Wessex Silvicultural Group: Field Meeting Notes
2015 Study ‘The Silviculture of Resilience’

Meeting 1: Resilience & Diversity – Managing the Threat of Disease

The first of this year’s meetings took place at Rushmore Estate, Wiltshire. Woodland management at the estate has aimed at developing structural diversity in both semi-natural stands and coniferous plantations. Within this setting, the group explored the role of stand structure manipulation in increasing resilience, one accepted definition of which is ‘the ability of a forest to absorb disturbances and re-organise under change to maintain similar functioning and structure’. With the expert input of Christine Reid, National Woodland and Forestry Specialist, Natural England, the group also addressed the management of ash-dominated stands in light of the threat from Chalara (Hymenoscyphus fraxineus).

Morning Session

Since the 1990’s, Rushmore forestry estate has been managed to produce complex stand types, and the resultant structural diversity is pronounced. There is a large area of irregular, broadleaved high forest (much of which has a low to moderate stocking density), a significant coppice component, and a relatively small coniferous resource. Ash dominates the broadleaved high forest stands, while Norway spruce and Douglas fir are abundant within the coniferous high forest.

Half Hide Wood, the first stop of the day, is effectively an irregular shelterwood, where there has been a slow transition to ash from hazel and birch. Here, there is a fine balance between biodiversity and timber production. Within the stand a shrub understorey has been maintained to suppress ground vegetation, while overstorey ash trees have been favoured for their large crowns. These large, vigorous trees are potentially more resistant to pathogens such as Chalara, whilst also representing the best timber trees. The stand has a relatively low basal area, and hence air flow through the canopy is good. Increasing air flow within stands of Corsican pine has been shown to be effective against the fungal pathogen Red Band Needle Blight Dothistroma septosporum by reducing spore loads and decreasing humidity (which is favoured by many fungal pathogens). The same may be true of Chalara and ash stands. Research currently being carried out at the estate (Poore et al., 2013) aims to investigate the relationship between forest structure and biodiversity in a range of woodland types (i.e. coppice stands vs limited intervention stands vs stands managed purely for timber production). This research also aims to identify a ‘biodiversity index’ i.e. an indicator species whose abundance can be used to monitor overall biodiversity.

Christine Reid took this opportunity to outline the importance of ash in the British landscape and the current knowledge base in relation to Chalara. In terms of the varying susceptibility of trees to Chalara, genetics appears to be the key factor, while environmental conditions may also interact (stressed trees being more susceptible to the pathogen). Studies indicate there is no resistance to Chalara in UK ash trees, but some tolerance. Recent research has identified a gene which confers tolerance to Chalara, and this appears to be present in around one sixth of UK ash trees - the distribution of these trees is now subject to further investigation. Maintaining a dense understorey may help to reduce the impact of Chalara, acting as a physical barrier to the fungal spores, which reach the canopy after being released from fruiting bodies on fallen leaf stems. Understorey species already present should be encouraged, while underplanting with species such as lime, cherry, or aspen may help achieve the desired structure. Establishing and maintaining mixed age and mixed species stands will provide greater opportunity for adaptation in the wake of the impact of Chalara..

Our next stop, an ash-dominated pole stage stand, represents the type of stand at greatest risk from Chalara. Danish research indicates that stand collapse (i.e. death of entire stands) appears only to affect dense pole stage trees, a result of high spore loads and small crowns. Removing 20-30% of trees in such stands has been shown to reduce this risk.

