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Disclaimer: The views and opinions expressed in this article are those of the author and do not necessarily reflect the views of the Arboricultural Association.

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We must plan ahead to win at the numbers game

Author:  Keith Sacre
  13/08/2018
Last Updated:  13/08/2018

‘Continuing to work with stakeholders to develop and implement a programme to plant 1 million trees in England’s towns and cities by 2022.’

This statement is from page 78 of the government’s 25-year environment plan. This, given that we are now mid-way through 2018, equates to approximately 250,000 trees per year beginning in the planting season 2018–19. Ignoring the fact that there is no mention of funding and given the assumption that a significant number of these urban trees will be standard trees and above, this is highly ambitious to say the least.

Talking Trees is a programme being delivered jointly by Bristol City Council, Bristol Tree Forum, Forest Avon Trust and the Woodland Trust. They have set themselves a target of doubling the city’s canopy cover from 15% to 30% by 2050.

These are just two examples of ambitious and very creditable aspirations. There are many, many more which are far too numerous to explore in full here.

Getting the target right

Forest Research has produced, in collaboration with Treeconomics and others, a desk-based study of canopy cover figures for 220 urban areas in England. The study suggests a mean urban tree cover of 16.3%. It also shows huge discrepancies, with a range of 45% cover in Fareham in Hampshire down to 3.3% in Fleetwood in Lancashire. The key message for decision makers suggested that there should be an average 20% target for towns and cities with 15% for coastal towns. The results of this study along with details of those other towns and cities that have completed canopy surveys can be found at www.urbantreecover.org.

The former mayor of London committed to the planting of 10,000 additional street trees for each of his two terms of office. Both targets were achieved.

In a paper entitled ‘Assessing Canopy Cover Over Streets and Sidewalks in Street Tree Populations’ published in the ISA Journal of Arboriculture in November 2002 Greg McPherson, one of the very early developers of UFORE, a precursor to the i-Tree model, and Scott Maco of the Davey Tree Company, suggested that

‘Tree canopy targets are difficult to specify broadly because the opportunities to create canopy are highly variable among cities even within a climatic region or land use class.’

They continue,

‘tree canopy percentage is just one of the many criteria to consider’

and list age class, species diversity and equitable distribution as just some of the other factors which need to be considered when planning a healthy and sustainable urban forest.

It has been suggested to me, in a private conversation, by Dave Nowak of the USDA Forest Service – recognised as the primary developer of the i-Tree model – that just planting large numbers of trees over a prescribed period of time, often in chasing canopy cover targets, can be detrimental to sustainable urban forest development as population bubbles and an eventual age class imbalance are created.

Both the number of trees planted and percentage canopy cover targets are useful and commendable – and certainly better than nothing – but is it possible to achieve both in a planned and manageable way? The discussion is a long one and beyond the space available but two factors might be considered.

In towns and cities approximately 40% of the urban forest estate is in public ownership and largely under the stewardship of local authorities. Most local authorities have an operational inventory of the trees they manage. This may be partial and incomplete or highly sophisticated and comprehensive. Developments in the i-Tree Eco 6 model facilitate the conversion of these inventories into i-Tree studies. The process is cheap and quick but the outcome offers comprehensive information which may not otherwise be available and enables full and detailed urban forest urban forest management plans, down to political ward level, to be prepared for the publicly owned estate.

Two London boroughs, Camden and Ealing, have already converted their existing inventory into i-Tree studies. Newcastle City Council and others are currently engaged in the process, while Grosvenor Estates have undertaken a similar exercise for their London estate.

Were this to be achieved on a national basis the government’s numeric planting aspiration would have a solid evidential base and canopy-cover targets could be founded on detailed knowledge and the tree populations being managed. For a fuller understanding of inventory conversion contact Kenton Rogers at kenton@treeconomics.co.uk.

A dilemma for tree nurseries: What to grow to meet future requirements

A dilemma for trees nurseries: what to grow to meet future requirements.

The growing challenge

The ‘numbers game’ as a basis for tree planting also has problems associated with it not least for the nursery industry and potentially biosecurity. Carefully planned and thought-out tree planting over a prescribed period of time enables collaboration with tree nurseries to take place and long-term contract growing is facilitated. The New York case study is now well known, but less publicised is the fact that the 22,000-per-annum tree planting programme over a 10-year period was achieved in collaboration with the nursery industry in the USA, with contract growing and detailed specification an essential part of that collaboration. Similar collaborative processes have been witnessed in Melbourne (Australia) and Greater Lyon in France where highly successful and acclaimed urban forest management plans are in evidence.

In addition, contract growing facilitates greater species choice and tree population resilience through diversity. Currently there is a ‘chicken and egg’ situation. Why should tree nurseries invest in a wider species list when there is no defined market and why should specifiers widen their palette when the trees they identify may not be available in the nursery industry? Who is going to go first?

The ‘numbers game’ also has the potential to produce irregular and erratic demand which is no basis for nursery investment in the UK. This begs the question as to where young trees are to come from if the supply of UK-grown trees is outstripped irregularly by huge fluctuations in demand fuelled by planting by numbers.

If anyone would like to discuss any of the above and particularly the potential and benefits of contract growing then please contact me at keith@barchamtrees.co.uk.


Article taken from Issue 182 of the ARB Magazine.