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Climate change could put tropical plant germination at risk: Study

  • Under a worst-case climate change scenario, more than 20% of plant species in the tropics may experience temperatures too high for their seeds to germinate by 2070, according to an analysis of seed germination data compiled by the UK’s Royal Botanic Gardens Kew.
  • Under that same worst-case scenario, over half of tropical species may see reduced rates of germination by 2070 as well, the study reports, while many temperate species at high latitudes will move closer to their optimum temperature and may experience increased germination success as a result.
  • The analysis shows that 26% of tropical species and 10% of temperate species are already experiencing temperatures above their optimum. Some plants are found living at sites where temperatures are already above their maximum, suggesting that their lineage in that location may be effectively extinct.
  • Plants that find themselves outside of optimum or tolerable temperature ranges may be able to migrate to higher latitudes or altitudes, and existing diversity can offer a reservoir of genetic variation for species to adapt, but physiological limits and long generation times may mean even diverse species struggle.
The germination of plant seeds is already being adversely affected by climate change, an impact expected to worsen throughout the 21st century. Image by Susan Everingham courtesy of Alexander Sentinella.

Tropical plants are at greater risk as the climate warms because soil temperatures may become too high for them to germinate, according to an analysis of over 10,000 laboratory studies recorded in the Millennium Seed Bank Partnership Data Warehouse. Researchers found a higher risk of germination failure for species living closer to the equator.

The study, published in the journal Global Ecology and Biogeography, used data from seed germination experiments compiled by the Royal Botanic Gardens Kew to calculate germination tolerance characteristics such as maximum, minimum, and optimum temperature for over 1,300 plant species, spanning every continent except Antarctica. They found that many species — particularly those in the tropics — are already living outside their optima, and likely suffering reduced germination rates.

The researchers combined these results with climate modelling to predict the risk of germination failure by 2070. “Over 20% of plant species in the tropics may face temperatures higher than their maximum germination temperature … [and] over half of tropical species may have reduced rates of germination,” warned study author Alexander Sentinella, a PhD student at the University of New South Wales in Sydney, Australia. By comparison, just 8% of species outside the tropics would exceed their maximum germination temperature in the next 50 years under the model projections.

Species living in high latitudes tend to be nearer their lower temperature limit, meaning that climate change may actually move them closer to their optima. “For species at higher latitudes, increased temperatures may increase rates of germination,” Sentinella explained.

Lauren Buckley, an evolutionary ecologist at the University of Washington who was not involved in the study, described the results as exciting, saying that the study “fills a data gap to show that tropical plants may [like animals] be more at risk with warming.”

“Early life stages are often particularly temperature sensitive, so germination is a good place to start when examining sensitivity to climate change,” she added.

Illustration from an 1860 botanical text book of complex plant structure. As climate change escalates, temperatures will alter optimal sites for plants, meaning that what grew at one location as drawn in this 1860 textbook, may already not be there, or could be gone from that place by 2070. Image in the Public Domain.

Plants that find themselves outside of optimum or tolerable temperature ranges may be able to migrate to higher latitudes or altitudes to avoid scorching temperatures. “The study highlights the need to keep tropical habitats intact so organisms can find thermal refuges and shift their distributions though climate warming,” said Buckley. Fragmentation of tropical biomes resulting from rampant deforestation, agribusiness, mining, and infrastructure construction could significantly reduce the potential for successful plant migration.

Existing diversity within species can offer a reservoir of genetic variation, providing resilience that may be crucial for species to adapt to climatic changes, says Sentinella, making it all the more important to conserve existing tropical ecosystems. However, physiological limits and long generation times may mean even diverse species struggle to adapt.

In fact, the study showed that some species have already exceeded their maximum germination temperature, suggesting that they may be in “extinction debt.” Long-lived individuals growing at sites where their seeds are no longer able to germinate will see their lineage at that location go extinct. The extent to which such local extinctions might disrupt tropical ecosystems was beyond the scope of the study.

Germination marks the first potential point of failure for a plant, and it is also comparatively easy to study. But global research into the impacts of climate at other stages in the lifecycle have been limited due to the challenges involved.

“Even if a seed germinates, it has to survive the seedling stage, then establish itself as a mature plant. These later stages are much harder to measure, but are crucial for understanding exactly how ecosystems will respond to climate change,” said Sentinella.

However, studies to date do suggest that climate change may be pushing plants to their physiological limits. For example, a 2018 study found that in both temperate and tropical forest canopies, taller trees are regularly experiencing temperatures high enough to hamper photosynthesis. Additional threats faced by tropical ecosystems, including deforestation and wildfires, can also increase soil temperatures, potentially exacerbating the negative heat impacts on 21st Century forests and savannas.

Seed germination experiments can provide data on maximum, minimum, or optimum germination temperatures and are comparatively quick and easy to carry out. Image by Susan Everingham courtesy of Alexander Sentinella.

The research team used data from the World Climate Research Programme’s Coupled Model Intercomparison Project Phase 5 (CMIP5) climate model under a high emission scenario, making these predictions a worst-case scenario for what plants may experience by 2070.

If we take action now to mitigate future climate change, says Sentinella, “the reality will not be as dire as our study predicts.”

Scientists have hypothesised for 50 years that plants and animals living in tropical climates — where annual variations in temperature are relatively small — would have evolved to tolerate a narrower range of temperatures compared to temperate species, which have to cope with large seasonal temperature variations.

Evidence of this temperature vulnerability relationship between regions has so far been largely supported by animal studies. But many researchers expect that tropical plant species will be similarly more vulnerable than temperate ones, because even moderate increases in soil and air temperatures can push them out of their tolerance range.

The new study is the first global test of this climate variability hypothesis for temperature tolerance in the plant kingdom. But contrary to that hypothesis, the results show that plant seeds of 443 different species in both tropical and temperate zones can tolerate a similar breadth of temperatures — roughly 15 degrees Celsius — albeit centred around different optimum temperatures.

“Tropical species are at a higher risk from warming [but] it’s not because they have narrower tolerances — they’re just closer to their upper limit,” Sentinella explained. A latitudinal gradient in temperature tolerances is a central assumption used in many other ecological theories, so this result “may have implications for patterns of evolution and latitudinal gradients in biodiversity,” said Buckley.

Citation: 

Sentinella AT, Warton DI, Sherwin WB, Offord CA, Moles AT. Tropical plants do not have narrower temperature tolerances, but are more at risk from warming because they are close to their upper thermal limits. Global Ecol Biogeogr. 2020;00:1–12.

Banner image: The germination of plant seeds is already being impacted by rising soil temperatures. Image by morningchores.com found on Pinterest.