New research helps us to better identify vulnerable plants

The UN projects that the majority of terrestrial species' will see their distribution range shrink drastically as a result of global changes, estimating that 1 million species are in danger of being extinguished over the next century. Factors determining species' range sizes are many. On a global scale, temperature is often used to study species' distribution patterns, in particular in plants.  More specifically, tropical plants are often thought to have narrower geographic distributions because of the small range of temperatures they are exposed to in the tropics. In contrast, temperate regions that expose plants to cold winters and a warm summers, would make species adapted to a broader range of conditions, consequently making their distribution range larger.

According to Arnaud Gallou, PhD candidate at the Department of Biological Sciences (BIO), University of Bergen and leading author of the new research paper Diurnal temperature range as a key predictor of plants’ elevation ranges globally in Nature Communications, there are a few flaws with this rationale.

"Firstly, previous hypotheses overlook avoidance strategies of plants in temperate regions, such as dormancy, as a way to escape winter temperatures. The influence of winter on plant range sizes may thus be overestimated. Secondly, and most importantly, these hypotheses neglect lethal extreme temperatures as a limit for species’ distribution ranges and that temperature variability differs from place to place. Plants growing in an area where temperatures vary greatly should be more likely to encounter temperatures that will kill them as they move to a different location with similar temperature regimes but slightly colder or warmer average temperatures", says Gallou.

Taking this into account, Gallou and colleagues, including John-Arvid Grytnes and Suzette Flantua at BIO, have proposed a new hypothesis suggesting that the distribution range of species should be smaller in places where temperatures vary more. The hypothesis is called ‘temperature range squeeze hypothesis’, because species’ distribution ranges would tend to squeeze as the range of temperature variation increases.

"The first study on the temperature range squeeze showed encouraging results when looking at the distribution range of plants in mountains globally. We found that species’ range sizes decreased toward regions exposed to greater variations in temperature, supporting the new hypothesis. The influence of temperature variation on species’ range sizes was greater when temperatures vary daily due to the impossibility of plant species to go dormant for a few hours only", says Gallou.

The study also includes an unexpected pattern that needs to be studied further. 

"In addition, we also found that large-range species were located in regions close to large bodies of water, such as the ocean or sea, where the high humidity of the air that results from evaporation likely buffers temperature variations. In contrast, small-range species were found in more inland regions and dry places. Further work needs to be done to confirm these results, but this is exciting because this study forces ecologists to rethink the way they view how species respond to temperature variations", says Gallou.

Contrary to previous ideas, it seems that the relationship between temperature variations and species’ distribution range follows an oceanic-continental gradient. 

"The implications are considerable, in particular for biogeographers who have been relying on previous hypotheses to study species dynamics and predict the response of plants to climate change. This research can help us recenter our efforts on plant communities in thermally variable places which might be much more vulnerable than previously thought. Identifying these populations and ecosystems is crucial so that stakeholders can take appropriate measures on time for effective conservation strategies", says Gallou.

The paper is a contribution to the i) Humans on Planet Earth - Long-term impacts on biosphere dynamics (HOPE) project (ERC funded), and ii) Past, Present, and Future of Alpine Biomes Worldwide (Trond Mohn Stiftelse and the University of Bergen start-up grant TMS 2022STG03) at the Department of Biological Sciences, UiB. The HOPE project is funded by a European Research Council Advanced Grant and the project’s goal is to addresses a critical question in Earth system science.