Effects of Urbanization on Bud Burst and Tree Leaf Development In

UNIVERSITY OF GOTHENBURG Department of Earth Sciences Geovetarcentrum/Earth Science Centre

Effects of urban

environment on budburst

and leaf development

on Tilia europaea trees

in Gothenburg, Sweden

Katarina Bergman Lyck

ISSN 1400-3821 B1023 Bachelor of Science thesis Göteborg 2018

Mailing address Address Telephone Geovetarcentrum Geovetarcentrum Geovetarcentrum 031-786 19 56 Göteborg University S 405 30 Göteborg Guldhedsgatan 5A S-405 30 Göteborg SWEDEN

I. Abstract

In cities there is a well-documented phenomenon called urban heat island (UHI), this means that the temperature inside the city is higher than the surrounding countryside. Today the artificial surface in the city consisting of buildings, pavements, cars and traffic, contributes to a temperature rise within the city. Trees and vegetation however, lower surface and air temperatures through evapotranspiration and by providing shade. The modern city generally only have vegetation in parks and a few trees for sun and wind coverage along the streets. Trees provide benefits such as emitting oxygen, screening out particulates and other air pollutants, minimize erosion by intercepting precipitation and modify the surface temperature by shading the ground. An important factor for these benefits is the timing of budburst and leaf development. This study aims to get a better understanding about how the intra-urban air temperatures affects the phenology of Tilia europaea (common lime). T. europaea is the most common street tree in Gothenburg, which is why it is chosen for observation in this study. The study was done through air temperature measurements and observations of budburst at eight sites within the city. Hourly, daily, day-time and night-time average temperatures all shows an increase in temperature during the investigated period. The sites closer to the centre are generally warmer and shows a stronger UHI effect during night than the sites further out. When it comes to budburst and leaf development there is no statistical significance between intra-urban air temperatures and onset of budburst and leaf development.. A possible factor influencing phenology is the growing conditions for the trees at the sites. Vegetation covered ground is favourable over paved ground for tree growth. Also the area the trees can grow in can influence the phenology.

Keywords: Phenology · Tilia europaea · Urban heat island · Gothenburg · Sweden

II. Sammanfattning

I städer finns ett väl dokumenterat fenomen som kallas stadens värmeö (UHI), vilket innebär att temperaturen i staden är högre än den omgivande landsbygden. I dag bidrar den konstgjorda ytan i staden som består av byggnader, trottoarer, vägar, bilar och trafik, till en temperaturökning inom staden. Träd och vegetation, sänker lufttemperaturer genom evapotranspiration och genom att ge skugga. Den moderna staden har i allmänhet bara vegetation i parker och några träd för sol och vindskydd längs gatorna. Träd ger fördelar som utsläpp av syre, filtrering av partiklar och andra luftföroreningar, minimerar erosion genom att hejda nederbörd och modifiera yttemperaturen genom att skugga marken. En viktig faktor för dessa fördelar är tidpunkten för knoppsprickning och lövutveckling. Denna studie syftar till att få en bättre förståelse om hur intra-urbana temperaturer påverkar fenologin av Tilia europaea (park lind) i Göteborg. T. europaea är det vanligaste trädet i städer vilket gjorde att det blev valt för denna studie. Studien genomfördes genom lufttemperaturmätningar och observationer av knoppsprickning vid åtta platser i Göteborg. Dygns-, tim-, dagtids- och nattids medeltemperaturer visar alla en temperatur ökning under undersökningsperioden. Platserna närmare centrum är vanligtvis varmare och visar en starkare UHI-effekt under nattetid än de längre bort från centrum. När det gäller knoppsprickning och lövutveckling finns det ingen statistisk signifikans mellan intra-urbana lufttemperaturer och tiden för knoppsprickning. En möjlig faktor som kan påverkar fenologin är växt förhållandena för träden. Mark täkt av vegetation är gynnsamt över asfalterad mark för trädtillväxt. Även storleken på ytan som träden kan växa i kan påverka fenologin.

