Plant communities are biochimically organized

The biochemical analysis of 100 Phanerogames (five vegetal community from fresh water to Mediterranean matorral) permits to see 1/a general gradient from mineral pool (metals and metalloids) to organic pool (micromolecules; phenols, tanins; macromolecules : lignin, cellulose), mainly based on C content of the photosynthetic apparatus. Synthetic expression is given by the DIN ratio. 2/ a contrast between phytocenoses with high (sub-alpine and mattoral) or low (meso-hygrophilic prairie, fresh-water communities) C content. The determinism is both endogenous (genetic and systematic capacities) and exogenous (mesological constraints). Lignosity, evergreeness and sclerophyllia connect leaf biochemical composition and mesological constraints (climate, grazers). The C/N ratio also supports the previous concept of << phytochemical tactics for deterring herbivores,, based on the allocation of resources between plants rich in tanins (therefore in C) against those rich in alkaloids (therefore in N). This phytochemical organization illustrate evolutive competition between woody and herbaceous habitus, and is part of the more general concept of adaptative strategies. Far from being due to chance, vegetal communities are subject to an organizing logic expressed on a biochemical level, to be compared with their conventional botanical structure.