Are bluebells too slow for climate change?

Slow demography and colonization rates 17,500 times lower than the current velocity of climate change make range shifts virtually impossible in the emblematic forest plant Bluebell.

Above: The Hallerbos in Belgium is nicknamed ‘the blue forest’ because of the carpets of spring-flowering bluebells (Hyacinthoides non-scripta), which attract yearly more than 100,000 visitors. (© Sanne Govaert).

Climate change causes many species to shift their distributions towards higher elevations and latitudes to track their optimal climatic conditions. The current rate of change in the climate system, however, is high. Possibly too high for many species, such that the optimal climate conditions are moving faster than the rate at which a species can disperse. If you know that the distribution limits of many plant species native to temperature forests of Europe are still singed by dispersal limitation since last glacial maximum c. 11,7000 year ago, can we actually expect these species to be able to keep pace with the current rate of climate change?

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Sanczuk, P., De Lombaerde, E., Haesen, S., Van Meerbeek, K., Van der Veken, B., Hermy, M., Verheyen, K., Vangansbeke, P. & De Frenne, P. (2022). Species distribution models and a 60-year-old transplant experiment reveal inhibited forest plant range shifts under climate change. Journal of Biogeography, 49, xxx–xxx. 

To answer this question, I revisited, to my knowledge, one of the longest running transplant experiments in the world: It was 1960 when the pioneer in Belgian forest ecology, Jules Register, decided to transplant bluebells (Hyacinthoides non-scripta) from a natural population within its range to several forest sites beyond its range in Belgium. Bluebell is an emblematic species growing in deciduous forests of Europe, and probably among the most famous forest herb in Europe for laymen. The species has its emblematic status due to its pale-blue flowers: in optimal conditions, bluebells form continuous carpets covering the understorey layer, resulting in magnificent views in Spring. For this reason, the Hallerbos in Belgium is nicknamed ‘the blue forest’ and attracts yearly more than 100,000 visitors during its flowering time. However, as it is true for many understorey plant species, bluebell has long life cycles and seed dispersal is extremely limited in space. It can take 5 to 10 year for bluebell seedlings to reach a reproductive state, and previous estimates of annual colonization rates in natural populations are low, varying between 100 cm to only 0.6 cm per year. With such slow colonization rates, it is highly questionable if bluebell will be able to track the 21st century climate change.

It was only 45 years after the installation of the transplant experiment that a first evaluation was conducted. The performance of the source (within bluebell’s natural range) and transplanted (beyond bluebell’s natural range) populations was measured to understand the main limiting factors of the species’ distribution. Recently, I revisited the experiment. I asked “couldn’t we gain new insights if we combine present-day modelling techniques with data from this extremely old experiment?”

Hence, in Spring 2020, I relocated all populations and performed the same measurements. Unfortunately, when pairing my data to the previous data, I found clear signals that the population performance has decreased: in both the source and transplanted populations, individual plant performance and also the population growth rates were lower in 2020 compared to the previous survey in 2005. To interpret these negative trends within a larger spatial context, I built species distribution models. These models indeed confirmed a gradual decrease of the habitat suitability within large parts of the species distribution under climate change. Thus, the decrease predicted by the models has likely already started in the study populations.

Two of the transplanted populations in 2020. Several traits were measured on 10 flowering individuals within each population. (© Pieter Sanczuk)

Moreover, based on the colonization distance of the main dense population front since 1960, I estimated that the average colonization rate is only 0.02 m per year. Currently, this is 17,500 times lower than the velocity of climate change (the isotherms in temperate broadleaf and mixed forests are shifting at a rate of 350 m per year). Especially in the highly fragmented landscape of north-west Europe, such low colonization rates make range shifts that are fast enough to track the shifting climate, virtually impossible. In essence, bluebell’s climatic envelope is currently running away from its natural distribution.

Where is the good news? Owing to the high structural complexity of forest canopies, temperature extremes experienced by organisms living in the shade of trees can be buffered. For instance, forest-floor maximum temperatures are on average 4.1 °C cooler compared to free-air measurements. This is a larger difference in temperature compared to the projected increase by the end of the 21st century due to climate change. Moreover, forest floor temperatures are even cooler in structurally complex forests with a closed canopy (up to 8.3 °C during warm summer days). If we optimize forest management towards cool and dark forest understorey conditions, we can make an important step forward to maintain bluebell in ancient deciduous forests across their entire range.

Written by:
Pieter Sanczuk, PhD candidate, Forest & Nature Lab, Department of Environment, Faculty of Bioscience Engineering, Ghent University, Belgium

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