Nyckelord: Fenologi · Tilia europaea · Stadens värmeö · Göteborg · Sverige

Table of content I. Abstract ...... I II. Sammanfattning ...... II 1 Introduction ...... 4 1.1 Urban Heat Island ...... 4 1.2 Urban vegetation ...... 5 1.3 Plant phenology and temperature ...... 6 1.4 Aim ...... 6 2 Method ...... 7 2.1 Study Area ...... 7 2.1.1 Gothenburg climate and its UHI ...... 7 2.1.2 Gothenburg vegetation and Tilia euopaea ...... 7 2.1.3 Site location ...... 8 2.2 Observations ...... 11 2.3 Analysis ...... 12 3 Results...... 13 3.1 Air temperature differences ...... 13 3.1.1 Average air temperature ...... 13 3.1.2 Average day-time and night-time air temperature ...... 14 3.1.3 Hourly average air temperature ...... 16 3.2 Budburst and leaf development ...... 17 3.3 Regression analysis ...... 18 4 Discussion ...... 19 4.1 Limitations...... 20 5 Conclusion ...... 20 Acknowledgements ...... 21 References ...... 22 Appendix ...... 25

III

1 Introduction

1.1 Urban Heat Island

In every city we have a phenomenon called Urban Heat Island (UHI), this means that the temperature inside the city is higher than the surrounding countryside (Oke, 1972, 1982, 1988). This well-documented phenomenon was discovered in 1818 by meteorologist Luke Howard. Intra-urban thermal variations within a neighbourhood depend on sky view factor (SVF) or amount of greenery. SVF is the proportion of visible sky at the investigated site (Svensson, 2006). Cooling during the night has two modes. The first mode is site dependent cooling during night-time – dense canyons cool less than open spaces. During this time the wind speeds are still relatively high which allows sensible heat flux to dominate. Also the radiative divergence is an important process in this mode. The geometry of the site determines the heat flux. In the second mode, which begins three to four hours after sunset, all places have the same cooling rate. In this mode the wind cease and develops an elevated inversion. During this mode the determining factor becomes the air layer above the rooftops (Holmer et al., 2007). Geometry is one of the most important factors affecting intra-urban sand urban-rural air temperature differences. SVF strongly controls differences of intra-urban surface temperature. SVF relationship with air temperature is, however, less evident (Holmer et al., 2007; Konarska et al., 2016a). Night-time air temperatures is strongly affected by SVF. Anthropogenic heat flux and heat storage increases in built-up areas (Konarska et al., 2016a). Cities consist of features that alter the energy balance within the urban canopy layer (UCL) by for example increasing the absorption of radiation, energy emittance, and sensible heat storage. Some of these features are: buildings, pavements, roads, traffic and vegetation. Urban surfaces keep the air temperatures high and reduces the cooling rate (Oke, 1987). UHI is in general most developed during the night, around three to five hours after sunset (Oke & Maxwell, 1975). Global climate change results in increasing temperatures. High temperatures is already common in urban areas and increasing temperatures may worsen the health impacts. Heat stroke, hyperthermia and increase in mortality rate have been linked to an increase in the intensity and frequency of heat waves (Stott et al., 2004; Tan et al., 2007; Luber & McGeehin, 2008).

