Fire and fire-adapted vegetation promoted C4 expansion in the late Miocene.
Large proportions of the Earth's land surface are covered by biomes dominated by C(4) grasses. These C(4)-dominated biomes originated during the late Miocene, 3-8 million years ago (Ma), but there is evidence that C(4) grasses evolved some 20 Ma earlier during the early Miocene/Oligocene. Explanations for this lag between evolution and expansion invoke changes in atmospheric CO(2), seasonality of climate and fire. However, there is still no consensus about which of these factors triggered C(4) grassland expansion. We use a vegetation model, the adaptive dynamic global vegetation model (aDGVM), to test how CO(2), temperature, precipitation, fire and the tolerance of vegetation to fire influence C(4) grassland expansion. Simulations are forced with late Miocene climates generated with the Hadley Centre coupled ocean-atmosphere-vegetation general circulation model. We show that physiological differences between the C(3) and C(4) photosynthetic pathways cannot explain C(4) grass invasion into forests, but that fire is a crucial driver. Fire-promoting plant traits serve to expand the climate space in which C(4)-dominated biomes can persist. We propose that three mechanisms were involved in C(4) expansion: the physiological advantage of C(4) grasses under low atmospheric CO(2) allowed them to invade C(3) grasslands; fire allowed grasses to invade forests; and the evolution of fire-resistant savanna trees expanded the climate space that savannas can invade.
Biodiversität und Klima Forschungszentrum (LOEWE BiK-F), Senckenberg Gesellschaft für Naturforschung, Senckenberganlage 25, 60325 Frankfurt am Main, Germany.
Simon
S
Scheiter
Institut für Physische Geographie, Johann Wolfgang Goethe-Universität Frankfurt am Main, Altenhöferallee 1, 60438 Frankfurt am Main, Germany.
Steven I
SI
Higgins
Department of Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN, UK.
Colin P
CP
Osborne
School of Geographical Sciences, University of Bristol, University Road, Clifton, Bristol BS8 1SS, UK.
Catherine
C
Bradshaw
School of Geographical Sciences, University of Bristol, University Road, Clifton, Bristol BS8 1SS, UK.
Dan
D
Lunt
Botany Department, Rhodes University, PO Box 94, Grahamstown 6140, South Africa.
Brad S
BS
Ripley
Department of Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN, UK.
Lyla L
LL
Taylor
Department of Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN, UK.
David J
DJ
Beerling
D000222:Adaptation, Physiological; D005075:Biological Evolution; D002245:Carbon Dioxide / Q000378:metabolism; D003198:Computer Simulation; D017753:Ecosystem; D005390:Fires; D008954:Models, Biological; D010788:Photosynthesis; D018515:Plant Leaves / Q000502:physiology; D006109:Poaceae / Q000378:metabolism / Q000502:physiology*; D012621:Seasons; D013696:Temperature; D014197:Trees / Q000378:metabolism / Q000502:physiology*
D002245:Carbon Dioxide