Revegetation of urban habitats with regionally native plant species is a commonly employed strategy to enhance ecosystem services in cities. However, little is known about how planted species will integrate into the pollination networks of already established spontaneous plant communities. As new “invaders” into disturbed urban habitats, restored natives may experience pollen limitation and may also lack evolved mechanisms to mitigate the deposition of heterospecific pollen from co-flowering urban plants, potentially jeopardizing the establishment of revegetation projects.
We documented patterns of pollen transfer in both planted native species and spontaneously growing species found in urban vacant lots in the city of Baltimore. A subset of the vacant lots was ‘restored’ by clearing and seeding with native plant seed mixes in 2014. These sites were mowed annually but not weeded. Additionally, nearby vacant lots, experiencing sporadic mowing by the city but otherwise unmanaged, were monitored. Inflorescences from both planted and spontaneous species were collected in 4 restored lots and 4 unmanaged lots in 2016, in both June and August. To determine how pollination quantity and quality differed between planted and spontaneous species and between site types (restored and unrestored), we acetolyzed stigmas, counted and identified pollen grains on each species.
Most spontaneous urban species received less conspecific pollen on restored sites that included co-flowering planted species, compared to unrestored sites. Spontaneous species generally received low proportions of heterospecific pollen across both site types, but there was substantial variation across species. Restored species tended to receive lower levels of conspecific pollen and higher levels of heterospecific pollen compared to spontaneous urban species. We observed pollen transfer between planted and spontaneous species, but only one planted species commonly donated pollen to other co-flowering species, whereas multiple spontaneous species acted as common pollen donors in the pollen transfer network. These results suggest that common spontaneous urban plant species may be preadapted to avoid heterospecific pollen receipt or to self-pollinate. Additionally, this suggests that pollen transfer from co-flowering spontaneous urban plants has the potential to negatively impact seed production of native species introduced to restore urban habitats.