MA Whitecross1, S Archibald, ETF Witkowski and AJ Chamberlain
1 University
of the Witwatersrand
School of Animal, Plant and Environmental Sciences
[email protected]
Private Bag 3
Wits 2050

Mopane distribution decreases where minimum
winter temps ↓ 4.8°C

But what happens at the southern limit where
frost events can happen?
Local scale study used to observe freeze impacts on
mopane
 Long term weather trends near southern mopane
range limit

(Ausperger 2009, Stevens et al. 2013)

16th June 2010: Severe freeze event

Freeze Event




Air temperature rapidly falls below 0°C
Extracellular water freezes within plant
Causes potentially fatal damage
Black frosts → freeze damage
(Snyder and Paulo de Melo-Abreu 2005; Schulze 2007)
Venetia Limpopo Nature Reserve, Limpopo Province, RSA
How does freeze damage impact mopane?
Is there evidence
of a ‘freeze trap’?
How does recovery
change with elevation?
Do low elevations have the
most severe damage ?
Slope
SEVERITY: High
Intermediate
Low
The LOWEST
elevations had
the HIGHEST
severity
Logistic regression (GLM):
RD=52.821, d.f.=174, p<0.001
High
Low elevation
High severity
4m
Threshold
Low
Increased Topkill
0 = full recovery
Little topkill measured on
trees > 4 m
Percentage Net Regrowth
100
Bottom
80
Middle
60
Top
40
20
0
-20
554
556
558
560
562
564
-40
566

-60
-80
-100

Elevation (m.a.s.l.)
568
570
High severity =
least recovery
‘Freeze Trap’
572
Percentage Net Regrowth
100
Bottom
80
Middle
60
Top
40
20
0
-20
554
556
558
560
562
564
566
568
-40
-60
-80
-100
Evidence of
freeze trap
Elevation (m.a.s.l.)
570
572
DECREASE in annual number of
days below 0°C
5
9
4
8
3
7
2
1
0
-11960 1970 1980 1990 2000 2010
Days below 0˚C
Temperature (˚C)
INCREASE in lowest annual
minimum temperatures
6
5
4
3
-2
2
-3
1
-4
0
-5
1960
Year P=0.00054
R² = 0.2269;
1970
1980
1990
2000
R² = 0.1609; P=0.004
2010
Data collected from: 1960 - 2010
Lowest Annual
Minimum
Temperature (°C)
Return Period
(years)
2
-2
1
2
9
-4
none
0
Musina
Whitecross et al. 2012 (South African Journal of Botany):

Topkill evident in tree heights < 4m



Driver for height diffs down slope gradient
Evidence of freeze trap at bottom of slope
Can escape freeze trap by reaching 4 m in 9 years

Need for regional definition of the ‘freeze trap’

Single freeze event can cause severe damage

CC: ↓ frost frequencies ≈ ↑ Mopane distribution

Freeze events are a noteworthy disturbance











Dr. Sally Archibald and Prof. Ed Witkowski
FABI (CTHB)
Diamond Route - E Oppenheimer & Son and the De Beers Group
Duncan MacFayden , Jamie Zylstra and Piet
South African Weather Services
Sumeshni Pillay
Dominique Prinsloo
Nikki Stevens
Helenor Whitecross
Robert Whitecross
Ashleigh Camberlain