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, 96 (3), 345-52

Leaf Evolution: Gases, Genes and Geochemistry


Leaf Evolution: Gases, Genes and Geochemistry

David J Beerling. Ann Bot.


Aims: This Botanical Briefing reviews how the integration of palaeontology, geochemistry and developmental biology is providing a new mechanistic framework for interpreting the 40- to 50-million-year gap between the origination of vascular land plants and the advent of large (megaphyll) leaves, a long-standing puzzle in evolutionary biology.

Scope: Molecular genetics indicates that the developmental mechanisms required for leaf production in vascular plants were recruited long before the advent of large megaphylls. According to theory, this morphogenetic potential was only realized as the concentration of atmospheric CO2 declined during the late Palaeozoic. Surprisingly, plants effectively policed their own evolution since the decrease in CO2 was brought about as terrestrial floras evolved accelerating the rate of silicate rock weathering and enhancing sedimentary organic carbon burial, both of which are long-term sinks for CO2.

Conclusions: The recognition that plant evolution responds to and influences CO(2) over millions of years reveals the existence of an intricate web of vegetation feedbacks regulating the long-term carbon cycle. Several of these feedbacks destabilized CO2 and climate during the late Palaeozoic but appear to have quickened the pace of terrestrial plant and animal evolution at that time.


F<sc>ig</sc>. 1.
Fig. 1.
Stages in the evolution of the megaphyll as documented by Zimmermann's telome theory (Zimmermann, 1930), with representative fossil plants for each stage illustrated below. Upper schematic: Osborne et al. (2004b), images of Rhynia, Psilophyton and Archaeopteris after Gifford and Foster (1988), image of Actinoxylon after Matten (1968).
F<sc>ig</sc>. 2.
Fig. 2.
Evolution of the atmosphere and land plants in the late Palaeozoic. (A) Changes in atmospheric CO2, modelled (open circles) or reconstructed from fossil soils (closed). (B) Observed increase in maximum width of megaphylls. Points indicate average maximum size for 5- or 10-Myr intervals. (C) Maximum plant height calculated from measurements of fossil stem diameter, assuming modern stem diameter–height relationships. (D) Changes in terrestrial carbon burial. Modified after Beerling and Berner (2005).
F<sc>ig</sc>. 3.
Fig. 3.
Systems analysis diagrams of the long-term carbon cycle with and without geophysiological feedbacks involving land plants. (A) The inorganic geochemical carbon cycle. (B and C) Geophysiological feedbacks introduced by plant evolutionary responses to changes in atmospheric CO2 that result in changes in silicate rock weathering and organic carbon burial, respectively. (D and E) Geophysiological feedbacks as in B and C but including the direct effects of CO2 on climate via the atmospheric greenhouse effect. Arrows originate at causes and end at effects. Blue, inorganic processes; green, organic processes.
F<sc>ig</sc>. 4.
Fig. 4.
Tropical weathering by deep-rooting trees on Kohala Mountain, Hawaii. The image shows the weathering ‘halo’ around the roots caused by mineral depletion. Depth of roots is approx. 7 m. Photograph courtesy of Carl Bowser. Reprinted from Berner et al. (Treatise on Geochemistry 5, p. 170; ©2003, with permission from Elsevier).

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