Late 20 th century mangrove encroachment in the coastal Australian monsoon tropics parallels the regional increase in woody biomass 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 Grant J Williamson (Corresponding author) School of Plant Science, University of Tasmania, Private Bag 55, Hobart 7001, Australia. Email: [email protected]Phone: +61 3 6226 1944 Fax: +61 3 6226 2698 Guy S Boggs Tropical Spatial Sciences Group, Charles Darwin University, Darwin NT 0909, Australia. Email: [email protected]David MJS Bowman School of Plant Science, University of Tasmania, Private Bag 55, Hobart 7001, Australia. Email: [email protected]Date of manuscript draft: 15 September 2009 1
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Late 20th century mangrove encroachment in the coastal Australian monsoon tropics parallels the regional increase in woody biomass
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Late 20th century mangrove encroachment in the coastal Australian monsoon tropics parallels the regional increase in woody biomass
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Grant J Williamson (Corresponding author) School of Plant Science, University of Tasmania, Private Bag 55, Hobart 7001, Australia. Email: [email protected] Phone: +61 3 6226 1944 Fax: +61 3 6226 2698
Guy S Boggs Tropical Spatial Sciences Group, Charles Darwin University, Darwin NT 0909, Australia. Email: [email protected]
David MJS Bowman School of Plant Science, University of Tasmania, Private Bag 55, Hobart 7001, Australia. Email: [email protected]
Date of manuscript draft: 15 September 2009
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ABSTRACT 24
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In Kakadu National Park, a World Heritage property in the Australian monsoon
tropics 250 km to the east of Darwin, a number of recent studies have shown that
woody encroachment (expansion of woody communities) and densification
(increased biomass in woody communities) has occurred in the last 40 years. The
cause of this increase in woody biomass is poorly understood, but possibly associated
with the control of invasive Asian water buffalo, trend to higher rainfall, and
increased frequency of fires. Mangroves provide an important context to understand
these landscape changes given that they are unaffected by fire or feral water buffalo.
We examine change in mangrove distribution in a series of coastal tropical swamps
fringing Darwin, Northern Territory, Australia over a 30-year period using a series of
7 aerial photographs spanning 23 years from 1974 and a 2004 high-resolution satellite
image. In late 1974 Darwin was impacted by an intense tropical cyclone. Vegetation
at 3,000 randomly placed points was manually classified, and a multinomial logistic
model was used to asses the impact of landscape position (coastal, intertidal and
upper-tidal) and swamp on mangrove change between 1974 and 2004. Over the study
period there was instability and slight mangrove loss at the coast, stability in the
intertidal zone and mangrove gain in the upper-tidal zone, with an overall increase in
mangrove presence of 16.2% above the pre-cyclone distribution. A swamp that was
impacted by drainage works for mosquito control and the construction of a sewage
treatment plant, showed a greater mangrove increase than the two unmodified
swamps. The mangrove expansion is consistent with woody encroachment observed
in nearby but ecologically distinct systems. Plausible causes for this change include
changed local hydrology, changes in sea level and elevated atmospheric CO2
Despite severe damage from a tropical cyclone the coastal swamps examined in this
study all show landward mangrove expansion over a 30-year period, primarily
replacing brackish reed swamps in the upper tidal zone. Expansion rate is particularly
high in the swamp that has undergone hydrological changes as the result of human
engineering works, but given the expansion is also occurring in the other swamps,
direct human interference alone cannot be established as the cause. While the
observed changes are similar to those expected to be seen with sea-level rise, this
cannot be confirmed as the primary driver of change given the fragmentary aerial
photographic record. The mangrove expansion is consistent with densification trends
in other ecologically distinct ecosystems in northern Australia, including savannas and
rainforests, suggesting regional-scale factors are driving woody expansion. Plausible
candidates for this change include changed local hydrology, changes in sea level and
elevated atmospheric CO2 concentrations.
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ACKNOWLEDGEMENTS
Funding for this project was provided by the Australian Research Council (ARC)
Linkage Grant (No. LP0667619), Northern Territory Department of Health and
Community Services, Australian Bureau of Meteorology, Northern Territory
Research and Innovation Fund, Australian Department of Defence and Charles
Darwin University. The authors would like to thank the Medical Entomology Branch
of the Northern Territory Health Department, especially Peter Whelan for expertise
and assistance, the Northern Territory Department of Planning and Infrastructure for
assistance with spatial data and equipment, and Lubomir Bisevac and Dimity Boggs
for assistance in the field.
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Table 1. Mean and standard deviation of environmental variables for three landscape zones in the three swamps. Elevation is based upon Australian Height Datum, creek distance is the distance from nearest tidal creek, shore distance is the nearest distance to the low tide point.
Table 2. Change in the percentage of all vegetation/landscape units for the three swamps broken down by coastal, intertidal and upper-tidal zones between 1974 and 2004. Salt flat includes all vegetation-free areas inundated by the highest tides. Typha, Eleocharis, and Schoenoplectus are graminoid dominated vegetation types that fall along a gradient of increasing salinity, while the grass/sedge category comprises the remaining treeless vegetation. Woodland comprises all non-mangrove woody vegetation, including savanna and coastal thicket.
Table 3. Vegetation transition matrix, showing proportion of points in each category converting to other categories between 1974 and 2004. Vegetation/landscape units are as in Table 2.
2004 Grass/Sedge Mangrove Salt Flat Schoenoplectus Woodland Eleocharis Water Urban Typha
Figure 1. Map of 2004 vegetation communities in the study. The coastal, inter-tidal and upper-tidal zones used in the analysis are shown. The zones were defined in respect of tidal regimes and vegetation types (Table 2).
Figure 2. Proportion of sample points with mangroves over time for the coastal, intertidal and upper-tidal zones across the three swamps for all intervals between 1974 and 2004.
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Figure 3. Mean proportion of points showing mangrove increase (a) and decrease (b) between 1974 and 2004. Error bars indicate standard deviation reported by multinomial logistic regression.
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Figure 4. Preceding 5-year mean sea level and mean annual rainfall for study intervals in Darwin.
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Figure 5. Change in extent of mangrove vegetation in the upper-tidal areas of the three swamps between 1974 and 2004.