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The conceptual framework of direct gradient analysis (DGA) is discussed in relation to the functional, factorial approach to vegetation. Both approaches use abstract simplified environment gradients with which to correlate vegetation response. Environmental scalars based on physical process models of environment and/or known biological growth processes can be incorporated to make analyses less location specific. An example of an environmental scalar (radiation index) for converting aspect and slope measurements to the more biologically relevant radiation input at a site is given.The problem of the shape of species response curves to environmental gradients is examined using a sample of I 286 plots from eucalypt forest in southern New South Wales. An important conclusion is that skewed or bimodal response curves may be due to unsatisfactory distribution of observations and/or unrecognized environmental factors. The use of Generalized Linear Modelling (GLM) as a method for providing a statistical basis for DGA is presented. Analyses using GLM, and presence/absence data are presented for a range of eucalypt species (Eucalyptus rossii, E. dalrympleana, E. fastigata etc.). Successful prediction of species distributions (realized niches) can be achieved with mean annual temperature, mean annual rainfall, radiation index and geology. Quadratic terms are required in many cases, indicating bell-shaped response curves. The major variability associated with species niches is shown to be related to a limited number (4) of environmental factors. DGA with biologically relevant scalars and appropriate statistical methods is suitable for studying many problems of species' realized niches and plant community composition. Preface

Increasing concentrations of atmospheric CO2 will alter regional rainfall and evapotranspiration regimes that drive groundwater recharge. Improved methods of simulation and analysis are needed for assessing the potential sensitivities of soil–water–vegetation systems to climate change. This study demonstrates methods for generating climates and simulating soil–water and vegetation dynamics in response to current and double CO2 climate sequences. Climate change scenarios came from dynamic equilibrium (constant CO2) runs of a general circulation model (GCM). Based on historical climate and GCM output, a stochastic point weather generator produced realizations of the cross‐correlated daily climate variables. A numerical model of infiltration, variably saturated flow, and evapotranspiration produced temporal distributions of groundwater recharge rates for various soil–vegetation environments. Climate change scenarios were simulated for two climatic zones in Australia: subtropical (North Stradbroke Island, Queensland) and Mediterranean (Gnangara, Swan Coastal Plain, Western Australia) having summer‐ and winter‐dominated rainfall regimes, respectively. In these simulations, groundwater recharge values were affected by the dynamic growth and senescence of vegetation, as changes in temperature and rainfall regimes affected growth rates and leaf areas. The temperature regime dominated the hydrologic response in the Mediterranean climate, and the rainfall frequency–duration regime dominated in the subtropical climate. For the simulated Mediterranean climate change (14% rainfall increase), changes in mean recharge values ranged from −34% to +119%, while subtropical climate change (37% rainfall increase) caused increases from 74 to > 500%. Changes in mean recharge rate, interannual variability, and temporal persistence were related to the soil and vegetation characteristics. The model was useful for quantifying complex, nonlinear responses to climate change that require further exploration.

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