A cool spot in a biodiversity hotspot: why do tall Eucalyptus forests in Southwest Australia exhibit low diversity?

Xue Meng Zhou, Kosala Ranathunge, Marion L. Cambridge, Kingsley W. Dixon, Patrick E. Hayes, Miroslav Nikolic, Qi Shen, Hongtao Zhong, Hans Lambers

Research output: Contribution to journalArticlepeer-review

1 Citation (Scopus)

Abstract

Background and aims Southwest Australia is a biodiversity hotspot, with greatest plant species diversity on the most severely phosphorus (P)-impoverished soils. Here, non-mycorrhizal species with highly-effective carboxylate-releasing P-acquisition strategies coexist with mycorrhizal species that are less effective at accessing P on these soils. Non-mycorrhizal carboxylate-releasing species facilitate P acquisition of mycorrhizal neighbours that are better defended against pathogens. In the Southwest Australian Biodiversity Hotspot, there are also 'cool spots' of low-diversity tall mycorrhizal Eucalyptus communities on P-impoverished soils. These Eucalyptus trees obviously do not require facilitation of their P acquisition by carboxylate-releasing neighbours, because these are only a minor component of the low-diversity communities. We hypothesised that in low-diversity tall Eucalyptus forests, mycorrhizal species release carboxylates to acquire P. Thus, they would not depend on facilitation, and must be strong competitors. However, because they would not depend on external mycorrhizal hyphae to acquire P, they would also not be able to access soil organic nitrogen (N), for which they would need external hyphae. Methods Since carboxylates not only mobilise P, but also manganese (Mn), we used leaf Mn concentrations ([Mn]) in the natural habitat to proxy rhizosphere carboxylates. To verify this proxy, we also measured carboxylate exudation of targeted species with high leaf [Mn] using seedlings grown in low-P nutrient solutions. Results Using these complementary approaches, we confirmed our hypothesis that dominant Eucalyptus species in 'cool spots' release carboxylates. Since mineralisation of organic N is associated with fractionation of N, enriching organic N with N-15 while nitrate is depleted in N-15, we measured the stable N isotope composition of leaf material. The results show that dominant Eucalyptus species did not access organic N, despite being ectomycorrhizal. Conclusions The low diversity of tall Eucalyptus forests in southwest Australia can be explained by dominant mycorrhizal species exhibiting a carboxylate-releasing strategy. The tall eucalypts are therefore strong competitors that do not require facilitation, but also do not access organic N.

Original languageEnglish
Pages (from-to)669-688
Number of pages20
JournalPlant and Soil
Volume476
Issue number1-2
Early online date2 Jul 2022
DOIs
Publication statusPublished - Jul 2022

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