Structural and functional changes in coffee trees after 4 years under free air CO2 enrichment

Climate forecasts suggest that [CO2] in the atmosphere will continue to increase. Structural and ecophysiological responses to elevated air [CO2] (e[CO2]) in tree species are contradictory due to species-dependent responses and relatively short-term experiments. It was hypothesized that long-term exposure (4 year) to e[CO2] would change canopy structure and function of Coffea arabica trees.

Coffee plants were grown in a FACE (free air CO2 enrichment) facility under two air [CO2]: actual and elevated (actual + approx. 200 μL CO2 L-1). Plants were codified following the VPlants methodology to obtain coffee mock-ups. Plant canopies were separated into three 50 cm thick layers over a vertical profile to evaluate their structure and photosynthesis, using functional-structural plant modelling.

Leaf area was strongly reduced on the bottom and upper canopy layers, and increased soil carbon concentration suggested changes in carbon partitioning of coffee trees under e[CO2]. Increased air [CO2] stimulated stomatal conductance and leaf photosynthesis at the middle and upper canopy layers, increasing water-use efficiency. Under e[CO2], plants showed reduced diameter of the second-order axes and higher investment in the youngest third to fifth-order axes.

The responses of Arabica coffee grown under long-term exposure to e[CO2] integrated structural and functional modifications, which balanced leaf area loss through improvements in leaf and whole-plant photosynthesis.