title: Light acquisition and use by individuals competing in a dense stand of an annual herb, Xanthium canadense.
abstract: The importance of light acquisition and utilization by individuals in intraspecific competition was evaluated by determining growth and photosynthesis of individual plants in a dense monospecific stand of an annual, Xanthium canadense. Photosynthesis of individual plants in the stand was calculated using a canopy photosynthesis model in which leaf photosynthesis was assumed to be function of leaf nitrogen content and light availability. The estimated photosynthetic rates of individuals were strongly correlated with the measured growth rates. Photosynthetic rates per unit aboveground mass (RPR, relative photosynthetic rate) increased with increasing aboveground mass, suggesting asymmetric (one-sided) competition in the stand. However, larger individuals had similar RPRs, suggesting symmetric (two-sided) competition. These results were consistent with the observation that size inequality over the whole stand increased with growth, but it remained stable among the larger individuals. The RPR of an individual was calculated as the product of absorbed photon flux per unit aboveground mass (Φmass) and light use efficiency (LUE, photosynthesis per unit absorbed photon flux). Φmassindicates the efficiency of light acquisition, and was higher in larger individuals in the stand, while LUE was highest in individuals with intermediate aboveground mass. LUE depends on leaf nitrogen content. At an early stage, leaf nitrogen contents of smaller individuals were similar to those that maximize LUE. Light availability to smaller individuals decreased as they grew, while their nitrogen contents did not change markedly, which decreased their LUE. We concluded that asymmetric competition among individuals in the stand resulted mainly from lower efficiencies in both light acquisition and light use by smaller individuals.
authors.affiliation: Biological Institute, Graduate School of Science, Tohoku University, Aoba, Sendai 980-8578, Japan e-mail: hikosaka@mail.cc.tohoku.ac.jp, Fax: +81-22-2176699, , , , , , JP.
authors.forename: Kouki
authors.initials: K
authors.lastname: Hikosaka
authors.affiliation: Biological Institute, Graduate School of Science, Tohoku University, Aoba, Sendai 980-8578, Japan e-mail: hikosaka@mail.cc.tohoku.ac.jp, Fax: +81-22-2176699, , , , , , JP.
authors.forename: Sinya
authors.initials: S
authors.lastname: Sudoh
authors.affiliation: Biological Institute, Graduate School of Science, Tohoku University, Aoba, Sendai 980-8578, Japan e-mail: hikosaka@mail.cc.tohoku.ac.jp, Fax: +81-22-2176699, , , , , , JP.
authors.forename: Tadaki
authors.initials: T
authors.lastname: Hirose
keywords: Key words Intraspecific competition; Light acquisition; Light use efficiency; Size inequality; Whole-plant photosynthesis