Pollination in a drier world: Even moderate drought alters floral scent

Coline Jaworski discusses her recent paper: Long-term experimental drought alters floral scent and pollinator visits in a Mediterranean plant community despite overall limited impacts on plant phenotype and reproduction. Find out more about how drought can affect floral scent and the implications of climate change for pollination.

Everyday, climate change is more present and dramatic in our daily lives, and 2022 has been no exception: heatwaves, drought, floods. Climate change also affects animals and plants, but, more importantly, it can have dramatic impacts on species interactions, which are the foundation of ecosystem functioning. Pollination, a key ecosystem function and indispensable for the reproduction of the vast majority of flowering plants (and therefore, for the production of primary biomass), relies on pollinators’ capacity to locate flowers in which they will collect floral resources (pollen and nectar).

Pollinators such as bees use both visual cues (flower colour and size) and olfactory cues: floral scent. Floral scent is a complex blend of dozens of volatiles and is very sensitive to environmental stress. As such, plant defence activation as a response to stress will cause the emissions of specific volatiles that mix up in floral scent. In this study, we investigated how long-term drought, a major predicted effect of climate change, affects floral scent and the consequences for plant-pollinator interactions in a shrubland, one of the most widespread Mediterranean habitats. We found a consistent alteration of floral scent in all species studied with increased emissions of volatiles related to stress. This led to modifications of pollinator visits.

Managed honeybees preferred to visit non-stressed plants, but their reduced visit frequency to drought-stressed plants was compensated by more visits by small wild bees. Changes in visits may be linked to modifications of floral resources. We did not find differences in the abundance of floral resources of the dominant species, but nectar production in thyme (a rare species at our study site) was divided by almost four! Also, drought may have changed the quality of floral resources (such as the sugar composition in nectar). Managed honeybees were by far the most abundant pollinator on site, both by number of individuals and by number of visits. Because social bees, like managed honeybees, are adapted to track floral resources most efficiently, they might be more efficient at locating the best resources possibly from non-stressed plants here.

In the Mediterranean Basin, climate change is causing an intensification of drought intensity and frequency. Our community-level in situ measures showed that drought altered floral scent with changes in plant-pollinator interactions and limited impacts on plant reproduction. We discuss why the context of our study – an exceptionally wet year – may have led to limited effects of drought. (A) Sampling of floral scent in the study site equipped with rainfall exclusion plots and control plots. (B,C) Two wild bee species of small body size and (D) Apis mellifera, the managed honeybee, which has a large body size, collecting floral resources on Salvia rosmarinus.

Despite the measured changes in flower scent and in pollinator visits, we measured limited impacts of drought on plant reproduction. This may be a consequence of the rather moderate drought we measured during the field of the study. If even moderate climate change such as tested here can alter floral scent plant-pollinator interactions, it is likely that the current climatic extreme events may have a profound impact on pollination and terrestrial ecosystem functioning. We need more research to predict impacts of these drastic events, and to evidence possible mitigation strategies. In particular, we only looked at one demonstration of climate change, drought. However, climate change simultaneously affects precipitation, temperatures and atmospheric pollution; the simultaneous effects on bee pollinators and pollination functions may therefore be much worse than predicted.

Coline Jaworski University of Cambridge

Read the full paper online: Long-term experimental drought alters floral scent and pollinator visits in a Mediterranean plant community despite overall limited impacts on plant phenotype and reproduction.

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