tough times for large trees: relative impacts of elephant … times for large trees: relative...

Tough times for large trees: Relative impacts of elephant and fire on

large trees in Kruger National Park

Graeme Shannon1, Maria Thaker1

Abi Tamim Vanak1, Bruce Page1,

Rina Grant2, Rob Slotow1

1University of KwaZulu-Natal, 2Scientific Services, SANParks

Shannon G., Thaker M, et al. 2011. Ecosystems 14: 1372-1381

Large trees in savanna ecosystems

• Key role in ecosystem functioning

– Keystone components

– Nutrient pumps

– Habitat heterogeneity

– Increase biodiversity

Damage to large trees: Role of elephant

• Foliage utilisation

• Breaking of large branches

• Debarking

• Pushing over

Damage to large trees: Role of fire

• Removal of lower crown biomass

• Damage to tissues

• Topkill

Effect of elephant and fire: are they additive?

• Elephant damage to trees

makes them more

susceptible to fire

• Opening up of canopy

increases fuel load

– Higher intensity fires

Understanding the patterns of damage

• Determine impact of elephant, fire (main ecological drivers) and disease on large trees over a 30-month period subsequent to initial description

• Particular focus on the independent and combined effects of previous impact on subsequent levels of impact and mortality

Surveys of large trees

• Transects: 2.5 years apart

(Apr 2006, Nov 2008)

• 22 Transects

(67 km total)

• Southern Kruger

• N = 2522 trees

(> 5 m height)

1st survey of large trees

• location of individual trees

(≥ 5 m height)

• species, dimensions

• use/impact by elephant

(proportion tree volume removed)

• fire damage (proportion tree volume removed)

• disease (presence of wood borer, heart rot, etc)

2nd survey of large trees

• each relocated tree assessed to determine additional level of impact by elephant, fire, disease

• New trees that progressed into > 5m height class were included and assessed

0

5

10

15

20

25

30

35

Elephant Fire Disease Mortality

Perc

enta

ge

All trees 5.0-6.5 m 6.6-8.0 m 8.1- 9.5 m > 9.6 m

Damages by height class

0

5

10

15

20

25

30

35

Elephant Fire Disease Mortality

Perc

enta

ge

All trees 5.0-6.5 m 6.6-8.0 m 8.1- 9.5 m > 9.6 m

Damages by height class

30%

16%

0

5

10

15

20

25

30

35

Elephant Fire Disease Mortality Progression into ≥ 5m height

class

Perc

enta

ge

Acacia nigrescens Spirostachys africanaSclerocarya birrea Combretum apiculatumTerminalia sericea

Damages by species

0

5

10

15

20

25

30

35


class

Perc

enta

ge


Heavy impact on Acacia, Combretum & Sclerocarya

*

* *

*

*

* *

*

*

* *

*

0

5

10

15

20

25

30

35


class

Perc

enta

ge


No worries for Terminalia sericea

* * * *

What have we learnt?

• Mortality levels of large trees were 4.6% per annum

• Mortality was double that of progression into the > 5 m height class

• Pattern was species-specific

– Mortality > progression for some palatable species (A. nigrescens, C. apiculatum, S. birrea)

– Progression > mortality for less palatable species (S. africana, T. sericea)

• At current rates of mortality and progression, composition and structure of large tree guild will change

• Pattern is heterogenous across landscape

- lowveld sour bushveld: highest levels of damage by fire and

elephant (highest mortality)

** 3rd survey in March 2011 (7 years after 1st survey)

Previous damage affects the amount of subsequent damage

(806)

(180)

(318) (1218)

Repeated damage by elephant and fire

Fire in 2006 No fire in 2006

Only fire

Only elephant

Both

No fire or

elephant

(806)

(180)

(318) (1218)

If no elephant and no fire damage in 2006, then little subsequent fire impact


(806)

(180)

(318) (1218)

If in 2006…fire only, elephant only or both, then subsequent impact by fire was similar


Only fire

Only elephant

Both

Previous elephant and fire damage are not additive…

• When A. nigrescens is utilised by only elephant, then subsequent tree volume removed by fire increases by 1.11 times – Elephant damage makes A. nigrescens vulnerable to fire damage

– Certain areas with A. nigrescens were prone to fires

• Tree volume removed by subsequent fire increases by 1.12 times for Spirostachys and 1.3 times for Combretum, but only if previously damaged by both elephant and fire – Double the type of damage results in similar further impact

• Little effect of previous damage on subsequent tree volume removed by fire for Sclerocarya and Terminalia

Subsequent fire impact depended on species…

(806) (1218)

(318)

(180)

Tree volume removed by elephant was highest for trees that were previously burned but not elephant utilised

Elephant do not repeatedly target the same trees


(806) (1218)

(318)

(180)

Both fire and

elephant

Effect of previous fire and elephant damage were not additive

Subsequent elephant utilisation was not double


Subsequent elephant impact depended on species…

• If a tree had previous fire damage, then tree volume subsequently removed by elephant increased – by 1.16 times in Sclerocarya and 1.23 times in A. nigrescens

Elephants targeted burnt trees!

• If a tree was previously utilised by elephant, then tree volume subsequently removed by elephant decreased – by 1.11 times in A. nigrescens and 1.18 times in Combretum

Elephants did not repeatedly target the same trees!

• Little effect of previous damage on subsequent tree volume removed by elephant for Spirostachys and Terminalia

Type of elephant damage matters…

0

20

40

60

80

― fire elephant disease ― fire elephant disease

pushed over (2006) debarked (2006)

Pro

ba

bili

ty o

f m

ort

alit

y (

Od

ds r

atio

) Trouble starts once you are pushed over…

25 times more likely to die


0

20

40

60

80



Pro

ba

bili

ty o

f m

ort

alit

y (

Od

ds r

atio



0

20

40

60

80



Pro

ba

bili

ty o

f m

ort

alit

y (

Od

ds r

atio

) Just debark me and leave please…


0

20

40

60

80



Pro

ba

bili

ty o

f m

ort

alit

y (

Od

ds r

atio



What have we learnt?

Repeated damage is a problem but sequence of events matters

• Elephant did not target individual trees repeatedly

– Diminishing returns to over-forage?

• Elephant removed more volume from trees that were burned but not previously elephant utilised

– fire events may be facilitating foraging behaviour at the plant-scale (post-fire flush?)

Repeated damage is a problem but sequence of events matters

• Intense utilisation by elephant is a bad start

– probability of mortality was much higher for trees that suffered subsequent damage after being pushed over or debarked

• Fire is the last straw

– Fire was the leading cause of mortality for trees previously damaged by elephant

Acknowledgements • Amarula Elephant Research Programme

• UKZN Research Office

• SANParks

• National Research Foundation

• Dave Druce

• Alain Smith

Strength of effect = odds ratio

0 1 10

Odds Ratio

Like

liho

od

10 9 8 7 6 5 4 3 2 1

5

Odds ratio 0.1 = 10 times less likely

Odds ratio 10 = 10 times more likely

tough times for large trees: relative impacts of elephant … times for large trees: relative...

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