title: Canopy leaf area of a mature evergreen Eucalyptus woodland does not respond to elevated atmospheric [CO2] but tracks water availability.
abstract: Canopy leaf area, quantified by the leaf area index (L), is a crucial driver of forest productivity, water use and energy balance. Because L responds to environmental drivers, it can represent an important feedback to climate change, but its responses to rising atmospheric [CO2] and water availability of forests have been poorly quantified. We studied canopy leaf area dynamics for 28 months in a native evergreen Eucalyptus woodland exposed to free-air CO2 enrichment (the EucFACE experiment), in a subtropical climate where water limitation is common. We hypothesized that, because of expected stimulation of productivity and water-use efficiency, L should increase with elevated [CO2]. We estimated L from diffuse canopy transmittance, and measured monthly leaf litter production. Contrary to expectation, L did not respond to elevated [CO2]. We found that L varied between 1.10 and 2.20 across the study period. The dynamics of L showed a quick increase after heavy rainfall and a steady decrease during periods of low rainfall. Leaf litter production was correlated to changes in L, both during periods of decreasing L (when no leaf growth occurred) and during periods of increasing L (active shedding of old foliage when new leaf growth occurred). Leaf lifespan, estimated from mean L and total annual litter production, was up to 2 months longer under elevated [CO2] (1.18 vs. 1.01 years; P = 0.05). Our main finding that L was not responsive to elevated CO2 is consistent with other forest FACE studies, but contrasts with the positive response of L commonly predicted by many ecosystem models.
authors.affiliation: Hawkesbury Institute for the Environment, Western Sydney University, Locked Bag 1797, Penrith, NSW, Australia.
authors.forename: Remko A
authors.identifier: http://orcid.org/0000-0002-8499-5580
authors.initials: RA
authors.lastname: Duursma
authors.affiliation: Hawkesbury Institute for the Environment, Western Sydney University, Locked Bag 1797, Penrith, NSW, Australia.|UMR 1391 ISPA, INRA, Villenave d'Ornon, 33140, France.
authors.forename: Teresa E
authors.initials: TE
authors.lastname: Gimeno
authors.affiliation: Hawkesbury Institute for the Environment, Western Sydney University, Locked Bag 1797, Penrith, NSW, Australia.
authors.forename: Matthias M
authors.initials: MM
authors.lastname: Boer
authors.affiliation: Hawkesbury Institute for the Environment, Western Sydney University, Locked Bag 1797, Penrith, NSW, Australia.
authors.forename: Kristine Y
authors.initials: KY
authors.lastname: Crous
authors.affiliation: Hawkesbury Institute for the Environment, Western Sydney University, Locked Bag 1797, Penrith, NSW, Australia.
authors.forename: Mark G
authors.initials: MG
authors.lastname: Tjoelker
authors.affiliation: Hawkesbury Institute for the Environment, Western Sydney University, Locked Bag 1797, Penrith, NSW, Australia.
authors.forename: David S
authors.initials: DS
authors.lastname: Ellsworth
mesh_terms: D001272:Atmosphere; D002245:Carbon Dioxide / Q000032:analysis*; D005052:Eucalyptus / Q000254:growth & development*; D065928:Forests; D009517:New South Wales; D018515:Plant Leaves / Q000254:growth & development*; D014867:Water / Q000032:analysis*
chemical_list: D014867:Water; D002245:Carbon Dioxide
keywords: drought; free-air CO2 enrichment; leaf area index; litter production; phenology