1.2 Urban vegetation

Urban parks have a cooling effect, referred to as park cool island (PCI). PCI has been reported in cities across the world (Spronken-Smith & Oke, 1999; Lindén, 2011; Holmer et al., 2013; Konarska et al., 2016a). Vegetation creates lower temperatures through a combination of increased latent heat flux due to evapotranspiration and shade (Spronken-Smith & Oke, 1999). In Gothenburg a clear, calm day in summer gives an intensive PCI. Vegetation will also enhance evening cooling in the day during this season, it has also an effect during winter. However, this is not as strong as the one during warmer seasons (Konarska et al., 2016a). City vegetation is generally located in parks and along streets for sun and wind coverage. Trees and vegetation benefit the city by lower air temperatures through evapotranspiration, and lower surface temperatures by providing shade, they also improve the air quality (Alexandri & Jones, 2008). A review study made by Bowler et al. (2010) investigate whether tree plantations, parks or green roofs affects the urban air temperature. Their results shows that the average temperature in parks was 0.94°C cooler in the day and 1.15°C in the night. However, previous studies is based on observations of small green areas, and the impact of green areas in the wider urban area has not yet been demonstrated. They also suggest urban greening programs to evaluate the benefits of green areas to human health through reduced temperature. However, trees can have negative impacts especially for those who are allergic to pollen. The timing of the phenological phases influences the timing of the pollen season. In Sweden the tree pollen season stretches from early March to the end of July (Pollenkoll, 2018). A late spring can make that many different species pollinate at the same time and give high pollen rates in the air. From a health perspective it is important to know intra-urban temperatures influence plant phenology. Timing of leaf development/budburst is also an important factor when valuating ecosystem services provided by trees. Leaves provide benefits such as emitting oxygen, screening out particulates and other air pollutants, minimize erosion by intercepting precipitation and modify the surface temperature by shading the ground. If the trees are defoliated they will have a harder time to providing these benefits (Alexandri & Jones, 2008). By observing Tilia europaea L. in Florence, Italy, investigated if phenological observations were related to local air temperature, distance to city centre and impervious surfaces and building density. Flowering dates for T. europaea in Florence showed a strong

variability. Massetti et al. (2015) found no significant relationship between distance from the city or the density of buildings and start or end of flowering. However, they observed that the flowering occurred first in areas with a higher fraction of impervious surfaces. Their findings suggest that built-up surfaces can be used to estimate flowering of T. europaea, and along with temperature records it can provide an estimation of changes in plant phenology. Some other studies investigated how urban and rural climate affected plant phenology, and if flowering is affected by UHI effect. These studies have not only been conducted in Europe but also worldwide e.g. Wielgolaski (2001), Neil and Wu (2006), Luo et al. (2007), Mimet et al. (2009), Schleip et al. (2009), Neil et al. (2010), and Jochner et al. (2012). Temperature appears to be the main driver of changes in plant physiology in urban areas (Wielgolaski, 2001; Neil & Wu, 2006; Schleip et al., 2009; Neil et al., 2010). Apart from air temperature Wielgolaski (2001) explained some phenological variances by air moisture and/or soil moisture, excessive water in the soil and soil nutrients. Vegetation growing in urban areas is exposed to warmer temperatures and therefore tend to bloom earlier in spring (Luo et al., 2007; Mimet et al., 2009; Jochner et al., 2012).

1.3 Plant phenology and temperature

Plant phenology is highly dependent on air temperature (Chmielewski & Rötzer, 2001; Olsson et al., 2013). Since 1950’s the growing season has been extended due to earlier spring and later autumns, especially at high latitudes. Species respond differently to temperature, and tree species in the same region get different responses. Many trees need a chilling period before spring to break winter rest. A chilling period requires cold temperatures to prevent early onset of growth. This is to prevent risk of frost damages due to backlashes of spring (Olsson et al., 2013). Studies have shown that higher temperatures during late winter and early spring promotes earlier onset of budburst, leaf development and flowering (Fitter et al., 1995; Walkovszky, 1998; Sparks et al., 2000).

1.4 Aim

The aim of this study was to get a better understanding about how the city affects the phenology of Tilia europaea, also known as common lime. Specific objective was to analyse the intra-urban air temperatures to see if it induce earlier onset of budburst and leaf development in Tilia europaea.

