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Forest expansion and grassland contraction within a Eucalyptus savanna matrix between 1941 and 1994 at Litchfield National Park in the Australian monsoon tropics


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Bowman, DMJS, Walsh, A and Milne, DJ 2001 , 'Forest expansion and grassland contraction within a Eucalyptus savanna matrix between 1941 and 1994 at Litchfield National Park in the Australian monsoon tropics' , Global Ecology & Biogeography, vol. 10, no. 5 , pp. 535-548 .

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Analysis of digitized aerial photographs taken
in 1941 and 1994, using image processing and geographical
information system technology, enabled
the quantification of change in the coverage of
forest and grassland patches that occur within
savanna matrix in a subcoastal
region of the Australian monsoon tropics. The
3058 ha study area was orientated along a low
escarpment that separated a sandstone plateau
from lowlands that comprised 58% and 42% of
the area, respectively.
In the 53-year period, humans modified less
than 1% of the study area, primarily for road
building, and primarily in savanna areas. More
than 85% of the study area at both sample times
was covered by savanna. However, over the same
period, the forest coverage increased from 5.03%
to 9.91% of the study area and coverage of grassland
decreased from 6.70% to 2.47%. The aerial
photography also showed that tree density in the
savanna had increased, although this was not
assessed quantitatively.
There was an increase in the number of forest
patches from 116 to 142. The number of grassland
patches decreased (particularly those > 1 ha) from
87 to 59, although the size class distribution of
forest and grassland patches was statistically similar
for both sample times.
A 50-m GIS buffer was used to distinguish
creek-lines environments from surrounding catchments.
Using this criterion, 14% of the study area
was classified as plateau creek-lines and 9% lowland
creek-lines. Although the expansion of forest
and loss of grassland varied significantly amongst
catchment and creek-lines on the plateau and
lowlands, the 1941 rank order of coverage of each
vegetation type was maintained in these four
landscape categories in 1994. In both years the
greatest extent of forest and grassland occurred on
the lowland catchments, despite their accounting
for only one-third of the study area.
Transition matrices for vegetation change
among the four landscape categories demonstrated
that, unlike the other vegetation types,
grasslands, particularly on the plateau, had a low
probability of remaining unchanged during the
study period.
The cause(s) of the overall increase of woody
biomass across the topographic and edaphic
gradient remains unclear but may be related to a
period of increased rainfall since the 1970s, as
well as to the cessation of Aboriginal landscape
burning at the beginning of the study period.

Item Type: Article
Authors/Creators:Bowman, DMJS and Walsh, A and Milne, DJ
Keywords: Aerial photography, Eucalyptus savanna, GIS, grassland, landscape change, monsoon tropics, northern Australia, tropical forest, vegetation dynamics.
Journal or Publication Title: Global Ecology & Biogeography
ISSN: 0960-7447
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