Abstract: Aims In the Core Cape Subregion (CCR), a Mediterranean-climate ecosystem with infertile soils, the legume species Podalyria calyptrata and P. burchellii are in a separate clade to P. leipoldtii and P. myrtillifolia. The closely related species are allopatric, and with the west-east climate gradient and variation in soil nutrient availability in the CCR, it was hypothesized that the two closely related allopatric species would differ in their ecological niche and root:shoot ratio, specific root length (SRL) and organic acid exudation responses to phosphorus (P) supply.
Methods With increasing P supply in the glasshouse, we measured plant biomass, leaf nitrogen ([N]), [P], root morphology and release of organic acids. We determined species soil and leaf [N] and [P] and climate in field sites.
Important findings At low P supply, P. calyptrata roots exuded more organic acids than P. burchellii which instead produced roots with a greater SRL, and P. myrtillifolia allocated more biomass to roots than P. leipoldtii. In the field, leaf [P] and climate suggested that P. leipoldtii occupied the most oligotrophic niche followed by P. burchellii and then P. calyptrata and P. myrtillifolia. Closely related allopatric species differed in their mechanisms for P-acquisition and ecological niche, indicating that the environment overrides phylogeny in determining P-acquisition traits for these species, and suggesting that climate regulates nutrient availability, driving distribution and speciation.

Key words: climate, distribution, nodules, organic acids, root:shoot ratio, specific root length