Organic-matter dynamics in the riparian zone of a tropical headwater stream in Southern Brasil

• CPOM is influenced by precipitation.
• Litter deliver to this headwater stream was highly seasonal and was higher at the end of the dry season.
• Much of the CPOM were leaves (74%).
• Almost all species present contributed to the leaf litter (188 out of 192) where 19 of 37 species are more important (mainly Protium spp., Cabralea canjerana, and Casearia decandra) around 40% of the CPOM.
• Cerrado, Atlantic Forest and Amazonian species were identified in the riparian zone.

We assessed the dynamics of allochthonous coarse particulate organic matter (CPOM) in a tropical headwater stream in the foothills of the Ouro Branco Sierra (S. Brazil; at 1300 m a.s.l.) over a yearly cycle, and the contribution of leaves, twigs, and other plant parts to the different types of input and stock. Additionally, we identified the key species that contribute to each type of input and to seasonal patterns in the input and storage of CPOM (particles > 1.00 mm). Our study tested (and rejected) the hypothesis that the majority of riparian plant species along tropical headwater streams are perennial, and therefore the input of CPOM tends to be continuous over the course of the year. Leaves contributed 74% of CPOM, with input increasing toward the end of the dry season (September to October). Of the total of 188 plant species identified in the riparian zone, 47 species together contributed over 28% of the detritus to the aquatic ecosystem. The species that contributed most were Protium spp., Cabralea canjerana (Vell.) Mart., and Casearia decandra Jacq. The CPOM dynamics of Garcia Stream showed a highly seasonal pattern due to the influence of precipitation. A number of species characteristic of the Amazon and Atlantic Rain forests were identified, together with species of the Cerrado, indicating that the riparian vegetation should not be considered part of a single forest domain, but rather a unique and integrated system that functions as an ecological corridor for species dispersal. The data obtained here confirmed the ecological complexity of the riparian tropical system, derived from the diversity of plant species and the seasonal dynamics of CPOM, related to the phenology of the plant community.