2 Method

2.1 Study Area

2.1.1 Gothenburg climate and its UHI

This study was conducted in Gothenburg city (57°42′N, 11°85′E) located at the west coast of Sweden. With its 564 039 inhabitants (SCB, 2018), it makes it the second largest city in the country. The city centre is characterized by dense and generally low building structures, around four to six stories high. However, there are a few exceptions, new built buildings tends to be higher than 6 stores. The proximity to the ocean gives Gothenburg a maritime temperate climate with warm winters and relatively cool summers. The annual average precipitation is 757.8 mm, and the annual average air temperature is 7.7°C, with highest mean air temperature in July and lowest in January (SMHI, 2009). Geomorphologically Gothenburg is described as a fissure valley landscape, with a few dominating larger valleys. Together with the proximity to the sea a special UHI pattern occurs. In the winter the temperature difference between the urban and rural areas are generally higher in the eastern areas than in the western due to the warmth of the sea. Gothenburg’s UHI is affected by the sea breeze from the west (Eliasson & Holmer, 1990). Studies have also shown that the sea breeze dislocate the UHI centre away from the coast and have a delaying effect on the UHI formation (Gedzelman et al., 2003; Freitas et al., 2007).

2.1.2 Gothenburg vegetation and Tilia euopaea

Gothenburg has vegetation located in parks, as street trees, in gardens, and in forest areas mainly in the rural areas. Urban trees is common along streets and in parks, there is also a few in private gardens. Trees in street conditions or in paved areas are exposed to stressful moments, e.g. heat, drought, limited soil volume, de-icing salt, and high soil pH (Lindgren et al., 2005; Sjöman et al., 2012; Konarska et al., 2016b). Gothenburg has a few larger parks and urban forests e.g Slottskogen, Trädgårdsföreningen, Kungsparken, Göteborgs botaninska trädgård and Ängårdsbergen. These green areas contains a high numbers of species and has rare species than normally do not grow in the city. Cities contain a high number of species in street environment, however, there is a few species dominating, with the most common one being Tilia europaea (Sjöman et al., 2012).

Urban trees need space to be able to survive both underground and above. Street trees are not provided reasonable conditions to thrive, and do not reach normal development. Trees need space for their rots and the city need place for pipes and wires. Trees grooving on parking lots undergoes a lot of stress that will affect their growth, rooting volume, and access to water and nutrient (Celestian & Martin, 2005; Konarska et al., 2016b). Specific guidelines prepared by the City of Gothenburg (Göteborgs Stad) is used when building new streets, planting new trees or fix the already existing streets with trees in. In a street environment this results in one or two rows of trees at the edges and pipes and wires in the middle. The trees can be placed on row that requires a width of at least 14 m or in two row with a width of 21.5 m. Every tree has a 4 m friendly zone with no wires in. Ground coverage differ between site locations. To get a looser soil ground-breaking perennials are used with the possibility to increase gas exchange. When it is not possible to use this, gravel is used or barked closest to the tree. An alternative is to use a plastic grid type filled with permeable grit fraction (Lindgren et al., 2005). Tilia is commonly used as a street tree and/or as a park tree in Gothenburg and is the most common specie in the city (Sjöman et al., 2012). This is the reason to why it is used in this study. T. europaea is an allergenic species, pollen exposure can induce allergic reaction and cough. Fortunately, it is a species with low pollen dispersal in the air (Massetti et al., 2015). A full grown T. europaea can be up to 25 m in height and have a 12-20 m broad crown (Lindgren et al., 2005).

2.1.3 Site location

Eight sites all with T. europaea, located with different distance from the centre were chosen for the study, (Figure 1). To avoid the effect from growing conditions all sites, aside from Lilla Olskroksgatan, were located at grass areas. The sites were chosen to represent different microclimates and different distances in the city (Table 1).

Figure 1: Map of study areas in Gothenburg.

Table 1: Description of measurement and observation sites. The sites are ordered by increasing distance from Nya Allén.

Distance Photograph Site name Site description (km)

Nya Allén Nya Allén, E – W street with deciduous trees growing on grass, including T. europaea. Located at the edge of 0 Kungsparken.

Gamla Ullevi Small green area (N – S) between Parkgatan and Nya Allén opposite to Gamla Ullevi. T. europaea growing 1.1 on grass at both sides of the road.

Ullevigatan Ullevigatan, E – W street with T. europaea trees, growing on grass, along the southern side. Located next 1.7 to one of the main streets through the city.