Farnham Woods, our next stop, was illustrative of the structural diversity prevalent at Rushmore Estate. An ash-dominated stand in which birch had been retained to give a middle-storey, and large diameter oak provides the financial value. Initially appearing understocked, stem distribution curves representing the stand highlight the diversity of stems present. Amongst this ash-dominated woodland, Christine Reid highlighted some of the issues surrounding management of Chalara in the UK. Some management strategies adopted elsewhere in the EU to mitigate the impact of Chalara have shown to be ineffective, and lessons should be learnt from the mistakes made. For example, in Lithuania, infected stands have been heavily thinned or removed completely. The effect of this has been two-fold. Firstly the water table has been raised (increasing humidity, and hence creating conditions favourable for Chalara), and secondly the growth of ground vegetation has been promoted, inhibiting natural regeneration of ash. Natural regeneration in Chalara affected stands is often compromised, even without suppression from ground vegetation, as ash seedlings show a high susceptibility to the fungus. Ash seeds have also been found to be infected.

Afternoon Session

Christine Reid began the afternoon session with a discussion of the ecological impact of Chalara (see Reid (2014)). Although tree species such as oak and beech support most ash-associated wildlife, ecological traits such as decomposition rate, litter quality, nutrient cycling and rooting depth differ markedly. Therefore loss of ash from ecosystems will impact on ecological functioning. Species most similar in terms of ecological function include alder, lime, rowan and to some degree, sycamore. Looking to the future, and to the potential regeneration of previously ash-dominated woodland, current advice is to encourage natural regeneration already present on site or if planting, to examine the NVC type or to mirror species composition in nearby natural woodlands. In the short term, felling of diseased ash is not advised, in an effort to protect associated biodiversity.

Chase Woods (Cpt. O18/19), our next stop, again represented a shelterwood system dominated by ash, with a smaller sycamore component. Although the sycamore component may offer a degree of stand resilience if Chalara takes effect, underplanting to achieve a more mixed stand may now be the most sensible management option.

A series of further stands visited at Chase Woods illustrated species diversification in Norway spruce- dominated stands and complementary stand management. At stop 6 (L24/25/26), a Norway spruce- dominated mixed species stand, theories behind the apparently low levels of natural regeneration of spruce where discussed. With explanations such as insufficient light levels and high browsing damage dismissed, could allelopathy (similar to that seen in black walnut) inhibit seedling growth in mixed stands, in a bid to prevent inbreeding? The next intervention at this stand will be to further diversify by creating gaps between racks and planting Douglas fir and western red cedar. Silver fir and Atlantic cedar have also been considered, although pure groups would need to be planted to account for their slow growth.

Stands at Rushmore Estate highlight the benefits of structural diversity in light of abiotic, as well as biotic threats. Several conifer-dominated compartments at Chase woods suffered significant windblow in 2014, particularly in areas topographically exposed. With patches of naturally-regenerated Douglas fir and Norway spruce remaining, there is now opportunity for stands to adapt to the disturbance and re-organise under change.

Concluding Remarks

The dominance of ash across the broadleaved high forest stands at Rushmore Estate stimulated discussion around the impact and management of Chalara in the UK. To summarise, there are several management strategies which are recommended:

  1. Maintain open stands to reduce spore loads and decrease humidity
  2. Develop an understorey which may help reduce re-infection
  3. Grow trees quickly – more vigorous trees appear less susceptible to infection
  4. Diversify – establish and maintain mixed stands
  5. Thin to promote large healthy crowns
  6. Do not fell trees pre-emptively
  7. Avoid very heavy thinning/clearfelling


Pre-meeting notes:
WSG 25 03 15 Rushmore Meeting Notes.pdf

Poore, A., Sterling, P., and Alder, D. (2013) Rushmore woodland biodiversity research project. Available at: http://www.selectfor.com/research/downloads/rushmoreproject_2013.pdf

Reid, C., Goldberg, E. & Alsop, J. (2015) What can we do about “Chalara” ash dieback (Hymenoscyphus fraxineus) on woodland SSSI? Joint advice from Natural England and the Forestry Commission. Forestry Commission/Natural England Report.

Reid, C. (2014) Potential ecological impacts of ash dieback. Available at: http://www.ccfg.org.uk/conferences/conference2014/resources/downloads/reid_poster.pdf