Effects of Edge Orientation and Internal Fragmentation on Cyclone

Disturbance Patterns in Mabi 5b Fragments

Jeffrey Allenby

School for Field Studies

Center for Rainforest Studies

Queensland, Australia

Faculty Advisor: Amanda Freeman

Background

• History of severe fragmentation of Mabi 5b forest through land use over the last century

• Edge effects have become more of a problem as a result

• Interior habitat has become critically endangered

Cyclone Larry

• 20 March 2006 Cyclone Larry passed over the Atherton Tablelands

• Category 3 cyclone when it passed south of the fragments studied

• Winds over 100 km/h recorded in the area

Other research

• Impact of cyclones on individual tree species

• Influences of topography on cyclone disturbance levels

• Impact on flora after cyclone disturbance

Purpose

• To determine the effects of Cyclone Larry on Mabi 5b fragments

• To determine the effects of edge orientation on the patterns of disturbance

• To determine the effects of internal fragmentation on the patterns of disturbance

Study Site

• 6 Fragments of Mabi 5b• Curtain Fig (260 ha)• Wongabel (250 ha)• Tolga Scrub (80 ha)• Thomas Road (19 ha)• Picnic Crossing (16ha)• Favier’s Plot (1 ha)

Field Methods

• Took visual assessments on a 50m grid using the Curran Scale at 564 sites

Level Description0 No Disturbance-Canopy is still largely intact and mostly closed

1 Light Disturbance-No tree fall or stems snapped in the canopy. Canopy predominantly still intact with <25% loss

2 Moderate Disturbance-Canopy foliage retention is 25-50%. Larger limbs will be lost and fallen along with an occasional tree fall in the canopy.

3 Heavy Disturbance-Widespread defoliation of canopy and 50% canopy foliage removed. Extensive treefall or trunks have been snapped off.

Lab Methods

• Inputted and coded disturbance observations into a Geographic Information System (GIS)

• Overlaid Atherton Shire cadastre, Stanton and Stanton Vegetation Mapping, and aerial photographs provided by QPWS

Analysis

• Frequency of overall disturbance levels was calculated

• Frequency of disturbance levels was calculated for edges facing each of the 8 inter-cardinal directions

• Performed a Chi-square test on wind direction and disturbance levels

Results

15%

36%

47%

2%

0

1

2

3

Level

• Disturbance levels overall were high:

Results

• Wind hit the southwestern, western, northwestern, and northern edges the hardest

78%

22%

9%

9%

36%

46%

4%

44%

39%

13%

8%

46%31%

15%

17%

63%

20%

4%

13%

30%

53%

5%

95%

22%

78%

E

N

S

W

0

1

2

3

Level

Results

• Chi-Square analysis shows a strong correlation between edge orientation and levels of disturbance.(χ2=134.493, P=.000, df=14)

Edge Orientation

• There is a higher than expected frequency of heavy disturbance on the western edges

• There is a higher than expected frequency of light to moderate disturbance on the eastern edges

• Consistent with previous findings of cyclone disturbance patterns

Internal Fragmentation

• If fragmenting element is narrow (road or stream) and parallel to the wind:

Tolga Scrub

Wongbel (southern section)

Internal Fragmentation

• If fragmenting element is narrow (road or stream) and curves:

Wongabel (northeastern)

Curtain Fig(southern)

Internal Fragmentation

• If fragmenting element is narrow (road or stream) and perpendicular to the wind:

• Consistent with information about windbreaks

Tolga Scrub

Internal Fragmentation

• If the fragmenting element is wide (forestry plot):

Wongabelinterior edge

(western section)

WongabelInterior edges

(northeastern section)

Limitations and Future Research

• Lower levels of data collected for inter-cardinal directions (NE-SE-SW-NW)

• Not enough examples of effects of internal fragmentation to be conclusive

• Further research into the effects of internal fragmentation on disturbance patterns needs to be done

Conclusions

• Edge orientation plays a large role in determining disturbance patterns

• Internal fragmentation appears to play a large role in determining interior disturbance patterns depending on orientation and shape

• Both edge orientation and internal fragmentation influence how far into the fragments edge effects are seen

Acknowledgements

Thanks to • My research partner Melissa Schlothan for helping me

collect, assemble and make sense of our data • The Plant and Bird DR groups for helping gather data• Amanda Freeman and Tim Curran for providing insight

and assistance at every stage of the work• Mabi Project: Scientific Purposes Permit WISP02094504• Queensland Department of Natural Resources, Mines

and Energy: Atherton Cadastre• Wet Tropics Management Authority: Stanton and

Stanton Vegetation Mapping