Fabriksgatan Fabriksgatan, N – S street canyon with T. europaea trees at the eastern side. Trees growing on grass. 1.7

Lilla Lilla Olskroksgatan, N – S Olskroksgatan street canyon with T. europaea trees at the western and eastern side. Trees not 2.5 growing in grass.

Distance Photograph Site name Site description (km)

Hökegatan Hökegatan, N – S street canyon with T. europaea on grass, growing at the western side. 2.8

Kålltorpsgatan Kålltorpsgatan, NW – SE street. T. europaea growing on grass at the north eastern side. 3.5

Sanatoriegatan Sanatoriegatan, E – W street canyon with T. europaea growing on grass at the northern and southern side. 4.1

2.2 Observations

Air temperature was recorded simultaneously at the eight sites using TinyTag Plus 2 loggers. TinyTag has an accuracy of ±0.5°C (Gemini Data Loggers, Chichester, UK). Loggers were placed in a naturally ventilated radiation shield on the north side of the tree trunks, at the height of approximately 2.2m above the ground. Air temperature measurements were conducted from March 23rd 2018 until May 7th 2018 every tenth minute. Data collection occurred every second to third week, by using the program EasyView 5.6.4.2. Field observation of bud burst and leaf development in T. europaea was conducted three times per week starting March 26th to May 7th. Eight T. europaea trees at each site were observed. Since April 23rd the development of ten apical leaf buds on the southern parts of a sample tree was assessed. At each occasion ten new buds was chosen. Each bud got an individual value (0-2); these ten values were summed up to give a leaf development score for

each sample tree (0-20). The observations stopped when a tree reached the maximum score or at the last observation day (May 7th). A leaf development index for each site was calculated by adding up the tree scores for each observation day per site. This produced an index with values ranging from 0 to 160. A protocol made by Wesołowski and Rowiński (2006) was used to determine which development stage bud/leaf were in. The buds were classified as: (0) Undeveloped, (1) Broken and (2) Developed (Figure 2).

2 Figure 2: Stages of bud/leaf development in Tilia europaea (common lime) classified as: 0 (undeveloped), 1 (broken) or 2 (developed).

2.3 Analysis

Data analysis were made in Microsoft Excel 2013. Five different graphs over temperature were produced for the eight sites. Regression analysis were made for average air temperature and budburst index. Analysis of budburst and leaf development index were also conducted in excel. A table over individual bud index were made over the period April 23rd to May 7th, this is the period for the individual observations.

3 Results

3.1 Air temperature differences

3.1.1 Average air temperature

In the beginning of the investigated period, March 23rd to April 3rd the temperature was between -1.3°C to 4.7°C with lowest temperatures at Kålltorpsgatan and Sanatoriegatan, and highest temperatures at Ullevigatan (Figure 3). In April 4th the temperature rise with around 6°C. There is a clear temperature increase during the investigated time period with large fluctuations, especially in the first half. At the end of April the temperature at the sites starts to differ more. 16,0 Nya Allén

14,0 Gamla Ullevi ° C) ( 12,0 Fabriksgatan Ullevigatan 10,0 Lilla Olskroksgatan 8,0 Hökegatan 6,0 Kålltorpsgatan Sanatoriegatan 4,0 2,0

Average air temperature temperature air Average 0,0 -2,0 2018-03-23 2018-03-25 2018-03-27 2018-03-29 2018-03-31 2018-04-02 2018-04-04 2018-04-06 2018-04-08 2018-04-10 2018-04-12 2018-04-14 2018-04-16 2018-04-18 2018-04-20 2018-04-22 2018-04-24 2018-04-26 2018-04-28 2018-04-30 2018-05-02 2018-05-04 2018-05-06 Figure 3: Daily average temperature at each site in Gothenburg, March 23rd to May 7th 2018. Categorized by distance from Nya Allén, dark red - closest, dark green - farthest.

