In recognition of National Tree Week 2016, Journal of Ecology Associate Editor Emily Lines and some of her students from Queen Mary University of London got together to think about the state of today’s urban trees…
London may not be the world’s largest city, but with 8 million trees covering 20% of its area, it is, according to the founder of the Greater London National Park campaign Daniel Raven-Ellison, the world’s largest urban forest. The historical significance of trees in London is all around us, with place names such as Poplar, for the local abundance of black poplar trees, and Seven Sisters, named after the seven elms that once stood in a circle there (Hidden London, 2016). Indeed the iconic London plane is so named because of its abundant planting by the Victorians (Street Tree, 2014) and is likely, appropriately for our multicultural city, a hybrid of two non-native species: the Oriental plane and American sycamore.
Enhancement of urban natural space often comes in the form of tree planting initiatives, such as the Mayor of London’s Street Tree programme, which are underpinned by common perceptions of ‘the more, the better’. However, amidst our preoccupation with planting, we may have overlooked the fact that urban trees are exposed to levels of stress in built-up environments that are a far cry from their natural habitats. Indeed, recent research on urban trees in the US highlights a potentially worrying future for our urban trees. There, an alarming drop in urban tree cover has been observed (Nowak & Greenfield, 2012), and newly planted urban trees rarely reach full maturity (Moll, 1989); one study found that for every 100 trees planted, only half would live for more than two decades (Lu et al., 2010).
As a group of East London students and residents we are lucky enough to have access to the green spaces of Victoria park, Mile End park, and Lea river valley, so we are well-placed to highlight the challenges faced by urban trees.
The destructive effect of urbanisation on habitats could be offset by planting and maintaining large trees that can hold many species. A single urban tree can provide habitat for hundreds of other organisms. For instance, oaks provide both nesting sites for birds such as the pied flycatcher and marsh tit and roosting sites for bats within holes in their bark (Woodland Trust, 2016). Non-native urban trees provide shelter and food to migrating birds such as waxwings that feed on berries (BTO Bird Migration, 2016; Greenspace Information for Greater London, 2016). Insects such as butterflies, beetles and moths are highly dependent on urban trees for their habitat, with London’s willows and oaks supporting over 400 insect species (Rogers et al., 2015). Urban trees also provide vital habitat for ‘hidden species’ such as lichens, with London’s trees alone harbouring over 60 species of lichen (Larsen et al., 2007). Lichens are known to be a biomonitoring species, found only in areas of limited pollution. While SO2 has often been cited as the main culprit for restricting lichen colonisation in cities, studies have now found that vehicular emissions (especially release of nitrogenous compounds) coupled with temperature changes may have a stronger impact on species distribution (Purvis et al., 2003; Bates et al., 2001).
Levels of SO2, nitrous oxides and other airborne pollutants can be reduced through urban tree planting. Trees reduce air pollution by trapping material particulates in the leaf structure and absorb noxious pollution in their leaves. The material is washed away by rainy weather so absorbed pollutants are incorporated into the soil and are broken down by microbes (Kane & Kirwin, 2009). However, the role of urban vegetation in air pollution reduction is not clear and is likely to depend on strongly on tree species, condition and location in relation to pollutant source (Vos et al. 2013). Recent research from Grote et al. (2016) has compared tree species by pollutant removal potential. Among trees studied in Newcastle-Upon-Tyne, the non-native oak, Quercus phellos was found to remove the most ozone while European beech removed the most gaseous pollutants and particles.
It’s tempting to see a simple solution to a perpetual problem – street plus tree equals cleaner air for humans and animals. Except, predictably, it’s not as simple as that. Other factors planners need to consider are a tree’s water use, shading capacity, hardiness, and even its propensity to release pollutants of its own, and all of this in the ever-present context of climate change. For example, spruce performs well for water use efficiency plus pollutant removal (Grote et al. 2016); but as a tree with a natural distribution in Northern Europe, it may become less useful to urban planners in a warmer future.
A better understanding of the response of different species to urban stresses is needed. Our urban forests are threatened by their own lack of diversity; of the 100 different species found across European cities and towns, 70% are made up of less than five genera (Pauleit et al., 2002). A homogeneous urban tree stock could be susceptible to introduced diseases and invasive pests (Nowak et al., 2010), echoing the arrival of Dutch elm disease that killed over 25 million of the iconic, pollution-tolerant English elm tree in the UK alone (Potter et al., 2011).
There is, therefore, a great need for an ecological perspective on urban planning and policies that lead to a diverse range of trees being planted. London has plans to maintain and increase the canopy cover in the city over time, but the need to implement an improved management plan that is more suitable for the current “urban forest” is increasingly being recognised. Any policies and legislation involving trees require a wide array of relevant information to be considered to inform effective decision-making, which in turn requires more rigorous studies into how threats to our urban trees can be anticipated and understood. In addition to this, improved methods of quantifying ecosystem services will increase the economic priority of caring for urban trees. A more diverse range of species, planted with a full understanding of the impact on them of urban stresses, and visa versa, will produce a resilient future urban forest that is able to support other urban species and provide air, soil and water regulation for the city’s inhabitants.
By increasing awareness of initiatives such as the Mayor of London’s plan to increase tree cover in the city to 25% and the campaign to make Greater London a National Park, we can engage with the public and politicians involved. London appears to be taking steps towards better management of its trees and the animals, plants and services that they sustain; but the future of urban trees depends on us all getting out and learning about what they have to offer and what we can offer them!
Mishma Abraham, Aqeeb Akram, Jack Daniels, Isabella Eddington, Yi Thong Koh, Callum Sykes and Emily Lines, School of Geography, Queen Mary University of London.
Find out more about National Tree Week on the Tree Council Website, including an article on this year’s activities, an interactive map of events near you, and a free downloadable poster. You can also keep track on what’s going on throughout the festival using the hashtags #NationalTreeWeek, #ChangingViews and #TreeCharter.