Natural England - How to interpret the short rotation coppice (src) yield maps

How to interpret the short rotation coppice (src) yield maps

The aim of these regional yield maps is to identify potential SRC yields within England.

The guidance below must be referred to when using the yield maps for assessing opportunities and optimum sitings of SRC. Information about how the maps have been generated is provided below along with the limitations of the model.

IMPORTANT - The information supplied by the SRC yield maps is for guidance only and is not intended to fully substitute for professional agricultural advice. Each new planting proposal should be considered on a case by case basis. The user is responsible for any commercial decisions taken based on information presented in the maps.

Background information on how the map was generated

This map is based on yield estimates obtained at a network of 49 field experiments established across the UK during the mid to late 1990s. These experiments were part of the research project, “Yield Models for Energy Coppice of Poplar and Willow” funded by Defra, DTI and Forestry Commission. Further details of this project are available from Forest Research (contact Ian Tubby tel: 01420 22255).

The yield model used to transform site specific yield estimates for five willow varieties into regional maps takes account of the following variables: annual rain fall; seasonal rain fall (March – October); growing degree days; frost days; soil pH and soil texture (sand, clay or loam). Environmental data sets based on 5 x 5 km grid squares representing 30 year averages for each climatic variable were used as was soil data (available from the National Soil Resources Institute).

Model predictions of the potential productivity of the following five willow varieties were considered (parent species in parenthesis): Tora (Salix viminalis x S. schwerinnii); Bjorn (Salix viminalis x S. schwerinnii); Stott 10 (Salix burjatica x S. viminalis); Jorunn (Salix viminalis x S. viminalis) and Jorr (Salix viminalis x S. viminalis). These varieties have either been planted commercially or are similar genetically to those planted commercially.

Average yield estimates for these five varieties grown for two three-year cutting cycles in each 5 x 5 km grid square were calculated. These yield estimates were then categorised into a ‘suitability index’ where ‘highly suitable’ refers to a yield in excess of 12 oven dry tonnes per hectare per year (odt ha-1 yr-1), a ‘suitable’ yield between 8 – 12 odt ha-1 yr-1 and an ‘unsuitable’ yield below 8 odt ha-1 yr-1.

A constraint was added to the model which classified sites at altitudes greater than 300 m above sea level as ‘unsuitable’.

Model limitations

  • Annual variations in weather patterns and climatic conditions can vary considerable from the 30 year average values used here. These variations may lead to actual crop performance deviating from the predictions displayed in the map.
  • New willow varieties become available on a regular basis. These varieties map offer improvements in yield over the varieties used to generate the map.
  • Yields can vary significantly on the local scale due to variations in soil type, moisture availability and soil nutrient status that are beyond the resolution of the map.
  • Yield in the first cutting cycle is often lower than yield in the second cutting cycle.
  • Yield estimates are based on experiments established with approximately 10,000 cuttings per hectare, managed on a three year rotation following cut back. No fertiliser, irrigation or disease control was applied to these experiments. Vigorous chemical and mechanical weed control was used. Variations in crop management from this regime could influence crop productivity significantly.

For further information on this map contact Robert Matthews at Forest Research (tel: 01420 22255).