UK ash (Fraxinus excelsior) host maps

Before we can investigate the risks faced by ash trees we first need to know where they are found and how many are growing in different regions of the country. This involves a little guesswork, but we are incredibly lucky in Great Britain as lots of information is available to inform our estimates.

Introduction

Ash, Fraxinus excelsior, is a native tree and its range covers the whole country. Ash trees readily self- seed through wind dispersed seeds and are found in many, if not all, land uses. They are most frequent in less managed woodland and hedgerows.

Land use

In order to map the ash distribution, we first combined existing land use maps and woodland cover maps to make our own simple seven category map. Each category in our map was chosen so that we could link it to previous published surveys describing tree composition.

1. All forestry commission land is included in a database that records all tree species present.
2. All woodland > 0.5 ha in size is sampled in the National Forest Inventory
3. Smaller woods are included in the countryside survey
4. Open grown trees and hedgerows are also recorded in the countryside survey
5. iTree surveys have been conducted in a growing number of towns and cities

Figure 1: An example of the Smarties land cover map. A single 50km by 50 km square containing the Forest of Dean is shown.

Broadleaf cover

We used Canopy Cover maps produced through Copernicus (a European Earth Observation program) to calculate the amount broad leaf tree cover in each 20m X 20m square of the land class map. We could then summarise the findings to give the total area of broadleaf in each land class across a one- kilometre grid. To add further detail, we repeated this process to include all small woody features (young trees, hedges and scrub).

Figure 2: Total broadleaf cover by land class. Maps show the estimated area of broadleaf cover in each 1 km by 1 km square across Great Britain.

Ash Abundance

To estimate the amount of ash in each square we combined our broadleaf cover maps with the relevant survey data which described the proportion of ash found within all broadleaved species. The survey data enabled us to map not only the average amount of ash expected, but also generate infinite possible ash maps based on the variation recorded within the survey samples.

Figure 3: Examples of the final ash map. Twenty randomised predictions of the Ash distribution are shown in this animation. Each map shows the total number of Ash trees in each 1 km by 1 km square. Many of these will be young trees, as the average diameter (at 1.3 m) of a UK ash is only 17.4 cm.