The first of this year’s meetings, which address the topic ‘Alternative Species in Situ’, took the Wessex Silvicultural Group to Westonbirt Arboretum. This offered a unique opportunity for the study group to examine mature examples of novel tree species with forestry potential, and visit several species trial plots.
The day began with a presentation of bespoke artwork to Graham Darrah in celebration of 50 years contribution to the WSG. Following an introduction to the arboretum by Simon Toomer, the study group proceeded to the old arboretum, where Richard Jinks (Project Leader for Species Research, Alice Holt) and John Weir (Advisor for Woodland Creation and Resilience, FC England) lead a discussion of the dendrology and silviculture of a range of species which have the potential to be used under predicted climate change models:
Japanese cedar Cryptomeria japonica – a major timber species in Japan, experience from the UK suggests that C. japonica can attain yield classes similar to some of our most widely planted spruce species (YC 16-22), when grown in dense stands. According to research carried out by Forest Research, timber produced is soft, light and durable, and is stronger than that or either Western red cedar Thuja plicata or Sitka spruce Picea sitchensis, although this depends on growing conditions. Forest plots have recently been established by the Forestry Commission using seed sourced from China, where 600-800 years of tree breeding has produced a 20-fold increase in tree quality.
Macedonian pine Pinus peuce – since the impact of Red Band Needle Blight Dothistroma septosporum on Corsian pine Pinus nigra var. maritima in the UK, contingency species such as Ponderosa pine Pinus ponderosa and Lodgepole pine Pinus contorta have also proved susceptible. Macedonian pine appears resilient to D. septosporum, and resistant to the Pinewood nematode Bursaphelenchus xylophilus and white pine blister rust Cronartium rubicola. This 5 needle pine regenerates well, and is productive on a range of soil types. Results of FC species trials in Wales indicate that P. peuce seed can be difficult to germinate, while seedlings grow and establish slowly, requiring much investment in establishment.
Dawn redwood Metasequoia glyptostroboides – demand among UK foresters for deciduous conifers seems unwavering. Selected cultivars of this species offer reduced fluting and high growth rates (Swamp cypress Taxodium distichum is a slower growing alternative).
Oriental spruce Picea orientalis – tolerates hotter, drier and more calcareous conditions than either Norway spruce Picea abies or Sitka spruce. Forest plots established by FR show similar performance to Norway spruce in terms of growth rate, while timber quality is similar to that of Norway/Sitka spruce. However, this species appears to be particularly susceptible to Great spruce bark beetle Dendroctonus micans. Two seed batches from two regions in the species native range are now being planted in the North and East of the country. There has been much interest in another alternative spruce, Picea omorika (Serbian spruce), although this species required very dense planting, and rarely attains high growth rates.
European silver fir Abies alba – has applications in continuous cover forestry due to its shade tolerance, will attain a yield class of 12-20 and compares with Abies grandis in terms of timber quality. Can be slow to develop, although this may represent an initial investment in root growth, eventually leading to a more stable form. Planting should be as part of mixed woodland to avoid problems with Adelges aphids. Trial plots of Abies concolor (White fir) are also in existence.
Cedars – French foresters have recently taken an interest in Atlas cedar Cedrus atlantica, while in the UK there appears to be a good market for Cedar of Lebanon Cedrus libani. Little information on provenance exists to date.
Monkey puzzle Araucaria araucana – planted at Westonbirt as part of the Conifer Conservation Project, we are currently uncertain as to the potential of A. araucana as a forestry species. Aside from the obvious amenity value offered, this species can be coppiced, and large specimens may be valued by woods turners. Monkey puzzle can however, be slow to establish, and appears to be highly susceptible to Honey fungus Armillaria spp.
Coast redwood Sequoia sempervirens – although the timber of this species contains a high proportion of bark and sapwood, and stains black when treated, the redwood market in counties such as New Zealand and America is vast. Experience at Longleat Forestry also indicates a thriving market here in the UK. Coast redwood is frost tender, and can be used effectively as a beat up crop with Douglas fir, and in underplanting with species such as Birch. Northern California seeds origins offer the best crop.
Wellingtonia Sequoiadendron giganteum – a species highly tolerant of both drought and extreme cold. While timber in old growth parkland trees can be brittle, plantation stands produce timber of similar quality to other commercial redwood species. Although there have been some issues with the health of stands on the West coast of the UK (apparently unexplained thinning of crown and tree death), Wellingtonia may be a suitable forestry species for Eastern parts of the UK.
Chinese mahogany Toona sinensis – a fast-growing mahogany, the timber of which is suited the furniture making market. Stourhead have recently planted around 50 trees as part of mixed woodland.
The morning’s discussion on species choice was supported by a resounding argument from many members of the group - the importance of planting mixed stands including a diverse range of species. This will be key in improving the resilience of forests to threats from climate change and from pests and diseases.
During the afternoon session, the study group undertook a walking tour of Silk Wood looking at past and present research on tree species at Westonbirt Arboretum:
Larch Progeny Trials (‘Westonbirt 31’) – looked at breeding and selection of Hybrid larch Larix × marschlinsii. Now underplanted.
Douglas Fir Clone Bank (‘Westonbirt 6’) – established in 1966 with the aim of investigating the characteristics of 500 different Douglas fir clones. Scions of each clone were grafted onto standard seedlings, the union of the two individuals still being evident today. The clone bank now acts as a reserve of genetic material, offering a source of material for breeding and improvement programmes.
Project Reinfforce – forms part of network of species trials, spanning 38 separate sites spread across the Western side of the EU. In total, 30 tree species have been planted. This includes 36 specimens of each species, from 3 different seed provenances (distributed across the climatic range of that species). Both native and non-native species are the subject of the trial, including: Chinese fir (Cunninghamia lanceolata), Loblolly pine (Pinus taeda), Stone pine (Pinus pinea), Large-leaved maple (Acer macrophyllum), Oriental spruce (Picea orientalis), a range of Eucalypts (including E. gunnii and E. dalrympleana), plus native British oaks and Birches to name a few. Analysis of results will include data from the entire network, and it is hoped that results will shed light on how different species and provenances respond to changing climatic conditions, so aiding future choice of forestry species.
Transformation to Continuous Cover Forestry (‘Westonbirt 29’) – an experiment aiming to investigate overstorey treatments, fencing treatments and herbicide treatments (plus their interactions) on the regeneration of ash and development of ground flora. The study found that, in terms of ground flora, treatments did not affect the species mixture - there were differences in species abundance, but not species diversity. This indicated that certain treatments resulted in species being supressed within the understorey, but not eliminated from it. Little data on Ash regeneration could be collected, due to poor growth throughout the study.
Ash Resistance Trials – aimed at exploring resistance within the Fraxinus genus to Ash dieback Chalara fraxinea. Scions of 52 taxa (species, sub-species and cultivars) will be grafted onto F. excelsior seedlings, planted at the arboretum, and variation in resistance to the pathogen looked at. To date there is some evidence that Asiatic ash (Fraxinus mandschurica) may have some degree of resistance.
The first field meeting of the year facilitated an important discussion of the choice and characteristics of potential forestry species and their silviculture and improvement. The resounding sentiment of the day centred on the unnecessary reliance of UK forestry on a very narrow range of species. There is an array of tree species with forestry potential, many possessing characteristics which rival those of our more traditional species, which may offer the forestry industry a brighter future in the face of current threats from climate change, and from pests and diseases.
Forestry Commission Bulletin 125 offers further information on forestry and climate change in the UK, including the impact of pathogens, and future provenance selection.