Ontogenic development of chemical defense by seedling resin birch: Energy cost of defense production.

Whether production of chemical defenses by plants is or is not an energetically costly process is an important, but unresolved, question in chemical ecology. We suggest studies of the ontogenetic development of plant defense systems can help resolve the question. As an example of this approach to the cost question, we explore the problems associated with production of immobile chemical defenses that defend juvenile resin birches against browsing by mammals. From this exploration we draw two conclusions: (1) Shortly after germination, production of chemical defenses by small-seeded species, such as birch, is energetically costly. (2) Opposing selection for defense versus competitive ability in the seedling stage of birch has resulted in a trade-off in allocation of carbon to production of immobile chemical defense versus allocation of carbon to production of storage reserves. We suggest this trade-off results in a large indirect cost of defense because carbon used for production of immobile chemical defenses is unavailable for support of growth in the future, but stored carbon can be used to support future growth.