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
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