Looking at average daily air temperature for the whole time period at each site against distance from city centre (Figure 4). Sanatoriegatan (7.2°C) and Kålltorpsgatan (6.9°C), the sites furthest away from the centre, are the coolest ones. Gamla Ullevi (7.6°C) and Ullevigatan (7.4°C) are warmest, Nya Allén and Fabriksgatan (7.4°C) has the same temperature and right after them comes Hökegatan (7.3 °C).

7,6

7,5 ° C)

7,4 Nya Allén Gamla Ullevi 7,3 Fabriksgatan Ullevigatan 7,2 Lilla Olskroksgatan 7,1 Hökegatan Kålltorpsgatan

Average air temperature ( temperature air Average 7,0 Sanatoriegatan

6,9 0,0 1,0 2,0 3,0 4,0 5,0 Distance (km) Figure 4. Average daily air temperature at each site against distance Categorized by distance from Nya Allén, dark red - closest, dark green - farthest.

3.1.2 Average day-time and night-time air temperature

Average day-time air temperature at each site against distance from city centre shows temperatures between 8.0°C – 8.6°C, (Figure 5). All sites, except from Kålltorpsgatan, shows a high average day-time air temperatures around the city. There is a relationship between distance and average day-time air temperatures.

8,7

° C) 8,6

8,5 Nya Allén Gamla Ullevi 8,4 Fabriksgatan 8,3 Ullevigatan time time ( temperature - 8,2 Lilla Olskroksgatan Hökegatan 8,1 Kålltorpsgatan 8,0 Sanatoriegatan Average day Average 7,9 0,0 1,0 2,0 3,0 4,0 5,0 Distance (km)

Figure 5. Average day-time air temperature at each site against distance. Categorized by distance from Nya Allén, dark red - closest, dark green - farthest.

Average night-time air temperature at each site against distance from Nya allén shows temperatures between 5.2°C – 5.8°C, (Figure 6). The sites closer to Nya Allén have much warmer temperatures (5.7°C – 5.8°C) than the sites furthest away (Kålltorpsgatan and Sanatoriegatan). A weak relationship between distance and night-time air temperatures can be seen.

5,9 ° C) 5,8

5,7 Nya Allén 5,6 Gamla Ullevi Fabriksgatan 5,5 Ullevigatan time time ( temperature - 5,4 Lilla Olskroksgatan

5,3 Hökegatan Kålltorpsgatan 5,2 Sanatoriegatan Average night Average 5,1 0,0 1,0 2,0 3,0 4,0 5,0 Distance (km)

Figure 6. Average night-time air temperature at each site against distance. Categorized by distance from Nya Allén, dark red - closest, dark green - farthest.

3.1.3 Hourly average air temperature

Kåltorpsgatan and Sanatoriegatan are the sites that is coolest during night-time (20-07), (Figure 7). During this time the other stations has nearly the same temperatures. Around sunrise (6-7) the temperature increases rapidly till noon. Between 12 and 18 the temperature differs around 1°C between the sites. Gamla Ullevi Fabriksgatan and Sanatoriegatan is the warmest sites and kålltorpsgatan is the coolest. After 16 the temperature starts to drop at all station except for Gamla Ullevi, which drops rapidly after 18.

12 Nya Allén

11 Gamla Ullevi C) ° 10 Fabriksgatan Ullevigatan 9 Lilla Olskroksgatan 8 Hökegatan 7 Kålltorpsgatan Sanatoriegatan 6

5 Average hourly air temperature ( air temperaturehourly Average 4 0 2 4 6 8 10 12 14 16 18 20 22 Hour Figure 7. Hourly average air temperature at each site in Gothenburg, for the whole investigated time period. Categorized by distance from city centre, dark red - closest, dark green - farthest.

3.2 Budburst and leaf development

Budburst and leaf development at each sites were slow in the beginning, where only Hökegatan had buds that had reach stage 1 (Table 2). Hökegatan is the site with highest index score and was the site with the most developed leaves when the observations ended. Lilla Olskroksgatan started to develop leaf last compared to the other sites, and got the lowest total index. The sites in the centre were slow starting, however most of the buds on the trees reached development stage 1 in the end of the observation period. Furthermore, Ullevigatan did not reach stage 1 with all the buds, this makes it to the least developed site near the city centre. Kålltorpsgatan and Sanatoriegatan started to develop early, however they were slow in the start, and most of the development occurred during the last days of observation.

