Thetford Forest Diversifying after Dothistroma Needle Blight
Heavy and early thinning of Corsican pine in Thetford Forest has been implemented to control the outbreak of a pathogen, and a range of other species underplanted to diversify stands.
Thetford Forest – Forestry Commission England, East England Forest District
RFS Woodlands for Climate Change Award – Winner and Gold Award
Background
Thetford Forest, part of the public forest estate managed by the East England Forest District of Forestry Commission England, contains substantial areas planted with monocultures of Corsican pine (Pinus nigra subsp. salzmannii). Outbreaks of the pathogen Dothistroma Needle Blight (DNB) have devastated many stands, with high mortality resulting from secondary pathogens.
Traditionally, stands of this species are thinned from 22 years onwards. Following evidence that close spacing in monoculture plantations of Corsican pine can result in high mortality in young stands, Forest Research scientists advocated heavy and early thinning. Local staff, led by Terry Jennings and Brian Greenacre instigated early thinning in an attempt to manage the pathogen.
Given a moratorium on planting Corsican pine (since 2006), and the widely accepted benefits of diversifying the range of species in a stand, district staff experimented with introducing underplantings of various alternative conifer species to these thinned stands.
Thinning and DNB outbreaks
To date, 192 hectares (ha) has been thinned and underplanted. Stands are thinned to 1200 stems per ha. Outbreaks of DNB appear to be reduced in frequency and severity. The district intends to implement a rolling programme to convert 100ha per year.
Underplanting
Following detailed consideration of soil type and drainage, local climate – particularly frost which can impact the area at almost any time of year – aided by the Ecological Site Classification decision support system, 10 species were selected for use in underplanting.
Species selected for underplanting
| Latin name | Common name |
|---|---|
| Abies alba | European silver fir |
| Abies grandis | grand fir |
| Cedrus atlantica | Atlas cedar |
| Cryptomeria japonica | Japanese red‐cedar |
| Picea omorika | Serbian spruce |
| Pinus peuce | Macedonian pine |
| Pinus strobus | Weymouth pine |
| Pseuditsuga menziesii | Douglas‐fir |
| Sequoia sempervirens | coast redwood |
| Thuja plicata | western red cedar |
Species were selected (see table) according to their shade tolerance, as they needed to be able to thrive under the canopies of the older Corsican pine. They are also moderately resistant to drought and considered resilient to projected climate change. In the future, broadleaves will be trialled. The benefits of underplanting at different densities ‒ low (1300 plants) and high (2500 plants) ‒ are being monitored. The benefits of fences to exclude browsing mammals are also being assessed.
Lessons learnt
Levels of mortality arising from outbreaks of DNB have fallen dramatically in these early‐thinned stands of Corsican pine. It is hoped that the more open stands with their diverse stand structure (containing trees of varying height), together with a greater range of species, will create forest stands more resilient to future outbreaks of DNB and other pests and diseases.
Forest managers will need to monitor the over‐storey closely to ensure that the underplanted crop thrives. The greater range of species present in the forest may assist in meeting the future needs of society, for example, timber products. The impacts of this new practice are being implemented to vast areas of forest; in the words of District Manager Brian Greenacre “We are changing the landscape”.
The 2014 Excellence in Forestry Climate Change
Award was supported by Forestry Commission
England in partnership with England’s Climate Ready Support Service hosted in the Environment Agency
Category judges:
Dr Gabriel Hemery FICFor, is Chief Executive of the Sylva Foundation.
John Weir MICFor, Advisor for Woodland Creation and Resilience, Forestry Commission England.
Text and Photos by the Sylva Foundation.
