Figure 57.6 FACE Experiment: Effects of CO₂ and Nutrients on NPP (A) The free-air CO₂ enrichment (FACE) experiment involved releasing CO₂ from towers constructed around pine trees. (B) The trees grew better under conditions of elevated CO₂ (550 parts per million) and nutrients compared with ambient conditions. Without added nutrients, CO₂ had a minimal effect on tree growth.