Plant energetics and the synthesis of population and ecosystem ecology

Abstract

Historical conceptual divisions in ecology are dissolving as ecologists seek general currencies for understanding plant function, population dynamics, species interactions and ecosystem processes. Foremost among such synthetic approaches are population-ecosystem models (PEMs), which treat the biomass dynamics of individuals as a foundation for coupled demographic and biogeochemical processes. Because plant productivity and tissue composition drive terrestrial energy and nutrient cycling, and plant growth itself is often used as a fitness proxy, PEMs predict that phenomena related to population dynamics, such as species coexistence, are associated with processes at the ecosystem level. I argue that this line of thought reflects a persistent misunderstanding of the relationship of plant growth—the creation of new structural biomass—and individual fitness. A survey of measured energetic costs of a wide variety of plant functions suggests that the majority of assimilated energy for many plants is allocated to survival rather than growth processes. This diversion of assimilate to survival—such as fighting infection, reducing potential tissue damage, or storing reserves in the face of defoliation risk—contributes to individual fitness but may not impact the flow of energy and nutrients in the environment or to other trophic levels. Synthesis. Using simple models of plant energy budgets, I demonstrate how incorporating survival allocation into PEMs using a bet-hedging approach can decouple population and ecosystem dynamics. In revisiting what little is known about how individual plants use energy, I suggest that recasting plant strategy theory in an energetic-demographic framework may offer a useful way forward in the drive to unify population and ecosystem processes.