Are fire resprouters more carbon limited than non-resprouters? Effects of elevated CO₂ on biomass, storage and allocation of woody species

Resprouting ability is a key functional trait determining plant responses and vegetation dynamics after disturbances such as fire that shape most global biomes. It is likely that rising atmospheric CO₂ concentrations will alter resource allocation patterns in plants which in turn will alter resprouting responses. In this study, we asked: (1) do resprouters have greater allocation to storage than non-resprouters?; (2) if so, do resprouters account for this negative carbon balance by having reduced growth?; and (3) do resprouters have a relatively weaker growth response compared to non-resprouters under elevated CO₂ levels due to their increased allocation to storage? To address these questions, we grew congeneric species-pairs of shrubs common to south-eastern Australia with contrasting resprouting abilities under ambient and elevated CO₂ levels. We found that resprouters in general allocated more resources to storage (root non-structural carbohydrates and biomass) and had less total biomass than non-resprouters. Under elevated CO₂ levels both sprouting types increased biomass production, suggesting they were carbon limited. Surprisingly, the resprouters allocated this additional carbon to biomass rather than to storage. This suggests that although elevated CO₂ levels may not affect resprouting ability directly in resprouters, it may enhance other aspects of persistence such as escapability and bud protection. Furthermore, non-resprouters may also benefit from the additional carbon by being able to set seed more quickly and increase seed production thus enhancing their recruitment after fire. Thus, the relative benefits of elevated CO₂ levels on resprouters versus non-resprouters remain equivocal.