Calorimetric Estimates of Construction Cost and Their use in Ecological Studies

Abstract

Photosynthetic carbon fixation is the most abundant form of autotrophic energy capture on earth, and thus plant biomass represents the single largest pool of energy available for trophic level transfers. Energy partitioning, allocation and content influence both plant form and function, setting the stage for trophic level transfers. In many ways energy is the most basic unit that can be used to compare organisms, spanning studies of biomolecules, cells, organs, tissues, individuals, populations, communities, ecosystems and global dynamics. It is not surprising that early ecologists devoted much attention to the direct measure of the energy contents of plants. What is more surprising is that so few recent studies have expanded on the early work to incorporate more mechanistic measures of energy requirements. The concept of construction cost, or the total substrate requirement for the biosynthesis of an endproduct, has been attributed to Pasteur (1875, as cited in Penning de Vries, Brunsting & van Laar 1974). In 1987 Williams et al. defined construction cost as a measure of the amount of glucose required to provide carbon skeletons, reductant and adenosine 5'-triphosphate (ATP) for the synthesis of organic compounds, and formulated a set of techniques and equations to estimate the construction cost of plants. These researchers found that the energy required to form plant tissues can easily be estimated from simple measurements of the heat of combustion, ash content and organic nitrogen content. Prior to publication, the method was used in an extensive study of common Spanish plants (Merino 1987), but since publication, the technique has been cited only 25 times in the primary literature and twice in textbooks (Science Citation Index to July 1993). Of these citations, only half actually used the technique. The ecological significance of construction cost has only begun to be studied. Of the 25 papers that have cited Williams et al. (1987), nearly one-third were primarily concerned with modelling (Nobel 1991; Nobel, Lopez & Alm 1991; Raich et al. 1991; Ryan 1991; Webb 1991a,b;