COS 221-5 - Urban heat islands affect the thermal tolerance and photosynthetic responses of urban trees.

Background/Question/Methods

We are living in a time of unprecedented rapid urbanization and climate change. To prepare cities for a warmer future, we should create better urban forest management plans to select tree species capable of tolerating or acclimating to warmer conditions. Here, we study how urban heat islands (UHI) affect species performance by testing whether heat tolerance and photosynthesis are higher in warmer growing conditions. To do so, we compare heat tolerance and photosynthetic acclimation of seven urban tree species in the city of Montreal under contrasting temperature conditions. We selected the hottest and the coldest spots within the city and sampled ten mature trees in each environment. We compared the temperature where photosystem II declined 50% of the maximum value (T50) for the seven species. In addition, we selected five of the seven species to measure in situ photosynthetic CO2 assimilation to determine the optimum temperature (Topt) and the carbon uptake in each environment. with this study we aim to go forward in urban planning using physiology to improve species performance and ecosystem services.

Results/Conclusions

Two of the seven species (A. platanoides and A. saccharinum) had T50 was higher in warming conditions (by ~1C), three species (Q. macrocarpa, Q. rubra, and T. Cordata) had higher T50 in colder conditions, and two species (C. occidentalis and G. Triacanthos) do not present any difference. Concerning Topt, four species from the five with in situ photosynthetic measurements (A. saccharinum, T. cordata, C. occidentalis, and G. triacanthos) had significative higher Topt in warmer conditions. Our results suggest that heat tolerance and photosynthesis response are highly species-specific. Not necessarily all the species present acclimation to warmer urban conditions, even testing this in mature trees with enough lifetime to acclimate to specific micro-climates. Higher performance in cold areas could be an indication of high heat sensitivity and lower acclimation potential for future conditions. We also noticed that not all the species showing higher T50 in warmer conditions show higher Topt. Thus, the short and long-term responses to warming can be independent. Our results suggest that species need to be examined individually in different environments to assess their physiological tolerance and acclimation potential to warmer temperatures and thus, create more efficient urban management plans.