CHANGES IN URBAN GREENERY DURING THE 2018 HEAT WAVE – A CASE STUDY FOR GÖTEBORG USING REMOTE SENSING

Sammanfattning: Urban vegetation plays a crucial role in diverse urban microclimates. The numerous ecosystem services provided by vegetation, particularly trees, ensure greater quality of life and thermal comfort regulation. Yet, not all trees are immune to extreme weather events, such as droughts and heat waves, especially in cities. The urban heat island effect, combined with human-induced climate change, adds significant stress to vulnerable urban ecosystems, especially in species that are not used to such extreme conditions. The 2018 summer heat wave which occurred simultaneously with a long period of drought from late spring to late July caused significant tree heat stress, vitality decline and subsequent mortality in urban habitats in Sweden. This study aimed to investigate the impact of this event on urban tree health in Gothenburg, Sweden. The Normalized Difference Vegetation Index (NDVI) was used as an indicator for tree health, which was obtained through remote sensing by using satellite imagery. An overall decline in NDVI across the entire urban area of Gothenburg was identified for late July 2018 when compared to the same period in 2017. Field measurements were conducted in spring 2023 on trees in central Gothenburg to find traits correlating to NDVI differences in July 2018 - July 2017, such as tree height, tree crown surface and permeable surface percentage. Tree height and tree crown area relatively correlated with NDVI changes in 2018, with NDVI declines being lower in larger trees. This indicates greater NDVI declines in smaller trees, implying them to be more drought sensitive. Additionally, NDVI declined x% in trees with a larger y% fraction of permeable surface under the crown. Due to a co-variance of NDVI decline with tree height, I conclude that tree size was the most important indicator for drought sensitivity, followed by area of permeable ground. Smaller trees are shown to generally have greater permeable surface fractions below the crown, due to their size alone. Hence, the advantages of larger trees compared to permeable surface fraction are a key finding. No correlation was found between tree species composition in affected vs. control areas, apart from linden trees (Tilia Europaea) being found more in grouped areas with no decline and less in single tree areas with NDVI decline. With linden trees being large, this confirms previously mentioned statements about tree size and drought resistance. These findings can be used as a basis for improving urban vegetation design, highlighting the importance of tree size, and improving planting design for greater tree growth, which can increase climate resilience and resistance towards extreme weather conditions that are likely to occur more frequently in the future.