Trees in cities provide important benefits (or ecosystem services), such as decreased local air temperatures, reduced air pollution and the attenuation of storm water. The range and volume of the benefits provided depends on the size of the trees and their canopy. While city locations can be good places for trees to grow, for example if a tree is growing alone and isolated from the competition of other trees, they can also be stressful places which present challenges to tree growth. For example a strong increase in air temperature and reduction in water availability in an urban location may restrict tree growth and in turn, the benefits it provides.

Knowledge of the rate at which trees grow in urban areas is an important consideration when calculating and valuing the benefits provided by urban trees. This paper reports on a study of growth variations in diameter and height for four common urban tree species (sycamore, silver birch, European ash and pedunculate oak) across five cities in Great Britain (Cardiff, Birmingham, Peterborough, Glasgow and Edinburgh). It also explains how climate affects the typical radial growth of ash and oak.

It explains that multiple factors affecting tree growth seem to influence different species in different ways, with for example sycamore trees showing overall the fastest growth in Peterborough but birch trees showing the slowest. Results therefore indicate a substantial variation in the mean annual growth rates across different cities. When calculating ecosystem service delivery it is therefore important to take into account the city and the age and species variations of trees within that city.

In terms of the effect of climate on the radial growth of ash and oak trees in Great Britain, rainfall and temperature where found to have an effect, but the strength and direction of influence varied with time of year, species and city. In particular, low rainfall at the start or during the growing season was found to be a significant factor limiting radial growth. A trend towards a reduction in growth was therefore identified in hot and dry years, primarily in south-eastern cities but in other cities too. This highlights the risk that a changing climate may have on the growth and, consequently, on the provision benefits by healthy urban trees. For example, if hot and dry years become the norm, the growth of both oak and ash could significantly reduce, especially in the southern regions of Great Britain.

This research was funded by the Forestry Commission and carried out in collaboration with Prof. Tom Levanic from the Slovenian Forestry Institute. It was conducted as part of Forest Research’s work quantifying the benefits of urban trees.

A full version of the paper is available.