Comparison of the carbon, water, and energy balances of mature stand and clear-fell stages in a British Sitka spruce forest and the impact of the 2018 drought

Abstract Sitka spruce is the major conifer species in British upland forests and is predominantly managed as even-aged, single-species plantations with rotation lengths of less than 50 years using a “patch clear-felling” system. Evidence on the impact of clear-felling on the carbon, water, and energy balances of plantation forestry is sparse and extreme weather events, such as the 2018 drought that occurred across Europe, are expected to have considerable impact. We used energy balance and gas flux data from a typical mature stand and a clear-felled site on peaty-gley soil at Harwood Forest in Northumberland, UK, over 4 years to quantify the impact of clear-felling and investigate how the drought of 2018 affected these fluxes. Felling resulted in an annual CO2 source (7.05  tC ha − 1 yr − 1 ) of similar magnitude in absolute terms to the mature stand sink (7.09  tC ha − 1 yr − 1 ). The source was reduced with the growth of ground vegetation, and after tree replanting two years post-felling, the site was a weak source. Felling increased reflected radiation by 92%, reduced evapotranspiration by 28% and therefore increased site wetness by 8%, while replanting and the spread of ground vegetation increased evapotranspiration by 16% on average. The warm, dry spring and summer in 2018 increased evapotranspiration and reduced net carbon uptake of the mature stand by 30% compared to the mean of the previous three years, because of a small decrease in photosynthetic uptake and a small increase in ecosystem respiration. The drought conditions also increased ecosystem losses of CO2 at the clear-felled site by 28%. The clear-fell site would have turned into a small CO2 sink in the second year after replanting, if there had not been a drought.