Light-use efficiency of native and hybrid poplar genotypes at high levels of intracanopy competition

In southern Wisconsin, U.S.A., tree growth and associated canopy traits wer e compared among five native and hybrid genotypes of poplar (Populus spp.) in replicated, monoclonal stands planted at a 1 x 1 m spacing. The overall objective of this study was to assess clonal suitability to cultural condit ions entailing high levels of intracanopy competition (such as high-density plantations or long rotations) and to identify selection criteria suitable to such conditions. Two of the clones were Populus deltoides Bartr., two w ere P. deltoides x Populus nigra L. (DN) crosses, and the fifth was a P. ni gra x Populus maximowiczii A. Henry (NM) cross. In the third year after est ablishment, variation in aboveground biomass gain (ANBG) was analyzed in re lation to canopy light interception (IPAR) and canopy light-use efficiency (LUE) during a 31-day period when growing conditions were most favorable (l ate June through late July). ANBG in this interval varied by twofold among genotypes (2.76-5.78 Mg.ha(1)), and it was highest in the two P. deltoides clones, followed by the NM and DN hybrids, respectively. Across genotypes, ANBG was unrelated to IPAR, which varied by only 5%. Instead, it was strong ly and positively related (r(2) = 0.99) to the twofold variation in LUE (1. 06-2.22 g.MJ(1)). Among measured canopy traits, the best predictor of LUE ( r(2) = 0.88) was an additive combination of factors associated to the optim ization of canopy photosynthesis: LUE was negatively related to both the ca nopy light-extinction coefficient and compensation irradiance at the canopy base. We infer from these findings that poplar genotypes can vary consider ably in LUE and, correspondingly, in the extent to which photosynthesis is optimized in dense canopies. Furthermore, the low LUE among hybrid genotype s at this level of intracanopy competition may reflect a bias in "tree impr ovement" efforts towards maximizing biomass production under conditions of relatively low competition.