To many people, the landscapes of north
Australia can seem unchanging, a constant backdrop against the
tumult of human affairs. New studies show however, that this is not
the case with many landscapes of the tropical savannas gradually
being transformed.
Vegetation that is thickening is one of the
major trends discovered by a suite of landscape change studies
associated with the Tropical Savannas CRC: a historical photo
comparison; a systematic aerial photo survey and an intensive
on-ground study. Together they reveal a dynamic landscape in
continual change. These remarkable findings may have wide
implications for the north ranging from how we manage fire to how
we measure carbon credits. By Peter Jacklyn
Top right, the ‘Leichhardt’ at the Depot Landing,
Victoria River, c 1914, and bottom, the Depot Landing in 1997.
Parkinsonia has replaced the riverside vegetation and vegetation of
the plains and hills appear to have increased in density. Photos:
Johns Collection (top), Darrell Lewis (bottom)
Click here
for a larger view of the images.
Introduction | Interpreting photos | Why is
vegetation changing | Figure
1 | Figure 2
| Figure 3
| | Figure 4 | |
More information |
The photos shown in this article are from a survey assembled by
historian Darrell Lewis. He relates how in 1980 he was doing
historical research on the VRD using photos from archives when
“in amongst them I saw old images of present day locations
which made me think ‘hang on, something’s changed
here’”.
“So I visited a few of the exact spots where the original
photographs were taken from and re-took the shot,” said
Darrell. These visits confirmed his suspicions that the
“something” that had changed was the tree cover, which
seemed to have increased in density.
Darrell and other researchers like Dr Jeremy Russell-Smith of
the Bush Fires Council of the NT realised that if widespread, such
a trend would have implications for land management. The TS-CRC
funded Darrell to conduct a more thorough comparison. He now has
collected more than 100 of these photo pairs taken at various
places across the Victoria River District.
According to Darrell “a lot of the photos show the
landscape has ‘densed-up’ a bit—there’s a
tendency for more trees or for trees having larger canopies in the
later shots.” (See Figure
1 and Figure 2
). “In only one or two comparisons has the trend gone the
other way, for example in Jasper Gorge” ( Figure 4 ).
The general trend seems to a thickening of trees in the flatter
country along rivers, with less obvious change in the rocky
uplands.
As Darrell points out, caution is needed when interpreting these
photos. For a start, the older “before” photos tend to
have been taken where the people were, on the flatter country and
there are few shots from the rest of the landscape. This is where a
more intensive on-ground study by Oxford University PhD student Ben
Sharp comes in. Ben surveyed hundreds of sites across all land
types in Bradshaw station in the northern VRD. He too recorded
thickening on the alluvial flats but they made up a relatively
minor portion of his study area. By contrast, on Bradshaw’s
sandstone plateaus 58 per cent of his sites had experienced
vegetation loss in the last 50 years compared to only 12 per cent
that had thickened up. Ben also found more complex changes in many
sites involving invasions by different plant species.
On a larger scale, clearer patterns were seen in a TS-CRC funded
systematic survey of aerial photographs which sampled the entire
VRD, and which dated back to the 1940s. Dr Rod Fensham and Russell
Fairfax of the Queensland Herbarium undertook this project over the
last two years. Preliminary results found increases in tree cover
across a broad range of landscape types although some sites showed
no change or decreases in cover.
The aerial photo survey showed that significant vegetation
change seems to have occurred in the last few decades—and
this is also supported by Darrell Lewis. A few years ago Timber
Creek in the VRD hosted a 50th anniversary celebration for soldiers
who had been stationed there during World War II. “When they
looked at the bush of the 1990s the visiting veterans were
shocked,” relates Darrell Lewis. “Then they looked at
some 1914 photos of the region and said ‘that’s what it
looked like’ (in the 1940s) very open country.”
Such stories imply much of the changes occurred after World War
II, as seen in Figure
1 and Figure 2
. The big question is of course: why is the vegetation changing? Dr
Fensham thinks that while altered fire and grazing patterns have
played a role, it is changes in rainfall that may be particularly
important. “Changes in the way country was burned and the way
it was grazed (due to the shift from Aboriginal to European
management) occurred fairly early on, but the biggest changes in
vegetation seem to have happened in the last 30 years,” said
Rod.
"This correlates with climate changes—there were major
droughts in the 30s and 60s and high rainfall in the 70s.”
What we may be seeing is partly a decades-long recovery cycle from
severe drought. (See article Nature's
Bulldozer: Tree Dieback in the Savannas .)
Other factors may also be involved, for example carbon dioxide
concentration in the air has increased by more than 20 per cent
since 1940 which may be encouraging plant growth. At the smaller
scale of Bradshaw station, the picture seems more complex and Ben
Sharp records clear correlations between grazing and fire history
and various types of vegetation change. While the exact nature and
extent of this landscape change is still unclear, what is plain is
that the landscape can no longer be regarded as a static backdrop.
Quite apart from the more obvious human impacts the passing decades
may see slowly changing fuel loads for fires; varying carbon
storage levels and altered impacts for animals in the tropical
savannas.
