NF8 Proportion and area of larch within Phytophthora ramorum Risk Zone 1

Larches are important timber species for Scotland and account for around 10% of conifer growing stock in Great Britain (35.6 million m³)[1]. Scotland accounts for approximately half of this (17.1 million m³), with the Forestry Commission Scotland’s National Forest Estate (NFE) holding around 28% of this volume (4.8 million m³) (Forestry Commission, 2014).

Phytophthora ramorum (Pr) is a fungus-like pathogen which is causing serious damage and mortality to trees and other plants.  Although the disease can infect around 150 plant and tree species, in Britain, larches (Japanese, European and Hybrid) have been found to be particularly susceptible and can die within one to two seasons after infection.

Studies indicate that the UK is currently climatically a very suitable area for Pr establishment and survival, but it is not yet clear how climate change will impact upon its prevalence (Moffat et al., 2012). However, there is an east-west climatic divide, with increased risk in the moister west of the UK, with the lowest risk noted for Eastern Scotland (DEFRA, 2012). This climatic division is reflected in the three Forestry Commission Risk Zones (Figure 1) and are used to guide forestry management (Forestry Commission Scotland, 2015a):

• Zone 1 encompasses the general area of higher climatic risk where infection has been, or is considered most likely to be, found on larch.
• Zone 2 remaining areas of higher climatic risk, but where isolated or no infection has yet been found
• Zone 3 Areas of lower climatic risk where isolated or no larch infections has yet been found

Figure 1 Map of Phytophthora ramorum Risk Zones

There are three species of larch in Scotland, the European (Larix decidua), the Japanese (Larix kaempferi) and the Hybrid (Larix x eurolepis). To date, it is Japanese larch which has shown by far the greatest susceptibility to the disease, as well as being the most prevalent species of the three (Figure 2). However data on all three species is presented here as it is known that both the European and Hybrid types are susceptible (Defra, 2014) and an outbreak was detected in European larch in Cornwall in 2011 (Forestry Commission, 2011a).

This indicator considers the risk to larch of infection based on both the absolute area and proportion of Scotland’s larch which lies within the Risk Zones. Areal data provides a metric of the amount of larch at high risk of exposure to the disease; whereas the proportional data enable the identification of any change in the potential threat to Scottish larch based forestry and timber products as a whole.  Larch area data are based upon the primary, secondary and tertiary species data available in the Forestry Commission Scotland Sub-compartment Database 2014.

Related indicators:

NF10 Forest area infected by Phytophthora ramorum

NF11/NF12 Number of forest sites served with a Statutory Plant Health Notice (SPHN) for Phytophthora ramorum (Pr); Area of forest felled under Special Plant Health Notices (SPHNs) for Phytophthora ramorum (Pr)

Prior to 2009, Phytophthora ramorum was limited in its extent and impact in the UK.  However during 2009 it was found infecting and killing large numbers of Japanese larch trees in South West England.  In 2010 it was found on Japanese larches in Wales, Northern Ireland and the Republic of Ireland, and in Scotland at one site on the Craignish peninsula. During 2011 further infection sites were found on Mull and in Dumfries and Galloway (Forestry Commission Scotland, 2015b). As a consequence, Risk Zones were revised in 2011, expanding Zone 1 to include north west England and the area around the island of Mull (Forestry Commission, 2011b).

There was a significant expansion of outbreaks in Dumfries and Galloway in 2012 and 2013, following a wet autumn and mild winter in 2011/2012 then extremely wet and windy conditions in summer and autumn 2013 (Forestry Commission, 2011c). The current extent of Risk Zone 1 reflects this expansion and the zone now covers a significant portion of Scotland (Figure 1).

The indicator is currently not able to include an analysis of these former extents in conjunction with sub-compartment data.

