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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] (392μmolmol−1 vs. 784μmolmol−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. [ABSTRACT FROM AUTHOR]

Global climate change is expected to affect how plants respond to their physical and biological environments. In this study, we examined the effects of elevated CO2 ([CO2]) and low soil moisture on the physiological responses of mountain maple (Acer spicatum L.) seedlings to light availability. The seedlings were grown at ambient (392 µmol mol−1) and elevated (784 µmol mol−1) [CO2], low and high soil moisture (M) regimes, at high light (100%) and low light (30%) in the greenhouse for one growing season. We measured net photosynthesis (A), stomatal conductance (gs), instantaneous water use efficiency (IWUE), maximum rate of carboxylation (Vcmax), rate of photosynthetic electron transport (J), triose phosphate utilization (TPU)), leaf respiration (Rd), light compensation point (LCP) and mid-day shoot water potential (Ψx). A and gs did not show significant responses to light treatment in seedlings grown at low soil moisture treatment, but the high light significantly decreased the Ci/Ca in those seedlings. IWUE was significantly higher in the elevated compared with the ambient [CO2], and the effect was greater at high than the low light treatment. LCP did not respond to the soil moisture treatments when seedlings were grown in high light under both [CO2]. The low soil moisture significantly reduced Ψx but had no significant effect on the responses of other physiological traits to light or [CO2]. These results suggest that as the atmospheric [CO2] rises, the physiological performance of mountain maple seedlings in high light environments may be enhanced, particularly when soil moisture conditions are favourable. [ABSTRACT FROM AUTHOR]

Northern white-cedar (Thuja occidentalis L.) regeneration originating from layering is generally considered to be more shade tolerant than that of seed origin. In this study, we examined photosynthetic light-use efficiency, an important component of shade tolerance, of these two forms of regeneration using light-response curves of 1-year-old rooted cuttings of layers and seedlings grown under low (20% of full sun) and high (>90% of full sun) light conditions Northern white-cedar showed a strong potential for photosynthetic acclimation to light availability, as indicated by significant differences in the light compensation point (LCP), dark respiration (Rd), apparent quantum efficiency (PE), and light saturation point (Qsat) but not light-saturated rate of net photosynthesis (Amax) between cuttings grown under high and low light. Values of LCP, and PE did not differ between layers and seedlings grown under high and low light, but layers had higher Amax and Qsat than seedlings when grown under high light. This result may reflect a comparatively lower capacity for photosynthetic acclimation of seedlings to high light conditions, perhaps because of their younger physiological age. A lower capacity for photosynthetic acclimation to light in seedlings could limit the repair and recovery of their photosynthetic systems from damage associated with the more extreme microenvironments of open, recently disturbed sites. [ABSTRACT FROM AUTHOR]