Proportions of a pine nurse influences overyielding in planted spruce forests of Atlantic Europe

•Growing Scots pine in mixture with Sitka spruce results in higher productivity on less fertile soils;•This ‘nursing’ of the spruce by the pine is due to improved nitrogen availability;•A high proportion of the pine nurse results in the highest mixture productivity. The wider use of mixtures in planted forests is desirable to increase their resilience against the impacts of climate change and the potential damage caused by abiotic and biotic disturbances. However, there is limited evidence about the long-term effects of mixtures upon productivity in planted forests. In western regions of the British Isles ‘nursing mixtures’ of pines (Pinus spp.) and Sitka spruce (Picea sitchensis) have been used since the 1930s to establish spruce forests on nutrient poor soils without expensive chemical inputs (herbicides, fertilisers). Despite this history, there are few reports either of the effect of these mixtures on long-term stand productivity or of any interaction between productivity and proportion of mixture. We explored these aspects using data from an experiment which compared a nursing mixture of Sitka spruce and Scots pine (Pinus sylvestris) arranged in five different mixture proportions ranging from 25:75 to 75:25 spruce:pine. There were also pure plots of Scots pine and Sitka spruce (the latter grown with and without nitrogen fertiliser) so allowing estimation of overyielding (i.e. where the mixture is more productive than expected from the performance of the component species in pure plots). After 30 years, all treatments had closed canopy and Sitka spruce formed the major component of all mixtures. All the nursing mixtures showed significant overyielding (calculated on the basis of measurements of basal area) when the pure Sitka spruce without nitrogen was used to predict performance. The highest overyielding was found in the mixture with 33 per cent Sitka spruce. By contrast, if the pure Sitka spruce with nitrogen was used to predict mixture performance, significant overyielding was only found in mixtures with a low proportion of spruce (25 and 33 per cent), while those with a higher proportion (66 and 75 per cent) showed significant underyielding. Examination of growth in earlier years, supported by foliage analysis of the spruce, suggests that the mixture performance reflects pine facilitation of the spruce in the stand initiation phase when nitrogen was limiting growth. Once canopy closure had occurred, competition between the species in these unthinn