Post on 11-Oct-2020
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