How these changes to Sitka spruce plantations could help improve biodiversity in Scotland
“Blocky mixes” are being put forward as an option according to a new report.
A new report has found how the most common conifer plantation can be managed in a more biodiversity-friendly way long-term.
Sitka spruce dominates the forestry sector, with the non-native tree making up about half of Scottish woodlands.
There have been concerns the plantations are less resilient to disease, severe weather and longer-term climatic shifts. The visual impact of the ‘serried ranks’ on the landscape and the lack of species diversity under its dense canopy have also been noted as issues.
Diversification of woodlands is fairly common now, with the Scottish Government only allowing up to 65 per cent of one tree variety in a woodland to avoid climate and disease related problems.
There can be issues, however, with some native broadleaves being unable to grow long-term with Sitka.
It is why some researchers have suggested planting natives in “blocky mixes” among the plantations.
A new paper from the DiversiTree project, a collaborative project aimed at increasing woodland resilience through greater tree species diversity, began the study identifying 564 species (12 birds, 147 bryophytes, 28 non-lichen-forming fungi, 123 invertebrates, 243 lichens and 11 mammals) which use the Sitka spruce for feeding or as a living space. However, many of these are non-specialists and can survive on a wide range of trees.
The authors then assessed 34 tree species which could be planted with Sitka spruce for their impacts on both biodiversity and ecosystem function.
This included checking their leaf litter decomposition traits, influence on the soil and leaf litter chemistry, as well as their ability to grow alongside Sitka spruce, support Sitka spruce-associated species and nurture additional biodiversity.
Authors said mixing Sitka forests with oak, sycamore, Scots pine, birch, beech and Norway spruce would provide the greatest biodiversity benefits, supporting both the species found on Sitka spruce and additional biodiversity not currently found in these forests.
However, the study explained all these tree species, except sycamore, are unable to grow in the long-term when planted in mixtures with Sitka spruce.
This is when authors suggested “blocky mixes” - small blocks of single tree species which connect the forest like a patchwork quilt. They claimed this would help diversify Sitka spruce plantations without major impact to timber production.
The paper said more research is needed to decide the optimal size and arrangement of these “blocks.”
Dr Ruth Mitchell, a plant soil ecologist at the James Hutton Institute and the paper's lead author, said: “As our forestry adapts to a changing climate and new tree pests and pathogens, it is crucial that we understand how this may impact forest associated biodiversity.
“This work shows how diversification of our Sitka spruce plantations with other tree species can bring many biodiversity benefits."
Forestry leaders insist Scotland needs to increase its timber production, often citing the UK being the second largest importer of timber products in the world.
They claim the Sitka industry has “become a lot wiser” in the past few decades. If used in the right place, the sector said the conifer remains the best tree to meet the “urgent need” to grow more UK timber on an environmental and productivity level, and to meet climate mitigation obligations.
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