By Alannah Burrill
In June of 2022, I took a trip to Mount Mitchell—the highest peak in North Carolina and west of the Mississippi River. The mountain has a unique ecosystem because of its high elevation and it hosts many unique plant species. When I visited, I found an informational plaque that was probably a few decades old about the wildflowers that could be found there and I took note of their listed flowering times. Oddly, many of the wildflowers that I observed while hiking the mountain in mid-June were already in bloom, but weren’t supposed to be until July-August. I remember thinking that this was likely an effect of climate change that I was witnessing first hand, and I wondered how this phenomenon may affect the pollinators on the mountain, as well as what all this would mean for the survival of such beautiful and unique wildflower species.
Figure 1: Bombus occidentalis, a species of bee pollinating a flower. Photograph: Rich Hatfield. (Xerces 2020)
As we know, plants are an essential part of the existence of life on this planet. Angiosperms, or flowering vascular plants, are particularly important for human existence, and many of these plant species are used as sources of food, medicine, and materials that benefit us greatly. These flowering plants are dependent on pollination for reproduction, and in nature, there is often a close synchronous relationship between angiosperms and their pollinators. In other words, many pollinators hungrily emerge from hibernation at the same time that flowering plants are ready to be pollinated, but the time of emergence varies from species to species. Pollination benefits both parties involved; pollinators get a sweet snack, and plants are able to produce seed-bearing fruits that allow for reproduction.
Figure 2: A Western Monarch butterfly flying above milkweed flower. Flickr Creative
Commons Attribution 2.0 Generic; Xerces Society (2020)
That being said, with increasing global temperatures, plants around the world are starting to flower earlier than they have in the past (by a few days to a few months), and pollinators are struggling to keep up with this new pace. The question is: how will altered flowering times affect plants’ pollination patterns and reproductive success? An intriguing peer-reviewed study aims to answer this question. The paper is titled “Pollinator effectiveness varies with experimental shifts in flowering time” (but I will be mostly focusing on the topic of flowering time and less on pollinator effectiveness).
In this study, two perennial wildflower species native to Wisconsin were examined; Tradescantia ohiensis (common spiderwort) and Asclepias incarnata (swamp milkweed). Scientists experimentally manipulated the flowering times of the two plant species to bloom 1-2 weeks before they normally would. They did this by utilizing two separate greenhouses; a warmer one (around 75-82 degrees Fahrenheit during the day, 70 degrees at night) and a cooler one (around 64-70 degrees Fahrenheit during the day, 61 degrees at night). The warmer greenhouse consisted of an area with average light conditions as well as an area with supplemental light.
Figure 3. Tradescantia ohiensis (common spiderwort)
Gallice, G. (2022). Tradescantia ohiensis Flower petals. NC State Extension. photograph. https://plants.ces.ncsu.edu/plants/tradescantia-ohiensis/.
All of the plants started out in the cooler greenhouse, then they were moved to the warmer greenhouse in normal light conditions, then they were moved within the warmer greenhouse to the area with supplemental light. Different groups of plants were moved through this sequence at different times. This mimicked the beginning of springtime, with some plants experiencing that seasonal change at a natural rate and others experiencing it more rapidly than others. As the plants began to flower, they were moved out to a prairie field to be pollinated by native bees, butterflies, and other pollinators. Reproductive success was determined by the amount of seeds found in the fruit formed from a pollinated flower.
As you might predict, the plants that were moved to the warmer greenhouse with supplemental light first were the first to flower due to their faster rates of photosynthesis. These plants that flowered early were not visited by as many, or as effective pollinators. The study concluded that the seed count (and fecundity, or reproductive success) increased with later flowering times in both species and decreased with earlier flowering times, indicating that there are indeed costs to plant reproduction associated with increased temperatures. There were, however, external factors that influenced the seed count, including the behaviors and morphological features of pollinators. As the study concludes, different species of insects have different levels of pollination effectiveness. While a plant that flowers early may still be visited by pollinators, those pollinators may not be as effective as those that have historically visited the plant during its flowering time, leading to lower seed count in fruits—an indicator of lower fecundity.
Figure 4: Asclepias incarnata (swamp milkweed)
Makin, J. (2016). Asclepias incarnata. Lady Bird Johnson Wildflower Center. photograph, Langton
That being said, it’s not all doom and gloom for angiosperms and their pollinators! Some wildflower species could actually benefit from earlier flowering because more effective pollinators may visit plants that have historically been more frequently visited by smaller or less effective pollinators. This study didn’t go into great detail about this aspect, but I wonder if some species could even start producing more seeds than they have in the past, leading to higher reproductive success.
This is a topic that still calls for a lot more research to be conducted, but this study is a good starting point. I would be really interested to see studies like this conducted in different parts of the world including my hometowns, Charleston, SC and Asheville, NC, as well as places closer to the poles that are experiencing climate change effects more rapidly and dramatically than more temperate regions. This type of study could also be partaken on Mount Mitchell—a high-elevation ecosystem with abundant wildflowers that is clearly being affected by climatic changes. Since humans rely so heavily on plants for survival, better understanding of the effects of climate change on pollination and plant reproduction will be essential in our adaptation to a warmer planet.
For further information, I recommend visiting here and here.
Sources:
Rafferty, N. E., & Ives, A. R. (2012). Pollinator effectiveness varies with experimental shifts in flowering time. Ecology, 93(4), 803–814. https://doi.org/10.1890/11-0967.1 (paywalled)