Abstract
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 (g s), instantaneous water use efficiency (IWUE), maximum rate of carboxylation (V cmax), rate of photosynthetic electron transport (J), triose phosphate utilization (TPU)), leaf respiration (R d), light compensation point (LCP) and mid-day shoot water potential (Ψx). A and g s did not show significant responses to light treatment in seedlings grown at low soil moisture treatment, but the high light significantly decreased the C i/C a 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.
Highlights
The responses of shade tolerant species to light availability in canopy gaps are at two endpoints
A, gs and Ci/Ca were 28%, 24% and 9% greater respectively, in the high than the low light treatment in seedlings grown at the high soil moisture treatment
This study shows that the net photosynthesis (A) of mountain maple seedling would be less responsive to canopy gaps when soil moisture is low and that seedlings would be more sensitive to moisture stress when grown in canopy gaps than when grown in shade of a forest canopy
Summary
The responses of shade tolerant species to light availability in canopy gaps are at two endpoints. Some shade tolerant species grow slowly and consistently in the understory and do not respond considerably to canopy gaps. Species responses to the occurrence of canopy gaps can be important in influencing forest dynamics. The responses of shade tolerant species to canopy gaps may be limited by physiological constraints such as decreased photosynthesis if other resources are limiting [4,5]. Shade tolerant species responses to canopy gaps may be contingent on their ability to alter physiological traits for positive carbon balance [6]. Any factors that enhance the ability of shade tolerant species to increase A in canopy gaps may play an important role in forest dynamics
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