High leaf mass per area of related species assemblages may reflect low rainfall and carbon isotope discrimination rather than low phosphorus and nitrogen concentrations

Authors

  • Byron B. Lamont,

    Corresponding author
    1. Department of Environmental Biology, Curtin University of Technology, PO Box U1987, Perth WA 6845, Australia,
      †Author to whom correspondence should be addressed. E-mail: rLamontB@curtin.edu.au
    Search for more papers by this author
  • Philip K. Groom,

    1. Department of Environmental Biology, Curtin University of Technology, PO Box U1987, Perth WA 6845, Australia,
    2. Centre for Ecosystem Management, School of Natural Sciences, Edith Cowan University, Joondalup WA 6027, Australia, and
    Search for more papers by this author
  • R. M. Cowling

    1. Department of Environmental Biology, Curtin University of Technology, PO Box U1987, Perth WA 6845, Australia,
    2. Terrestrial Ecology Research Unit, Department of Botany, University of Port Elizabeth, PO Box 1600, Port Elizabeth, South Africa
    Search for more papers by this author

†Author to whom correspondence should be addressed. E-mail: rLamontB@curtin.edu.au

Summary

1. Leaf morphology at the site/species level should reflect environmental constraints on plant growth. One of the oldest controversies in ecology is the environmental basis for sclerophylly. The dominant view (Beadle’s theory) is that it has a nutritional, rather than a drought, basis, especially low phosphorus.

2. Using leaf mass per area (LMA) as an index of sclerophylly, we assessed its relationship with leaf phosphorus (P) and nitrogen (N) along extensive rainfall gradients in southwestern Australia and the Cape of South Africa. Leaf 13C/12C discrimination (Δ13C), as an index of intrinsic water-use efficiency, was also examined in the Cape. All Hakea species (Proteaceae) were sampled at 10 sites in Australia (96 species), and all Proteaceae at 14 sites in the Cape (82 species). All were evergreen shrubs with iso(bi)-lateral leaves.

3. In each region there was a strong (inverse) curvilinear relationship between mean LMA per site and mean annual rainfall and Δ13C, but none with mean P or N on a mass basis (although P and N on an area basis declined with rainfall). The Cape study was a particularly good test of Beadle’s theory, as P varied as much between sites as rainfall, and more between sites than within sites.

4. Leaf thickness and dry density were not as well correlated with rainfall as LMA, and leaf area and mass showed no relationship with rainfall. Area and mass had much greater variation within sites than between sites, limiting their value in plant–environment studies, while LMA was the most site-stable of the eight leaf attributes measured, except for Δ13C.

5. For all species considered individually in each region, there was a similar pattern as the site level, with LMA most strongly correlated (negatively) with rainfall and Δ13C and (positively) with leaf thickness, but no consistent relationship with P, N or density.

6. We conclude that when water and nutrient supply vary independently in the field, rainfall (as an index of water status) and Δ13C may be more closely correlated (inversely) with level of sclerophylly than nutrient status among evergreens, so that the role of sclerophylly as a drought adaptation warrants further consideration.

Ancillary