Climate Change Boosts Urban Tree Growth in Cities Worldwide

Based on tree ring analyses in ten metropolises worldwide, we show that, in general, urban trees have undergone accelerated growth since the 1960s. In addition, urban trees tend to grow more quickly than their counterparts in the rural surroundings. However, our analysis shows that climate change seems to enhance the growth of rural trees more than that of urban trees. The benefits of growing in an urban environment seem to outweigh known negative effects, however, accelerated growth may also mean more rapid ageing and shortened lifetime. Thus, city planners should adapt to the changed dynamics in order to secure the ecosystem services provided by urban trees.

Numerous studies show substantial effects of climate change on plant growth^1,2,3,4,5. The combination of increasing temperatures, extended growing seasons and reduced or intra-annually redistributed precipitation can increase tree growth in boreal and temperate climate zones, often in higher altitudes⁵. The same factors, however, reduce growth in warmer and drier zones where plant growth becomes water limited due to these changes^6,7. The latter may apply particularly to plants in city centres where the urban heat island effect can aggravate this limitation^8,9,10. The average annual daytime surface urban heat island effect (mean of 419 large cities) is 1.5 ± 1.2 °C¹¹. Other studies report that the air temperature in urban areas may be as much as 2°–10 °C higher than in the surrounding nonurban areas¹². These warmer temperatures may substantially affect the living conditions of plants, particularly if precipitation patterns change simultaneously¹². Therefore, many studies address how climate change, on top of the heat island effect, modifies the manifold functions and services provided by urban plants – especially urban trees – as the most prominent and long-lived elements of urban ecosystems. These previous studies are mainly focused on how climate change alters urban species composition⁹, carbon storage^13,14, and biodiversity¹⁵. These studies were primarily based on model predictions and investigated various adaptation measures^7,16 such as choice of drought resistant species and various environmental provenances^17,18. However, less is known about how climate change and the urban heat island in combination affect the vitality and growth of urban plants. While the impacts of climate change on tree growth have been extensively studied in forests^19,20,21, only limited information is available for urban environments²².

This study analyzes urban tree growth under climate change based on a worldwide increment core sampling and dendrometric tree ring analysis, dating back 150 years. In ten metropolises in boreal, temperate, Mediterranean, and subtropical climate (Fig. 1, Table 1, Figure S1) we sampled a total of 1383 mostly mature trees (see Table 2for sub-sample sizes) using dendrometric measurements and by taking increment cores for dendrochronological analyses. In each city we selected one common species which occurred from the urban centre to the rural outskirts (Figure S1). The rationale behind this selection was to obtain an overarching collection of species which typically thrive under the respective site and climate conditions, and to obtain sufficient samples of trees both suffering and not suffering from the urban heat island effect. Only trees showing no visible signs of damage or disease were taken into account for sampling (see Methods section for a detailed description of the sampling procedure).

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