New research reveals the impacts of forest management for timber production on the carbon sequestration of a Sitka spruce forest and shows the impact of the 2018 drought.

Our climate is changing with global temperatures potentially rising 2 – 4 °C above pre-industrial levels by the end of this century.  Trees and forests have an important role in mitigating climate change, and in helping reach ‘net -zero’ emissions targets. In the UK, it is estimated that in 2018 there was a net carbon dioxide (CO₂) uptake (i.e. a sink) of 18 million tonnes by forests.

Coniferous trees, such as pines, spruces or firs, comprise approximately half of the UK’s forests; around half of those are made up of fast-growing Sitka spruce grown and harvested for a range of timber products.  These forests are predominantly managed as even-aged, single-species plantations, using ‘patch clear-felling’ for harvesting and a growth cycle duration of less than 50 years.

This intensive management affects the forest carbon cycle, through changing the CO₂ uptake of the trees and the release of CO₂ and other greenhouse gases from the soil, and may determine how large a carbon sink forests are.

Research conducted by Forest Research recently published in the journal ‘Agricultural and Forest Meteorology’ quantifies the impact of the clear-felling and replanting management system on a typical upland Sitka spruce plantation on shallow peat soils. The researchers assessed the carbon balance, comparing a mature stand and a clear-felled site over a four-year period. The low rainfall of 2018 provided insights into the impacts of drought on the forest and felled areas.

The research shows that clear-felling resulted in a large initial CO₂ release in the year post-felling, and this was quantitatively similar to the net annual CO₂ uptake of the mature stand which was a strong sink.  However, the growth of regenerating seedlings and ground vegetation reduced the net release of CO₂ from the clear-felled area in subsequent years.

Importantly, in the third year after felling, the year of replanting, the clear-fell site reduced to being only a weak net source of CO₂.  We conclude that using the clear-felling and replanting management system for productive Sitka spruce stands on similar shallow peat soils is likely to start to recreate a carbon sink within 5 years of felling. However, the drought of 2018 in the fourth year after felling increased the net losses of CO₂ at the clear-felled site by 28%, slowing down the carbon sink recovery. The drought also reduced the annual net CO₂ uptake of the mature Sitka spruce stand by 30% compared to the previous 3 years, although it remained a strong carbon sink.

This research shows that the conifer timber production cycle on shallow peat soils can contribute to net-zero emissions targets. Further droughts may reduce the contribution of our forests to these targets, so understanding drought sensitivity is important in managing climate change risk.

Citation: Xenakis, G., Ash, A., Siebicke, L., Perks, M., Morison, J.I.L, 2021. 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. Agricultural and Forest Meteorology, Volume 306, 108437 https://doi.org/10.1016/j.agrformet.2021.108437.

Image: Monitoring greenhouse gas fluxes within a mature stand of Sitka spruce at Harwood Forest.