Summary1. Determining the relative importance of anthropogenic and non-anthropogenic in¯uences on structure is essential for informed management of savannas and for carbon accounting under greenhouse obligations. 2. The magnitude of drought and dieback was examined using the rainfall records for Queensland and historical records of dieback. Tree dieback was examined in Eucalyptus savanna in north Queensland by random sampling after a recent drought. 3. Analysis of rainfall records revealed that particularly severe droughts occurred three times this century in inland Queensland, while more local droughts of similar intensity had been less frequent elsewhere. A review of historical records con®rmed extensive tree death following past droughts. 4. Approximately 29% of trees were dead or nearly dead over a sampled area of about 55 000 square kilometres. Dieback was greatest on alkaline igneous rocks, intermediate on metamorphics, sedimentary rocks and acid igneous rocks, and lowest on alluvia. 5. Of the widespread dominants, the Eucalyptus crebra±E. xanthoclada species complex was highly susceptible to dieback, E. brownii and E. melanophloia±E. whitei moderately aected, and Corymbia clarksoniana and Melaleuca nervosa less severely aected. Preferential death of large over small size classes was signi®cant for only E. crebra±E. xanthoclada. 6. The 1990s drought was especially intense in the vicinity of the North Queensland study area. However, within the study area there were only weak correlations between dieback and rainfall de®cits as derived from modelled data. A classi®cation of seasonal rainfall revealed no evidence of localized aberrant rainfall events, such as might result from heavy localized storms, within the study area during the drought. Thus the substantial patchiness in dieback within the study area was only poorly related to local rainfall patterns. Signi®cant correlations of the dieback of some taxonomic groups with predrought basal area suggested that the competitive in¯uence of trees may be a partial cause of the patchiness of dieback. 7. The magnitude of drought should be included in functional models predicting tree±grass ratios and must be accounted for if the magnitude and cause of structural trends in Eucalyptus and other evergreen savanna vegetation are to be deciphered.

Abstract. Floristic data from paired roadside‐paddock analyses from grassland in central Queensland, Australia, were ordinated. The mean direction of the vectors between these pairs was almost perfectly aligned with the indirect gradient represented by the first axis of Non‐metric Multi‐dimensional Scaling. It confirms anecdotal evidence of a trend from infrequently grazed roadsides to constantly grazed paddocks. The increasing abundance of annual herbs and grasses along this putative gradient is consistent with documented trends from elsewhere in the world. The response patterns of individual species along the disturbance gradient is consistent with ecological theory predicting unimodal peaks in abundance along physical environmental gradients. The ancestral perennial dominants of the grasslands, Dichanthium sericeum and D. queenslandicum, exhibited a declining response to grazing disturbance. Even the generally unpalatable perennial grass Aristida leptopoda declined considerably in the upper segments of the grazing disturbance gradient. A suite of herbaceous trailing legumes had peaks in their abundance near the middle of the grazing disturbance gradient, trends that can be readily explained by the combination of their palatability and intolerance to competition from tall perennial grasses. Several species including the noxious exotic herb Parthenium hysterophorus showed increasing abundance along the grazing disturbance gradient. The methodology may have application as a rapid method of assessing disturbance impacts elsewhere, and is most suited where a management differential between paired plots can be reliably generalized and where the physical environment is relatively monotonous.

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The putative hybrid zone between Eucalyptus populnea and E. brownii is examined using morphological and molecular techniques. This species complex displays continuous morphological variation across the study area, which has been previously interpreted as the product of hybridization between allopatric species. A microsatellite analysis indicates that there was little genetic structuring across the morphological cline and only low levels of population differentiation. The nested clade analysis of the JLA+ region of the chloroplast DNA (cpDNA) indicates that the geographical distribution of cpDNA haplotypes is unlikely to be the result of historical hybridization events, and that restricted seed-mediated gene flow with isolation by distance is responsible for the phylogeographical distribution. A more plausible explanation for the origin and persistence of the morphological cline is that the process of continuous morphological diversification has been promoted by a directional selection gradient. This study addresses species status within Eucalyptus and the belief that hybridization is widespread and is an important process in the group's evolution.