EXOTIC WOODY WEEDS Use of simulation models to predict future ...

3.3 Prickly acacia population dynamics and management using the
DYMEX model
A vast number of model simulations have been run and only representative results are presented here to
illustrate the nature of the simulations and results. Most model runs were carried out using weather
data sequences and paddock conditions representative of Marathon Station, near Hughenden (143º 34'E
20º 54' S). The modelled patterns of phenology, germination, individual development, survival,
competition, and self-thinning are both qualitatively and quantitatively consistent with field studies.
Model runs have been limited to 40 year periods (approximately the lifespan of prickly acacia) due to
available weather data and computation power. Over such short runs, the model is sensitive to starting
conditions. This is consistent with field observations of similar aged invasions that show marked
differences in infestation density and population structure that can not be attributed to biophysical or
land use differences, suggesting invasion history as the proximal cause.
3.3.1 Impacts of prickly acacia invasion
In order to estimate the effects of invasion by prickly acacia on pasture productivity, the relationship
between the density of prickly acacia and pasture production (Mooy et al. 1992) was used in a
simulation of the effect of planting trees along a bore drain on a cattle property. The results (Figure
3.20) show the dramatic decline in pasture production after 40 years (only the last 30 years of the
simulation are shown).
Basal Area Density (m 2 /ha)
5
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
2000
1800
1600
1400
1200
1000
800
600
400
200
0
Pasture Production Density (kg/ha)
Upland Basal
Area Density
(m^2 ha^-1)
1957
1959
1961
1963
1965
1967
1969
1972
1974
1976
1978
1980
1982
1985
1987
1989
1991
Upland Pasture
Production
Density (kg ha^-
1)
Figure 3.20 Simulated impact of increasing prickly acacia basal area density on pasture production at
Hughenden, north Queensland. Pasture production estimates were derived using a regression equation
from Mooy et al. (1992).
3.3.2 Effects of climate change on prickly acacia population dynamics
In Australia, the projected effect of increases in temperature and water use efficiency is to intensify the
population dynamics processes of prickly acacia. The consequences of this are to increase the rate of
invasion of uplands, and to increase the amount of prickly acacia biomass that can be sustained,
particularly in uplands. It is important to note that due to self-thinning processes (intra-specific
competition) and the law of constant yield, stem density can vary widely for a given biomass density.
Thus stem density is not a good indicator of the state of a paddock with respect to weediness. Biomass
density of woody plants is of interest to climate change research because it is related to the amount of
carbon sequestered in the tree and shrub material. It is also closely related to basal area density, which
in turn is related to pasture production. Unfortunately, neither of these measures is easily perceived
through casual inspection. Canopy cover of prickly acacia ie proportion of the paddock covered by the
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