Evolutionary divergences in leaf structure and chemistry, comparing rainfall and soil nutrient gradients

Leaf structural and chemical characteristics were compared in pairs of species that were phylogenetically independently contrasted along rainfall gradients (10 pairs) or soil nutrient gradients (9 pairs), using perennial plants in New South Wales, Australia. The objective was to test hypotheses regarding leaf attributes that should be successful in sustaining populations in vegetation at lower vs. higher rainfall or lower vs. higher soil nutrients, and especially to assess the proposition that lower rainfall and lower soil nutrients favor similar shifts in leaf attributes, and in this sense can be grouped together as sources of 'stress.' Some evolutionary divergences in leaf structure occurred repeatedly toward the lower end of both rainfall and soil nutrient gradients. These include narrower leaves, lower specific leaf area (SLA), thicker lamina, and denser leaf tissue. In other respects, the different resource gradients appeared to favor different leaf attributes. In 8 of 10 contrasts, plants of low-water environments had more sclerified vasculature than their relatives on higher rainfall sites. This may be an adaptation to resist wilting and so mitigate cell damage. In 9 of 10 contrasts, plants of low-water environments had more nitrogen per unit leaf area than their relatives in high-rainfall sites. This may be to exploit higher light availability in arid communities. In 7 of 9 contrasts, plants from low soil nutrient environments had thicker epidermis than their relatives on higher nutrient soils. In 8 of 9 soil nutrient contrasts, leaf nitrogen concentration was lower in the low-nutrient species, and consequently, the ratios of phenol/N and tannin/N were higher. Thicker epidermis and increased defense/N ratios may reduce herbivory and thereby reduce demand on soil nutrients required to replace leaves.