Quiver trees must get on the move to escape human-caused climate change

‘If you want to know the future, look at the past,” Albert Einstein famously said. That’s exactly what a team of ecologists at Stellenbosch University did to try to understand and predict the response of the quiver tree to ongoing and future climate change.

The researchers went 22 000 years into the past and projected 50 years into the future to simulate the paleo and future geographic range of the rare desert succulent, which is found in arid South Africa and Namibia and seen as a sentinel of human-caused climate change.

The researchers describe how the “link between past, present, and future may be extremely valuable when investigating climate change risks to species under anthropogenic [human-caused] climate change” in their paper, published in the journal Frontiers in Ecology and Evolution, in work led by Lara Brodie, of the global change research group of the department of botany and zoology.

Around the world, a large body of research and observation shows that biological systems and species are responding to human-made climate change, said Guy Midgley, the head of the research group and a co-author of the paper.

A common prediction for plants is that they will shift their geographic ranges in response to warming, either poleward or upward in elevation, will adapt through genetic changes, changes in biology or behaviour — or they will go locally extinct. 

Globally, trees are most vulnerable to human-caused climate change. “While trees may have been able to migrate or adapt to previous changes in the paleo climate, taking place over thousands of years, they may not be able to adapt or migrate as fast as is needed to survive climate change,” he said. 

The concern for slow-growing and long-lived tree species such as the quiver tree “is whether they will be able to shift range fast enough to keep pace with the ever-increasing rates of climate change and the likelihood of significant lags of range shifts behind changing climates”, according to the paper.

The researchers say that over the past 10 000 years the surface air temperature in southwestern Africa probably rose by 5°C during glacial to interglacial warming. 

But the latest August report of the Intergovernmental Panel on Climate Change shows that surface air temperatures in Africa are projected to rise more than 5°C in the dry, subtropical areas in less than a century.

Over the past 20 years, Midgley and his postgraduate students have been documenting how quiver tree populations respond to climate change. In 2007, research by conservation biologist Wendy Foden showed signs of a coming southward and upward shift, with individual trees dying off in the warmer parts of the range towards the equator and at lower elevations throughout the northern half of the range. 

Brodie used species distribution modelling to reconstruct the likely spatial extent of the quiver tree’s ranges during glacial periods, and tested the results using population genetic methods. 

She found that about 22 000 years ago, the models predicted that two-thirds of suitable habitat for quiver trees was outside the current range — about 650km further to the north. As temperatures became more favourable, the trees started expanding their range polewards at a rate of 0.4km a decade. 

About 6 000 years ago, the quiver tree range was more or less in the same location as the current range. It took quiver trees up to 18 000 years to migrate 650km poleward. 

By 2070, the models predict that quiver trees will have to shift 191km eastwards to adapt to a changing climate — 42km in 70 years or 6km a decade. This is roughly 15 times faster than what was expected of them 18 000 years ago. 

Overall, according to the paper, the outlook for the persistence of the species in the wild is positive, because most of its current range appears to remain habitable into the future. 

“However, the northerly [equator-ward] populations appear to be at risk of local extirpation, as the calculated migration rate of the species is unlikely to permit them to track the projected warming rate over the next few decades. If this occurs, valuable adaptive genetic diversity of populations possibly selected under warmer and drier conditions may be lost.”

The team recommends the proactive conservation of the genetic diversity of the northern populations by safeguarding individuals in botanical gardens and “carefully considered” assisted colonisation in suitable areas, noting how their findings can be used to help conserve other endemic and near endemic species in the species-rich desert region.

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Sheree Bega
Sheree Bega is an environment reporter at the Mail & Guardian.

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