The breakdown of Blue Gum (Eucalyptus globulus Labill.) bark in a Portuguese stream

Abstract

Eucalypt forests produce large amounts of bark, which potentially accumulate in streams, constituting an important carbon and nutrient source for benthic food webs. In this study, we compared the breakdown (and associated microbial activity and diversity and invertebrate abundance) of Eucalyptus globulus bark and leaves, enclosed in coarse and fine mesh bags, in a 3 rd order stream of central Portugal. Biofilm development on bark was also analyzed with scanning electronic microscopy and respiration rates quantified. After 90 days of incubation, bark lost 21-51 % of its initial mass while leaves lost 48-57 %. Fungal biomass (as determined from ergosterol concentrations) increased over time and was higher in leaves than in bark (79 vs. 50 mg [g AFDM] -1 at day 90). Sporulation by aquatic hyphomycetes was only observed after 2 weeks (leaves and bark in coarse mesh bags) or 2 months (bark in fine mesh bags). The initial litter mass converted into conidia in leaves was 7-45 fold the values found in bark. Fungal communities were dissimilar in the two substrates with bark presenting the lowest number of species. Lunulospora curvula and Anguillospora crassa dominated the fungal communities in bark, while L. curvula and Tetrachaetum elegans were the dominant species in leaves. Respiration rates, as a measurement of microbial activity, were lower in bark than in leaves (0.10 vs. 0.25 mg O 2 [g AFDM] -1 h -1 in fine mesh and 0.11 vs. 0.39 mg O 2 [g -1 AFDM] -1 h -1 in coarse mesh). Biofilms in bark clearly increased after 15 days of immersion and contributed to 6-85 % of total oxygen consumption. Overall, the results suggest that microbial decomposition pathways dominate the processing of eucalyptus leaves and bark, although leaching and physical fragmentation may stimulate and facilitate the breakdown of bark.