Effects of experimental drought on the fine root system of mature Norway spruce Dirk Gaul a, *, Dietrich Hertel a , Werner Borken b , Egbert Matzner b , Christoph Leuschner a a Plant Ecology, Albrecht-von-Haller-Institute for Plant Sciences, University of Go ¨ttingen, D-37073 Go ¨ttingen, Germany b Department of Soil Ecology, BayCEER, University of Bayreuth, Dr.-Hans-Frisch-Straße 1-3, D-95448 Bayreuth, Germany 1. Introduction Fine roots play an important role in global carbon cycling because they are short-lived organs characterised by rapid turnover (Hendrick and Pregitzer, 1993; Gill and Jackson, 2000). Assuming that fine root longevity is 1 year, fine root production may constitute about 30% of the global net primary production (Jackson et al., 1997). Together with aboveground litter fall and the turnover of mycorrhizal hyphae, root production and mortality represent an important path of organic carbon input to soils. Thus, soil organic matter and root dynamics are tightly linked (Guo et al., 2005; Rasse et al., 2005). However, our understanding of below- ground C transfer processes is still limited especially with respect to the regulation of root dynamics by abiotic stressors such as drought. This knowledge will become increasingly important since current scenarios of global climate change predict an increase in frequency and intensity of drought periods for various regions of the world (Rowell and Jones, 2006; IPCC, 2007). In Central Europe, a significant increase in the duration of summer drought periods is expected to take place in particular in the more continental regions (Hulme and Sheard, 1999; Christensen and Christensen, 2003). Drought-induced changes in fine root dynamics may affect tree growth and vitality. Altered growth dynamics could also affect carbon sequestration in forests. A deeper understanding of the drought tolerance of the fine root system of the main tree species is urgently needed. Many reports on drought-induced dieback of fine roots in temperate and also tropical forests exist (e.g. Srivastava et al., 1986; Kummerow et al., 1990; Leuschner et al., 2001; Davies and Bacon, 2003). In one of the few throughfall exclusion experiments in temperate forests, Joslin and Wolfe (2003) found a reduction in fine root biomass and a displacement of root biomass to greater soil depth. On the other hand, a limited number of studies suggest Forest Ecology and Management 256 (2008) 1151–1159 ARTICLE INFO Article history: Received 17 March 2008 Received in revised form 9 June 2008 Accepted 10 June 2008 Keywords: Decomposition Fine root mortality Minirhizotrons Production Soil coring Throughfall exclusion ABSTRACT Norway spruce (Picea abies (L.) Karst.) is an economically important, but relatively drought-sensitive tree species that might suffer from increasing drought intensities and frequencies, which are predicted to occur in parts of central Europe under future climatic change. In a throughfall exclusion experiment using sub-canopy roofs, we tested the hypothesis that enhanced drought leads to an increase in fine root mortality, and also to a higher, subsequent fine root growth. Fine root production and mortality were assessed using two independent approaches, sequential soil coring (organic layer) and direct observations in minirhizotrons (organic layer plus upper mineral soil). Six weeks of throughfall exclusion resulted in mild drought stress, i.e. a reduction in average soil moisture from 20 to 12 vol.% during the treatment. Based on the sequential coring data, experimental drought did not result in significant changes in fine root biomass during the 6-week treatment period, but caused an increase in fine root mortality by 61% in the 6 weeks following the drought treatment. Remarkably, fine root production showed a synchronous increase in this period, which more than compensated for the loss due to increased mortality. The minirhizotron data confirmed that the drought treatment increased fine root loss in the organic layer. Based on this method, however, root loss occurred during the drought period and was not compensated by increased root production. The mild drought stress was mainly restricted to the organic layer and did not significantly influence fine root dynamics in the mineral soil. We calculated that the drought event resulted in an extra input of about 28 g C m À2 and 1.1 g N m À2 to the soil due to increased fine root mortality. We conclude that even periods of mild drought significantly increase fine root mortality and the associated input of root-derived C to the soil organic matter pool in temperate Norway spruce forests. ß 2008 Elsevier B.V. All rights reserved. * Corresponding author at: Grisebachstr. 1, D-37077 Go ¨ ttingen, Germany. Tel.: +49 551 395702; fax: +49 551 3922029. E-mail address: dgaul@gwdg.de (D. Gaul). Contents lists available at ScienceDirect Forest Ecology and Management journal homepage: www.elsevier.com/locate/foreco 0378-1127/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2008.06.016