Table 2. Budburst and leaf development index (0-160). Starting in April 23rd when individual observation began and ending in May 7th.

Nya Gamla Ullevi- Fabriks- Lilla Höke- Kålltorps Sanatorie- allén Ullevi gatan gatan Olskroksgatan gatan -gatan gatan 23 Apr 0 0 0 0 0 60 0 0 25 Apr 0 0 0 2 0 60 3 10 27 Apr 33 0 0 2 0 63 3 10 30 Apr 59 33 9 24 0 72 3 10 2 May 62 32 9 24 0 80 8 15 4 May 67 54 48 41 0 97 22 37 7 May 83 80 56 80 53 144 80 57

In this study Lilla Olskroksgatan site had trees growing on gravel and asphalt. This gave the site an influencing factor that did not existed at the other sites. Because of the different growing conditions Lilla Olskroksgatan is treated in a different way.

3.3 Regression analysis

Regression analysis were made between average air temperatures (daily, day-time, night-time, hourly, and max) and budburst index and between average ait temperatures and days till first budburst (Table 3). None of the tested regression analysis showed a statistical significance.

Table 3. P-values from regression analysis.

Budburst index Days till budburst P-value P-value Average daily temperature 0.95 0.14 Average day-time temperature 0.82 0.08

Average night-time 0.67 0.38 temperature Average max temperature 0.32 0.12

4 Discussion

Variation among the observed trees was minimised by they all growing on similar ground (grass). However, Lilla Olskroksatan did not have grass covered ground. This could be one reason to the late leaf development at this site. Trees need nutrition and water to be able to grow properly. Ground covered by asphalt and paving is not as permeable to water as grass is (Celestian & Martin, 2005; Lindgren et al., 2005; Konarska et al., 2016b). Therefore, Lilla Olskroksgatan was treated differently and not compared to the other sites to see if intra-urban temperatures affects the onset of budburst. Another site that developed late was Ullevigatan. It had high temperatures in the beginning of the period, in the end it were in the middle (Figure 3). A reason for the late development can be the open, generally flat street area with sparse vegetation and low to non-buildings. Open areas tend to loose heat during the night because there is nothing to trap it (Spronken- Smith & Oke, 1999). However, the air temperature at Ullevigatan was one of the highest during night-time and day-time (Figure 5 and Figure 6). This coincide with Celestian and Martin (2005) result. Hökegatan was the fastest developing site (Table 2), fairly high average temperatures (Figure 3) and warm night-time temperatures (Figure 6). The site was located in an N – S street canyon with an open area in the most northern part. The surrounding buildings have warmed up the air during the day and in the night the heat got trapped inside the canyon. Comparing the results at Hökegatan with Massetti et al. (2015) and Luo et al. (2007), it concludes that built-up surfaces and UHI can be a main driver to earlier budburst and leaf development. Two similar sites were Nya Allén and Gamla Ullevi, both sites were located in the city centre in a park like area with a street running through. They had similar average air temperatures and similar budburst and leaf development (Table 2 and Figure 4-7). The reason for this similarity can be the similar growing conditions for the trees and/or the high night- time temperatures. Both sites had trees growing on grass which favour tree growth. This result confirm that trees growing on grass develop earlier than those that do not. Hökegatan and Lilla Olskroksgatan were two locations (near each other) further away from the city centre (Table 1). They had similar air temperatures, but not similar growing conditions for the trees. This can be the reason to the extreme differentiation in terms of phenology timing (Table 2).

No statistically significant relationship were found for any tested variable (Table 3). Previous studies (Fitter et al., 1995; Walkovszky, 1998; Sparks et al., 2000) shows that higher temperatures during early spring and late winter affects the onset of budburst and leaf development. Even though, this study does not show this an effect of air temperature may still exist on a larger scale. Temperature differences at each site were too small to show if intra- urban temperatures affected budburst. Also, all measurement site were close to the centre. More data were needed to investigate if the previous month temperature affected budburst.

