Abstract
Aim
It is usually assumed that low biodiversity of arid regions is due to the inability of most lineages to adapt to their low/unreliable rainfall. However, species requiring fire-stimulated seed-dormancy release are also at a fitness disadvantage in deserts that rarely burn. We hypothesized that a hard-seeded, temperate-climate genus is not absent from Australia's deserts because it is insufficiently adapted to aridity but that the dearth of fire means its germination requirements cannot be met.
Location
Earth, mainly Australia.
Taxa
Many fire-prone and fire-free clades, with Cryptandra (Rhamnaceae) as a case study.
Methods
We document clades with/lacking fire-stimulated-seed-dormancy release and compare their occurrences under different climates and degrees of fire exposure. We compare drought-related traits of Cryptandra species in southwestern Australia with matched species in Central Australia. Grid cells were overlain onto maps to document co-occurrences of Cryptandras and climate type/vegetation cover/rainfall reliability and fire activity, and a distribution model was created. The phylogenetic signal was quantified, and ancestral trait reconstruction was undertaken for that part of the Rhamnaceae bearing Cryptandras.
Results
Species fire-proneness peaks at intermediate levels of aridity. A sample of eight worldwide lineages is drought-adapted but not fire-dependent and thus colonises arid regions; 23 clades with fire-stimulated-seed-dormancy release possess species that no longer require fire and have migrated into arid regions; and eight clades, where non-fire-stimulated-seed-dormancy release has not evolved, remain in fire-prone regions, despite apparent arid-tolerant traits. Leaf/stem/habitat traits of selected Cryptandras appear as drought-tolerant as matched species in arid Australia. Abundance of fire-dependent Cryptandras in Western Australia is greatest in the well-vegetated zones with a mediterranean climate and rare/absent in the fire-free deserts.
Conclusions
Many xeromorphic clades with fire-stimulated-seed-dormancy release in temperate/(sub)tropical regions possess recent sister lineages with non-fire-stimulated-seed-dormancy release in arid regions, whereas others appear incapable of evolving lineages that no longer benefit from fire. As drought-adapted, hard-seeded Cryptandras and other Rhamnaceae universally require occasional fires to break seed dormancy, this may explain their absence in rarely burnt deserts that cannot satisfy this requirement.