Photosynthetic responses to understory shade and elevated carbon dioxide concentration in four northern hardwood tree species

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Tree Physiology, 26:1589–1599

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Photosynthetic responses to understory shade and elevated carbon dioxide concentration in four northern hardwood tree species

Lesley T. Sefcik (1,2), Donald R. Zak (1) and David S. Ellsworth (1)

1. School of Natural Resources and Environment, 440 Church Street, University of Michigan, Ann Arbor, MI 48109-1041, USA / 2. Corresponding author (ltsefcik@umich.edu) / Received December 9, 2005; accepted March 25, 2006; published online September 1, 2006

Summary

Seedling responses to elevated atmospheric CO₂ concentration ([CO₂]) and solar irradiance were measured over two growing seasons in shade-tolerant Acer saccharum Marsh. and Fagus grandifolia J.F. Ehrh. and shade-intolerant Prunus serotina, a J.F. Ehrh. and Betula papyrifera Marsh. Seedlings were exposed to a factorial combination of [CO₂] (ambient and elevated (658 µmol mol^–1)) and understory shade (deep and moderate) in open-top chambers placed in a forest understory. The elevated [CO₂] treatment increased mean light-saturated net photosynthetic rate by 63% in the shade-tolerant species and 67% in the shade-intolerant species. However, when measured at the elevated [CO₂], long-term enhancement of photosynthesis was 10% lower than the instantaneous enhancement seen in ambient-[CO₂]-grown plants (P < 0.021). Overall, growth light environment affected long-term photosynthetic enhancement by elevated [CO₂]: as the growth irradiance increased, proportional enhancement due to elevated [CO₂] decreased from 97% for plants grown in deep shade to 47% for plants grown in moderate shade. Results suggest that in N-limited northern temperate forests, trees grown in deep shade may display greater photosynthetic gains from a CO₂-enriched atmosphere than trees growing in more moderate shade, because of greater downregulation in the latter environment. If photosynthetic gains by deep-shade-grown plants in response to elevated [CO₂] translate into improved growth and survival of shade-intolerant species, it could alter the future composition and dynamics of successional forest communities.

Keywords: Acer saccharum, Betula papyrifera, deep shade, downregulation, Fagus grandifolia, nitrogen limitation, photosynthetic enhancement, Prunus serotina .