Calluna vulgaris-dominated upland heathland sequesters more CO₂ annually than grass-dominated upland heathland

It has been shown in many habitats worldwide, that a shift in vegetation composition between woody shrub and graminoid dominance can influence carbon (C) cycling. Due to land management practices and environmental change, UK upland heath vegetation has historically undergone shifts in dominance from the woody dwarf shrub Calluna vulgaris (Calluna) to species poor graminoid swards. The consequences of this for C sequestration are unknown. We compared annual net ecosystem exchange (NEE) of carbon dioxide (CO₂) between building phase Calluna- and grass-dominated communities within three upland heaths in Scotland, measuring c. monthly over a year. Light and temperature response curves were generated, and the parameters derived were applied to continuous light and temperature data to extrapolate CO₂ fluxes over the full year and generate estimates of annual CO₂ sequestration for each vegetation type. Grass-dominated communities had higher ecosystem respiration rates than Calluna-dominated communities, attributed to graminoids having greater metabolic demands and producing more labile litter which decomposes readily. Both communities had similar gross primary productivity over the year; the net result being higher NEE within the Calluna-dominated than the grass-dominated community (-2.36 ± 0.23 and -1.78 ± 0.18 μmol CO₂m(-2)s(-1) respectively). Modelled CO₂ fluxes over a year showed both communities to be CO₂ sinks. The Calluna-dominated community sequesters -3.45 ± 0.96 t C ha(-1)yr(-1), double that sequestered by the grass-dominated community at 1.61 ± 0.57 t C ha(-1)yr(-1). Potential rate of C sequestration by upland heath is comparable to that of woodland and the increase in total sequestration that could be gained from habitat restoration may equate to c. 60% of the annual UK C sink attributed to forest land management. National C sequestration by heathlands is also more than double that by peatlands. Management of graminoid-dominated upland heath should promote Calluna re-establishment, thus providing a C benefit in addition to benefits to biodiversity, grazing and sporting interests.