Soil depth shapes plant functional diversity in granite outcrops vegetation of Southwestern Australia

Abstract

Background: The arid and nutrient-poor Southwestern Australia is one of the global biodiversity hotspots. Embedded in this landscape, granite outcrops are considered terrestrial insular habitats supporting habitat heterogeneity when compared to the more homogenous surrounds. Ecology of plant species and communities on granite outcrops has been addressed in numerous studies. However, functional diversity (FD) in context of the environmental heterogeneity remained unexplored.

Aims: We tested whether mesic deep-soil habitats on granite outcrops can sustain larger FD than dry shallow-soil habitats.

Methods: We calculated FD for dominant species for five single traits (leaf dry matter content, foliar δ¹³C, foliar C:N ratio, plant height and specific leaf area) and their combinations. We employed Generalized Additive Mixed Models to quantify the relationship between selected climate and soil depth variables, and FD.

Results: More benign (deep-soil) habitats supported larger FD for foliar C:N, plant height and for multiple traits than did shallow-soil habitats.

Conclusions: We suggest that: (1) functional diversification, likely aimed at avoiding intra- and interspecific competition for light and nutrients acquisition, might be the important factor in deep-soil habitats; (2) deep-soils patches on and around granite outcrops may serve as ecological microrefugia for biota associated with resource-rich environments.

Keywords:

• aridity
• functional diversity
• gradient-analysis
• mass ratio hypothesis
• physiological tolerance hypothesis
• refugia
• soil depth

Acknowledgements

This research stemmed from a broader project aimed at furthering the ecological knowledge of vegetation on the putative refugial granite outcrops of Southwestern Australia. We greatly thank Gunnar Keppel (University of South Australia, Adelaide), Etienne Laliberté (The University of Western Australia, Perth and University of Montreal), Marko Spasojevic (University of California, Riverside), and James Tsakalos (The University of Western Australia, Perth) for providing insightful comments in previous versions of the manuscripts. Paul Macintyre (The University of Western Australia, Perth) kindly produced Figure 1. Giulio Molinari, Grant Wardell-Johnson, Romina Savini, Ross Young, Salah Dehum, and Tran-Duc Thien crucially assisted in the fieldwork. Gianluigi Ottaviani was supported by CIPRS (Curtin University, Perth, Australia), Endeavour Europe Award (Australian Government), SIRF, UPAIS, Safety-Net Top-Up, Ad-hoc (The University of Western Australia, Perth, Australia) scholarships during his PhD project. Matteo Marcantonio’s Ph.D. research is funded by FIRS > T scholarship (FEM International Research School and Trentino, Trento, Italy). Prof. Ladislav Mucina is supported by the Iluka Chair in Vegetation Science and Biogeography at The University of Western Australia, Perth, Australia.

Disclosure statement

No potential conflict of interest was reported by the authors.

Supplemental data

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Additional information

Funding

Gianluigi Ottaviani was supported by CIPRS (Curtin University, Perth, Australia), Endeavour Europe Award (Australian Government), SIRF, UPAIS, Safety-Net Top-Up, Ad-hoc (The University of Western Australia, Perth, Australia) scholarships during his PhD project. Matteo Marcantonio’s PhD research is funded by FIRS > T scholarship (FEM International Research School and Trentino, Trento, Italy). Prof Ladislav Mucina is supported by the Iluka Chair in Vegetation Science and Biogeography at The University of Western Australia, Perth, Australia

Notes on contributors

Gianluigi Ottaviani

Gianluigi Ottaviani is a research assistant. His research focuses on using trait-based approaches to advance mechanistic and functional understanding of plant community assembly and of refugia.

Matteo Marcantonio

Matteo Marcantonio is a Ph.D. student. The major theme of his research is to understand the processes that shape the distribution of invasive vector species, with focus on their spread in urban environments.

Ladislav Mucina

Ladislav Mucina is a professor at The University of Western Australia, Perth, Australia, and at Stellenbosch University, Stellenbosch, South Africa. His research investigates the ecological and evolutionary drivers of plant community patterns using phylogenetics, functional traits and vegetation mapping approaches.