In Spring 2012 London and the south east of England were officially declared to be in drought conditions. Just as our trees were coming into leaf, water suppliers imposed heavy restrictions which affected how and when newly-planted specimens could be watered. The London Tree Officers Association (LTOA) feels that the current attitude toward trees in time of drought urgently needs to be challenged and revised. Rather than contributing to the problems associated with water management, trees are part of the solution.
Those who manage trees are well aware of the immediate and long-term environmental benefits that they bring. Trees are an integral part of urban water cycles, slowing the percolation rate of water by interrupting rainfall and releasing it more gradually to ground level, and in many cases channelling water down the stem and into tree pits instead of the surrounding hard landscape (Xiao et al 2000). The risk of flooding is further alleviated by the increased permeability of soil caused by roots penetrating previously compacted areas (Denman et al 2012). Soil moisture is steadily replenished and the constant process of transpiration helps to prevent waterlogging. In the absence of trees, rainfall hits our paved surfaces and is directed straight into the drainage systems (where capacity allows) to be lost without benefit.
The high (and increasing) ratio of hard, reflective surfaces in cities such as London – combined with high levels of heat and fuel emissions – make them particularly susceptible to the Urban Heat Island effect, exacerbating local drought conditions and elevating temperatures above acceptable levels for the resident population. It is now well recognised that urban environments can be several degrees warmer than surrounding rural areas (Ennos 2012); policy makers are therefore having to respond to future climate impacts in order to mitigate risks to service delivery, the public, infrastructure, businesses and the environment.
Climate change forecasters predict more weather extremes in the future, featuring both heavy rainfall and long periods of drought with associated temperature increases. Trees – and the canopy cover they provide – play a crucial role in alleviating the effects of these conditions (Ray et al 2010) by regulating temperatures through the provision of shade, reducing wind speeds, through the discernible effects of transpiration and by reducing the risk of flooding in the ways already described. Successful tree planting to sustain tree stocks and guarantee the condition of the urban forest of the future is a crucial part of this process, but without an investment of water in the first few years of establishment new trees will not survive to contribute to dealing with the wider issues.
The LTOA is keen to emphasise the importance of ensuring that every young tree in London has access to the water that it needs to survive and establish. Every litre moving slowly through the canopy and seeping into the soil is a litre which will not end up flowing into the Thames, flooding the High Street on its way. The more healthy and vigorous a tree is, the more effectively it will be able to intercept and conserve water. This document aims to provide practical recommendations to tree managers with regard to water sourcing, usage and conservation which will help the arboricultural industry to deal with future drought conditions.
During the 2012 drought there was considerable confusion over what could or could not be irrigated. Watering ‘gardens’ – defined as including parks, lawns and grass verges – was prohibited, but no specific mention was made of tree pits. At the same time, washing cars for commercial purposes using drinking water was exempted from the ban (Thames Water 2012). To address the concerns arising from the 2012 Temporary Usage Ban, in addition to releasing this document the LTOA have also written to the major water suppliers to ask for clarification regarding trees, and to insist that tree watering is given dispensation from future restrictions. This letter is reproduced in Appendix I.
This document was produced by the LTOA Sustainable Water Management Working Party, comprised of Rupert Bentley Walls (LB Hackney), Cameron Brown (LB Camden), David Lofthouse (LB Merton), John Parker (Transport for London), Katie Roberts (Trees for Cities) and Grayham Tindal (LB Islington).