Crown humus: Part I - The chemistry of the canopy organic matter of rain forests in Costa Rica

In some tropical rain forests organic matter, derived from epiphytic plants , accumulates as heavy mats on tall tree tops (Crown), trunks and branches. Upon microbial and abiotic (chemical, geological) interactions, the canopy organic matter (COM) is transformed into Crown humus. Detailed chemical, c hromatographic and spectroscopic analyses of seven COM samples (COM-I to -7 ), collected from five different tree tops, and one soil sample collected f rom the corresponding forest floor (FF-1), from Monte Verde Reserve Forest, in Costa Rica, have been carried out in respect of their low M-w organic c ompounds and the contained humus. The nature and chemical characteristics o f these arboreal humic substances are compared with those of their terrestr ial and aquatic counterparts. Some striking similarities, particularly in t he core structures of fulvic acids (FAs), occurring in three different natu ral habitats (arboreal, terrestrial and aquatic), are observed. Like those of terrestrial and aquatic -FAs, the molecular architecture of the Crown-FA s constitute metallo-organic complexes of mono-, di- and oligomeric oxygena ted dibenzo-alpha-pyrones (1-4, 9a, b, 11, Chart 1). However, the ecologica l and geochemical conditions of the arboreal mini-ecosystem being different , are reflected in the simpler structures of a large variety of its seconda ry metabolites (Tables I & II) entrapped in the inner core and outer-surfac e of the Crown humus. The role of these compounds in the microbial (chemohe terotroph) metabolic sequence enroute to Crown humus is appraised.