Contents lists available at ScienceDirect Forest Ecology and Management journal homepage: www.elsevier.com/locate/foreco The carbon balance of a six-year-old Scots pine (Pinus sylvestris L.) ecosystem estimated by different methods Veiko Uri a, ⁎ , Mai Kukumägi b , Jürgen Aosaar a , Mats Varik a , Hardo Becker a , Kristiina Aun a , Alisa Krasnova a,b , Gunnar Morozov a , Ivika Ostonen b , Ülo Mander b , Krista Lõhmus b , Katrin Rosenvald b , Kaie Kriiska b , Kaido Soosaar b a Estonian University of Life Sciences, Institute of Forestry and Rural Engineering, Kreutzwaldi 5, 51014 Tartu, Estonia b University of Tartu, Institute of Ecology and Earth Sciences, Vanemuise 46, 51014 Tartu, Estonia ARTICLE INFO Keywords: Carbon balance Scots pine Eddy covariance Net ecosystem exchange Net ecosystem productivity Soil respiration ABSTRACT Clear-cutting is a conventional method of forest management which significantly changes carbon (C) cycling at the ecosystem level for a long time. Estimation of the interim period during which the ecosystem turns from a C source to a C sink is crucial for clarifying the environmental effects of management on forest C cycling. The current study provided new knowledge of C cycling in young pine stand and demonstrated the recovery of C sequestration of the forest ecosystem during the post harvesting period. We estimated the C balance in a 6-year-old Scots pine stand by using two different methods: carbon bud- geting, for estimating annual net ecosystem production (NEP), and eddy covariance (EC), for estimating net ecosystem exchange (NEE). For C budgeting, the above- and belowground biomass production of the ecosystem, as well as the soil heterotrophic respiration efflux at the studied site was estimated. Annual NEE at the studied young forest ecosystem was 1.19 ± 0.36 t C ha -1 , gross primary ecosystem production was 9.87 and total ecosystem respiration was 11.06 t C ha -1 . Estimated NEE was in good accordance with the results of NEP (1.37 t C ha -1 ), which confirms the relevance of the C budgeting method. Increased annual woody biomass production is the main factor which induced the young Scots pine ecosystem to act as a C sink: annual C accumulation in tree biomass in a 6-year-old stand was 1.0 t C ha -1 but reached already 2.4 t C ha -1 in the following year. Assuming that the annual R h flux is of the same magnitude in the subsequent years, the ecosystem will become a C sink already during a short period after clear-cut. Annual soil respiration (R s ) and heterotrophic soil respiration (R h ) were 6.0 and 4.2 t C ha -1 , respectively and the R h /R s ratio was 0.70. However, at this stage also the understorey vegetation contributed essentially to NEP, making up 56% of the annual C uptake accumulated in the plants. The methane flux and the leached C flux were negligible, 0.004 and 0.015 t C ha -1 yr -1 , respectively. Our results demonstrate that well regenerated young Scots pine stand on a former clear-cut area will be able to turn into a C sequestering ecosystem already before ten years after cutting. 1. Introduction During recent decades, the concentration of atmospheric carbon dioxide (CO 2 ) has rapidly increased inducing climate change. According to the Paris Agreement (2015), governments should sig- nificantly reduce carbon (C) emission and increase the share of bioe- nergy in the nearest future (UNFCCC eHandbook). In this aspect, it is essential to focus on forest ecosystems at the global and regional scales since forests mitigate climate change (Schimel et al., 2001) by accu- mulating carbon (C) in plant biomass as well as in soil (Cannell, 1999; Mund et al., 2002; Pussinen et al., 2002; Ågren and Hyvonen, 2003; Laiho et al., 2003). In general, boreal and hemiboreal forests have been reported as ecosystems with high C sink capacity (Dixon et al., 1994; Valentini et al., 2000; Liski et al., 2002, 2003; Gough et al., 2008). As changes in the C stocks may affect the balance between terres- trial and atmospheric C (Keith et al., 2009), clarification of C budgets at the ecosystem level is essential for understanding the driving forces behind the C cycle and sequestration capacity (Fang et al., 2007) which is very variable due to the high heterogeneity of different forest eco- systems. https://doi.org/10.1016/j.foreco.2018.11.012 Received 3 September 2018; Received in revised form 7 November 2018; Accepted 9 November 2018 ⁎ Corresponding author. E-mail address: veiko.uri@emu.ee (V. Uri). Forest Ecology and Management 433 (2019) 248–262 0378-1127/ © 2018 Elsevier B.V. All rights reserved. T