The evolution of flowering phenology: an example from the wind-pollinated African Restionaceae

Abstract

Background and aims: Flowering phenology is arguably the most striking angiosperm phenophase. Although the response of species to climate change and the environmental correlates of the communities have received much attention, the interspecific evolution of flowering phenology has hardly been investigated. I explored this in the wind-pollinated dioecious Restionaceae (restios) of the hyperdiverse Cape flora, to disentangle the effects of phylogeny, traits, and biotic and abiotic environments on flowering time shifts.

Methods: I recorded the flowering times of 347 of the 351 species, mapped these over a 98 % complete phylogeny and inferred the evolutionary pattern and abiotic correlates of flowering time shifts. The patterns and biotic/abiotic correlates of restio community mean flowering time were explored using 934 plots.

Key results: Restios flower throughout the year, with large spring and smaller autumn peaks. Species flowering time is evolutionarily labile, poorly explained by either the environment or traits of the species, with half of all sister species allochronic. Community mean flowering time is related to elevation, temperature and rainfall.

Conclusions: Flowering time shifts may result from assortative mating and allochronic speciation, possibly leading to non-adaptive radiation. However, community mean flowering time may be environmentally selected. Diversification of flowering time may be non-adaptive, but species could be filtered through survival in suitable communities.

Keywords: Allochronic speciation; Cape flora; Restionaceae; flowering time; phenological niche; phenology.

Fig. 1.

(A) Geographical distribution of plots used to infer spatial, climatic and ecological drivers of flowering time. The plots are colour coded by community peak flowering month: red – January and February; blue – March to May; green – June to August; yellow – September to December. Many plots are spatially clustered; consequently the dots overlap. (B) Female Elegia mucronata, (C) male E. mucronata, (D) female Restio strobilifer, (E) male R. strobilifer.

Fig. 2.

Community peak flowering distributed through the year, with wetland (black) and well-drained (grey) plots separated. The error bars reflect 100 random samples from the plots with more than 1 month of community peak flowering; the monthly distribution differs significantly (ANOVA: d.f. = 1, F = 122.9, P < 0.001). The monthly distribution of community peak flowering also differs significantly between wetland and well-drained habitats (χ ² = 54.495, d.f. = 11, P < 0.001).

Fig. 3.

Restio community peak flowering, simplified to seasons (summer = December, January, February; autumn = March, April, May; winter = June, July, August; spring = September, October, November) plotted against the first three principal component axes, giving ranges, quartiles and medians.

Fig. 4.

Number of restio species flowering in each month, based on the full flowering range of each species.

Fig. 5.

Phylogeny of Restionaceae showing reconstruction of ancestral flowering times, colour coded by month, over the maximum clade credibility tree, using parsimony optimization with a stepmatrix that weights transitions dependent on the temporal separation between the months. The ancestral flowering time (brown) is optimized to be October; shifts to earlier flowering times are coloured yellow (September) and several shades of green (August to May), whereas red (November), black (December), white (January) and shades of blue (February to April) indicate shifts to later flowering times.