If a plant can’t germinate, it’s heading for extinction. For many tropical trees, conditions could soon become too hot to procreate.

LONDON, 14 July, 2020 – There could soon be real trouble for tropical trees and other plants. As global average temperatures rise, in response to ever more profligate use of fossil fuels, it may for some species become too hot to successfully germinate.

The foliage most at risk from this thermal barrier to reproduction is certain to be in the tropics, where tens of thousands of plant species have already adapted to very nearly the limits of their tolerance.

Australian researchers report in the journal Global Ecology and Biogeography that they looked at 9,737 records for 1,312 species worldwide from the Kew Gardens’ global germination database, to work out the temperature ranges that suit germination.

They report that the closer to the equator, the more the risk that by 2070 temperatures could rise high enough to exceed the ceiling below which germination is possible.

More than half of all the tropical seedlings tested – 79 out of 142 – would experience temperatures higher than the optimum for breeding. And 41 out of 190 would meet temperatures that would be higher than the maximum at which seeds would germinate.

Survival impossible

“These plants are more at risk because they are near their upper limits. So even a small increase in temperature from climate change could push them over the edge,” said Alexander Sentinella, of the University of New South Wales, who led the study.

“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.”

The world’s tropical forests are already in trouble. Altogether there could be three trillion trees on the planet, and humans are already removing 15 billion a year. The richest habitats are in the tropics.

There could be 40,000 species of tree that flourish in the equatorial forests, and half of these have already been pronounced threatened. The hot moist forests provide cover for myriad smaller shrubs and plants under the canopy, and they flourish alongside a mosaic of wetland and grassland habitats to support some of the richest biodiversity on the planet.

The forests and the plants in them absorb a high proportion of the extra carbon dioxide emitted from power station and vehicle exhausts, they serve as a sponge to store rainy season water, and they recycle the planet’s oxygen. They are under increasing stress from human exploitation and climate change.

“These plants are more at risk because they are near their upper limits. So even a small increase in temperature from climate change could push them over the edge”

Higher temperatures mean greater extremes of windstorm that can severely damage whole forests; higher temperatures mean more intense droughts and greater fire hazard; climate change has begun to alter the mix, variety and abundance of tree species both in the tropics and worldwide; and where they can, tropical species have already begun to colonise higher ground to stay within suitable temperature boundaries.

So the realisation that plant species – like animals on land and fish in the oceans – may be most vulnerable at a key moment of the life cycle is even more bad news. Many species will still be able to reproduce, but if they have already gone beyond the optimum for germination, then the success rate will be smaller.

The news on a global scale is more encouraging: the researchers found that 95% of species at latitudes higher than 45° could actually benefit from global warming, because temperatures could shift more closely to the optimum for many temperate and cool zone plants. And some plant species may evolve as temperatures rise. But many will not adapt in time to rapidly-rising global temperatures.

“There are almost 400,000 plant species worldwide – so we would expect a number of them to fail to germinate between now and 2070,” Sentinella said.

“Humans have known about the dangers of climate change for decades, and we already have the answers to tackle it. Hopefully, our study will encourage people and policymakers to take action now.” – Climate News Network