4.1 Limitations

The results and conclusion of the study is affected by one main limitation, which is the lack of study sites in rural areas. Without a rural site it gets hard to investigate if the timing of tree budburst and leaf development occurs earlier in the most urban parts and if the intra-urban variations in air temperature induce earlier onset of budburst. During this study the weather in early March were cold and snowy and when the spring started the temperature rise high to almost summer temperatures at all locations. Also the wind speed and direction could have an impact on tree phenology. Wind speed and wind direction is something that is not measured in this study, however this is not included in previous studies either. In further studies it can be god to include wind speed and wind direction. Another limitation is the individual bud observation method. At each observation day different buds were chosen. It would have been better to observe the same buds at each occasion instead of choosing different. When selecting new buds it could have been an unconscious selection, so the right type of bud were chosen. Therefore, the buds were not randomly chosen. In further investigations same buds needs to be observed to minimise errors.

5 Conclusion

This study shows that: 1) The intra-urban temperature variations in Gothenburg have no effect on the phenology. No statistically significant relationship between air temperature and budburst were found. The intra-urban temperatures can still affect the onset of budburst however, it occurs in a larger scale.

2) Temperature is not the only factor influencing tree phenology. Looking at Lilla Olskroksgatan, growing condition affected the tree phenology more than temperature.

Acknowledgements

I would like to thank my advisor Janina Konarska at the Department of Earth Sciences University of Gothenburg, for guidance and advice thru this project. I would also want to thank Frida Nilsson for the help with installing the temperature loggers and collect temperature data. Lastly I want to thanks my friends and classmates for advice and meaningful discussions during this thesis and these three years of studies together.

References

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Appendix Graphs

16,00 Nya Allén 14,00 C)

° Gamla Ullevi 12,00 Fabriksgatan 10,00 Ullevigatan 8,00 Lilla Olskroksgatan 6,00 Hökegatan time temperature (

- Kålltorpsgatan 4,00 Sanatoriegatan 2,00 0,00

Average nightAverage -2,00 -4,00 2018-03-23 2018-03-25 2018-03-27 2018-03-29 2018-03-31 2018-04-02 2018-04-04 2018-04-06 2018-04-08 2018-04-10 2018-04-12 2018-04-14 2018-04-16 2018-04-18 2018-04-20 2018-04-22 2018-04-24 2018-04-26 2018-04-28 2018-04-30 2018-05-02 2018-05-04 2018-05-06

Figure 8: Average night-time temperature at each site in Gothenburg, March 23rd to April 7th 2018. Categorized by distance from city centre, dark red - closest, dark green - farthest. 18,00 Nya Allén

16,00 Gamla Ullevi C) ° 14,00 Fabriksgatan 12,00 Ullevigatan Lilla Olskroksgatan 10,00 Hökegatan 8,00 Kålltorpsgatan time temperature ( - 6,00 Sanatoriegatan 4,00 2,00 Average day 0,00 -2,00 2018-03-23 2018-03-25 2018-03-27 2018-03-29 2018-03-31 2018-04-02 2018-04-04 2018-04-06 2018-04-08 2018-04-10 2018-04-12 2018-04-14 2018-04-16 2018-04-18 2018-04-20 2018-04-22 2018-04-24 2018-04-26 2018-04-28 2018-04-30 2018-05-02 2018-05-04 2018-05-06 Figure 9: Average day-time temperature at each site in Gothenburg, March 23rd to May 7th 2018. Categorized by distance from city centre, dark red - closest, dark green - farthest.

closest, closest, 2018. Categorized by distance from city centre, dark red - dark city centre, from distance by 2018. Categorized th to May 7

rd at each site in Gothenburg, March 23 March in Gothenburg, site each at

farthest. dark green - green dark temperature air average Hourly 10: Figure 26