This database has been compiled from a desktop survey of currently available information on tree species and their relative resilience and tolerance to climatic changes in their environment. These environmental changes have been condensed into the future potential for drought, waterlogging and incidences of snap frosts. The database throws up which tree species possess the necessary qualities to be less or more resilient to these expected conditions.
It should not be considered the definitive tree species list for climate adaptation and should not be used as such, it will be changed and amended as the knowledge base is extended . Its use should always be in conjunction with sound arboricultural advice. This data-base is designed to be flexible; the ability to update records as new information becomes available is an advantage of using a data-base over rigid lists. This data-base is a tool for exploring possibilities and is best used to produce shortlists of candidate species for particular situations that can then be researched further using other sources of information. The data-base is an aid to help users make decisions and is not intended to make the decisions for them.
The objectives of the database are:
- to provide a facility to explore potential species suited to particular locations or planting objectives.
- to ensure that planting for climate change resilience does not over-ride other aspects of species choice, including impacts on human health and the environment.
- to provide guidance on ultimate size to ensure the ‘Right Place, Right Tree’ principle.
- to provide information on known problems associated with particular species.
- to provide a facility for practitioners to make informed decisions.
- to provide the evidence base on which ‘species lists’ are drawn up.
- to allow for future development and the inclusion of additional factors that should be included in the decision-making process.
Drought and Waterlogging Tolerance Categories
Tolerance of trees to drought is achieved in different ways and so is difficult to measure precisely using simple scales. Nevertheless, species can be ranked based on information on strategies to cope with limited water availability, knowledge of the characteristics of the sites where species grow naturally, and based on experience of tolerances when planted in the landscape. We have adopted the five-point scale used by Niinemets and Valladares (2006) for assigning species to particular tolerance ranks. These approximately correspond to the following situations in terms of duration of drought and dryness of soils during the growing season:
- Very intolerant: species needing a constant moisture supply throughout the growing season.
- Intolerant: species that are only able to tolerate short periods of drought and limited soil drying.
- Moderately tolerant: species able to tolerate up to about one month of drought, with soil moisture falling to around the permanent wilting point.
- Tolerant: species able to survive longer periods of drought of about two to three months.
- Very tolerant: species able to tolerate lengthy periods of drought.
Further details of the scale in terms of climatic and soil moisture variables can be found in Niinemets and Valladares (2006).
The waterlogging tolerance categories are also the ones used by Niinemets and Valladares (2006), and correspond to the following periods:
- Very intolerant: species unable to tolerate waterlogging for more than a few days in the growing season.
- Intolerant: species tolerate one to two weeks waterlogging during the growing season.
- Moderately tolerant: species able to survive up to 30 days of consecutive waterlogging.
- Tolerant: species can survive waterlogging for much of the growing season.
- Very tolerant: species able to survive extended periods of deep waterlogging.
We have based our drought tolerance categories using information from two sources, both of which summarise information from a range of other sources: Niinemets and Valladares (2006), which contains drought and waterlogging tolerances for a wide range of woody species collated from several sources, and from the Plants for a Future data-base (Morris, 2005) for species not listed by Niinemets and Valladares (2006). We have rounded or converted tolerance scores from both these sources to assign species to categories.
Niinemets, U., and Valladares, F. 2006. Tolerance to shade, drought, and waterlogging of temperate northern hemisphere trees and shrubs. Ecological Monographs 76:521-547.
Morris, R., (ed.) 2005. Plants for a future. Edible medicinal and useful plants for a healthier world. Plants for a Future Foundation. Charity No. 1057719, Lerryn, Lostwithiel, Cornwall. PL22 0QJ. http://pfaf.org/
The information given in the data-base is, to a large extent, subjective being based on observation, opinion and experience rather than direct scientific measurement. Much of the information may not relate to conditions in London (or the UK in general). Once all information has been collated to provide an objective view of published opinion, expert opinion for UK (London) conditions will be applied through a so-called Delphi analysis. Up to ten experts will discuss the information provided on a species-by-species basis, providing a consensus view for all. Where agreement cannot be reached, more detailed literature review together with a wider trawl of expert opinion will be applied. Strong interest in involvement in such a process was shown from a number of key organisations at a recent meeting of the Tree Council Communications Group.
