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

The model incorporates size-dependent intraspecific competition (Thomas and Weiner 1989) using a
function adapted from (Aikman and Watkinson 1980). This mechanism incorporates the concepts of
the zone of influence (Mitchell 1969) and ecological field theory (Walker and Dowling 1991).
The surface roots of trees in the uplands frequently extend well beyond the canopy perimeters (pers.
obs.). Upland adult trees are widely spaced with respect to their canopies, though seedling and juvenile
densities can be quite high. It was assumed that plants compete exclusively for space well outside their
canopies during dry periods, but can tolerate higher densities during wetter periods, and at moist sites
(eg alongside bore drains and at coastal sites).
The lifecycles include mechanisms for germination, seed decay, plant desiccation, growth, maturation,
density-dependent effects on growth and mortality, reproduction (including selective abortion),
herbivory, interspecific competition with pasture species, and differential seed dispersal due to stocking
patterns.
The model is driven by weekly averages of daily maximum and minimum temperature, and weekly
totals of rainfall and evaporation (Queensland Centre for Climate Applications, 1998). It incorporates
the effects of climate change through mechanisms affecting the growth rates of plants, and their wateruse
efficiency. The atmospheric CO 2 concentration, climate sensitivity (increase in global temperature
at 2 x CO 2 ), and rainfall can each be adjusted to simulate future climate scenarios. The mechanisms
accounting for effects of alterations in CO 2 were derived from a literature review of the effects of CO 2
alone, and in concert with temperature, nutrient and moisture availability on plant factors
(photosynthesis, growth rates, water use efficiency, and competitiveness).
Where possible, driving functions and parameters were derived from published empirical relations and
manipulative experiments (eg seedling drought tolerance), inferred from the plant's geographical
distribution, or derived from analyses of field observations. Remaining parameters were derived
through ad hoc iterative parameter-fitting techniques (Starfield and Bleloch 1991). The model was
verified with data from the field ecology research program (Radford et al. 1999).
In one experiment, seedlings were grown in pots under controlled environmental conditions designed to
mimic field conditions as closely as possible. After two weeks growth, the seedlings were subjected to a
soil dry-down regime that eventually resulted in death. Plant size, soil moisture level and time to death
were noted.
2.3.3 The effects of climate change on prickly acacia population dynamics
To assess the effect of climate change on the paddock populations of prickly acacia, the population
dynamics model was run with a series of standard scenarios using meteorological data for a range of
selected sites that represent a broad range of suitability under current climate. This method
standardised factors that would be affected by management decisions. The decision to utilise this
approach was taken after consideration of such factors as the longevity of prickly acacia, the limited
availability of historical meteorological data, and the time taken to run the model for moderate lengths
of time (40 years). The long-lived nature of prickly acacia (35 years) means that the model is sensitive
to initialisation conditions ie the model state at the end of a 40 year run can be highly dependent upon
the number and size of plants at the beginning of the run. The model is also highly sensitive to the
livestock species (cattle or sheep). When comparing effects between sites, the model was run with
100% cattle. The paddock configuration used for all runs was 1000 ha of uplands and 1 ha of high
quality habitat surrounding watering points. The rationale for this was that there is a bore-capping
program in place in Queensland at present, and bore drains are only present in Queensland. Thus, the
most relevant scenario to use is one in which there is a limited number of watering points in the paddock
that provide moist habitat for prickly acacia. All model runs were initialised with the same number of
adult plants. The chosen initial adult plant densities simulate a moderately invaded paddock in the
10