Flowering time likely an important driver of speciation in South African Restionaceae

Flowering time in angiosperms is a complex phenomenon that is influenced by biotic and abiotic factors in the plant’s environment, such as rainfall, temperature, and pollinator life histories. Flowering must allow for enough time beforehand to accumulate resources, coincide with pollinator availability and still leave enough time for fruit and seeds to develop and be dispersed under favourable conditions. While some plant families flower earlier or later than others, a phenomenon known as phylogenetic temporal niche conservatism, it is unknown whether this holds true at the species level.

In a new study published in Annals of Botany, author H. Peter Linder tested this possibility by recording the flowering times of 347 species of Restionaceae (‘restios’) in the South African Cape flora region using his own observations over a period of 25 years and around 12,000 herbarium specimens. Linder then mapped them to a phylogeny, allowing him to investigate the patterns in flowering time, its relationship to evolution in the group, and the abiotic factors that may play a major role in determining it. Plots were also used to study the plants at the level of the community.

In contrast to most Cape species, which peak in spring and gradually taper until winter, the restios flowered throughout the year, with a major peak in spring and a smaller one in autumn. The lowest points were the hottest (January) and coldest (June) months of the year. On a community level, flowering time was influenced by temperature, water availability, and altitude. On a phylogenetic level, however, data aren’t well explained by either environmental factors or individual species’ traits.

Half of all sister-species had dissimilar flowering times, suggesting the trait is evolutionarily labile and likely an important driver of speciation in the group. “For wind pollinated taxa, flowering time shifts are one of the few ways to achieve reproductive isolation,” Linder writes, though he notes that range expansion after speciation could erase evidence of allopatry having been a factor as well. 

Linder explains that there are two distinct processes at work; one on the community level and another at the species level. “Most likely these different processes operate on different time scales, with flowering time evolution associated with speciation probably on a million year time-scale, whereas community assembly could operate on a much shorter decadal or century time-scale,” he writes. “Perhaps more important is that the contexts are totally different: flowering time evolution occurs in a phylogenetic framework, whereas community peak flowering [is] in an ecological-spatial framework. Linking these two frameworks to account for both ecological and phylogenetic results presents an interesting challenge.”