Direct and legacy effects of long-term elevated CO₂ on fine root growth and plant-insect interactions.

Increasing atmospheric CO₂ concentrations alter leaf physiology, with effects that cascade to communities and ecosystems. Yet, responses over cycles of disturbance and recovery are not well known, because most experiments span limited ecological time. We examined the effects of CO₂ on root growth, herbivory and arthropod biodiversity in a woodland from 1996 to 2006, and the legacy of CO₂ enrichment on these processes during the year after the CO₂ treatment ceased. We used minirhizotrons to study root growth, leaf censuses to study herbivory and pitfall traps to determine the effects of elevated CO₂ on arthropod biodiversity. Elevated CO₂ increased fine root biomass, but decreased foliar nitrogen and herbivory on all plant species. Insect biodiversity was unchanged in elevated CO₂. Legacy effects of elevated CO₂ disappeared quickly as fine root growth, foliar nitrogen and herbivory levels recovered in the next growing season following the cessation of elevated CO₂. Although the effects of elevated CO₂ cascade through plants to herbivores, they do not reach other trophic levels, and biodiversity remains unchanged. The legacy of 10 yr of elevated CO₂ on plant-herbivore interactions in this system appear to be minimal, indicating that the effects of elevated CO₂ may not accumulate over cycles of disturbance and recovery.