Crown Carbon Gain and Elevated [CO2] Responses of Understorey Saplings with Differing Allometry and Architecture

1. Attempts at determining the physiological basis of species' differences, such as the ability to grow in deep shade, have been of limited success. However, this basis is fundamental to predicting species' responses to rising atmospheric CO2 in the forest understorey. We linked a leaf photosynthesis and a tree architecture model to predict the effects of dynamic and steady state photosynthetic characteristics, crown architecture and elevated atmospheric CO2 concentration ([CO2]) on crown-level carbon gain (Acrown). Twenty-four-h Acrown was modelled for shade-tolerant Acer rubrum and shade-intolerant Liriodendron tulipifera saplings growing for three years in a forest understorey under ambient and elevated [CO2] in free-air CO2 enrichment. 2. Two factors best explained Acrown in ambient [CO2]: tree light environment and sapling allometry. Microsite light environment influenced carbon gain via daily photosynthetic photon flux (PFD), average diffuse PFD and sunfleck characteristics. Species differences in specific leaf area (SLA) and size-related biomass allocation to leaves affected the effective leaf area and hence Acrown. 3. At a common above-ground biomass, small saplings (100 g above-ground dry mass) of L. tulipifera had higher Acrown than A. rubrum samples due to larger SLA and greater biomass allocation to leaves. Larger saplings of the two species had similar Acrown due to greater carbon allocation to leaves with increasing plant size in A. rubrum vs L. tulipifera. For saplings > 800 g, Acrown was greater in A. rubrum than in L. tulipifera. Enhancement of Acrown by elevated [CO2] on sunny days was similar for both species. 4. Overall, though the shade-tolerant species had lower Acrown than the shade-intolerant species at a common small size, our results indicate that the relative performance of these species can reverse at larger sizes due to allocational differences. These results suggest that elevated [CO2] may accelerate competition for light between A. rubrum and L. tulipifera as these species grow larger in the understorey.