Synchronous pulsed flowering: analysis of the flowering phenology in Juncus (Juncaceae)

The timing of flowering within and among individuals is of fundamental biological importance because of its influence on total seed production and, ultimately, fitness. Traditional descriptive parameters of flowering phenology focus on onset and duration of flowering and on synchrony among individuals. These parameters do not adequately account for variability in flowering across the flowering duration at individual and population level. This study aims to analyse the flowering phenology of wind-pollinated Juncus species that has been described as temporally highly variable ('pulsed flowering'). Additionally, an attempt is made to identify proximate environmental factors that may cue the flowering, and ultimate causes for the flowering patterns are discussed.

Flowering phenology was examined in populations of nine Juncus species by estimating flowering synchrony and by using the coefficient of variation (CV) to describe the temporal variation in flowering on individual and population levels. Phenologies were compared with null models to test which patterns deviate from random flowering. All parameters assessed were compared with each other and the performance of the parameters in response to randomization and varying synchrony was evaluated using a model population. Flowering patterns were correlated with temperature and humidity.

Most flowering patterns of Juncus were best described as synchronous pulsed flowering, characterized as population-wide concerted flowering events separated by days with no or few open flowers. Flowering synchrony and variability differed from a random pattern in most cases. CV values in combination with a measure of synchrony differentiated among flowering patterns found. Synchrony varied among species and was independent from variability in flowering. Neither temperature nor humidity could be determined as potential cues for the flowering pulses.

The results indicate that selection may act independently on synchrony and variability. We propose that synchronous pulsed flowering in Juncus is an evolved strategy that provides selective benefits by increasing outcrossing and by spreading the risk of reproductive failure.