The Functional Significance of Leaf Shape Variation - Towards a Consensus from Gene to Community
Smith, William K. , Jackson, Stephen T , Johnson, Daniel M. .
Functional Significance of Conifer Leaf Shape in an Evolutionary Context.
Since the Late Cretaceous expansion of angiosperms, coniferous trees have been dominant primarily in boreal forests and montane forests at higher altitudes. In contrast to the angiosperm-dominated forests of interglacial periods and the Tertiary, the extensive glacial phases of the Quaternary were showed an expansion of conifer species that is probably attributable to both winter and summer stress factors related to cooler temperatures. However, this does not explain the low representation of boreal and cool-temperate, broadleaf deciduous species such as Betula spp. and Alnus spp. These glacial periods of the Quaternary also had the lowest atmospheric CO2 concentrations than any time during the Cenozoic. To compensate for colder temperatures and the severe winter conditions experienced by evergreen species, the evolution of the needle-like leaf of conifers was favored. A more radial cross-sectional geometry provided increased mechanical strength, a greater leaf surface curvature and less sunlight absorption (low-temperature photoinhibition), a greater area-to-volume ratio that reduced water stress. Radial diffusion maximized CO2 absorption throughout the mesophyll without the need of increased stomatal pore area per unit leaf area. Although sunlight incidence was reduced by the more curved leaf surface, more cylindrical x-sections and radial diffusion also resulted in a diminished need for palisade cell layers as evidenced by their disappearance in sun leaves of certain conifer taxa. Important advantages of the needle-like shape also opened new venues for adaptive function at the shoot level such as greater leaf-packing per unit stem length. These stem-level characteristics enabled mechanical protection from blowing snow, as well as light-saturated photosynthesis at warmer temperatures, due to reduced abrasive and convective air flow. Effectively, the advantage of uncoupling from air temperature characteristic of more laminar broadleaves was accomplished by having a densely packed arrangement of leaves, which could only be accomplished by evolving a needle-like leaf shape.
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1 - Wake Forest University, Department of Biology, PO Box 7325, 226 Winston Hall, Winston-Salem, North Carolina, 27109-7325, USA
2 - University of Wyoming, Department of Botany, Laramie, Wyoming, 82071, USA
3 - USDA Forest Service, Pacific Northwest Research Station, 3200 Jefferson Way, Corvallis, OR, 97331
Presentation Type: Symposium or Colloquium Presentation
Location: Williford B/Hilton
Date: Monday, July 9th, 2007
Time: 3:30 PM