Crown Architecture of Populus spp. Is Differentially Modified by Free-Air CO2 Enrichment (POPFACE)

• Although canopy architecture is a prime determinant of forest functioning and productivity, it has received little attention when examining forest responses to rising atmospheric CO2 concentrations. In this study different characteristics of crown architecture of two Populus species and one hybrid were investigated within a highdensity poplar plantation. A free-air CO2 enrichment (FACE) facility was used to mimic future elevated CO2 concentrations. • Canopy depth and branching patterns were studied, and detailed branch characteristics such as branch dimension, inclination, and internodal length were assessed for the three poplar species in the FACE and control treatments. • Effects of elevated CO2 were restricted to a significantly increased canopy depth and longer internodal lengths after two years of CO2 enrichment. Additionally, branch dimensions and sylleptic branch numbers were increased by FACE, but responses were variable among growing seasons and species. However, FACE did not affect branch angles of origin and termination. • Crown architecture was modified mainly through a growth stimulation in response to FACE. Nevertheless, important differences among species were observed which may influence future CO2 responses when competition will become more important.