Early successional processes of basaltic lava ecosystems on Mt. Etna (Sicily) with additional comparative studies of Mauna Loa (Hawaii)
Primary succession on the basaltic lava flows of Mt. Etna was studied usmg chronosequence theory to investigate the first 500 years of ecosystem development. Separate experiments were conducted to look at how plant species, nutrient availability and lichen activity on the lava changed over time under different conditions based on the site location (age, aspect and altitude on the volcano). By comparing the results of these different areas of study, close links were observed between soil development and nutrient availability. Lichens were found to be an important stage in primary succession introducing biomass to form a developing soil as well as weathering the lava surface. The plant species present on the lava were found to change as plants first colonised the lava and were then replaced as further species appeared over time. Nutrient availability was investigated in living plant material by measurement of the enzyme nitrate reductase and also in the developing soil. Two large inputs of nitrogen were observed in the chronosequences. An early input believed to be lichen derived and another steadily increasing input associated with the soil. The biomass of the nitrogen fixing lichen Stereocaulofl vesuvianum on the lava flows was found to change over time with a rapid increase over the first 100 years of the chronosequence followed by a slower decline as competition and shading from vascular plants covered available habitat. S. vesuvianum was also found to be an efficient weathering agent on the lava altering the surface morphology. This weathering was observed qualitatively by detailed visual examination of the lava surface by scanning electron microscopy. Weathering was also measured quantitatively using an intelligent machine vision computer system, to collate the surface changes of many images simultaneously and compare surface change to a baseline chronosequence, allowing discrimination of fine differences in the extent of weathering. Two of the experiments conducted on Mt. Etna (nitrate reductase activity and lichen weathering) were repeated on a second volcano, Mauna Loa (Hawaii). This tested if the trends observed on Etna were typical of primary succession on lava and the impact of a different climate regime (tropical) compared to Etna (temperate). Nitrate reductase activity was found to be very low in the primary colonising species studied on Hawaii indicating that nitrogen is limited on the early lava flows. Lichen weathering by Stereocaulon vulcani on Hawaii was found to occur in a comparable manner to S. vesuvianum on Etna, and was similarly controlled by the lichen biomass and associated climatic conditions.