Abstract
Elevated atmospheric CO2 (eCO2) often increases photosynthetic CO2 assimilation (A) in field studies of temperate tree species. However, there is evidence that A may decline through time due to biochemical and morphological acclimation, and environmental constraints. Indeed, at the free-air CO2 enrichment (FACE) study in Oak Ridge, Tennessee, A was increased in 12-year-old sweetgum trees following 2 years of ∼40 % enhancement of CO2. A was re-assessed a decade later to determine if the initial enhancement of photosynthesis by eCO2 was sustained through time. Measurements were conducted at prevailing CO2 and temperature on detached, re-hydrated branches using a portable gas exchange system. Photosynthetic CO2 response curves (A versus the CO2 concentration in the intercellular air space (Ci); or A-Ci curves) were contrasted with earlier measurements using leaf photosynthesis model equations. Relationships between light-saturated photosynthesis (Asat), maximum electron transport rate (Jmax), maximum Rubisco activity (Vcmax), chlorophyll content and foliar nitrogen (N) were assessed. In 1999, Asat for eCO2 treatments was 15.4 ± 0.8 μmol m(-2) s(-1), 22 % higher than aCO2 treatments (P < 0.01). By 2009, Asat declined to <50 % of 1999 values, and there was no longer a significant effect of eCO2 (Asat = 6.9 or 5.7 ± 0.7 μmol m(-2) s(-1) for eCO2 or aCO2, respectively). In 1999, there was no treatment effect on area-based foliar N; however, by 2008, N content in eCO2 foliage was 17 % less than that in aCO2 foliage. Photosynthetic N-use efficiency (Asat : N) was greater in eCO2 in 1999 resulting in greater Asat despite similar N content, but the enhanced efficiency in eCO2 trees was lost as foliar N declined to sub-optimal levels. There was no treatment difference in the declining linear relationships between Jmax or Vcmax with declining N, or in the ratio of Jmax : Vcmax through time. Results suggest that the initial enhancement of photosynthesis to elevated CO2 will not be sustained through time if N becomes limited.
Highlights
In 2013, annual atmospheric CO2 concentration exceeded 396 ppm at Mauna Loa, 2.6 ppm greater than in 2012 and 25 % greater than the initial measurements in 1959 (Keeling et al 2014)
The main goal of this project was to determine if the initial enhancement of photosynthesis by elevated CO2 (eCO2) was sustained through time, and if responses were linked to the progressive decline in site resource availability (Garten et al 2011), foliar N content (Norby et al 2010) and shift in internal plant C allocation (Norby et al 2004)
In 1999, light-saturated photosynthesis (Asat) was significantly greater in eCO2 than in aCO2 foliage, but when re-measured in 2008, Asat was similar for the two treatments
Summary
In 2013, annual atmospheric CO2 concentration exceeded 396 ppm at Mauna Loa, 2.6 ppm greater than in 2012 and 25 % greater than the initial measurements in 1959 (Keeling et al 2014). ECO2 induced significant down-regulation and loss of photosynthetic capacity (e.g. in Arctic tundra grasses and shrubs (Tissue and Oechel 1987; Oechel et al 1994), beech (Epron et al 1996) and spruce (Marek et al 1995)), indicating that interaction with soil resources, longer term feedbacks and progressive plant acclimation remained a key uncertainty (Curtis and Wang 1998). Larger experiments using free-air CO2 enrichment (FACE) technology (Norby et al 2001) have allowed field crops, grasses and, in particular, woody forest vegetation to be exposed to eCO2 over many years. Such studies allow distinction of long-term responses from transient responses due to leaf plasticity, stand development or inter-annual environmental variability
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
