What is short rotation forestry?

Short rotation forestry (SRF) involves growing trees on rotations that produce material of 10-20 cm diameter at breast height, over a rotation of between 10 and 20 years.  Due to their faster initial growth, the species used are normally broadleaves, rather than conifers that dominate conventional production forestry in Great Britain.

SRF may be an attractive option for landowners, as it provides more immediate financial returns than conventional forestry.  Also, the ability to change genetic material more rapidly at rotation end, may make it more resilient to a changing climate than productive conifers and broadleaves.

The recent focus on SRF has been to provide biomass to be used for energy and heat generation.  However, there are likely to be other industrial applications for SRF, such as production of composite wood materials, biofuels and extractives. Using SRF species that have a range of end-uses will lower the risk of changes in markets.

How does it compare to short rotation coppice?

Short rotation coppice (SRC) involves establishing cuttings, normally of willow at very high stocking densities of 10,000 to 20,000 stems ha^-1, with the crop being harvested every two to four years.

Establishment and harvesting is highly mechanised and SRC is normally planted on relatively flat, agricultural land.  The harvested material is normally chipped on site  and is usually used as a biomass fuel.

Drawbacks of the system as a supply of biomass fuel are that the small dimension material produced has a high moisture content and bark:wood ratio.  Guidance on SRC, from Ireland is available from Teagasc.

What is the ideal SRF tree?

There are certain silvicultural, wood property and environmental characteristics that define an ideal SRF tree.  These are described in the following table.

Which species should be planted for SRF?

An initial review of species with potential, based on expert knowledge was produced in 2006.  This was superseded by a review of growth and environmental impacts in 2011.

There are historical trials that contribute to our knowledge of SRF species, such as the trials of eucalypts, Nothofagus and red alder established in the 1980s.  However, between 2010 and 2014 dedicated SRF species trials were established in Scotland, Wales and England, to compare a range of species.

The species tested in each country are described in the following table.

There are early but useful results from the Welsh and Scottish SRF trials and these are presented in the following reports:

• Parratt, M. (2019a) Short Rotation Forestry in Wales, Progress Report 2019, March 2019
• Parratt, M. (2019b) Short Rotation Forestry in Scotland, Progress Report 2018, April 2019

Some of the species planted in these trials should now be excluded from consideration due to concerns about their susceptibility to pathogens.  These include ash, larch and Nothofagus.

You can view an analysis of growth and survival results at seven years old from the England trials, on the Oxford Academic website. An overview of the performance of the species where data has been analysed is presented in the following table.

Survival is based only on trials in Wales and England but growth from trials in all three countries:

¹ poor survival at a trial at Brecon due to Neonectria, ² In Wales showed excellent survival, but eucalypts had very high mortality in all trials in Scotland, following cold winters at planting and also in England except at the most southerly trial, ³ Mixed performance across trials, ⁴ England data

The most productive genus by volume growth is Eucalyptus, but there is a risk from cold damage and cold winters soon after planting will result in high mortality.

The greatest potential for eucalypts is in the southwest of England and southern Wales, where the combination of a warm and high rainfall climate result in impressive growth.  Moving eucalypts northwards reduces productivity due to a drop in warmth, while moving eastwards results in a reduction in yield due to lower rainfall.

However, provided they are not subjected to excessive cold they will still grow faster than most other genera.  The photo below shows cider gum (left) and oak and birch (right), all at age 5 years at an energy forest established by Nottinghamshire County Council.

Alders in general exhibit high growth rates when grown on short rotations, with excellent survival.  However red alder has been attacked by Neonectria at one of the trials in Wales and Phytophthora alni, if it becomes common, could be very damaging.  Spring frost damage has also been identified as a risk with red alder across a range of provenances tested in GB.

Hybrid aspen has proven to be a very productive tree, but it can reproduce with native aspen, so could be a threat to the genetic resources of native populations.  However, aspen will only reproduce sexually in very warm summers and so this rarely happened in the north of GB, but it still remains a risk.

Compared with hybrid aspen, aspen is much slower growing but is still relatively fast for a native species and a naturally occurring polyploid, known as giant aspen is thought to be faster growing.

Research to date is available in a Forest Research report on trials in Scotland of aspen clones and another report by Eadha, which usefully compares aspen with other SRF species.

How do we grow SRF?

It is expected that SRF stands will be grown on rotations of between 10 and 20 years.  These short rotations mean that it is important that the trees capture the site rapidly.  This is best achieved through applying more intensive establishment techniques than are used in conventional forestry.  There are few examples of operational SRF in GB, but techniques normally involve intensive cultivation and rigorous weed control, including use of mulches and also application of fertiliser to compensate for any soil nutritional deficiencies.

The stocking density in SRF should reflect the growth rate of the species and also the end use of the material, which in turn influences the rotation.  In general, higher stocking rates will produce more biomass, but each stem will be of smaller dimensions, and this can increase handling and processing costs.

Thinning is not normally a feature of SRF, so the initial spacing and final crop spacing are the same.  There are also operational considerations, for example inter-row spacing might be chosen to enable mechanised weed control.  Cold tolerant eucalypts grown on a ten-year rotation in France for pulp are established at 1,000 to 1,250 stems ha^-1 corresponding to spacings of 4 m x 2.5 m and 4 m x 2 m respectively.

In contrast, the planting at Daneshill in Nottinghamshire established cider gum at 2,500 stems ha^-1 and shining gum, a faster growing species at 2,000 stems ha^-1.

What are the estimated yields for SRF species?

There are few examples of SRF species being grown over large areas and under the intensive conditions that are likely to maximise yields. Most of the predictions have been made by extrapolating results from small plots to estimate volumes, which are then converted to biomass using general estimates of wood density.

The following table provides some estimates collated from the literature of yields from potential SRF species.

What products or services can SRF provide?

The interest in SRF in GB in the last two decades has been directed at finding a carbon-lean source of fuel which could substitute fossil fuels for electricity and heat generation.

However, there has not been a massive uptake of SRF, woody biomass for heating and electricity generation has come from conventional forestry or from imports of pellets from North America and Europe.