No photosynthetic down‐regulation in sweetgum trees ( Liquidambar styraciflua L.) after three years of CO 2 enrichment at the Duke Forest FACE experiment

Photosynthetic capacity and leaf properties of sun and shade leaves of overstorey sweetgum trees ( Liquidambar styraciflua L.) were compared over the first 3 years of growth in ambient or ambient + 200 μL L − 1 CO 2 at the Duke Forest Free Air CO 2 Enrichment (FACE) experiment. We were interested in whether photosynthetic down‐regulation to CO 2 occurred in sweetgum trees growing in a forest ecosystem, whether shade leaves down‐regulated to a greater extent than sun leaves, and if there was a seasonal component to photosynthetic down‐regulation. During June and September of each year, we measured net photosynthesis ( A ) versus the calculated intercellular CO 2 concentration ( C i ) in situ and analysed these response curves using a biochemical model that described the limitations imposed by the amount and activity of ribulose‐1,5‐bisphosphate carboxylase/oxygenase ( Vc max ) and by the rate of ribulose‐1,5‐bisphosphate (RuBP) regeneration mediated by electron transport ( J max ). There was no evidence of photosynthetic down‐regulation to CO 2 in either sun or shade leaves of sweetgum trees over the 3 years of measurements. Elevated CO 2 did not significantly affect Vc max or J max . The ratio of Vc max to J max was relatively constant, averaging 2·12, and was not affected by CO 2 treatment, position in the canopy, or measurement period. Furthermore, CO 2 enrichment did not affect leaf nitrogen per unit leaf area ( N a ), chlorophyll or total non‐structural carbohydrates of sun or shade leaves. We did, however, find a strong relationship between N a and the modelled components of photosynthetic capacity, Vc max and J max . Our data over the first 3 years of this experiment corroborate observations that trees rooted in the ground may not exhibit symptoms of photosynthetic down‐regulation as quickly as tree seedlings growing in pots. There was a strong sustained enhancement of photosynthesis by CO 2 enrichment whereby light‐saturated net photosynthesis of sun leaves was stimulated by 63% and light‐saturated net photosynthesis of shade leaves was stimulated by 48% when averaged over the 3 years. This study suggests that this CO 2 enhancement of photosynthesis will be sustained in the Duke Forest FACE experiment as long as soil N availability keeps pace with photosynthetic and growth processes.