The dynamics of Acacia mellifera, implications for bush encroachment management.

Joubert, D.F., Smit, G.N

Conceptual model

21Rain - lots of it in 3 seasons

No fire 77

9

10

Xcompetition browsing

Joubert, D. F., Rothauge, A. Smit, G.N. A conceptual model of vegetation dynamics in the semi-arid Highland savanna

of Namibia, with particular reference to bush thickening by Acacia mellifera. (2008). Journal of Arid Environments.

return to open grassy state (drought, fungal dieback, clearing, seeding)

seed production

Hypotheses:

1. En masse seed production occurs in only excellent rainfall years

2.

Vigorously growing climax grasses can outcompete seedlings (reduce vigour; increase mortality)

3.

Fire is effective at an early stage of establishment in preventing the transition, ineffective as a control measure with established shrubs

4.

Browsing by small herbivores may significantly thin establishing thickets

-300

-200

-100

0

100

200

300

400

500

1892

/189

3

1902

/190

3

1912

/191

3

1922

/192

3

1932

/193

3

1942

/194

3

1952

/195

3

1962

/196

3

1972

/197

3

1982

/198

3

1992

/199

3

2002

/200

3

Potential bush encroachment initiation events very rare

Fire (preventor) and excess water (promoter) main drivers

1897-1899 1937-1940 1953-1958 1974-19801986-1989 2000-2002

= transition 3 events if 3 years are required

FIRES IN NAMIBIA SINCE 2000

Study site

Highland Savanna

MethodologyFire Control

½ clipped

Competition/browsing

X 4 (“climax”

veld)

Chi square

Soil water potential (gypsym

blocks)Soil nutrientscontrol

Results: Seed production

Percentage of trees with fruits and rainfall from 1998 to 2003

0.00%

10.00%

20.00%

30.00%

40.00%

50.00%

1998 1999 2000 2001 2002 2003

Year

% o

f tre

es w

ith

fruits

per

plo

t

0

200

400

600

800

1000

rain

fall

(mm

)

Rainfall(mm)

Mean % oftrees w ith

rainfall and pod production per year

0

100

200

300

400

500

600

700

1998 1999 2001 2002 2003 2004 2006

year

rain

fall

(mm

)

0

5

10

15

20

25

pods

/ tre

e

Pods / tree in relation to rainfall

y = 0.0504x - 12.169R2 = 0.9052

0

5

10

15

20

25

0 100 200 300 400 500 600 700

rainfall (mm)

pods

/ tr

ee

Available seeds per tree after pre dispersal predation for 2001 and 2002

Seeds in 2001 and 2002 per labelled tree

173.9

5.3 0.3

20.3 3.45.3

0.0

50.0

100.0

150.0

200.0

250.0

1 2

year

seed

s nonviableavailablepredated

2001 2002

Results: competition

Results: competition (A. pubescens unclipped)

Stem diameter = 3.6248mm + 0.00389*distance(cm)r = 0.0601

Stem

dia

met

er 2

009

(mm

)

Results: competition (A. pubescens clipped)

Stem diameter = 3.6838mm + 0.00052*distance(cm)r = 0.009

Stem

dia

met

er 2

009

(mm

)

Results: competition (S. pappaphoroides unclipped)

Stem diameter = 4.0083mm + 0.00688*distance(cm)r = 0.076

Stem

dia

met

er 2

009

(mm

)

Results: competition (S. pappaphoroides clipped)

Stem diameter = 4.5732mm + 0.0162*distance(cm)r = -0.1572

Stem

dia

met

er 2

009

(mm

)

Results: competition (A. pubescens)

3

4

1 2 3 4

Stem

dia

met

er 2

009

(mm

)

Results: competition (S. pappaphoroides)

2,3

11

Stem

dia

met

er 2

009

(mm

)

1 2 3 4

Results: different grasses provide different competitive effects?

A. pubescens S. pappaphoroides

Results: competition

•Soil moisture

•Soil nutrients

•Root distribution and architecture

•“minor” perennials

tuft

intertuft

Results fire

Larkin Powell

30.00

3.00

31

98

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

1 2

treatment

surv

ival

Results: fire + seedlings

firecontrol

P = 0.0000

Results fire

98

8

1 1 10

3

7

14 12 812

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

seed

lings

0-10

<20 <30 <40 >40

stem diameter (mm)

%

alivedead

Results browsing: seedlings

Survival Feb 2008 –

June 2009

SEEDLINGS

•

June 2009•

survival in controls significantly lower (64 %) exclosures

(84 %) (p < 0.001) (Chi square analysis).

•

21 % of seedlings in control confirmed browsed by hare and 21 % of these died.

•

Seedling mortality through browsing by hares likely to be much higher than this.

•

Recently, 12 % of 292 surviving seedlings in the controls browsed by hares within ±

40 days!!!

•

suggests that a very high proportion could be browsed in 2009 (study ongoing).

•

Browsing appears to increase in the dry season.

•

cumulative increase in the number of browsed saplings (stem diameters = 7.2 mm; SD 2.14 cm) from 15 % (2007) to 36 % (2008) (n = 60). Suggests possibly ALL saplings are browsed between recruitment events. Measurements will be repeated in 2009.

•

18 % mortality of saplings through the year NOT ascribed to browsing by hares (WATER STRESS?).

•

Hare browsing appears to not negatively affect saplings of such a large size (≥

8 years).

•

Browsing MAY PROMOTE growth at this stage.

SAPLINGS SAPLINGS

Conclusion•

Hypotheses:

•

En masse seed production occurs in only excellent rainfall years

•

Vigorously growing climax grasses can outcompete

seedlings (reduce vigour; increase

mortality)•

Fire is effective at an early stage of establishment, ineffective as a control measure with established shrubs

•

Browsing by small herbivores may significantly thin establishing thickets

Not convincing! Subtle reduction of vigour?

Major driver!

Thins thickets out!

Generally true, situation more complex

Adaptive and preventitive

managementdynamics management

1.

Infrequent establishment (fruit production, short term seed banks)

Monitor

2.

Grass competition generally “weak”

Monitor; maintain a healthy sward for..

3.

Fire effective in killing seedlings and saplings

Fire, Should be used where seedlings and saplings are present

4.

Hares and other small browsers could significantly thin out establishing thickets

Maintain healthy populations of small browsers (habitat, reduced poaching)

5.

Saplings and mature trees are drought sensitive and prone to fungal infections

Use these opportunistically to reduce costs of, for example, arboricides

6.

Slow growing Fire may be effective for longer periods

Acknowledgements•

Polytechnic of Namibia

•

BIOTA South•

University of Namibia

•

Neudamm•

Krumhuk

Farm

•

Researchers who have contributed to the knowledge of bush encroachment

•

All the people that helped me in the field, and with advice

•

Co-authors of a paper on the conceptual model

Thank you