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Paleo-climate simulations of Namibia’s quiver tree populations show significant range shift over past 22 000 y
Author: Wiida Fourie-Basson
Published: 03/11/2021

​​​Going 22 000 years into the past and projecting 50 years into the future, ecologists from Stellenbosch University (SU) simulated the paleo- and future geographic range of arid South Africa and Namibia's iconic quiver tree, Aloidendron dichotumum.

The aim was to establish its response to past climate change, and how human-made climate change could affect this response into the future.

Prof Guy Midgley, head of the global change research group in SU's Department of Botany and Zoology, says worldwide a large body of research and observation show that biological systems and species are already responding to human-made climate change.

“A common prediction for plants is that they will shift their geographic ranges in response to warming, either poleward or upward in elevation. The other options are on site adaptation by genetic changes or changes in biology or behaviour. The non-adaptive outcome is local extinction," he explains.

In the case of long-lived trees, however, the adaptive options are limited.

Globally, trees are the most vulnerable to man-made climate change, as they are long-lived with long generation times. In other words, 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 man-made climate change.

For example, over the past 10 000 years the surface air temperature in southwestern Africa likely rose by 5°C during glacial to interglacial warming at the end of the Pleistocene. In contrast, according to the latest report from the Intergovernmental Panel on Climate Change (IPCC), surface air temperatures in Africa is projected to rise in excess of 5°C in the dry, subtropical areas in less than a century.

It is therefore becoming imperative for the future conservation of endemic species such as the quiver tree, to try to understand and predict their response to ongoing and future climate change.

Sentinel of man-made climate change?

Over the past two decades, Prof Midgley and his postgraduate students have been observing and documenting quiver tree populations' response to climate change. In 2007, Prof Wendy Foden was able to show 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. Repeated observational studies over the years are needed to confirm the findings that such a range shift may be underway.

In the latest study, Prof Midgley's postgraduate student Lara Brodie used species distribution modelling to reconstruct the likely spatial extent of the quiver tree's ranges during glacial periods. She then used population genetic methods to test the results. The findings have recently been published in the journal Frontiers in Ecology and Evolution.

What are the main findings?

During the Last Glacial Maximum (LGM) (22 000 years ago), the models predict that two-thirds of suitable habitat for A. dichotomum was outside the current range – around 650 kilometres further to the north. As temperatures became more favourable, the trees started expanding their range polewards at a rate of 0.4 kilometres per decade. During the mid-Holocene, about 6000 years ago, the available quiver tree range was more or less in the same location as the current range.

This means that it took A. dichotomum up to 18 000 years to migrate 650 kilometres poleward.

This migration was indirectly confirmed by the results from the genetic population analyses: on average, the older northern populations support higher levels of genetic diversity than the more recently established populations in the southern and south-eastern regions (biologists call this phenomenon the 'founder effect').

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

According to the researchers, A. dichotomum will likely survive in the wild, but its geographic range would be much narrower. Southwards populations may also struggle to adapt and survive due to lower levels of genetic variation.

Furthermore, the researchers expect the northern reaches to become unsuitable in the future, and this range will likely be lost, along with its unique genetic diversity unless action is taken.

They recommend the proactive conservation of the genetic diversity of the northern populations of quiver trees by, for example, safeguarding individuals in botanical gardens, as well as carefully considered assisted colonisation in suitable areas.

After decades of research, the iconic quiver tree still stands as a sentinel for anthropogenic climate change. The latest findings will not only inform conservation responses for this species, but potentially also for a wide range of endemics in this species-rich region, Prof Midgley concludes.

The article 'Broadening predictive understanding of species' range responses to climate change: the case of Aloidendron dichotomum' was published in Frontiers in Ecology and Evolution in October 2021.

 On the photo: A quiver tree forest in the Karas region of Namibia. Photo credit: Zairon, CC BY-SA 4.0 Wikimedia Commons