The role of metals in soils and accumulation in the laurel forest of La Gomera (Canary Islands, Spain): preliminary results

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 (NH4Ac-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 NH4Ac-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.