title: Carbon cycle responses of semi-arid ecosystems to positive asymmetry in rainfall.
abstract: Recent evidence shows that warm semi-arid ecosystems are playing a disproportionate role in the interannual variability and greening trend of the global carbon cycle given their mean lower productivity when compared with other biomes (Ahlström et al. 2015 Science, 348, 895). Using multiple observations (land-atmosphere fluxes, biomass, streamflow and remotely sensed vegetation cover) and two state-of-the-art biospheric models, we show that climate variability and extremes lead to positive or negative responses in the biosphere, depending on vegetation type. We find Australia to be a global hot spot for variability, with semi-arid ecosystems in that country exhibiting increased carbon uptake due to both asymmetry in the interannual distribution of rainfall (extrinsic forcing), and asymmetry in the response of gross primary production (GPP) to rainfall change (intrinsic response). The latter is attributable to the pulse-response behaviour of the drought-adapted biota of these systems, a response that is estimated to be as much as half of that from the CO2fertilization effect during 1990-2013. Mesic ecosystems, lacking drought-adapted species, did not show an intrinsic asymmetric response. Our findings suggest that a future more variable climate will induce large but contrasting ecosystem responses, differing among biomes globally, independent of changes in mean precipitation alone. The most significant changes are occurring in the extensive arid and semi-arid regions, and we suggest that the reported increased carbon uptake in response to asymmetric responses might be contributing to the observed greening trends there.
authors.affiliation: CSIRO Oceans and Atmosphere, GPO Box 3023, Canberra, ACT, 2601, Australia.
authors.forename: Vanessa
authors.initials: V
authors.lastname: Haverd
authors.affiliation: Department of Earth System Science, School of Earth, Energy and Environmental Sciences, Stanford University, Stanford, CA, 94305, USA.|Department of Physical Geography and Ecosystem Science, Lund University, Lund, 223 62, Sweden.
authors.forename: Anders
authors.initials: A
authors.lastname: Ahlström
authors.affiliation: Department of Physical Geography and Ecosystem Science, Lund University, Lund, 223 62, Sweden.
authors.forename: Benjamin
authors.initials: B
authors.lastname: Smith
authors.affiliation: CSIRO Oceans and Atmosphere, GPO Box 3023, Canberra, ACT, 2601, Australia.
authors.forename: Josep G
authors.initials: JG
authors.lastname: Canadell
mesh_terms: D001315:Australia; D057486:Carbon Cycle; D003889:Desert Climate; D055864:Droughts; D017753:Ecosystem
keywords: Australia; global carbon cycle; greening trend; rainfall asymmetry; semi-arid ecosystems