Effects of elevated [CO2] and soil temperature on photosynthetic responses of mountain maple (Acer spicatum L.) seedlings to light

• We exposed mountain maple seedlings to climate change factors at high and low light environments.
• High soil temperature increased net A at the low-light but not at high-light treatment.
• Elevated [CO2] stimulated net A by 100% at the warmer Tsoil and by 48% at the low Tsoil.
• IWUE increased substantially at the high-light in response to high-light in elevated [CO2].
• Soil warming may enhance shade-tolerance in mountain maple but negate growth in high-light conditions.

Predicting the future distribution of plants requires a mechanistic understanding of how plants cope with their new environmental conditions under a changed climate. We examined the effects of elevated carbon concentration ([CO2]) and elevated Tsoil on the physiological responses to light of a shade-tolerant woody species, mountain maple (Acer spicatum L.). Seedlings were exposed to ambient and elevated [CO2] (392 μmol mol−1 vs. 784 μmol mol−1), current and elevated soil temperatures (Tsoil 17 vs. 22 °C) at high and low (100% vs. 30%) light conditions for one growing season. It was found that elevated Tsoil stimulated net photosynthesis (A) by 18% in the low-light treatment but tended to reduce A in the high-light treatment. Elevated [CO2] increased A by 100% under the elevated Tsoil and by 48% at the current Tsoil. The effect of elevated Tsoil also varied with [CO2]: it increased A by 13% under the elevated [CO2] but reduced A by 17% under the ambient [CO2]. We observed a significant increase in the instantaneous water-use efficiency of photosynthesis (IWUE) in response to the high-light treatment under the elevated [CO2] but not under the ambient [CO2]. Our data indicate that the elevated [CO2] markedly increased nitrogen (N) allocation to the photochemical apparatus of A in seedlings grown under the high-light condition. These results suggest that the predicted increases in atmospheric [CO2] will likely enhance the growth of mountain maple in canopy gaps in the forest, while the predicted elevated Tsoil will negate the benefit of high-light conditions.