A mismatch between germination requirements and environmental conditions: Niche conservatism in xeric subtropical thicket canopy species?

• The subtropical thicket currently occupies a predominantly semi-arid climate.
• Thicket lineages arose during past wetter and warmer climates.
• We examine the moisture requirements for germination of dominant canopy species.
• The majority of species had high moisture requirements for germination.
• We suggest that this is due to niche conservatism.

Seedlings of woody canopy species in the xeric forms of subtropical thicket are rare. Here we argue that the low number of seedlings is a consequence of niche conservatism where thicket species have retained germination and seedling establishment requirements associated with their ancestral origins in the warm, wet forests of the early Cenozoic. We test this hypothesis by evaluating the germination success of 12 arid and valley thicket species – representing a range of growth forms and dispersal modes – using a factorial germination experiment that sought to simulate permanently moist and deeply-shaded conditions (i.e. a wet forest environment) and sparsely-shaded and intermittently dry conditions (i.e. open microsites during the rainy season) in contemporary thicket environments. Germination success was significantly higher under the more mesic soil-moisture conditions for all species except Pappea capensis and Jatropha capensis. These results suggest that germination of thicket species requires long periods of high soil moisture supporting the niche conservatism hypothesis.