Sweetgum tree canopy during leaf expansion in early spring 2009 at Oak Ridge National Laboratory. Towers hold pipes that deliver Free Air CO2 Enrichment (FACE) to the canopy. Photo: Jeff Warren
Sweetgum tree canopy during leaf expansion in early spring 2009 at Oak Ridge National Laboratory. Towers hold pipes that deliver Free Air CO2 Enrichment (FACE) to the canopy. Photo: Jeff Warren

Atmospheric carbon dioxide levels have increased by ~ 25% over the last 50 years. While more carbon dioxide can initially stimulate plant photosynthesis, in a new study published in AoB PLANTS, Warren et al. found that long-term (12 years) exposure of sweetgum trees to elevated carbon dioxide resulted in no stimulation of photosynthesis. The loss of initial increases in photosynthesis was due to low leaf nitrogen levels, which suggests other limiting resources may moderate future impacts of elevated carbon dioxide on photosynthesis.