Pollinator dependence but no pollen limitation for eight plants occurring north of the Arctic Circle

Written by Viviane Koch, Leana Zoller, Joanne M. Bennett & Tiffany M. Knight. Read the full paper here.

We arrive amid a wooded surrounding – far away from intensive agricultural and anthropogenically affected land. Our landscape is characterised by green forests, bold reindeer, and harsh environmental conditions. Here, above the Arctic Circle in the Finnish part of Lapland, weather conditions can rapidly change, affecting the behaviour and mode of life of many species and consequently their interactions. Many of the plants are expected to reproduce auto-fertile, as not to be dependent on animal pollinators or wind in such rough climatic conditions. But are they? We observe numerous pollinators heading towards different plant species. For the plants that do rely on animal pollinators, do they receive adequate pollination service in this arctic location? These were the questions my colleagues and I strived to answer.

For my Master Thesis, I travelled to this remote location to study these questions. Leana Zoller (PhD student) and I measured pollinator dependence by comparing natural pollinated plants with those excluded from pollinator visits with fine-meshed bags. If bagged plants produced fewer seeds, plants were declared as pollinator dependent.

Lapland, Finland – north of the Arctic Circle (credit: Viviane Koch)

The plants that can´t set any seed without pollinators are the most pollinator dependent ones. These are also the ones, which should be the most affected by pollen limitation – reduced reproduction due to insufficient receipt of pollen by pollinators. Pollen limitation is measured by comparing naturally pollinated plants with plants that are additionally pollinated by using a hand-brush with external pollen. If the plants with additional pollen produce a higher number of seeds compared to the only naturally pollinated ones, plants experience pollen limitation. According to our colleagues, Drs. Joanne Bennett and Tiffany Knight, pollen limitation experiments have been conducted worldwide, but are still rare in the Arctic. Therefore, our research fills the gap for Arctic regions.

Measure of pollinator dependence and pollen limitation: bagged treatment, natural treatment, supplemental treatment (Credit: Viviane Koch)

In the context of climate change, the Arctic is experiencing a rapid change in climatic conditions which in turn could rapidly change the pollination services plants receive. We expected these harsh environmental conditions might cause low pollinator abundance or insufficient pollination service, leading us to find pollen limitation for our pollinator-dependent plant species.

Leana, a bumblebee and I at work (Credit: Leana Zoller, Viviane Koch)

Surprisingly, all plant species were pollinator dependent but not pollen limited. In contrast to past studies, we found that none of our focal plant species could fully reproduce in the absence of pollinators. Further, none of these pollinator-dependent plant species were pollen limited, suggesting that they receive sufficient amount of pollen despite the harsh environmental conditions. Therefore, our results suggest that other mechanisms must counteract these changes to maintain successful pollinator-plant interactions and reproduction at this high latitude site. One factor could be cold temperatures that may constrain plant reproductive success more than pollen availability, by for example limiting photosynthetic activity. Second, many of our plant species are phenotypically and ecologically generalised in their pollination. This buffers plants from an absence or insufficient services of any one pollinator. Our more specialised plant species may avoid pollen limitation by providing high rewards. For example, our focal plant species Dianthus superbus is nectar rewarded and is thereby highly attractive to pollinators. Third, migrating pollinators towards higher latitudes in response to increasing temperatures at lower latitudes might act as a new interaction partner with the Arctic plants and buffer them from pollen limitation. Finally, climate change might be less detrimental to pollination than land use change. However, at our field site, there has been little land use change.

Dianthus superbus (Credit: Viviane Koch)

Our findings provide a baseline to examine how pollen limitation of our focal plant species might change across space and time with land use and climate change. There is a need for future studies that quantify the identity and abundance of visiting pollinators and pollination services to plants to understand the causes and consequences of pollen limitation or lack thereof.

Our study provides new data of pollinator dependence and first experimental data of pollen limitation for many plant species and the first experimental data for the plant family Menyanthaceae. Therefore, our data contributes to the global knowledge on plant reproduction and plant-pollinator interactions. And above all, we found some answers to our questions in this remote area of Finnish Lapland.

Little land-use change in Kittilä, Lapland, Finland. (Credit: Viviane Koch)

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