NF9 Forest area and proportion of stands infected by Dothistroma needle blight

Forestpests and pathogens can cause extensive and catastrophic damage to productive forestry stocks. Affected woodlands may experience sufficiently severe outbreaks to reduce productivity, timber quality and/or to require changing forest management. The majority of insect pests that currently affect UK forestry are likely to benefit from climate change as a result of increased activity and reduced winter mortality. If climatic conditions influence the success of a pest or pathogen establishing, breeding and spreading, then a change in climatic conditions is likely to influence their prevalence and severity (Moffat et al., 2012). Other effects may be more indirect and result from increased susceptibility to infection due to damage or stress to the trees as a result of storms, drought or temperature extremes.

The fungal disease Dothistroma needle blight (DNB), often referred to as ‘Red Band Needle Blight’, has become the most significant disease affecting coniferous trees in the UK. The fungus affects the needles of the infected tree, which eventually shed. As this continues, year on year, gradually the tree will weaken, significantly reducing timber yields. It can also eventually lead to mortality. Whilst it was first recorded in the UK in the 1950s, it was not known in Scotland until 2002 and has now been identified in Corsican, Lodgepole and Scots pine stands (though primarily a disease of pines, five spruce species, European larch and Douglas fir are also known to host the disease) (Forestry Commission, 2010).

This indicator monitors the abundance of DNB in the Forestry Commission Scotland National Forest Estate (NFE) which holds over 35% of Scotland’s woodland. The indicator is based on DNB surveys which have taken place annually since 2006 to monitor the distribution and spread of the disease. Although the data covers the NFE only, changes in the abundance of DNB in these areas is likely to be indicative of DNB abundance across forestry in Scotland as a whole.

Related indicators:

NB36 Proportion and area of Caledonian pine woodland exposed to Dothistroma needle blight (DNB)

NF7 Proportion and area of pine woodland exposed to Dothistroma needle blight (DNB)

The current extent of pine woodland in the NFE that is known to have been infected with DNB is 13722 ha from 1661 infected sub-compartments (see Figure 1).

The impacts on timber yields are currently most severe on Corsican and Lodgepole pine (particularly those of ‘Inland’ origin), with trees infected by DNB becoming unmarketable over time (Forestry Commission Scotland 2013).

Figure 1 Total pine area (ha) in FCS sub-compartments with positive DNB survey results (2006-2013). Positive status is accumulative and therefore results include data for compartments which have been surveyed as positive for DNB either in the latest year or in an earlier. Data provided by FCS Dec 2013 (2013 data incomplete).

Due to changes in annual surveying effort over this period (most notably a significant increase in area surveyed between 2010-2011), it is important to consider the positive survey results as a proportion of total area surveyed. Figure 2 shows that over this period there has been an average annual increase of positive DNB sub-compartments of around 1% of total number surveyed, with the latest results showing that approximately 10% of all surveyed pine forestry in the NFE is positive for DNB.

Figure 2 Proportion (%) of total (accumulative)sub-compartments surveyed which show a positive result for DNB (results for 2013 are incomplete).

The majority of DNB infected forestry is in the north and north east of Scotland, with over half of infected pine in the North Highland district (see Table 1 and Figure 4a). This distribution largely reflects the distribution of pine in general across Scotland.

Figure 4b highlights that over a third of all forestry surveyed in the North Highland district is now infected with DNB. However, the detected infection proportion  does not show quite such a clear relationship with pine forestry distribution as Figure 4a, with the Scottish Lowlands having a relatively high proportion of infection (16.8%), though this district holds only approximately 2% of the National Forest Estate pine.

Table 1 Changing DNB distribution in FCS Forest Districts (area (ha) of surveyed FCS sub-compartments with positive DNB survey results). Table also provides 2013 figure for total pine area in each district. Positive status is accumulative and therefore results include data for compartments which have been surveyed as positive for DNB either in the latest year or in an earlier. Data provided by FCS Dec 2013, results for 2013 are incomplete.

Figure 4 Comparison of extent of pine woodland containing DNB in Forestry Commission districts: a) Total pine area (ha) in FCS sub-compartments with positive DNB survey results (2006-13); b) Proportion (%) of total pine area surveyed with positive DNB survey results (2006-13)

Whilst reasons for the increase in the disease are presently unclear, there is some evidence to suggest that increased rainfall in spring and summer coupled with a trend towards warmer springs is optimising conditions for spore dispersal and infection (Brown and Webber, 2008).

The increase in geographic extent and intensity of the disease across the whole of Britain is probably due to a combination of factors including an increase in favourable climatic conditions, as well as availability of suitable hosts, a genetically diverse fungal population, and movement of the pathogen through the plant trade.

The survey methodology targets priority areas, and survey effort has varied considerably over the period (Figure 5). There was a significant increase in 2011 in the amount of forestry included in the annual survey, with 2012 also maintaining this extent (NB the data presented here for 2013 does not represent the full data for the year).

Figure 5 Annual DNB survey extent (number of sub-compartments surveyed) in the National Forest Estate forestry districts 2006-13.

• Data covers FCS National Forest Estate only, however, changes in the abundance of DNB in this forestry is likely to be indicative of DNB abundance across Scotland.
• Survey effort has varied considerably over the period.
• The survey methodology targets priority areas, therefore positive results should not be seen as a random subset of the National Forest Estate.
• Positive survey results are applied to the whole sub-compartment, though detection may only have been at the edge of a sub-compartment.
• Positive status has been assumed to remain for a compartment for all subsequent years. Additional analysis could begin to establish re-surveying effort and what change is detected in these areas, though it is potentially difficult to infer complete change in a sub-compartment due to the issues with survey methodology mentioned above.

Forestry Commission Scotland- Ben Griffin, Glenn Wilson for provision of FCS data and reviewing; Colin Edwards for review and advice.

Forest Research- Anna Brown, Kate Beauchamp for advice regarding indicator development.

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.