Increased tree densities in South African savannas

>50 years of data suggests CO2 as a driver

Robert Buitenwerf1, William J. Bond2, Nicola Stevens3 and Winston S. W. Trollope4

1. UCT Botany, Goethe University Frankfurt 2. UCT Botany

3. CSIR NRE, Wits University, UCT Botany 4. Working on Fire International

Long-term fire experiments: the best we have

The usual suspects for driving tree cover, both grassland/savanna/forest transitions and woody thickening within savannas, are:

1. Land use: fire regime, grazing pressure, wood harvesting (e.g.

Staver et al. 2011)

2. Climate: rainfall (e.g. Sankaran et al. 2005)

Detecting effects from global drivers such as CO2 is difficult as they are usually confounded by ever-changing land use

Except in long-term fire experiments…

Kruger Experimental Burn Plots

Sampling scheme:

1950s: Diagonal transects

1970s: Survey of ½ Ha plots on 1 m2 grid

1990s: Diagonal transects

2000s: Partial repeat of 1970s plots

Variables:

• Species ID of each individual (Density)

• Tree height (1970s, 1990s, 2000s)

Pretoriuskop

Satara

Adapted from Biggs et al. 2003, Koedoe

MAP (mm)

500 - 550

550 - 600

600 - 650

650 - 700

700 - 750

Tree density over time

Semi-arid Mesic

Structural change of dominant species

Semi-arid

Acacia nigrescens

0 200 400 600 800 1000 1200

0-0.5

0.5-1

1-2

2-3

3-4

4-6

6-8

Siz

e c

lass m

Change in density %

Dichrostachys cinerea

0 200 400 600 800 1000 1200

0-0.5

0.5-1

1-2

2-3

3-4

4-6

6-8

Siz

e c

lass m

Change in density %

Structural change of dominant species

Terminalia sericea

0 200 400 600 800 1000 1200

0-0.25 m

0.25-0.5 m

0.5-1 m

1-2 m

2-3 m

3-4 m

4-6 m

6-8 m

> 8m

Siz

e c

las

s

Change in density, %

Dichrostachys cinerea

0 200 400 600 800 1000 1200

0-0.25 m

0.25-0.5 m

0.5-1 m

1-2 m

2-3 m

3-4 m

4-6 m

6-8 m

> 8m

Change in density, %

B.A.

Mesic

Eastern Cape Burn Plots

Albany thicket

Grassland

Savanna

University of Fort Hare Experimental Farm

Eastern Cape Burn Plots

• Started in 1980 by Winston Trollope

• Plots cleared of all trees at the start

• 6 treatments (1, 2, 3, 4, 6 yr, no burn )

• 2 replicate ½ Ha plots per treatment

• Analysed every 4th annual survey

• Variables: tree density and height

Eastern Cape tree density and structure

All burn treatments pooled except for fire exclusion

Increased tree cover in Eastern Cape

1973 2007

1 % tree cover 50 % tree cover

Eastern Cape 1925 Thanks to Timm Hoffmann, James Puttick PCU, UCT

Eastern Cape 1993 Thanks to Timm Hoffmann, James Puttick PCU, UCT

Eastern Cape 2011 Thanks to Timm Hoffmann, James Puttick PCU, UCT

Photo: William Bond

Conclusion

Tree density increased by 300% in mesic savannas despite constant fire regimes and climate over the past 60 years

However increased atmospheric CO2 is consistent with tree encroachment

70 % of total increase since preindustrial times took place during the Kruger EBP experiment Plants have not dealt with such high CO2 for at least 65 kyr and probably not for 3 million years

Acacia karroo

0

20

40

60

80

100

120

140

180 280 370

CO2, ppm

Dry

ma

ss

, g

resprout

shoot

root

Growth response to experimentally elevated CO2

See: Kgope et al. 2010, Austral Ecology Bond and Midgley 2012, Phil Trans Roy Soc. B.

Terminalia sericea

0

50

100

150

200

250

180 215 260 310 375 450

CO2 ppm

Dry

ma

ss

, g

resprout

shoot

root

Growth response in real life…

LGM 180 ppm

Pre-industrial 260 ppm

Ambient 375 ppm

Above-ambient 450 ppm

Thanks To everyone who has helped establish and maintain the

KNP Experimental Burn Plots over many decades and spent countless hours collecting data, particularly

Andre Potgieter and his team (1970s) and Doug Euston-Brown (2000s).

Wellington Shabangu helped collect immaculate data on the EC Burn Plots for 30 years.

Why only in mesic systems? Some speculations:

1. Dominant species in dryer, less fire impacted systems, have smaller carbon sinks than dominant species in wetter, fire dominated systems

2. High grazing pressure in dryer areas prevents seedling recruitment

3. Grasses are superior competitors for the additional soil moisture

4. Trees compete worse in clayey soils