Regional and local patterns of ectomycorrhizal fungal diversity and community structure along an altitudinal gradient in the Hyrcanian forests of northern Iran Mohammad Bahram 1 , Sergei Po ˜lme 1,2 , Urmas Ko ˜ljalg 1,2 , Shahin Zarre 3 and Leho Tedersoo 1,2 1 Institute of Ecology and Earth Sciences, University of Tartu, 40 Lai, 51005 Tartu, Estonia; 2 Natural History Museum of Tartu University, 46 Vanemuise, 50014 Tartu, Estonia; 3 Department of Plant Sciences, School of Biology, College of Science, University of Tehran, PO Box 14155-6455, Tehran, Iran Author for correspondence: Mohammad Bahram Tel: +372 737 6222 Email: bahram@ut.ee Received: 12 August 2011 Accepted: 2 September 2011 New Phytologist (2012) 193: 465–473 doi: 10.1111/j.1469-8137.2011.03927.x Key words: climate change, ectomycorrhizal symbiosis, elevation, precipitation, species richness, temperature. Summary • Altitudinal gradients strongly affect the diversity of plants and animals, yet little is known about the altitudinal effects on the distribution of microorganisms, including ectomycorrhizal fungi. • By combining morphological and molecular identification methods, we addressed the rela- tive effects of altitude, temperature, precipitation, host community and soil nutrient concen- trations on species richness and community composition of ectomycorrhizal fungi in one of the last remaining temperate old-growth forests in Eurasia. • Molecular analyses revealed 367 species of ectomycorrhizal fungi along three altitudinal transects. Species richness declined monotonically with increasing altitude. Host species and altitude were the main drivers of the ectomycorrhizal fungal community composition at both the local and regional scales. The mean annual temperature and precipitation were strongly correlated with altitude and accounted for the observed patterns of richness and community. • The decline of ectomycorrhizal fungal richness with increasing altitude is consistent with the general altitudinal richness patterns of macroorganisms. Low environmental energy reduces the competitive ability of rare species and thus has a negative effect on the richness of ectomycorrhizal fungi. Because of multicollinearity with altitude, the direct effects of climatic variables and their seasonality warrant further investigation at the regional and continental scales. Introduction Climate is the main factor determining biodiversity patterns at the global scale (Hawkins et al., 2003; Currie et al., 2004). Changes in the global climate have shifted the geographical distribution of plant and animal species both latitudinally and altitudinally (Parmesan & Yohe, 2003; Parmesan, 2006), causing extinctions in taxa that fail to migrate (Thomas et al., 2004). Information on diversity patterns along climatic gradients may enable the prediction of the response of species and entire communities to climate change (Grytnes & McCain, 2007). Because the same climatic factors largely explain both the latitudinal and altitudinal gradients of diversity, altitudinal patterns are commonly used to predict large-scale latitudinal pat- terns (Lomolino et al., 2006) to avoid the confounding historical and geographical factors. Compared with latitudinal gradients, seasonality and temperature fluctuate less strongly along altitudi- nal gradients (Rahbek, 1995). Changes in ecosystems are more abrupt along altitudinal gradients which permit the migration of species within a few generations (Hewitt, 1996). However, low oxygen concentration, low air pressure and high UV radiation constrain climatic effects at high altitude. In addition, the land area at higher altitudes is severely reduced and may promote the extinction of critically small populations. Although the species diversity of various organisms usually declines with altitude, con- trasting patterns occur in certain vascular plants, bryophytes and lichens (e.g., Bhattarai & Vetaas, 2003; Bruun et al., 2006; Desalegn & Beierkuhnlein, 2010). Therefore, the extrapolation of altitudinal diversity patterns across taxa seems premature. Most of the current knowledge on altitudinal and latitudinal diversity patterns relies on macroorganisms, whereas microorgan- isms have received little attention. The few studies that have been performed have revealed contrasting altitudinal diversity patterns in soil and aquatic microorganisms: a decline in Acidobacteria (Bryant et al., 2008) but an increase in aquatic bacteria and diatoms (Wang et al., 2011) at higher altitudes. These studies suggest that different processes may drive the altitudinal diversity of microorganisms: for example, soil temperature has the strong- est effect on Acidobacteria, whereas nutrients influence diatoms. Changes in climate may affect microorganisms directly or indi- rectly through shifts in vegetation. There is increasing evidence that biological interactions, including associations between Research Ó 2011 The Authors New Phytologist Ó 2011 New Phytologist Trust New Phytologist (2012) 193: 465–473 465 www.newphytologist.com