Experimental manipulation of flowering phenology alters reproductive success in a bumblebee-pollinated species
Earlier, warmer springs are changing the timing of plant reproduction and pollinator emergence. As plant and pollinator species respond in different ways to climate change, historic plant-pollinator mutualisms may be disrupted, potentially altering community composition and ecosystem functioning. In order to determine the extent to which phenological mismatches between plants and pollinators impact plant reproduction, we experimentally manipulated flowering phenology of the tall-fringed bluebell, Mertensia ciliata(Boraginaceae). This herbaceous perennial is native to the Rocky and Sierra Nevada Mountains, is pollinated by multiple generalist bumblebee species, and has been shown to advance its flowering phenology in years with early snowmelt and warmer springs.
By manipulating temperature and light, plants were induced to flower at different times (early, control, or late) and then placed in the field. We observed pollinator visits (pollinator identity, visit length, and number of flowers visited) to assess whether phenology mismatches occurred. Seed set data was collected for individual plants in each group at the end of the summer to assess reproductive success. Sites were located at the Rocky Mountain Biological Laboratory (RMBL) in Gothic, Gunnison County, Colorado, USA (latitude: 38°57’29” N, longitude: 106°59’06” W, altitude: 2,900 m a.s.l.).
We found that early-flowering plants received fewer pollinator visits than late-flowering plants. Moreover, the proportion of open flowers visited by pollinators more than doubled in late-flowering plants as compared to early-flowering plants. Yet despite receiving fewer visits, early-flowering plants produced more seeds than late-flowering plants. This higher seed set in early-flowering plants may be due to changes in pollinator identity, and thus pollinator effectiveness, or due to changes in resources (although water was held constant in this experiment). Interestingly, the seeds of late-flowering plants, while fewer in number, tended to have a greater mass, potentially due to a higher rate of outcrossing in late bloomers, or an increase in resource availability later in the summer.
To date, there have been few field studies which have directly tested the disruptive effects on plant reproduction associated with temporal mismatches between plant and pollinator phenologies. While previous studies have shown that specialist species may be at higher risk of loosing pollinator services due to phenological shifts , our research demonstrates that temporal mismatches between plant and pollinator phenologies are likely to also have important impacts on generalist species as well.