Background/Question/Methods   Understanding how plant species are assembled into communities and coexist continues to present a substantial challenge in the community ecology. Increasingly, trait-based approaches have offered the promise of being able to reveal general patterns and processes in community ecology and to be better able to mechanistically scale from organisms to ecosystems. The ‘holy grail' is to develop a general theory able to link key details of how variation in demography is influenced by the presence of competitors, and then to tie this competition-sensitivity of demographic performance all the way to the functional traits that determine it. Here we argue that, two recent extensions of metabolic scaling theory, MST, comprise many of the key pieces of such a general and predictive theory. Recent extensions of MST builds upon past insights from relative growth rate theory, plant thinning, and functional trait spectra. Together, these foundations provide a basis to link traits with performance and many aspects of community structure. We first overview a generalized trait-based scaling model of plant growth. Extending MST uniquely points to the key traits that interact to regulate so as to predict variation in relative growth rate and unit leaf biomass production. We show how variability of light environments can be incorporated in this framework. Second, we overview how ecological extensions of metabolic scaling theory can also provide a baseline to predict size, spatial structuring, and mortality rate of plant communities under resource steady state. We provide new analyses to assess predictions of extending MST to link traits with variation in plant growth and community structure.

Results/Conclusions   Together, these findings present an interesting paradox. On the one hand, extensions of MST shows that shared general principles of scaling plant form and function result in general patterns of the scaling of plant growth, demography, and community structure. On the other hand, trait based assembly and environmental differences leads to differences in species composition. We show that MST also highlights that the principle of zero-sum dynamics appears to have the potential to unite these seemingly opposing viewpoints of community ecology. Ultimately, zero-sum dynamics, that results from a set of shared metabolic and ecological principles, constrain all species interactions and responses to the environment. Linking of zero-sum community dynamics with traditional niche and trait-based approaches appears to be both the central problem and the most general principle in a general predictive theory of species coexistance.