HEAT TOLERANCE AND HEAT ACCLIMATION IN POTENTIAL FUTURE URBAN TREE SPECIES IN SWEDEN

The full effect on urban greenery due to global warming and future increased intensity and frequency of heatwaves is still unclear. Harsh urban conditions, like the urban heat islandeffect, may require the introduction of new tree species to maintain urban greenery. This study investigated the heat tolerance of five tree species not endemic to Sweden (Koelreuteria paniculata, Liquidambar styraciflua, Liriodendron tulipifera, Magnolia kobus, and Nyssa sylvatica). The research questions were: (1) Will the trees see an increased stress level with increased leaf temperatures? (2) Will the trees acclimate and become more stress-resilient after exposure to heat waves?, and (3) How does the ability to acclimate to heat stress vary among species? During the experiment, trees were exposed to three heat waves. Control treatments were exposed to 23°C in the first two heatwaves, while heat-treated trees were exposed to 33°C. In the third heatwave, both control and heat-treated trees were exposed to 38°C. To evaluate the heat stress tolerance, measurements for net photosynthesis, stomatal conductance, and photosystem II functionality (evaluated as the temperature at which the functionality was reduced 50% of unstressed values, T50) were taken after each heatwave. Additional leaf measurements of leaf temperatures, angles, light intake, and size were also taken for each heatwave. The results show a mixed response between species, indicating species-specific ability to handle heat. No significant results were found to show that trees exposed to multiple heat waves acclimate to become more heat stress resilient. However, trees exposed to repeated heat waves had fewer cases where leaf temperatures exceeded their T50 temperatures. Koelreuteria paniculata stood out as the previously heatwave-exposed individuals managed to maintain higher stomatal conductance and net photosynthesis in heatwave 3, compared to individuals exposed to a single strong heatwave. Another finding was the steady decline of Liriodendron tulipifera throughout the heat waves, as the heat-treated individuals significantly decreased in T50, stomatal conductance, and net photosynthesis over the three heat waves. As a whole, the results indicate that the five tree species may be able to handle moderate heat stress (33°C), but that severe heat stress (38°C) may be too much for them to handle. This suggests that the studied species may not be suitable for planting in the most heat-exposed urban locations.