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In Australia, New Guinea and the southwestern Pacific seven taxa are recognised in Sonneratia of which three species are redescribed in view of their Indo-Malesian counterparts (S. alba, S. caseolaris, S. ovata), one species is redescribed from its most likely Ind o-Malesian equivalent (S. lanceolata, being distinct from S. caseolaris), one widespread hybrid is fully described (S. x gulngai being the putative synonym of S. alba x S. caseolaris), and two other hybrids (S. alba x S. gulngai, S. alba x S. lanceolata ) of very limited occurrence are described as sub-units of their closest ‘parental’ forms. A key, descriptions and full synonymy are given as well as 2 distribution maps, 6 other figures and one table.

Backer & Van Steenis (1951) compiled a thorough review of the Sonneratiaceae, a family of the order Myrtales. Two genera were described and include Duabanga, a small evergreen rainforest genus, and Sonneratia. The latter was made up of five mangrove tree species: S. alba, S. caseolaris, S. ovata, S. apetala , and S. griffithii. Their distributions were described as tropical and ranging through the Indo-West Pacific region. However, the account was not complete and new observations were reported in various appendices up to 1972. Some observations included those of additional characters, e.g., leaf mucronate tips or stamen colour; but most importantly there was the possibility of undescribed taxa. These included putative hybrids discovered in NW. Borneo (Muller & Hou-Liu, 1966). In Australia, only one taxon (S. alba) was initially reported (Backer & Van Steenis, 1951; Jones, 1971), although there was some evidence that suggested the presence of others, including hybrids (Van Steenis, 1968; Muller & Van Steenis, 1968). Additional taxa were later discovered in extensive field surveys across northern Australia (Wells, 1982; Bunt et al., 1982). However, problems were encountered in classifying them using keys of Backer & Van Steenis (1951). These problems (Bunt et al., 1982; Duke et al., 1984) presence or absence of petals used to disting S.alba and S. ovata; 2) the occurrence of two forms o S.caseolaris; and 3) the possibility of a hybrid between one of the S.c a seolaris forms and S. alba (or S. ovata?). Therefore classification of Sonneratia species in the study region (described as Australia, New Guinea and the southwestern Pacific) was incomplete, and no taxa were adequately defined.
Van Steenis (1968) indicated that he expected a greater variety of forms of Sonneratia in Australia and New Guinea, including some of the putative hybrids observed in NW. Borneo (Muller & Van Steenis, 1968). Since then three other major taxa have been discovered.
Sonneratia lanceolata and S. caseolaris have never been observed to co-inhabit the same estuary, although their geographical ranges overlap. Sonneratia caseolaris is only found on the northeast coast of Australia, but S. 1anceolata also occurs in the Northern Territory (although it is limited to three separate areas).
In New Guinea the distribution of these two taxa is also disjunct, with S. lanceolata occupying most larger estuaries on the mainland south coast, and S. caseolaris in similar habitats on the north coast.

Features which characterise populations of the hybrid, S. x gulngai (Duke, 1984) include morphological attributes which are either intermediate or alternately shared between S. alba and S, caseolaris. These characters include: high levels of pollen infertility and poor fruit set; a floral development cycle which is complex; generally luxuriant foliage and greater tree size. This latter factor allows this common hybrid form to be easily recognised in the field. The regional distribution of the hybrid and the least common parent, S. caseolaris, are equivalent in Australia. Both taxa are found on the east coast from the southern limit, Murray River (18º05'S, 146º0l'E), north to the Olive River (12ºl0'S, 143º05'E). It is anticipated that in New Guinea additional populations will be located where the parental ranges also overlap.

BLUMEA 32 (1987) 277-302

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