Journal Article

Exploring rock fissures: does a specialized root morphology explain endemism on granite outcrops?

Pieter Poot, Stephen D. Hopper and Josepha M.H. van Diggelen

in Annals of Botany

Published on behalf of The Annals of Botany Company

Volume 110, issue 2, pages 291-300
Published in print July 2012 | ISSN: 0305-7364
Published online January 2012 | e-ISSN: 1095-8290 | DOI: http://dx.doi.org/10.1093/aob/mcr322
Exploring rock fissures: does a specialized root morphology explain endemism on granite outcrops?

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Background and Aims

Worldwide, many plant species are confined to open, shallow-soil, rocky habitats. Although several hypotheses have been proposed to explain this habitat specificity, none has been convincing. We suggest that the high level of endemism on shallow soils is related to the edaphic specialization needed to survive in these often extremely drought-prone habitats. Previous research has shown that species endemic to ironstone communities in SW Australia have a specialized root morphology that enhances their chance to access fissures in the underlying rock. Here we test the generality of these findings for species that are confined to a shallow-soil habitat that is of much greater global significance: granite outcrops.

Methods

We compared temporal and spatial root growth and allocation of three endemic woody perennials of SW Australian granite outcrop communities with those of congeners occurring on nearby deeper soils. Seedlings of all species were grown in 1·2 m long custom-made containers with a transparent bottom that allowed monitoring of root growth over time.

Key Results

The granite outcrop endemics mostly differed in a predictable way from their congeners from deeper soils. They generally invested a larger portion of their biomass in roots, distributed their roots faster and more evenly over the container and had a lower specific root length. In different species pairs the outcrop endemics achieved their apparent advantage by a different combination of the aforementioned traits.

Conclusions

Our results are consistent with earlier work, indicating that species restricted to different types of drought-prone shallow-soil communities have undergone similar selection pressures. Although adaptive in their own habitat in terms of obtaining access to fissures in the underlying rock, these root system traits are likely to be maladaptive in deeper soil habitats. Therefore, our results may provide an explanation for the narrow endemism of many shallow-soil endemics.

Keywords: Drought; ecophysiology; endemism; granite outcrops; Mediterranean climate; rock fissures; root foraging; root system architecture; shallow soils; water availability; woody perennials; Allocasuarina huegeliana; A. humilis; Calothamnus rupestris; C. quadrifidus; Eucalyptus caesia; E. drummondii

Journal Article.  6490 words.  Illustrated.

Subjects: Ecology and Conservation ; Evolutionary Biology ; Plant Sciences and Forestry

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