Toward a general theory of plant carbon economics.

Plant life-history variation reflects different outcomes of natural selection given the strictures of resource allocation trade-offs. However, there is limited theory of selection predicting how leaves, stems, roots, and reproductive organs should evolve in concert across environments. Here, we synthesize two optimality theories to offer a general theory of plant carbon economics, named as Gmax theory, that shows how life-history variation is limited to phenotypes that have an approximately similar lifetime net carbon gain per body mass. In consequence, fast–slow economics spectra are the result of trait combinations obtaining similar lifetime net carbon gains from leaves and similar net carbon investment costs in stems, roots, and reproductive organs. Gmax theory also helps explain ecosystem and crop productivity and even helps guide carbon conservation strategies. A key element of functional ecology is understanding why some combinations of organismal traits are observed whereas others are rare or absent. Compelling hypotheses explain why certain traits associate with one another into fast–slow economics spectra. Yet, biologists would also like to have a general theory explaining how these trait combinations connect with individual fitness. Merging existing leaf optimal lifespan theory and metabolic scaling theory yields Gmax theory as an example of formalizing a metabolic definition of fitness that explain how life-history variation and economics trait correlation patterns emerge from natural selection favoring individuals with the highest lifetime net carbon gain per body mass. Gmax theory shows that life-history diversity is the manifestation of a myriad of evolutionary ways to obtain a similar individual lifetime net resource gain per body mass across and within species living in a given environment. [ABSTRACT FROM AUTHOR]