Tannins are believed to be particularly abundant in tropical tree foliage. Their ecological role was mainly associated to plant herbivore defence, and very little is known on the quantity, variation, and potential role of tannins in tropical leaf litter and its decomposition. A main obstacle for studying variation in tannin concentration across species, are multiple analytical difficulties related to the highly species-specific tannin composition. With a focus on condensed tannins (CT), we addressed the questions (1) How variable is litter CT concentration among a range of co-occurring tropical tree species? (2) Can relatively easy and cheap analytical methods yield reliable results on CT concentrations across species? (3) How important are interspecific differences in litter CT concentrations for the decomposition of this litter? We used a HPLC based method, coupled with CT degradation by phloroglucinolysis, to quantify litter CTs, and to determine their chemical structure in leaf litter from 16 tropical tree species. These data were compared to data from some classical methods in the ecological literature, such as the broad estimate of total phenolics (Folin assay), and the CT specific acid butanol assay. Litter of the 16 species was exposed in a tropical forest either in fine-mesh width litterbags excluding fauna access, or in coarse-mesh width litterbags allowing access to fauna.
Results/Conclusions
We found large inter-specific differences in litter HPLC determined polymerization degree and concentration of CT, which didn’t correlate with Folin total phenolics, but correlated reasonably well with acid butanol CT. Litter decomposition correlated well with initial HPLC based CT concentration, but not with initial nitrogen or phosphorus concentration. Particularly strong correlations between decomposition and HPLC based CT concentrations were observed in the presence of fauna. Litter mass loss decreased with increasing extractable CT concentrations. Our results demonstrated large inter-specific variation in leaf litter CT concentration and structure among a range of tropical forest tree species. This variation in quantity and structure of CTs apparently explains best the variation observed in litter mass loss rates, in particular in presence of soil fauna. Our data suggest that the understanding of the ecological role of inter-specific variation in CTs requires accurate analytical methods, and that the commonly used assays provide only limited information. We conclude that leaf litter tannins play a key role in decomposition and nutrient cycling in the studied tropical rainforest.
