Sprucing up the UK’s Sitka spruce (<i>Picea sitchensis</i>) forests: can tree species diversification benefit biodiversity?

Abstract Sitka spruce (Picea sitchensis (Bong.) Carr.), is the most common tree species in UK’s commercial forests. Tree species diversification of the UK’s commercial forests is currently occurring to make them more resilient to new pests and pathogens and climate change. The potential ecosystem implications of this transition are poorly understood. Our aims are to, (i) list the biodiversity known to be supported by Sitka spruce as this has not been collated previously, and (ii) assess the changes, in both ecosystem functioning and biodiversity supported, if Sitka spruce forests are diversified with any of 34 other tree species. Using records from across the UK, we identified 564 species that use the Sitka spruce trees as a living space or for feeding. Most of these associated species were non-specialist and found on a wide range of other trees. Data from an extensive literature review showed that diversification with broad-leaved trees would potentially result in faster nutrient cycling and litter decomposition with potential implications for carbon storage in the litter layer. Diversification with Oak (Quercus petraea/robur (Matt.) Liebl.), sycamore (Acer pseudoplatanus L.), Scots pine (Pinus sylvestris L.), birch (Betula pendula/pubescens Roth), beech (Fagus sylvatica L.), and Norway spruce (Picea abies (L.) H. Karst.) provide the greatest biodiversity benefits both through supporting the biodiversity found on Sitka spruce and additional biodiversity thought to be absent on Sitka spruce. However, except for sycamore, none of these tree species will grow in long-term intimate mixed stands with Sitka spruce. Therefore, we propose that diversification of Sitka spruce plantations should be carried out as ‘blocky mixes’, i.e. small blocks of single tree species, with tree diversity occurring within a management unit, as a pragmatic approach to support timber production and provide biodiversity benefits. However, the optimal size and spatial arrangement of such blocks requires further assessment.