Effects of Climate and the Urban Heat Island Effect on Urban Tree Growth in Houston

The growing conditions of urban trees differ substantially from forest sites and are mainly characterized by small planting pits with less water, nutrient and aeration availability, high temperatures and radiation inputs as well as pollution and soil compaction. Especially, global warming can amplify the negative effects of urban microclimates on tree growth, health and well-being of citizens. To quantify the growth of urban trees influenced by the urban climate, ten urban tree species in four climate zones were assessed in an overarching worldwide dendrochronological study. The focus of this analysis was the species water oak (Quercus nigra L.) in Houston, Texas, USA. Similar to the overall growth trend, we found in urban trees, water oaks displayed an accelerated growth during the last decades. Moreover, water oaks in the city center grew better than the water oaks growing in the rural surroundings of Houston, though this trend was reversed with high age. Growth habitat (urban, suburban, rural and forest) significantly affected tree growth (p < 0.001) with urban trees growing faster than rural growing trees and forest trees, though a younger age of urban trees might influence the found growth patterns. Growing site in terms of cardinal direction did not markedly influence tree growth, which was more influenced by the prevalent climatic conditions of Houston and the urban climate. Higher temperatures, an extended growing season and eutrophication can cause an accelerated growth of trees in urban regions across, across all climatic zones. However, an accelerated growth rate can have negative consequences like quicker ageing and tree death resulting in higher costs for new plantings and tree management as well as the decrease in ecosystem services due to a lack of old trees providing greatest benefits for mitigating the negative effects of the urban climate.

Effects of the urban heat island and climate change on the growth of Khaya senegalensis in Hanoi,Vietnam

Background: Recent projections expect that Vietnam will be affected most severely by climate change with higher temperatures, more precipitation and rising sea levels. Especially increased temperatures will worsen the situations in cities, amplifying the urban heat island effect. Green infrastructures, i.e. urban trees are a common rtool to improve the urban micro-climate for humans. Vital and well growing trees provide greatest benefits such as evaporative cooling, shading, air filtering and carbon storage. However, urban tree growth is often negatively affected by urban growing conditions such as high soil sealing with compacted tree pits providing small growing spaces with limited water, nutrient and oxygen supply, further warm temperatures and high pollution emissions. This study analyzed the growth of urban and rural African mahogany(Khaya senegalensis(Desr.) A. Juss.) trees in the city of Hanoi, Vietnam and the effects of the surrounding climate conditions on tree growth.Results: The results showed that rural African mahogany trees grew better than trees situated in the city center, which is contrary to other results on tree growth of temperate and subtropical cities worldwide. Moreover tree growth was similar regardless of the time of growth. Other results regarding stem growth of African mahogany located in different areas of Hanoi(east, west, north, city center) revealed a better growth in the northern and western outskirts of the city compared to the growth of trees in the city center.Conclusion: African mahogany trees in the urban centers of Hanoi showed a decreased growth compared to rural trees,which was likely induced by a low ground-water level and high pollution rates. In view of climate change and global warming, the decreased tree growth in the city center may also affect tree service provision such as shading and cooling. Those climate mitigation solutions are strongly needed in areas severely affected by climate change and global warming such as Vietnam.