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

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 15N while nitrate is depleted in 15N, 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.

Role of mycorrhizas and root exudates in plant uptake of soil nutrients (calcium, iron, magnesium, and potassium): has the puzzle been completely solved?

“…Phenolic compounds in microbial siderophores are also involved in the mobilization and plant uptake of Fe in the rhizosphere (Nuzzo et al, 2018). In addition, grasses secrete phytosiderophores that act as strong chelators of Fe 3+ , triggering the mobilization of Fe (Chen, Wang, & Yeh, 2017;Kobayashi & Nishizawa, 2012), as well as P (Zhou et al, 2022). Vempati et al (1995) observed that Glycine max root exudates reduce Fe 3+ in soil to Fe 2+ , thus increasing Fe availability in the rhizosphere; this reduction was likely caused by phenolics in root exudates, which were oxidized to diquinones.…”

“…Phenolic compounds in microbial siderophores are also involved in the mobilization and plant uptake of Fe in the rhizosphere (Nuzzo et al., 2018). In addition, grasses secrete phytosiderophores that act as strong chelators of Fe 3+ , triggering the mobilization of Fe (Chen, Wang, & Yeh, 2017; Kobayashi & Nishizawa, 2012), as well as P (Zhou et al., 2022). Vempati et al.…”

Scientific impact, direction and highlights of Plant and Soil in the 30 years since Professor Hans Lambers became Editor in Chief

“…Hans says, "I gradually realised that the carboxylate-releasing strategy was far more important than I had even dared to believe at the very beginning. To quantify that, we proposed a tool to screen for species with carboxylate-releasing strategies and the leaf [Mn] approach has become an important screening tool for us, both in ecological studies and in screening crop genotypes" (Lambers et al 2015(Lambers et al , 2021Pang et al 2018;White and Neugebauer 2021;Zhou et al 2022). The approach has been used in ecosystems across the world, not least in China (Tian et al 2021;Yu et al 2020;Zhou et al 2021).…”

Shifts from an extensive to an intensive root nutrient‐acquisition mode with stand development of three Pinus species

“…In contrast to this expectation, previous studies have not shown a trade‐off between absorptive fine roots and mycorrhizal extraradical hyphae during stand development (Wallander et al., 2010; Wasyliw et al., 2020). However, it should be noted that these two studies did not consider root exudation, which may also play critical roles in nutrient acquisition, especially for mature stands since stand development may induce N and P deficiency (De Schrijver et al., 2012; DeLuca et al., 2002; Zhou et al., 2022). Therefore, more research is needed to fully understand whether and how tree species coordinate diverse nutrient‐acquisition strategies to acclimate to changes in biotic and abiotic conditions during stand development.…”