The geochemical fingerprints, the anthropogenic impacts and the plant essential nutrient contents of the laurel forest in Tenerife (Canary Islands, Spain).

Abstract

The laurel forest ecosystem is a good example for an endangered and critical environment due to its interface position in which various geochemical processes collide and interact with regional and global impacts. For example, different volcanic rocks, geogenic aerosols (e.g. Saharan dust, sea spray), anthropogenically released emissions (e.g. air and car traffic, industrial and urban developments etc.) and socioeconomic changes (e.g. expansion of cropland and fallow agricultural areas) affect and interact with the pedological laurel forest interface and with the surrounding ecosystem. The determination, interpretation and discussion of several geochemical processes and geogenic or anthropogenic impacts build the main content and scope of this research work. To achieve this goal, different kinds of samples are collected at 26 sampling sites on the northern slope of Tenerife, including volcanic rocks, soils, roots and leaves. The vegetation samples belong to the laurel forest tree species Laurus novocanariensis. The sampling sites spread over the main laurel forest areas (Anaga Massif, Orotava area, Teno Massif); remote forest areas, touristically influenced areas as well as former farmland near the border of laurel forest vegetation zone were given special attention. The geochemical composition determined for each sample consists of 22 elements (Si, Al, Fe, Ca, Na, Mg, K, Ti, P, Mn, Sr, Stot, Ctot, Zr, Ni, Zn, Cu, Pb, Mo, Sb, Cd, Hg). The first goal of research gravitates around the question whether it is possible to distinguish the Anaga, Orotava and Teno laurel forest topsoils from each other as to specific element ratios, which can be considered as typical geochemical fingerprints. The results reveal, that the Orotava topsoils are distinguishable from the Anaga and Teno topsoils due to plotted Sr/Ca, Na/Ca and Na/K ratios and due to calculated element-to-element ratios (e.g. Sr-to-Ca, Na-to-K). The Orotava rocks affected even the uppermost soil horizons and it is clearly recognizable that the rocks and soils affect also the associated leaf samples. Orotava leaves, for example, contain the highest Sr levels of all samples (up to 148.4 ± 8.7 µg/g), but the Sr leaf contents depend not only on high Sr soil levels (up to 788.0 ± 46.0 µg/g), but on specific Sr/Ca soil ratios. Several other element combinations can almost be used as typical geochemical fingerprints, such as Mg/Ni, Fe/Ti or Mg/Cu ratios. However, various topsoil samples have a similar plotted or calculated element ratio, which makes it impossible to determine unique Anaga or Teno fingerprints. However, Anaga, Orotava and Teno topsoils are distinguishable from each other due to specific Na-to-P ratios. Anaga Na-to-P ratios, for example, range from 1:1 to 1:4.1, Orotava ratios from 3.8:1 to 5.9:1 and Teno ratios from 1.2:1 to 2.7:1. The second research goal consists in answering the question whether the laurel forest ecosystem is affected by anthropogenic Hg, Pb, Cd and Sb impacts, focussing on the topsoils and the L. novocanariensis tree species. In this context it is necessary to differentiate between anthropogenically and lithogenically affected element contents. The results reveal that the slightly enriched Pb and Hg topsoil levels are caused by natural geochemical processes. Linear trends indicate that Hg and Pb are associated with other elements, such as Ca, Fe, Ti, Mn, Al and Zn. The Cd and Sb levels are present in normal amounts, but it is not possible to determine whether natural geochemical processes or anthropogenic impacts caused the observed trends and area-specific differences. The most popular touristic area of the Anaga Mountains (Pico del Ingles) nevertheless contains always the highest Sb, Cd, and Pb topsoil (e.g. up to 58.3 ± 4.2 µg/g of Pb) and leaf values (e.g. up to 0.70 ± 0.03 µg/g of Sb). The third research goal focuses on the essential nutrient contents of the laurel forest and the fallow agricultural areas. The soil and vegetation results of both areas are compared with each other and with reported nutrient levels in order to find out whether the laurel forest and the fallow agricultural areas contain sufficient nutrient levels. The results reveal that the sampling sites contain sufficient amounts of K, P, Ca, Mg, Stot, Fe, Mn, Zn, Cu, Mo and Ni within their leaves and soils. Only the K soil levels are sometimes slightly below the reported nutrient ranges. The fallow agricultural nutrient levels are mostly within the range of the laurel forest levels and they are more or less homogenously distributed within the fallow agricultural topsoils. This is one of the main differences compared to the laurel forest areas. Soil fertilizer residues are not determined in the examined fallow agricultural topsoils.