ORIGINAL PAPER Water status and drought stress in experimental gaps in managed and semi-natural silver fir–beech forests Urs ˇa Vilhar • Primoz ˇ Simonc ˇic ˇ Received: 4 January 2011 / Revised: 26 October 2011 / Accepted: 29 December 2011 / Published online: 18 February 2012 Ó Springer-Verlag 2012 Abstract The effect of gap characteristics, including gap age, vegetation cover, and stage of regeneration, on water balance and drought stress was studied over a four-year period in natural and experimental gaps of various sizes in Dinaric silver fir–beech forest in Slovenia. A medium gap, two experimental clear-cut gaps, and a closed stand were selected in a traditional irregular shelterwood managed forest, and an irregular-shaped natural gap and part of the stand were selected in a semi-natural forest. Soil moisture, throughfall, and stemflow were measured in the gaps and stands, and incident precipitation was monitored in the open area. Evapotranspiration and drainage water fluxes were estimated using the water balance model BROOK90. To estimate drought stress, the ratio between simulated actual and potential evapotranspiration was used. Precipi- tation in the 2001 and 2003 growing seasons was consid- erably less than that in 2002 and 2004. The highest drainage fluxes were in the newly created clear-cut gaps with sparse ground vegetation cover, followed by the nat- ural gap and medium gap, which had older and denser natural tree regeneration. On average, the clear-cut gap drainage fluxes were 18% (164 mm) higher than those in the forest stands. Evapotranspiration was lower in the natural gap and higher in the managed gap compared with the experimental clear-cut gaps. Water supply stress was greater for the forest stands than for the vegetation in the gaps. Drought stress was indicated to be lowest in the natural gap, which had patches of varying development phases and the highest water storage capacity of the soil and vegetation. Keywords Water balance Á Drought stress Á Experimental canopy gaps Á BROOK90 model Á Dinaric silver fir–beech forest Á Slovenia Introduction Water supply is a key factor affecting the stability, pro- ductivity, and health of forest ecosystems. Over the next few decades, global climate change is expected to increase the frequency and severity of both drought events and periods of water logging in Europe. This is due to the combination of two trends: an increase in temperature and a shift in precipitation regimes characterized by more precipitation in winter and less rain in summer (Berge `s and Balandier 2010). The exceptional climatic conditions of summer 2003 that caused a severe drought throughout much of Western and Southern Europe (Renaud and Reb- etez 2009) are expected to occur more frequently in the twenty-first-century. Repeated droughts are assumed to be responsible for altered water and carbon fluxes (Granier et al. 2007) and an increase in the rate of tree decline and mortality in Europe (Bre ´da et al. 2006). As a consequence, the productivity of forest ecosystems declines (Briceno- Elizondo et al. 2006). There is ample evidence that the water balance of forest ecosystems changes after clear-felling (Bormann and Lik- ens 1986; Katzensteiner 2003; Ritter et al. 2005b). It has been suggested that drainage water fluxes and N losses can be controlled by using harvesting practices that cause only small-scale disturbances rather than conventional clear-felling in which large areas of forest are harvested Communicated by C. Ammer. U. Vilhar (&) Á P. Simonc ˇic ˇ Slovenian Forestry Institute, Ljubljana, Slovenia e-mail: ursa.vilhar@gozdis.si 123 Eur J Forest Res (2012) 131:1381–1397 DOI 10.1007/s10342-012-0605-x