It is now considered that the eradication of Phytophthora ramorum on larch in Scotland is ‘no longer achievable and the aim now is to contain and slow down new outbreaks’ (Forestry Commission, 2015c). In order to manage the spread of Pr, Statutory Plant Health Notices (SPHNs) are issued by the Forestry Commission in Pr affected areas and require the felling of infected trees and those in a surrounding buffer zone. It is recommended that the felling is followed by a fallow period to further minimise risk of reinfection. Infected timber can still be used commercially, but early felling reduces the potential yield from the stock. Whilst restocking can occur after this period, it is recommended that larch (of any type) is not planted (Forestry Commission, 2011c[1]). New legislation introduced in 2014, brought in a change in restrictions to a smaller area of Risk Zone 1. Within this ‘Management Zone’ (Figure 3), an area where Pr is particularly prevalent, SPHNs are no longer issued for infected trees as this had become an impracticable way to manage the disease. However, movement of infected timber can only occur freely within the Management Zone, with strict controls on whether and how timber is moved outside of the area (Forestry Commission Scotland, 2015d). It is intended that this approach will make procedures in the most infected areas simpler and less time consuming, and will help in the management of Pr not only here but also in the rest of Scotland.

Figure 3 The legal boundary of the Phytophthora ramorum management zone. Source: Forestry Commission Scotland (2015d).

Over the period of 2010-2014, approximately 800 ha of forestry was felled under SPHNs served for Pr(see indicator NF11/NF12). As well as the reduction of larch due to this felling, it is possible that at some point forest managers will assess that the risk of larch trees not reaching economic maturity has become too high to justify planting them, as well as a reduction in nurseries continuing to stock Japanese larch (Moffat, 2015). Forestry Commission guidance for replanting of sites affected by the disease (Forestry Commission, 2010), along with an established strategy to increase the overall resilience of forestry by increasing diversification (FCS, 2013), is also likely to see a change in the planting dynamics for larch (particularly Japanese larch).  As a consequence it is likely that there will be a significant reduction in the area of larch within Risk Zone 1 (including the Management Zone) and an overall shift in the proportional distribution of larch across Scotland.

As the climate changes, so too will the suitability of sites for growing particular timber species (Figure 4). The use of such maps and site-specific tools such as ESC (Ecological Site Classification) by forest managers may well begin to see corresponding shifts in the spatial distribution of larch (along with other species). Whilst Figure 4 shows that much of the most suitable areas for Japanese larch correspond to Risk Zone 1, there are many smaller areas across the East and North of Scotland which will also see increases in suitability. However, as the Risk Zones are in large part determined by climate, it is likely that there will be a need to revise the Risk Zone boundaries as the climate changes.

Figure 4 Projected change in suitability for Japanese larch in Scotland (2050) under a low emission scenario (Forest Research, 2015)

n order to provide further detail to assess any change to the proportion of Scotland’s larch within Risk Zone 1, this indicator will also be informed by analysis of the proportion of new planting (including restocking) of larch species within the NFE that occurs within each Zone (Figure 5) and monitor the changing levels of new planting of larch species within Risk Zone 1 (Figure 6).

Figure 6 shows a decline in the planting of the most susceptible species (Japanese larch) from a high of around 200 ha in 2011, but it is currently too early to assess whether the downward trend is a response to the increased risk from Pr as there was a peak in planting at the start of this period.

Figure 5 New planting of larch (2009-13) in the National Forest Estate

Figure 6 New planting of larch (2009-13) in Phytophthora ramorum Risk Zone 1 (NFE data only).

Whilst the prevalence of Phytophthora ramorum is showing an increasing trend (Forestry Commission, 2015b), the indicator analysis is currently unable to show whether there has been any change in the amount of NFE larch that is situated within the climatically vulnerable zone, nor how this has changed as a proportion of all NFE larch.

Silvicultural dynamics that may drive any change in these indicators include:

• a reduction in area of established larch in plantations as a result of sanitary felling when Pr has been identified;
• a reduction of restocking and new planting with larch, given the risk of Pr infection
• the diversification of species composition within the whole National Forest Estate- an overall strategy identified by FCS as critical to “help the Estate to adapt to climate change and become more resilient to pressure”  (FCS, 2013)

Phytophthora ramorum:

http://www.forestry.gov.uk/forestry/INFD-5UBESN#isthereanything

Advice regarding indicator development and review- Forest Research: Anna Brown  and Andy Moffat; Forestry Commission Scotland: Colin Edwards.

Contains, or is based on, information supplied by the Forestry Commission. © Crown copyright and database right 2014. All rights reserved. Ordnance Survey Licence number 100021242.