USDA Hardiness Zone
The USDA hardiness zone system is a widely used index for expressing the cold tolerance, or hardiness, of cultivated plants. In this system, North America is divided into 11 zones categorised by the average annual minimum temperatures within a zone. Each zone represents the lowest temperature range for the survival of a given plant without protection, and the lowest tolerable zone number is frequently included in descriptions of plants, enabling gardeners to choose plants appropriate for their location. The overall temperature ranges, converted from degrees Fahrenheit to degrees Celsius, for the zones are given below:
- Zone 1: Below -45.5 °C
- Zone 2: -45.5 to -40.1 °C
- Zone 3: -40 to -34.5 °C
- Zone 4: -34.4 to -28.9 °C
- Zone 5: -28.8 to -23.4 °C
- Zone 6: -23.3 to -17.8 °C
- Zone 7: -17.7 to -12.3 °C
- Zone 8: -12.2 to -6.7 °C
- Zone 9: -6.6 to -1.2 °C
- Zone 10: -1.1 to 4.4 °C
- Zone 11: Above 4.4 °C
However, this system has drawbacks: hardiness is a complex phenomenon involving several interacting factors that may not be fully represented by a simple climatic measure; minimum temperatures within zones are not uniform but are subject to local climatic variability; and other factors besides average annual minimum temperatures, such as heat , drought and extreme cold snaps, affect survival. Nevertheless, the system has been extended to other regions such as Europe, and zone information for plants is widely publicised and is reproduced in this database for completeness. The British Isles generally experience mild winters due to the moderating influence of the Gulf Stream, and so our hardiness zones are relatively high ranging from Zone 7 in parts of Scotland to Zone 10 in the Isles of Scilly. It should be noted that urban areas are generally warmer – by up to 2°C in winter – than surrounding rural areas as a result of the ‘urban heat island effect’. Taking this into account can increase the range of suitable species, but the unpredictability of weather must always be borne in mind. Further information on the micro-climate of urban areas is available from The Met Office Factsheet on Micro Climates.
In considering which tree or trees to choose, if you are on the boundary between zones the tree which tolerates colder temperatures should be chosen to ensure survival.
Similarly, as climate change begins to affect the UK's hardiness zone designation (mainly Zone 8 at present) it may be necessary to amend tree choice based on which emissions scenario is likely to be encountered in future years. Although, erring on the side of caution is prudent, so planting trees that can tolerate unexpectedly severe frosts would be considered to be best practice if only to ensure that such events do not create considerable losses in the overall tree population of an area.
When selected the results page for a particular tree species gives an overview of the qualities and characteristics of the tree. A ticked box indicates the tree is suitable for the chosen location/soil type or has the listed attributes.
If the box is left blank this indicates that either there is no information currently available on this species, or in cases where a subset of valid boxes are ticked, the particular attribute has not been recorded against this species. As the database is developed these fields will be expanded and added to with the expansion of the knowledge base.
In the context of new developments in London where clay soil predominates one of the most important uses of this database will be to inform the design and engineering teams of the need to follow expert advice on designing foundations of commercial and residential properties to accommodate the presence of the chosen tree species for the location.
If the database is only used to facilitate the planting of smaller ornamental climate resilient trees on new developments rather than securing the positions of larger climate resilient tree species, it will, to a large extent have failed to climate proof London against future scenarios. It is therefore imperative that the use of the database is integrated into the decision making process at the very earliest stages of development design in its broadest terms and that professionals do not shy away from specifying large species trees in their designs.
Further Information on Root Spread
Users are advised to obtain further information on root spread from The Institution of Structual Engineers document Subsidence of Low Rise Buildings 2nd Ed. 2000. ISBN 1 874266 54 9. Careful consideration of the explanatory text in Chapter 8 and also considered use of the information supplied in Table 8.1 is required prior to planting a tree in proximity to an existing building.
In the context of new development this information can be used to support the case for engineered foundations when selecting tree species.