The second of this year’s field meetings took the WSG to Little Wittenham, Oxfordshire, where Jo Clark, Forestry Research Manager of Earth Trust hosted the day. The visit enabled the group to study the role of tree improvement in the choice of species for future use in the forestry sector. Paradise Wood was the destination for the day, a national research woodland home to the largest collection of hardwood tree improvement trials in the UK, largely comprising ash, oak, walnut and cherry orchards.
At our first stop, the principles of tree improvement programmes were discussed, including the four categories of Forest Reproductive Material (FRM) used for forestry purposes: Source-identified, Selected, Qualified and Tested (see FC publication: Forest Reproductive Material for further info). At Paradise Wood, results of breeding programmes with ash using ‘plus trees’ (i.e. the best available trees selected by phenotype in the wild) have given around 8-10% gain in performance, measured in terms of form and growth rate. Tested seed was destined to be entered onto the FC’s register of suppliers, but with the onset of Chalara fraxinea in 2012, developments were halted.
Subsequent to the arrival of Chalara in the UK, in 2013 the Earth Trust and partners (Forest Research, Sylva Foundation and Future Trees Trust) was granted funding from DEFRA to invest in 6 research programmes (the Living Ash Project) with the aim of securing Chalara-resistant ash trees for the future:
Alarmist reports compare the threat posed by Chalara to that of Dutch Elm disease. Although the threat from Chalara is real, it is important to remember that our ash resource is unlike our historic elm resource in that it is genetically very diverse. At Paradise Wood alone, there is a good, varied genetic stock, the culmination of 20 years of research.
Trials looking at fertilizer treatment of common and black walnut have shown no difference between treatments over many years. Results suggest that site management, rather than genetics, may play an important part in walnut growth, with more sheltered crops showing better form. Walnut has the largest crown diameter to stem diameter ratio of all broadleaves, and hence requires very wide spacings, but in the case of black walnut, growth rates are typically one third faster than oak, and the timber three times more valuable.
The clonal cherry orchard at Paradise Wood is composed of successive clonal lines (all qualified material), sourced for form, vigour and resistance to bacterial canker. Variation between clones was evident in terms of degree of branching, canker resistance and growth rates. Eight of these clones went into forming ‘Wildstar’, a product which did not live up to its reputation. The stand is now ready for thinning, which the WSG were able to advise on.
Reciprocal Transplant Experiments (RTEs) were utilized by Jo as part of her PhD to look at adaptation of ash to climate change. Results suggested that local populations of ash perform below average when compared to other provenances. In addition, the best performing provenances tended to grow best at all sites across a 2000km transect of Europe. This indicates that from a productivity perspective, it may not be wise to plant local stock, but rather to source provenances from slightly further south. Most importantly, it is crucial to plant a wide variety of genetic stock, to ensure resilience to climate change, and to pests and diseases such as Chalara.
The first walnut silviculture trial looked at during the afternoon highlighted what happens when environmental effects are too great - genetics cannot be investigated. The shockingly bad 16 year-old growth of the common walnut trial did however, allow Jo to establish the effect of stumping treatments on walnut. Firstly, walnut grows well if stumped. Secondly, growth is better if trees are stumped in the winter.
The second walnut trial examined the effect of nurse crops on tree growth, and yielded some fascinating results. Walnut was grown with nurse trees including birch, cherry, and Italian alder, and with shrub nurses including hazel and autumn olive (Elaeagnus umbellata). Results showed that one nurse species in particular, Elaeagnus umbellata, had a statistically significant positive effect on walnut growth, possibly due to the microclimate created and its nitrogen fixing ability (see article in the Quarterly Journal of Forestry).
Planted in 1995, the ash provenance trials have recently undergone phenology assessments, showing vast differences in flushing date (Romanian provenances flushing early and Welsh provenance flushing late). Form also varied considerably between provenances, with Yorkshire and Champagne provenances showing best results.
The oak BSO, established in 2003, contains seedlings from 65 superior phenotypes (plus trees), the aim being to produce improved acorns for stocking. Phenology assessments will be made (including flushing date, which correlates with incidence of shake), and juvenile and mature characteristics compared. This trial is one of a series of eight across the country, looking at performance of Q. robur and Q. petraea on a regional basis.
In conclusion of the day’s events, some key points were made relating to tree improvement: