Trees get frosty reception in Karoo

By Joh Henschel, Kayleigh Muller and Tim O’Connor, SAEON Tweet

While sweltering temperatures elicit hot debates, it is opportune to think of the opposite - bitter cold. In the midst of a general trend of warming, sharp drops in temperatures occasionally shock the system...

Eighteen months ago, a freak cold spell gripped the eastern Karoo. The thermometer dropped further below zero than it had in a decade or two. The results were only too visible at the onset of the next summer when frost-burnt trees failed to green. Ever-ready on the lookout for environmental changes and their causes, SAEON was on the spot to investigate.

Hill-to-valley view of frost damage at the bottom of the hill (Picture: Kayleigh Muller)   Severe frost damage in a tree thicket in a valley in the Karoo, showing taller trees towering above the frost layer and trees on hillslopes undamaged by frost (Picture: Joh Henschel)

SAEON researchers found that the severe frost had affected the eastern boundary of the Karoo, supporting the notion that frost is a contributing factor towards delimiting the Nama-Karoo. In fact, frost could explain why trees are nearly absent in the Nama-Karoo, except on hills, though present in the adjacent savanna and grassland biomes.

Data from the South African Weather Service (SAWS) station revealed that occurrences of temperatures below minus 7 degrees Celsius have increased in Kimberley, now every decade compared to every two decades before 1977. A possible explanation is that with climate change, crystal-clear calm nights – cloudless, low humidity conditions retain less heat, cold air skinks into valleys – may have become more prevalent during winters in the Nama-Karoo. Such frigid winter nights contrast strongly with the progressively more balmy winter days, intensifying the stress.

Regrowth on a frost-damaged tree (Picture: Kayleigh Muller)

Internal freezing

During frost events, minimum temperatures occur at one to two metres above the ground, the level of young trees. Internal freezing damages tree cells; subsequent differential thawing of sun-exposed and sheltered parts exacerbates the stress. The result is death, if not of the entire tree, then of parts.

SAEON researchers measured damage caused by the 2014 frost events on trees at study sites in the Northern Cape near Kimberley and Schmidtsdrift and in the Eastern Cape near Middelburg and Hofmeyr. They examined the effects of frost on trees in relation to their elevation along the slope between valley floor and hilltop, also, how different species or sizes of trees differ in susceptibility to frost damage, and recorded the trees’ extent of regrowth during the first subsequent growth season in early 2015.

At each locality, tree damage varied. About 5% of all trees at three of the sites were killed by frost, comparable to the impacts caused by fire. The degree of frost damage declined up the slope. Typically, trees lost some 60% volume low on the slope, some 30% at 30 m higher elevation, and remained undamaged 100 m above the valley floor.

Most susceptible species

The most frost-susceptible species were three acacias with a tendency to expand into boundary areas of the Karoo during periods without severe frost, namely sweet-thorn (Acacia karroo), black-thorn (Acacia mellifera) and the umbrella-thorn (Acacia tortilis). The Karoo Kuni-bush (Searsia burchelli), which occurs on hills across the Nama-Karoo, may be confined to hills by frost, and is damaged or dies at the base.

Hardly any of the frost-damaged plants could regrow during the subsequent summer to near their previous size. It may take them several years to recover their former size… provided there is not another frost. Thus, overall growth of populations of trees along the Karoo boundary appears to be limited by frost, which may, in the long run, restrain the expansion of trees into the Karoo from adjacent biomes.

Kayleigh Muller, a research assistant at the SAEON Arid Lands Node, studies a frost-damaged Searsia burchelli at the bottom of a hill (Picture: Joh Henschel) Kayleigh examined the effects of frost on trees in the Karoo in relation to their elevation along the slope between valley floor and hilltop (Picture: Kayleigh Muller)

Bigger picture

As much as the 2014 frost event is part of a bigger picture, including other species, places and times, the SAEON study merely served to touch this little-studied subject, requiring further research. Frost is one of several infrequent events which SAEON endeavours to understand better, and be ready to observe as and when they occur.

We wish to thank the Schmidtsdrift community, Justin du Toit, Roelf and Teresa Opperman and Trevor Marrick for their assistance with fieldwork, and the South African Weather Service for providing weather data.