Do mixtures improve the productivity and quality of Sitka spruce stands in the uplands?

摘要

Productive conifer stands are the backbone of British forestry, and in an era of increasing economic and climatic uncertainty there is a vital need to understand interspecies interactions in our forests and how species mixtures may impact financial returns. Productive forest resilience and resistance can be enhanced by diversifying tree species, and therefore the development and management of mixed-species forests and the impacts upon productivity have been a focus of recent research. On very nutrient-poor soils in the UK, ‘nurse species’ have long been employed to improve the establishment of Sitka spruce (Picea sitchensis) and forego the need for chemical fertilisers. Few studies, however, have analysed the development of these mixtures over time, especially on less impoverished soils. This study aimed to address this gap by analysing the survival, productivity and stem straightness of a ‘nursing’ mixtures experiment planted in 1986 in poor- to medium-nutrient soils, in which Japanese larch (Larix kaempferi), lodgepole pine (Pinus contorta') or Scots pine (Pinus sylvestris) were mixed with Sitka spruce in a ‘nurse:spruce’ ratio of 1:3,2:2 or 3:1. These mixtures were compared against pure Sitka spruce plots.The results showed that the interactions between facilitation and interspecies competition affected the development of each mixture differently. Sitka spruce came to dominate each mixture, but the extent of that dominance and the resulting éffect on productivity was dependent on the functional traits of the ‘nurse’ species and the ratio of the mixture at planting. By Year 25, a large majority of the pine had succumbed in all treatments. After 35 years, the highest basal areas (when considering both species) were recorded in the 1:3 lodgepole pine mixtures, due to their combination of increased tree diameter and stand density. These produced an estimated i2m2ha-1 more than the pure Sitka spruce plots. The productivity results of the other mixtures planted with 50% or higher of the nurse were also encouraging. Therefore, growing Sitka spruce in certain mixtures may increase the carbon capture potential of some upland conifer stands. Although the Sitka spruce in mixtures tended to have straighter stems than in the pure Sitka spruce plots, analysis showed the difference was not significant. The results suggest that Sitka spruce can be grown in more diverse mixed forests without causing a reduction in productivity or timber quality.