No photosynthetic down-regulation in sweetgum trees (Liquidambar styraciflua L.) after three years of CO2 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 muL L-1 CO2 at the Duke Forest Free Air CO2 Enrichment (FACE) experiment, We were interested in whether photosynthetic downregulation to CO2 occurred in sweetgum frees 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 CO2 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 CO2 in either sun or shade leaves of sweetgum trees over the 3 years of measurements. Elevated CO2 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 CO2 treatment, position in the canopy, or measurement period. Furthermore, CO2 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 ka 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 frees rooted in the ground may not exhibit symptoms of photosynthetic down-regulation as quickly as free seedlings growing in pots. There was a strong sustained enhancement of photosynthesis by CO2 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 CO2 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.