Journal of Tropical Ecology (2004) 20:125–129. Copyright © 2004 Cambridge University Press DOI: 10.1017/S0266467403001020 Printed in the United Kingdom SHORT COMMUNICATION Arbuscular endomycorrhizas are dominant in the organic soil of a neotropical montane cloud forest Ingrid Kottke ∗1 , Adela Beck ∗ , Franz Oberwinkler ∗ ,J¨ urgen Homeier† and David Neill‡ ∗ Institute of Systematic Botany and Mycology, Eberhard-Karls-University of T ¨ ubingen, Auf der Morgenstelle 1, D-72076 T ¨ ubingen, Germany † Department of Ecology, University of Bielefeld, Germany ‡ Herbario National de Ecuador, Quito, Ecuador (Accepted 10 December 2002) Key Words: acid humus, arbuscular mycorrhiza, ectomycorrhiza, Ecuador, Graffenrieda emarginata, Neea sp., trees The ecology and forest management of tropical montane cloud forests of the Neotropics have attracted little scientific attention so far, and understanding of the ecosystem is still fragmentary (Churchill et al. 1995). The montane forests of southern Ecuador are an outstanding biodiversity hotspot for vascular plants (Barthlott et al. 1996), including a multitude of tree species (Madsen & Øllgard 1994). However, identification of species, studies of phenology, productivity, seed production and growth of seedlings have only just begun (Homeier & Breckle 2002). None of the trees has been investigated with respect to their mycorrhizal status, although it is well established that mycorrhizal symbioses are not only important for survival of trees in nutrient-poor habitats (Read 1991) and play a key role for nutrient cycling and nutrient retention in the humus layers (Medina & Cuevas 1993, Rilling et al. 2001), but also have an important impact on the composition of plant communities (Allen et al. 1995, Kottke 2002, van der Heijden et al. 1998). Two types of mycorrhizal associations are formed by forest trees: the arbuscular mycorrhiza (AM) and the ectomycorrhiza (ECM) (Smith & Read 1997). The AM- type is formed by members of the Glomeromycota (sensu Sch ¨ ußler et al. 2001) while the ECM-type is formed by members of the Asco- and Basidiomycota. Consequently, structure, physiology and ecology of the interaction types are diverse (Smith & Read 1997). Tropical forests rich in species from diverse families are considered to be dominated by AM-forming trees, while the temperate and boreal forests, the Eucalyptus forests and the dipterocarp 1 Corresponding author. forests are dominated by relatively few tree genera or species that have ectomycorrhizal associations. However, ECM-forming trees have also been found in patches or larger stands between AM-forming trees in the humid tropics of Africa (Fassi & Fontana 1962, H¨ ogberg 1982, 1986; H ¨ ogberg & Nylund 1981, H ¨ ogberg & Piearce 1986, Onguene & Kuyper 2001), in the Amazonian lowland forest (Meyer 1991, Singer et al. 1983), in Cuban coastal regions (Kreisel 1971), and recently in the Guianas (B´ ereau et al. 1997, Henkel 1999). The predominance of AM-associated vegetatation has been considered to be correlated to soil horizons low in organic matter while ectomycorrhizal vegetation was thought to cope better with nutrient-poor organic soils, as only the latter can utilize organically bound nitrogen (Janos 1980, Read 1991). More recent investigations have shown, however, that AM dominate in poor soils of lowland tropical forests in French Guiana (B´ ereau et al. 1997), and occur at the same frequency in horizons of low and high organic matter content in tropical heath forests of Borneo (Moyersoen et al. 2001). Forest stands in Puerto Rico dominated by Dacryodes excelsa also form thick mats of roots and organic matter, though they are also strictly associated with AM fungi (Lodge 1996). In our study sites under consideration in southern Ecuador, a very thick (30–100 cm) but loose layer of pure organic material covers the shallow silty soils on top of highly weathered quartzitic or loamy sandstone. The soil is acidic but varies locally between pH 2.4 and 4.0. Extensive root development was observed in the humus fraction only. The organic forest floor contains the largest nitrogen pool and a relatively high proportion of basic cations such as Ca, Mg and K, most probably stimulating root development in this horizon (Wilcke et al. 2002). The