Urbanised landscape and microhabitat differences can influence flowering phenology and synchrony in an annual herb

1. Flowering phenology, a crucial determinant of plant reproductive success and biotic interactions, is susceptible to urbanisation. Numerous studies have shown the impact of urbanised landscapes on flowering phenology based on comparisons along urban–rural gradients. Phenological patterns among microenvironments in the urban ecosystem have received less attention, although they often offer unique habitats with varying artificial influences, such as roadsides, drainage ditches and vacant lots. If differences in microenvironments diversify flowering phenology, the urban matrix might reduce flowering synchrony with neighbouring populations, limiting outcrossing opportunities and therefore reducing reproductive success.
2. We investigated the flowering phenology and synchrony of the native annual herb Commelina communis in approximately 250 populations at two rural and two urban sites over 3 years. To determine the effect of microhabitat differences, we categorised the microhabitats of C. communis populations into five types: drains, roadsides, vacant land, farmland and forest edge. In some study populations, we investigated reproductive success (seed set) to estimate the degree of outcross pollination limitation.
3. Our findings revealed that populations in urban sites exhibited earlier flowering onset and longer flowering duration compared to rural locations. Besides, we did not detect consistent patterns of flowering onset, peak and duration among the different microhabitat types. For flowering synchrony, we found that the population in urban sites, growing in drain habitats, and with artificial disturbances exhibited relatively lower interpopulation flowering synchrony, suggesting their phenology differed from neighbouring populations within the same landscape. Additionally, populations in urban sites, especially those growing in drain and roadside habitats, suffered severe outcross pollen limitation compared to those in rural landscapes.
4. Synthesis and applications. In conclusion, our results indicate that in addition to landscape changes associated with urbanisation, variations in local microhabitats also influence the flowering phenology and synchrony of C. communis populations. Urbanised landscapes and differences in microhabitats could contribute to the diversification of phenological patterns between populations, potentially having a negative impact on the reproductive success of native plant species. These findings highlight the need to consider not only spatial but also temporal fragmentation from diversified flowering phenology when addressing conservation in the urban matrix.