Will Elevated CO2 Increase Forest Productivity? Evidence from an Australian FACE Experiment

Abstract:

Rising atmospheric CO₂ may enhance forest productivity via CO₂ fertilisation and increased soil moisture associated with water savings.  Quantification of the response of forest productivity to rising CO₂ concentrations is important, as increased forest productivity may contribute to the mitigation of anthropogenic climate change. Vegetation greenness indices derived from digital photographs have been correlated with a number of measures of ecosystem productivity including total biomass, leaf area index and gross primary productivity. Our study examines the effect of elevated CO₂ on patterns in overstorey and understorey vegetation greenness at a Free Air CO₂ Enrichment facility (EucFACE) situated within a temperate eucalypt forest in Sydney, Australia. EucFACE consists of six treatment areas, three subjected to ambient CO₂ (‘ambient’) and three with ambient plus 150 ppm CO₂ (‘elevated’). Each treatment area had one camera monitoring canopy greenness for a 12 month period and four cameras monitoring one understorey vegetation plot (2.25 m²) each for a 15 month period. Vegetation greenness was measured daily using the green chromatic coordinate (GCC).

Understorey and overstorey GCC and rates of understorey greening and browning were not affected by elevated CO₂. Periodic differences in canopy greening and browning between CO₂ treatments were observed, though these probably reflect an insect defoliation event in one treatment area. Increases in canopy and understorey GCC were associated with a combination of extended periods of high soil volumetric water content (VWC) (>0.1) and high maximum temperatures (>25 °C). Browning appeared to be associated with a combination of periods of high maximum temperatures and low VWC or low minimum temperatures. Our short term findings suggest that eucalypt forest productivity will be sensitive to changes in climate, but may be relatively insensitive to changes in CO₂ in the near future.