Effect of thinning on turbulence structure, energy and gas exchange in a boreal forest

Thinning is performed primarily to manage between-tree competition and allocate growth-limiting resources (e.g. light, water, nutrients) to the remaining trees and to increase their growth rate and vitality. From biophysical point of view, thinning changes tree spacing, number, and size distribution. Our hypothesis is that altered stand structure and decreased foliage density cause modifications of the microclimate, radiation budget and turbulence characteristics within the canopy. Jointly, these physical constraints change the dynamics of biogeochemical cycles and affect mass and energy exchange between soil and vegetation components and the atmosphere.

Here, we investigate the short-term response (i.e. one-two years post thinning) to the thinning done at Hyytiälä forest located in southern Finland. We present results using eddy covariance (EC) fluxes of NEE and ET at both ecosystem level (i.e. above canopy EC) and ground vegetation level (i.e. sub-canopy EC). We found that the forest became a source of carbon during the first post thinning year (+55 gC m^-2), while in the second post-thinning year (2021), the ecosystem has only partly recovered showing annual NEE value of -152 gC m^-2 which is somehow far from the long-term net uptake measured at the site (-252 gC m^-2). Preliminary results show that the thinning had less impacts on ET fluxes. We also report the effect of thinning on ecosystem surface fluxes of carbonyl sulfide (COS), carbon monoxide (CO) and ozone (O3). Here, we hypothesize an increase of CO biogenic emission, due to an increase of amount of litter and light on the forest floor, and a decrease of COS and O3 deposition rates, related to foliage removal.

Finally, the functional response of the flux components are analysed by using clustering and modelling approaches in order to disentangle the roles of thinning and weather on measured fluxes and budgets.