlit and shaded leaves, respectively. With an estimated night-time latent heat flux of 24 W m−2, tree transpiration significantly increased the cooling rate around and shortly after sunset, but not later in the night. Despite a strong midday latent heat flux of 206 W m−2, a cooling effect of tree transpiration was not observed during the day.
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| 10. |
- Konarska, Janina, 1986, et al.
(författare)
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Transpiration of urban trees and its impact on nocturnal cooling in Gothenburg, Sweden
- 2015
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Ingår i: ICUC9 – 9 th International Conference on Urban Climate jointly with 12th Symposium on the Urban Environment. 20-24 July 2015, Toulouse, France.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- One of the ecosystem services provided by urban trees is the cooling effect caused by their transpiration. However, while the transpiration of forest trees has been widely studied, little research has been conducted on the daytime and night-time transpirational cooling effect of mature urban trees. Knowledge about the transpiration of street and park trees and its response to different environmental factors can prove useful in estimating the thermal influence of urban greenery as well as in urban planning and management. The aim of this study is to i) quantify the magnitude and diurnal variation of transpiration of common urban tree species in a high latitude city (Gothenburg, Sweden), ii) analyse the influence of weather conditions and fraction of permeable surfaces within the vertically projected crown area on tree transpiration, and iii) find out whether transpiration of urban trees remains active during the night and therefore contributes to nocturnal cooling. Measurements were conducted on mature street and park trees of seven tree species common in Gothenburg: Tilia europaea (Common lime), Quercus robur (English oak), Betula pendula (Silver birch), Acer platanoides (Norway maple), Aesculus hippocastanum (Horse chestnut), Fagus sylvatica (European beech) and Prunus serrulata (Japanese cherry). Stomatal conductance and leaf transpiration were measured using a LI-6400XT Portable Photosynthesis System (LI-COR Biosciences) at daytime and night-time on warm summer days of 2012-2013 in Gothenburg. Leaf area index (LAI) of the studied trees was measured with a LAI-2200 Plant Canopy Analyser (LI-COR Biosciences) in order to estimate the latent heat flux due to tree transpiration. Leaf transpiration was found to increase with vapour pressure deficit and photosynthetically active radiation, with on average 22% of the midday incoming solar radiation being converted into latent heat flux. Midday rates of sunlit leaves varied between species, ranging from less than 1 mmol m-2 s-1 (B. pendula) to over 3 mmol m-2 s-1 (Q. robur). Daytime stomatal conductance was positively related to the fraction of permeable surfaces within the vertically projected tree crown area. A simple estimate of available rainwater, comprising of precipitation sum and a fractional surface permeability within the tree crown area, was found to explain 68% of variation in midday stomatal conductance. The results indicate that a high fractional surface permeability can minimize the frequency of water stress experienced by urban trees and enhance their transpirational cooling. Night-time transpiration was observed in all studied species and was positively related to daytime tree water use. Nocturnal transpiration amounted to 7% and 20% of midday transpiration of sunlit and shaded leaves, respectively. With an estimated latent heat flux of 27 W m-2, evening tree transpiration enhanced the cooling rates around and 1-2 hours after sunset, but not later in the night. The results of transpiration measurements will be combined with vegetation data derived from LIDAR and LAI measurements to estimate neighbourhood- to city-scale cooling effect provided by urban trees.
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