ORIGINAL ARTICLE Edgard A. Bontempo e Silva • Shigeaki F. Hasegawa Kiyomi Ono • Akihiro Sumida • Shigeru Uemura Toshihiko Hara Differential photosynthetic characteristics between seedlings and saplings of Abies sachalinensis and Picea glehnii, in the field Received: 21 October 2011 / Accepted: 3 July 2012 / Published online: 27 July 2012 Ó The Ecological Society of Japan 2012 Abstract Abies sachalinensis and Picea glehnii are co- dominant tree species and major components of the forests of Hokkaido, Japan. Recent work suggests that a reversal in potential competitive superiority at different developmental stages could be important to explain their coexistence. Such shifts in competitive advantage can be mechanistically understood by studying the corre- sponding physiological differences between distinct life stages. Accordingly, our objective was to investigate in the field the photosynthesis of shade-growing juveniles of these species from two different size-classes, seedlings and saplings. Our results show that seedlings of both species had higher concentrations of photoprotective xanthophylls than saplings, especially in spring, and suggest that seedlings have a lower threshold of stress tolerance than saplings. Photosynthetic capacity per needle area and lateral shoot extension rate decreased from the seedling to the sapling stage in A. sachalinensis, while in P. glehnii, both increased from the seedling to the sapling stage. Abies sachalinensis had higher photo- synthetic rates at the seedling stage but lower rates at the sapling stage than P. glehnii. Nevertheless, A. sachalin- ensis had a higher lateral shoot extension rate than P. glehnii at both stages. Our physiological results support previous ecological observations that A. sachalinensis is a superior competitor to P. glehnii in the understory, and show that its competitive advantage is higher at the seedling stage than at the sapling stage. Keywords Life-history strategies Æ Photoadaptation Æ Photosynthetic pigments Æ Photosynthetic rate Æ Physiological ecology Introduction Spruces and firs dominate many of the sub-boreal, boreal, and alpine forests of the world, which hold more than 25 % of the carbon stored in terrestrial ecosystems and are likely to be affected by global climate change (Easterling et al. 2007). The sub-boreal forests of Hok- kaido, in northern Japan, are ecologically similar to strict-sense boreal forests even though they are well be- low the boreal latitudes (Kojima 1991; McCarthy 2001). Abies sachalinensis (Fr. Schm.) Masters and Picea glehnii Masters are dominant canopy trees in many of Hokkaido’s forests (Kojima 1991; Uemura 1994) and their coexistence and co-dominance in these forests have received considerable attention (Kubota and Hara 1995, 1996; Takahashi 1997; Takahashi and Kohyama 1999). Although both species are acknowledged to be shade- tolerant and to share regeneration sites, A. sachalinensis seems to regenerate more vigorously than P. glehnii on logs or forest ground (Takahashi 1994). Abies sachalin- ensis generally grows faster and has a shorter longevity than P. glehnii (Uemura 1994; Umeki 2001). It has re- cently been proposed that a reversal in competitive superiority at different life stages plays an important role in the coexistence of these species (Nishimura et al. 2010). In their work, Nishimura et al. (2010) observed that A. sachalinensis is a superior competitor to P. glehnii at the sapling stage but that the superiority is reversed in adult trees at the upper-canopy level. To understand such shifts in competitive superiority and growth patterns of trees, it is necessary to study the corresponding physiological changes that take place at Electronic supplementary material The online version of this article (doi:10.1007/s11284-012-0973-1) contains supplementary material, which is available to authorized users. E. A. Bontempo e Silva (&) Æ S. F. Hasegawa Æ K. Ono Æ A. Sumida Æ T. Hara Institute of Low Temperature Science, Hokkaido University, Kita-19, Nishi-8, Kita-ku, Sapporo 060-0819, Japan E-mail: silva@lowtem.hokudai.ac.jp Tel.: +81-011-7067660 Fax: +81-011-7067660 S. Uemura Forest Research Station, Field Science Center for Northern Biosphere, Hokkaido University, Nayoro, Japan Ecol Res (2012) 27: 933–943 DOI 10.1007/s11284-012-0973-1