Mitigation of drought by thinning: Short-term and long-term effects on growth and physiological performance of Norway spruce (Picea abies) Julia A. Sohn a,1 , Timo Gebhardt b,c,⇑,1 , Christian Ammer b , Jürgen Bauhus a , Karl-Heinz Häberle c , Rainer Matyssek c , Thorsten E.E. Grams c a Chair of Silviculture, Faculty of Environment and Natural Resources, University of Freiburg, 79085 Freiburg, Germany b Dept. of Silviculture and Forest Ecology of the Temperate Zones, Georg-August-Universität- Göttingen, 37077 Göttingen, Germany c Dept. of Ecology and Ecosystem Management/Ecophysiology of Plants, Technische Universität München, D-85354 Freising, Germany article info Article history: Received 4 June 2013 Received in revised form 25 July 2013 Accepted 25 July 2013 Available online 31 August 2013 Keywords: Stable carbon and oxygen isotopes Radial growth Water stress Tree rings Resistance Recovery abstract We hypothesize that reductions in stand density through thinning improve the recovery of radial stem growth in Norway spruce trees (Picea abies) from severe drought. However, thinning may not lead to higher relative radial growth during drought. Annual stem growth and stable carbon and oxygen isotopes in early- and latewood were assessed in trees from heavily thinned (HT), moderately thinned (MT) and un-thinned control stands at two sites in southern Germany. Physiological performance of trees as inferred from stable isotope analysis was used to interpret annual stem growth in response to the drought events in 1976 and 2003. Only in recently thinned stands, trees maintained growth probably through higher soil water availability during the drought year when compared to controls. In contrast, thinning improved the growth recovery in the years following the drought irrespective of the time span between thinning and drought. We conclude that thinning improves drought recovery response in the short and long term and should be considered as an effective management strategy to increase drought tolerance of Norway spruce stands. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Global climate change will distinctly alter ecosystem conditions through rising temperatures, reduced precipitation and more fre- quent weather extremes (IPCC, 2007; Lasch et al., 2002; Meehl and Tebaldi, 2004). In Europe, this became evident during and after the extreme drought of 2003, which was characterized by prolonged water shortage in combination with significant heat waves (Ciais et al., 2005; Rennenberg et al., 2006; Rebetez et al., 2008). Severe drought episodes instantly affect physiological processes in trees such as transpiration, photosynthesis and carbon (C) allocation followed by resulting in increased tree susceptibility to secondary stressors such as by phytophagous insects or parasitic fungi (Bréda et al., 2006; Desprez-Loustau et al., 2006; Rouault et al., 2006). This may lead to reduced primary productivity and eventually whole-stand decline (Bréda et al., 2006; Jyske et al., 2009). In Germany, Norway spruce (Picea abies [L.] Karst.), planted pre- dominately in even-aged and mono-specific stands, is economically the most important tree species and occupies 30% of the total forest area (BMELV, 2008). Yet, large tracts of Norway spruce forests are growing near the perceived drought limit of the species, and with climate change, may be exposed to even more unfavorable condi- tions in the future (Koelling et al., 2009). As a consequence, Norway spruce has been already lost a substantial proportion of the previ- ously cultivated area (Polley et al., 2009). Most strategies to adapt forest ecosystems to climate change have long-term goals such as diversification of species composition or introduction of drought tolerant species (Kazda and Pichler, 1998; Ammer et al., 2008). However, there are few silvicultural adaptation approaches for immature and juvenile stands. For exist- ing stands, intensive thinning has been discussed as an option to mitigate drought impacts (e.g. Cescatti and Piutti, 1998; Laurent et al., 2003; Martin-Benito et al., 2010). In the short-term, thinning has been found to reduce stand transpiration (Bréda et al., 1995; Lagergren et al., 2008), intercep- tion of precipitation and competition aboveground and below- ground (Aussenac and Granier, 1988; Bréda et al., 1995) so that soil water content is increased (Stogsdill et al., 1992; McDowell et al., 2003). However, increasing within-stand radiation and air flow in the canopy in recently thinned stands can also increase transpiration of remaining trees and promote the development of understory vegetation, causing a decrease in soil water availability 0378-1127/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.foreco.2013.07.048 ⇑ Corresponding author at: Dept. of Silviculture and Forest Ecology of the Temperate Zones, Georg-August-Universität- Göttingen, 37077 Göttingen, Ger- many. Tel.: +49 8161 714872; fax: +49 8161 714576. E-mail address: Timo.Gebhardt@forst.uni-goettingen.de (T. Gebhardt). 1 These authors contributed equally to this paper. Forest Ecology and Management 308 (2013) 188–197 Contents lists available at ScienceDirect Forest Ecology and Management journal homepage: www.elsevier.com/locate/foreco