Climate trouble in the tropics

Tropical plants close to the equator are most at risk from climate change because it will likely become too hot for many species to germinate, new research suggests.

Conventional wisdom has been that tropical species are at risk because they are used to – and can only cope with – a narrow temperature range.

However, a study by Australia’s University of NSW shows the greatest threat comes from already being close to their upper thermal limits.

“The figures are quite shocking because by 2070 more than 20% of tropical plant species, we predict, will face temperatures above their upper limit, which means they won’t germinate, and so can’t survive,” says Alex Sentinella, lead author of a paper in the journal Global Ecology and Biogeography.

In contrast, the researchers found that 95% of plant species at latitudes above 45 degrees are likely to benefit from warming, at least in relation to germination, because temperatures are expected to shift closer to what is optimal.

The study analysed 9737 records for 1312 species held in the Millennium Seed Bank at London’s Kew Royal Botanic Gardens. They included plants from every continent except Antarctica, excluding agricultural crops.

Seeds were used, Sentinella says, because “you can experiment on them quickly, there are a lot of studies about them and, importantly, germination directly relates to how a species will survive”.

He and colleagues also examined climate data for the relevant locations, using the average temperature of the warmest three months from 1970 to 2000 to predict temperatures for 2070.

They found that more than half of tropical species are expected to experience temperatures exceeding their optimum germination temperatures. Those that can survive will do so at a reduced rate of germination, and are likely to be less successful.

However, the big surprise, Sentinella says, was that the hypothesis often used for animals – that those near the equator will struggle to survive the impact of climate change because they have narrower temperature tolerances – is not true for plants.

“We found that regardless of latitude, plant species can germinate at roughly the same breadth of temperatures, which does not align with the animal studies.”

Sentinella says it is possible some plants could slowly adapt and evolve to increasing temperatures, but it is difficult to predict which ones would survive.

“Sometimes plants can migrate by starting to grow further away from the equator or, up a mountain slope where it’s cooler. But if a species can’t do that it will become extinct.”