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Dissatisfaction persists with current approaches to range condition and trend assessment. Sometimes assessed condition does not truly represent the past or the potential of range. One of the likely causes is a failure to reexamine and change if necessary the theoretical basis of assessment, in line with developing understanding of ecological processes. The concept of thresholds of environmental change appears to provide a reasonable alternative in some circumstances to the concepts of gradual retrogression and secondary succession which are currently accepted. I suggest that environmental change can be discontinuous, with thresholds between alternative states. Once a threshold is crossed to a more degraded state, the former state cannot be attained without significant management effort, such as prescribed burning, ploughing, or herbicide application, rather than simple grazing control. Examination of data from extensive monitoring programs and from a study of grazing impact, as well as more general sources of information, indicates that thresholds of change may be identifiable in arid rangelands. A practical means of monitoring proximity to thresholds is available and, with the aid of multivariate analysis, the effects of spatial variability and season can be separated from those of management. The potential of this approach deserves investigation in a wider variety of environments.

Buffel grass (Cenchrus ciliaris L.) has invaded extensive areas of arid and semi-arid Australia following its introduction as a pasture species and for erosion control. It has been suggested that buffel grass has initiated a positive fire-invasion feedback in central Australia, disrupting existing fire regimes, encouraging further buffel grass invasion, and disadvantaging the native woody flora in particular, but this hypothesis has not been tested quantitatively. This study investigated recently burnt woodland areas near Alice Springs for evidence of a fire-invasion feedback, including the impact of changing fire behaviour (intensity) on the native woodland overstorey flora. Despite the limitations inherent in a short study of ecological processes in a highly heterogeneous environment, substantial field evidence was found to support the existence of a buffel grass-initiated fire-invasion feedback. Buffel grass invasion was significantly correlated with increased fuel loads. Increased fuel loads were significantly correlated with increased burn severity, although the direct relationship between the proportion of buffel grass and increased burn severity was marginally non-significant. High field variance resulted in inadequate power to test whether or not the relative abundance of buffel grass had increased in the post-fire community. Burn severity was significantly correlated with the mortality of woodland overstorey species, and with the proportion of fire survivors that were reduced to basal resprouts. Seedling density of canopy species was low. It appears likely that future recruitment of canopy species will be hindered by the dense post-fire reestablishment of buffel grass cover at some sites. The overstorey flora is thus likely to be adversely affected by increased severity of fire associated with buffel grass invasion. As a result, there may be major change in the structure and composition of some woodlands.

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I vllL@l!y UL L U~I~G~V~L~U I IZommission of the Northern Territory, Alice Springs, NT 0871. AbstractWe developed two sets of regression models for flowering and fruiting of arid zone trees and shrubs, based on (i) rainfall in the current and preceding seasons and (ii) soil moisture lagged over varying time periods combined with mean maximum temperature and daylength in the month prior to phenological observations. Using up to 4 years of flowering and fruiting records, we found that both approaches identified responses matching those reported in two other long-term data sets or in the literature, for some species but not for all. The second approach appeared to provide better correlations than the first but we were unable to predict flowering and fruiting effectively.Flowering and fruiting of central Australian trees and shrubs were least in late summer, creating potential limitations on animal populations dependent on them for food. With better predictive capabilities, there is some scope for managing the trees and shrubs for particular purposes. However, very long-term phenological records are needed to improve predictions.

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