Early weed control can increase long-term growth, yield and carbon sequestration of Sitka spruce stands in Britain

Six experiments, initially established to investigate the effects of weed control on establishment of Sitka spruce (Picea sitchensis (Bong) Carr.) were revisited 15-25 years after planting, and 6-22 years after weed control ceased. At three of the six experiments weed control resulted in initial benefits to growth and in some cases survival at Age 3, while no effect was evident at the other three sites. At mid-rotation, 15-25 years after planting, although there were no persistent effects of weed control on height or diameter growth, at three of the experiments there was a positive effect on survival. Projection of the semid-rotation survival effects to full rotation using a growth and yield model indicated that weed control may have the potential to increase final yield and carbon storage at some of these sites in the longer term. The existence and magnitude of benefits from weed control were highly site dependent. This study demonstrates that there is clear potential for early weed control to increase the cost-effective carbon storage and hence the climate change mitigation potential of some British Sitka spruce forests, particularly on nutrient rich sites and with stands that are subsequently thinned. The development of dedicated long-term weeding trials in the UK, including thinned stands where cumulative volume removed in thinning is recorded, is required to confirm this.