European Journal of Soil Science, October 2012, 63, 674–684 doi: 10.1111/j.1365-2389.2012.01444.x The role of metals in soils and accumulation in the laurel forest of La Gomera (Canary Islands, Spain): preliminary results J. L. Mora a , C. D. Arbelo b , A. Rodr´ ıguez-Rodr´ ıguez b , J. S. Notario b , J. A. Guerra c & C. M. Armas b a Departamento de Agricultura y Econom´ ıa Agraria, Universidad de Zaragoza, 50013 Zaragoza, Spain, b Departamento de Edafolog´ ıa y Geolog´ ıa, Universidad de La Laguna, 38204 La Laguna, Spain, and c Gesti´ on del Medio Rural de Canarias, 38110 Santa Cruz de Tenerife, Spain Summary The present study investigated the content of Al and various heavy metals (Cr, Cu, Fe, Mn, Ni and Zn) in soils and plant tissues of a sub-tropical laurel forest in the Canary Islands and their relationship to the composition and structure of the forest. The content of different Al-fractions and bioavailable (NH 4 Ac-EDTA extractable) metals was analysed in a large number of soil samples and related to lithological and topographic factors. Seasonal variation of the metal content in the soil, phytomass and litter was studied at sites with different degrees of maturity or sites affected by forest dieback. Extremely large contents of bioavailable Al, Fe and Mn were observed in soils. The Al and Mn contents exert a considerable influence on ecosystem composition. The results of this study reveal the bioaccumulation of Al in Ilex canariensis Poir. and Viburnum rigidum Vent. and suggest the accumulation of Mn in Laurus novocanariensis Rivas Mart. The bioaccumulation patterns of Al were closely related to the NH 4 Ac-EDTA-extractable soil content but not as closely tied to KCl-extractable Al, suggesting the greater importance of humic complexes in the regulation of aluminium toxicity in comparison with hydroxy-Al interlayered 2:1 silicates. Bioaccumulation gives rise to Al-rich litterfall inputs, allowing the exclusion of Al-intolerant species from the understory or neighbouring vegetation. Introduction The presence of toxic metals in certain soils and their harm- ful effects on most plants are well known phenomena. Plants growing in soils that are rich in potentially toxic metals have developed different mechanisms to resist or tolerate toxicity. Hyper-accumulation is defined as the ability of certain plants to accumulate extremely large concentrations of metals in their tissues. Depending on the accumulated element, the hyper- accumulation threshold is set above 100 mg kg −1 (cadmium, Cd; cobalt, Co; chromium, C; lead, Pb), 1000 mg kg −1 (alu- minium, Al; arsenic, As; copper, Cu; nickel, Ni; selenium, Se) or 10 000 mg kg −1 (manganese; Mn, zinc, Zn) (Boyd, 2004). The distinctive feature of Andosols is a colloidal fraction in which short-range ordered minerals or organo-aluminic complexes are predominant. A distinction is usually made between allophanic (Silandic) Andosols rich in allophanic clay and non-allophanic Correspondence: J. L. Mora. E-mail: jlmorah@unizar.es Received 20 February 2012; revised version accepted 20 February 2012 (Aluandic) Andosols in which clay-sized mineralogy is dominated by Al/Fe humus complexes and hydroxy-Al interlayered 2:1 minerals (IUSS Working Group WRB, 2006). Non-allophanic Andosols are often strongly acidic and produce aluminium toxicity in the roots of sensitive plants. This toxicity is mainly attributed to Al 3+ from hydroxy-Al interlayered 2:1 silicates because of its rapid release kinetics (Dahlgren & Saigusa, 1994; Ito et al., 2009). The risk of Mn toxicity is also greater in acidic Andosols (Qafoku et al., 2004), but it also occurs at neutral pH (Lynch & St. Clair, 2004). Atlantic laurel forest is currently found growing on Andosols in the archipelagos of the Azores, Canaries and Madeira and is thought to be a relict of the tropical flora inhabiting the Mediter- ranean Basin during the Miocene-Pliocene. The canopy of the laurel forest is composed of approximately 20 tree species, which are mostly perennial laurel-leaved species (Fern´ andez, 2009). K ¨ ohl et al. (1996) studied the metal content in the leaves of 15 woody species of the Canary laurel forest and found large levels of Al, Fe and Mn, particularly in the leaves of Viburnum rigidum Vent., Ilex perado Aiton ssp. platyphylla (Webb & Berthel.) Tutin and  2012 The Authors 674 Journal compilation  2012 British Society of Soil Science