Tree Trials: Testing trees for the future
With more frequent occurences of extreme weather, and increased spread in tree-based diseases, how do we prepare forests throughout Europe to meet the strain of climate change? Back in 2012 several trials were started throughout the UK to monitor how different tree species coped in the changing British climate with the aim of diversifying future planting to create more resilient forests. Here, Matt Parratt from Forest Research, the UK's principal forestry and tree related research organisation, discusses the initial findings from one trial based in the Scottish Borders.
Forestry in Britain is reliant on a small number of tree species – approximately 96% of coniferous forest consists of just nine species, and one species in particular, Sitka spruce, makes up 52% of conifer forest cover. Broadleaf forests face similar issues with oak, birch and ash making up 46% of broadleaved woodland, but with diseases like Ash Dieback spreading across the UK and Europe, this reliance on certain species creates vulnerability in our forests. Viewed through the twin lenses of climate change and forest health, this ‘eggs in one basket’ approach is potentially problematic. One of the policy responses to this has been to promote more resilient forests by encouraging greater species and genetic diversity. But how do we decide which species will be suitable for future forestry?
The answer is trials – long term tests that look at survival, growth and issues of pests and diseases. After a gap in large scale species trials of around 40 years, a new series of trials was established in 2012 comprising two sites in England, one in Wales, and two in Scotland, including a trial in Glentress forest. The Glentress trial contains 14 distinct species, with between 1 and 3 provenances (the location of the trees that produced the seed) of each present. For example, Scots pine (Pinus sylvestris) is represented by trees from Scotland, Spain and Poland. Other species present include Atlas cedar (Cedrus atlantica), red oak (Quercus rubra), Silver birch (Betula pendula), Oriental spruce (Picea orientalis) and Nordmann fir (Abies nordmanniana). Sitka spruce (Picea sitchensis) was included as a standard species for comparison.
Each combination of species and provenance was planted in plots of 49 trees, giving a total of 32 plots. This was repeated twice more on the site to give 3 separate blocks. Replication like this allows us to apply statistical analyses to be more certain that the observed results are ‘true’ and not down to chance.
Survival and height have been assessed each year at the end of the growing season, as well as checks of each plot for signs of pests and disease. The Sitka spruce suffered with green spruce aphid (Elatobium abietinum) in 2019, with high levels of defoliation, and a number of silver birch were almost totally defoliated in the second year by sawfly larvae. Otherwise, all of the species have been healthy.
It’s early days yet, but there are hints of some interesting comparisons between species and provenances. After seven growing seasons, silver birch is the front-runner in terms of height (with an average of 562 cm), followed by European larch (378 cm), and Douglas fir from coastal Oregon (355 cm).
Looking at one species in more detail, we can start to see differences based on provenance. Douglas fir from the Siskyou mountains in Oregon had an average height of 278 cm, and can be compared with 325 cm for trees from Washington state, and 355 cm from coastal Oregon. Survival-wise they were 69%, 74%, and 80% respectively.
Comparisons at this stage must be taken with caution, but as the trial goes on the data obtained from this trial and others will help to guide our selection of species and provenances for resilient and sustainable forestry into the next century.