Scots pine (Pinus sylvestris) and silver birch (Betula pendula) mixed-species growth dynamics in the North York moors

Tree planting initiatives in the United Kingdom actively encourage the establishment of native, mixed forests. Scots pine (Pinus sylvestris L.) and silver birch (Betula pendula L.) are important components of the Caledonian pinewoods of Scotland, a focal habitat for native woodland restoration and conservation targets. Mixture proportions is an important variable in forest management as competition between species may impact productivity of either species. The objective of this study was to determine the effects of mixing Scots pine and silver birch on individual species productivity relative to each single-species, using data from a long-term silvicultural experiment in the North York Moors. Our results show how Scots pine continues to exhibit positive mixing effects at the expense of the silver birch component of the mixture. This effect was mostly evident with higher proportions of birch in a 1:1 as opposed to a 3:1 pine to birch ratio. Mean pine tree diameter was higher in mixture than in pure pine treatments, although gross stand basal area was always lower in mixtures. However, basal area of the component species in mixtures tended to be higher than in pure treatments. These findings highlight the importance of investigating the role that species mixture proportions play in individual species growth. Related information can be used to support forest management regimes which encourage the use of multiple species and to develop robust species mixtures. Further research is needed to fully understand the complex relationships and interactions underpinning these results.