Keen of Hamar Nutrient Addition Trials 1981-2013

Nutrient addition experiments initiated in 1980 on the Keen of Hamar, Shetland Isles, have produced a unique dataset of long-term vegetation response to amendments of major plant growth nutrients (N, P, NP, NPK and NPKCa). Previous studies have reported the notable impact of phosphorus on the ‘serpentine debris’ community, and the negligible effect of nitrogen. However, a survey in 2010 provided our first indication that the experimentally-induced phosphorus effect was weakening and this was consolidated by further surveys of vegetation cover and community composition. This community shift might have been different had the local climate acted synergistically with phosphorus additions: in the last few years of the study the Shetland Isles experienced particularly low spring rainfall – the dry spell may have been a well-timed environmental filter driving community recovery rather than a permanent change of state to heathland on an organic soil. The longevity of our investigation is a unique opportunity to explore vegetation response to the key drivers of global environmental change, namely climate change, eutrophication as a result of agricultural intensification, and the potential for invasion of species as new resource-rich niches become available.