Population Differences    Several previously described species, southern bluegum (E.  bicostata Maiden et al.), Maiden's gum (E.  maidenii F. Muell.), and E. pseudoglobulus Naudin ex  Maiden, have been reduced to subspecies of bluegum eucalyptus  (E. globulus ssp. globulus) (12). Steep clines are found  in many fruit and vegetative characteristics across the  subspecies boundaries, and more gradual changes appear within the  ranges of the four subspecies in Australia. The ssp. pseudoglobulus  is central, grading on different borders into each of the  other three subspecies. The most frost-hardy seedlings originate  from populations above 450 m (1,475 ft) elevation in the ranges  of ssp. bicostata and ssp. maidenii, but these  tend to be the oldest growing (13). Tasmanian bluegum eucalyptus,  ssp. globulus, originating near sea level in the southern  part of the species range, is the most rapidly growing. Within  taxa, drought tolerance of seedlings is associated with  populations native to the driest sites. Variation in glaucous  bloom of the leaves is correlated with elevation and the "bluer"  forms are more frost hardy and more drought tolerant than the "greener"  forms. Variations are known, such as California bluegurn  eucalyptus var. compacta (Hort.), a cultivar propagated in the  nursery trade for its compact habit and widely used along  California highways (2,20).

    Hybrids    Natural or controlled hybrids of bluegum eucalyptus with E.  blakelyi, E. botryoides, E. cinerea, E. cypellocarpa, E. ovata,  E. rudis, E. tereticornis, E. urnigera, and E. viminalis  are known (8,14,18).