Hotter Summers Could Break Up Forest Neighbourhoods

An international team of scientists led by Radosław Puchałka at Nicolaus Copernicus University in Poland found that common forest floor flowers currently blooming across the continent may respond very differently to warming temperatures in the years ahead. The study, published in Science of the Total Environment, looked at four woodland favourites – Anemone nemorosa, Anemone ranunculoides, Convallaria majalis and Maianthemum bifolium. Even though these flowers share the cool, shady forest habitat and have overlapping ranges across Europe now, they might not in the future. The surprise finding suggests that small differences in the flowers’ climate needs and tolerances could make one species vanish from an area while another thrives.

Summer Rains Critical for Forest Flowers

The researchers focused on four flowers that often grow together in shady European woodlands and forests—Wood Anemone (Anemone nemorosa), Yellow Anemone (Anemone ranunculoides), Lily of the Valley (Convallaria majalis) and Common May Lily (Maianthemum bifolium). These perennial herbs sprout up early each spring to carpet the forest floor before the trees above leaf out. All four prefer moist, humus-rich soils and indirect sunlight filtering down through the canopy. These shared habitat preferences and life cycle traits lead botanists to expect the flowers would respond similarly to climate shifts.

Puchałka’s team uncovered that precipitation levels during the warmest quarter of the year were most important in determining where these flowers can thrive. This period of peak summer heat and humidity appears to be a make-or-break factor in suitable habitat. Even minor variations between species in the amount of moisture needed to survive and flower during the summer could cause substantial divergences in range shifts as precipitation patterns change alongside warming.

When the researchers modelled the flowers’ range shifts under climate change scenarios, they found that all four species are projected to experience significant range losses by 2080, especially under more pessimistic warming scenarios. Among the four, Common May Lily is expected to experience the most severe range contractions.

Modelling the Flowers’ Future

The researchers used sophisticated computer models to predict how the climatic ranges of these flowers might shift as the climate changes. Specifically, they used what ecologists call “species distribution models.” This approach looks at known locations where a species currently exists, along with the climate conditions in each of those places. The model then estimates where else the same climate conditions exist geographically to map out the species’ potential suitable habitat.

By feeding different global warming scenarios into these models, the scientists can forecast how the climate envelope where the flowers can thrive might shift across Europe over time. Even though the flowers grow in the same forests now, differences in their modelled future habitat suitability could reveal diverging responses to climate change.

Puchałka’s team compared two future time periods in their models – the 2041-2060 period and the 2061-2080 period. They also assessed a range of climate projections, from more optimistic to more pessimistic greenhouse gas scenarios. Comparing multiple future time frames across various possible warming trajectories allowed them to gauge the variability and consistency of the flowers’ predicted range shifts.

The four flowers examined in the study – Wood Anemone, Yellow Anemone, Lily of the Valley, and Common May Lily – are all currently widespread and abundant across the nemoral zone of Europe. This is the temperate forested region that stretches across much of continental Europe. The researchers focused on these particular species because they not only share this broad European nemoral range today, but also have comparable traits and habitat preferences. All four grow as herbaceous perennials on the shady forest floor and bloom in the spring.

The precarious future of forest flowers

The four species examined in the study all prefer the cool, moist conditions found in shady European woodlands and forests. However, the differences uncovered in how their climatically suitable ranges may shift and contract in the future could lead to less overlap and fewer regions hosting all four flowers together.

This could be especially true if hotter, drier summers start to impact species like the moisture-loving Common May Lily more severely. Areas becoming unfavourable for one flower but not another could essentially decouple their currently shared ranges.

These findings highlight how even plant species that are well-adapted to the same habitat today may not be immune to distributional shifts as climate change impacts increase. For the spring-blooming anemones, lilies, and may lilies carpeting Europe’s forest floors, maintaining diversity could require conservation efforts to preserve suitable climatic microrefugia across their ranges.

Puchałka and colleagues also note that the plants may need assistance escaping threats and colonising new territories. They write:

In the absence of adaptation to long-distance dispersion, the magnitude of range shifts in all climate change scenarios suggests that despite gaining new optimal areas for colonisation in the future, all of them will be losers of climate change. In addition, their migration may be limited by the lack of appropriate soils, which applies particularly to anemones, for which the acidic soils of boreal ecosystems will be unsuitable for colonisation. Without assisted migration, these species will be unable to saturate new niches. Together with predicted losses of the climatic optima in the southern parts of their ranges, this will contribute to a decline in biodiversity and changes in the functioning of European forests.

Puchałka et al. 2023.

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Puchałka, R., Paź-Dyderska, S., Dylewski, Ł., Czortek, P., Vítková, M., Sádlo, J., Klisz, M., Koniakin, S., Čarni, A., Rašomavičius, V., De Sanctis, M. and Dyderski, M.K. (2023) “Forest herb species with similar European geographic ranges may respond differently to climate change,” The Science of the Total Environment, 905(167303), p. 167303. Available at: https://doi.org/10.1016/j.scitotenv.2023.167303.

Cover: Maianthemum bifolium. Image: Canva.