HEAT OF COMBUSTION, DEGREE OF REDUCTION AND CARBON CONTENT .3. INTERRELATED METHODS OF ESTIMATING THE CONSTRUCTION COST OF PLANT-TISSUES

During the last decade, degree of reduction (calculated from elemental composition), heat of combustion and carbon content have been proposed as 3 ways of estimating the construction cost of plant tissues. There is a fairly good agreement among these 3 methods, as they are all based on the link between the energy content of biomass and its reduction level. This relationship is not absolute and the regression coefficient between heat of combustion and degree of reduction may vary according to the chemical composition of biomass. The relation between degree of reduction and carbon content is implicitly based on the consequences in terms of molecular weight of the replacement of 1 oxygen atom by 2 hydrogen atoms in the process of reduction of photoassimilates (carbohydrates) into the various biomass compounds. Different trends can be observed on woody and non-woody tissues, which get richer in energy by increasing their content in lignin and lipids, respectively. Finally, the biosynthetic efficiency is also affected by the chemical composition of biomass. Therefore, while these 3 methods provide efficient and fairly simple tools for the estimation of the construction cost of biomass, they should not be used without considering what the dominant compounds of the plant tissues under study are and without choosing proper parameters in consequence.