Carboxylate-releasing Eucalyptus patens in a low-diversity spot in a biodiversity hotspot in Southwest Australia is not a ‘Darwinian demon’

Background and aims: Plants in severely phosphorus (P)-impoverished environments in southwest Australia are highly P efficient. In a Eucalyptus patens forest on P-impoverished soils in this region, leaf P concentrations ([P]) are very low in common species, which release carboxylates to acquire P. We hypothesised that these species exhibit a high photosynthetic P-use efficiency (PPUE). Being mycorrhizal and releasing carboxylates, the eucalypts would be very strong competitors, also against N₂-fixing neighbours. Therefore, we hypothesised that the dominant species show relatively low leaf nitrogen concentrations ([N]) and a high photosynthetic N-use efficiency (PNUE). Conversely, we expected these species to have a low intrinsic water-use efficiency (iWUE). We also tested the hypothesis that E. patens would not show P-toxicity symptoms at high P availability, and its carboxylate release would not decline at high P supply. Methods: We selected common species such as Eucalyptus patens, E. marginata, Corymbia calophylla, Banksia grandis and Bossiaea aquifolium. We analysed their leaf [P], [N], photosynthesis rates, stomatal conductance, PPUE, PNUE, δ¹³C, WUE and iWUE. We grew E. patens seedlings in nutrient solution with increasing P supply and measured their carboxylate-exudation rates to test if it declined at high P supply. Results: Eucalyptus patens released large amounts of carboxylates without a decrease in carboxylate release with increasing P supply. Eucalyptus patens and co-occurring common species had similarly low leaf [P] and [N], high PPUE and PNUE, but low WUE and iWUE, when compared with literature values. Conclusions: The low leaf [N] of dominant species was compensated for by high stomatal conductance. This strategy would be viable only in habitats with abundant water. Therefore, P-efficient water-inefficient species would be restricted in their distribution to habitats with high water availability and not be ‘Darwinian demons’.