Silver Fir Decline in Mixed Old-Growth Forests in Slovenia: an Interaction of Air Pollution, Changing Forest Matrix and Climate

Abstract

Silver fir (Abies alba Mill.) is from ecological, economical and social point of view one of the most important conifer species in Europe (Kramer, 1992; Prpic et al., 2001). Many organisms are closely linked to silver fir dominated habitats. Silver fir (hereafter fir) can be very productive tree species in optimal site conditions. On siliceous, heavy soils in the lowlands it is more resistant to drought than Norway spruce (Picea abies (L.) Karst.). However, fir is extremely susceptible to environmental change (Schutt, 1978), including climate change (Brinar, 1964; Wick & Mohl, 2006; Anic et al., 2009), wild ungulate browsing (Motta, 1996; Senn & Suter, 2003), management regime (Mlinsek, 1964; Hockenjos, 2008) and air pollution (Eckstein et al., 1983; Krause et al. 1986; Elling et al., 2009). It is especially vulnerable to SO2 emissions. This was recognized by analyses of needles composition, comparative bioindication of sites with different emission loads and tree-ring studies (e.g. Wentzel, 1980; Elling, 1987; Elling et al., 2009). In Slovenia in 1980 the SO2 emissions amounted to about 235,795 tonnes/year and have significantly decreased until 2007 to 14,245 tonnes/year, which is almost 94% (ARSO, 2010). The decrease of emissions in Slovenia started in late 1980s and influenced recovery of silver fir vitality and growth (Prelc et al., 1993; Ficko & Boncina, 2006). Similar processes were reported also from other countries (Dobrowolska, 1998). Beside air pollution also the climatic conditions, especially warm summers and repeated droughts had a significant impact on fir health (Mlinsek, 1964; Leibundgut, 1974; Becker et al., 1989; Bert, 1993; Thomas et al., 2002; Ficko et al., 2011). The first reports of local and regional fir decline in mixed fir-beech forests of the Dinaric Mountains date from the end of the 1920s (Safar, 1951). Sun exposed, rockier, and drier slopes were especially affected. The fir decline was probably triggered by harsh winters and hot, dry summers (e.g. 1950) and it was accompanied by bark beetle calamities. Overbrowsing is another factor that can cause fir density decrease (Gill, 1992; Motta, 1996; Klopcic et al., 2010). Namely, fir combines several features that make it vulnerable to browsing: it is one of the most palatable species, it grows very slowly in shaded old-growth conditions, and it recovers poorly from browsing damage. Old-growth forests are often game reserves and are thus characterised by excessive game densities (Korpel, 1995; Kenderes et al., 2008; Vrska et al., 2009; Diaci et al., 2010).