Agricultural and Forest Meteorology 157 (2012) 1–10 Contents lists available at SciVerse ScienceDirect Agricultural and Forest Meteorology jou rn al h om epa g e: www.elsevier.com/locate/agrformet Thinning effects on the net ecosystem carbon exchange of a Sitka spruce forest are temperature-dependent M. Saunders a,∗ , B. Tobin b , K. Black b,c , M. Gioria a , M. Nieuwenhuis b , B.A. Osborne a a University College Dublin, UCD School of Biology and Environmental Science, Belfield, D4, Dublin, Ireland b University College Dublin, UCD Forestry, School of Agriculture and Food Science, Belfield, D4, Dublin, Ireland c FERS Ltd. 117 East Courtyard, Cabinteely, Dublin 18, Ireland a r t i c l e i n f o Article history: Received 5 July 2011 Received in revised form 4 January 2012 Accepted 10 January 2012 Keywords: Sitka spruce Forest thinning Carbon stocks and fluxes Net primary productivity Eddy covariance Net ecosystem exchange Gross primary productivity Ecosystem respiration a b s t r a c t Commercial forest plantations need to be actively managed, through tree removal, in order to improve wood quality, maintain productivity and provide an economic return, although this could compromise an important role for forests in carbon sequestration and greenhouse gas mitigation. The impact of for- est thinning on net primary productivity (NPP) and net ecosystem exchange (NEE) was assessed using a combination of biometric and eddy covariance (EC) techniques. Two thinning operations were per- formed in close succession, which reduced the basal area of the stand by 17% and 11% and removed a timber volume of 48 m 3 ha -1 and 50 m 3 ha -1 , respectively. Annual rates of NPP ranged from 13.24 (±3.96) to 18.94 (±4.88) t C ha -1 and 13.22 (±3.72) to 17.77 (±5.30) t C ha -1 for the pre- and post-thinning peri- ods, respectively. Estimates of NEE varied between 8.44 (±1.34) to 8.87 (±1.48) t C ha -1 and 6.75 (±1.19) to 10.33 (±1.41) t C ha -1 in the pre- and post-thinning periods. Forest thinning did not have a significant impact on carbon stocks or fluxes when pre-thinning (2002–2006) and post-thinning (2007–2009) esti- mates of NPP and NEE were compared, however the range of inter-annual variability in NEE increased after thinning. The partitioning of annual NEE carbon budgets into gross primary productivity (GPP) and ecosystem respiration (R eco ) together with an analysis of key physiological parameters suggested that the impacts of forest thinning are largely dependent on temperature. An expected decrease in GPP after the initial thinning in 2007 was not observed due, in part, to the higher mean annual air temperatures and incident photosynthetic active radiation (PAR) and a compensatory increase in photosynthesis by the remaining trees. A continual decline in R eco , was observed in the years subsequent to the first thinning and was attributed to both biomass removal and climatic factors. Inter-annual variations in climate had a significant impact on NEE, GPP and R eco . Annual mean air temperature, total precipitation and total incident PAR were all shown to influence the processes driving CO 2 exchange. Overall, these results suggest that the impacts of the thinning practices, as implemented in this study, are dependent on climate and under similar conditions are unlikely, in the short-term, to compromise a role for forest ecosystems in carbon sequestration and greenhouse gas mitigation. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Forest ecosystems represent a significant terrestrial carbon store, where the majority of carbon sequestered is held within the woody biomass (Scott et al., 2004). Current forest manage- ment strategies that involve variable thinning regimes have been designed to enhance the timber yield and economic returns over the length of the forest rotation. In addition, policy-driven research has identified a potential for using forests to mitigate and off- set greenhouse gas (GHG) emissions through increased carbon sequestration and the utilisation of forest products and residues. ∗ Corresponding author. Tel.: +353 1 7162245; fax: +353 1 7161152. E-mail address: matthew.saunders@ucd.ie (M. Saunders). For example, the replacement of carbon intensive construction materials with timber-based products and the use of forests or for- est residues for bioenergy production may reduce anthropogenic carbon dioxide (CO 2 ) emissions (Eriksson, 2006). Forest planta- tions, therefore, have the ability to play an important role in the mitigation of, and adaptation to, global climate change. However, management strategies, such as thinning that reduce the amount of standing biomass, may in the short-term compromise rates of car- bon assimilation and negate a dual role for forests for both timber production and carbon sequestration. In Europe, Sitka spruce (Picea sitchensis (Bong.) Carr.) represents an important commercial tree species due mainly to its high pro- ductivity (Berhofer et al., 2003). This is particularly the case in Ireland where Sitka spruce accounts for approximately 50% of the total forest area as either mono-species or mixed stands (National 0168-1923/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.agrformet.2012.01.008