Wessex Silvicultural Group: Field Meeting Notes

2017 Study ‘Regeneration: Renewal and Creation’

Meeting 1: Underplanting (Ringwood Forest, March 15th 2017)

Jeremy Hunter

This year’s theme of regeneration through renewal and creation started with a visit to Ringwood Forest, hosted by Mike Abraham and preceded by a presentation from Terry Jennings, visiting from Thetford in the East England Forest District, and outlining on the work undertaken for regeneration there. Resilience in the face of red band needle blight (Dothistroma septosporum), or ‘DNB’, was a recurring theme throughout the day.

Presentation by Terry Jennings

The presentation from Terry Jennings detailed the Forest Resilience Programme being undertaken at Thetford. The forest is 19,211ha in size, with 17,65ha under SPA or SSSI designation, and none is classified as ancient woodland or PAWS. Over 13,000ha are pine (Corsican pine constituting 10,044ha and Scots pine 2,979ha) with around 8,000ha of the Corsican pine being under 40 years old. 150,000 tons are harvested annually in this forest. There has not been planting of Corsican pine since 2006, due to the outbreak of DNB. The disease is causing considerable loss in timber volume, with 8,600ha of the Corsican growing at yield class 4, rather than the expected yield class 14, resulting in the loss of 70,000m3obs/yr.

The replacement of Corsican pine is proving to be a challenge, due to the calcareous soil, high summer temperatures, low rainfall and late frost, however some shade tolerant species are currently showing promise. The management framework being created at Thetford is designed to help the forest to become more resilient. There are ten live projects involved as part of the Forest Resilience Programme (FRP), and Project 3, Underplanting, was detailed. Criteria for an underplanted site had been discussed and established, including that the stand be less than 20 years old, have a high stocking density, be on acidic soil and should not be worked within two years of an adjacent stand to help prevent damage from Hylobius abietis. Some of the key factors considered in selecting appropriate site treatment included timing (underplanting should be undertaken as soon as possible to pre-empt competition), soil pH, canopy density, the number and quality of overstorey trees, rack density and mammal pressure. Many conifer species were considered, mainly firs and pines.

In terms of results to date, one major and site specific benefit of underplanting that has been established is frost prevention, an issue at the Thetford site where many of the species planted do not cope well in the open or with frost. Douglas fir (which does not appear to suffer deer damage at the site) and white fir (Abies concolor) are growing healthily as an undercrop, as is the Eucalyptus (E. gunnii) at six years growth. Although DNB caused Corsican pine to become financially unviable as a crop on full rotation, the species was found to grow well for the first six years before slowing down, and as such it was considered whether this would might make it a good nurse crop. Thinning has been the only method undertaken to control the spread of DNB.

After the presentation, we moved out to visit the scheduled stops across Ringwood Forest.

Stop 1

The first stop was to compare one stand with another stand opposite which had received a heavier thinning prior to underplanting. Both stands had been thinned in 2013, and were in their first rotation. The stocking density was around 150 stems, or 207m3 per ha. The average top height of the stand was 23m with a mean DBH of 38cm. A soil pit had been dug to demonstrate the build-up of soil over 65 years with a very sandy stratum. The top soil had developed to a depth of around 14”. Due to the species of conifers present in the overstorey, it was clear that the shade tolerance of the species selected for underplanting would be important. Whilst bracken was prevalent, it did not pose a significant problem in terms of competition for light due to its growth remaining low to the ground.

Stop 2

The second stop was very similar to the first, but more advanced in terms of its development as a result of underplanting being undertaken. The stand had been marked and felled in 2014 with mulching and underplanting having taken place in late 2015. The volume had been reduced to 106m3, or 156 stems per ha, with the remaining trees attaining a yield class 10. The overstorey was composed of the same species as the first stop, with underplanting of 60% western hemlock and 40% coast redwood (and other species) having taken place. Beating up occurred in 2016 and 2017 with Atlas cedar and coast redwood. Despite being left unfenced, it was remarked that little deer damage was evident. It was also noted that some Scots pine natural regeneration may be possible. The low yield class of the pines could be attributed to both loss of foliage caused by DNB and the poor soil, which was still building up a top soil over the high sand content. However, growth of the underplanted trees was considered to be good.

Stop 3

The third stop was a site of Scots pine where natural regeneration had occurred. The stand is 85 years old and has a basal area of 16m2. It holds 110 stems per hectare, or about 140m3. The natural regeneration is quite dense but the quality appears good. It is undecided whether the stand should be left or thinned soon, although it was suggested by many that the site be left as it is, for now. Respacing of the natural regeneration is also being considered.

Stop 4

This 2.2ha site was clear felled in 2013, raked and burned in 2014, with scarification and planting taking place in 2015. Western hemlock, Sitka spruce and Scots pine were selected for the site and planted at 2m x 2m spacing. The western hemlock was planted in a corner between mature trees due to its shade tolerance and the Sitka spruce in a wet area. They have, so far, been growing well, however whether this growth rate will continue in the future remains to be seen. The plot had undergone some damage from H. abietis.

Stop 5

The final stop was at a 1.3ha compartment felled in 2014, which was mulched and planted in 2015. Whilst western hemlock made up the majority of the stock due to the sheltered nature of the site, Atlas cedar, Sitka spruce and Scots Pine was also included. It was still a young site, and so there was not much progress to report, however it appeared promising.

Concluding Remarks

Whilst Ringwood Forest has suffered in places from the impact of DNB, action has been taken over recent years to make the forest more resilient. Evidence suggests that thinning and underplanting with a variety of species can reduce the impact of DNB (due to a combination of change in local microclimate and change of species composition making an environment less hospitable to the disease). Meanwhile, stands of Corsican pine can themselves prove to be useful where there is not an immediate desire to fell them, providing cover for the undercrop, and potentially improving the quality and quantity of timber produced by the underplanted species.

It will be interesting to monitor the growth of young trees planted at various sites across Ringwood Forest into the future. It is likely that the growth rate and vitality of young trees will differ markedly between sites in which DNB infected pine has been retained (and underplanting has been carried out under a mature canopy, i.e. Stops 1 and 2) and sites that are more open (i.e. Stops 4 and 5).

References/Further Information

Wessex Silvicultural Group: Notes from previous meetings can be found here.

Forestry Commission (2016) Successful Underplanting: A Silvicultural Guide. HMSO, London. 42pp.