Giant water gum

By Graham Watson

Giant Water Gum (Syzygium francisii) is a member of the Australian rainforest tree aristocracy. It grows to huge proportions and exhibits a number of unique characteristics. First described back in 1911, it was named Eugenia francisii to honour the 22-year-old botanist W.D. (”Bill”) Francis, who would later become Australia’s first great rainforest botanist. Three years after Francis’ death in 1959, the tree was moved to its new home in the Syzygium genus.

The name “Water Gum” was coined by the early tree loggers who noticed that quantities of a watery sap were sometimes contained in a central cavity in the trunk and which poured out when the trees were felled. The “Giant” part of the name distinguishes this species from other smaller rainforest trees described as Water Gums.

This species is reasonably widespread, but not so much in the rainforests of our area. Magnificent specimens can be encountered among the rainforests of the Cambridge Plateau, including Richmond Range National Park, especially Bundoozle Flora Reserve and Toonumbar National Park. Nevertheless, some individuals can be found in Nightcap National Park but, importantly, also in Wanganui Gorge and Huonbrook. The rainforest remnants in the headwater areas of Coopers Creek that support this species are an important ecological asset to Byron Shire and it is gratifying to see a number of landowners working with Landcare and other sponsors to rid these remnants of invasive exotic plants.

A particularly impressive Giant Water Gum was recently encountered in Huonbrook and this individual has so far reached a height of over 30 metres. The buttressing on this specimen is prominent but not yet fully developed, indicating that it still has a lot of living and growing ahead of it (see attached pictures).

When in the rainforest, you can’t confuse this species with any other tree. The bark is a distinctive fawn colour and the trunk is decorated with numerous craterous depressions left by bark shedding. Sometimes in older trees, the trunk develops very large swollen protuberances up to a metre across.

Unlike almost all of its congeners, the leaves of Giant Water Gum do not have a spattering of translucent oil dots and are further distinctive by the close lateral venation, prominent intramarginal vein and wavy leaf margins.

The fruiting frequency is reasonably regular during late summer to autumn but the tree irregularly produces a massive fruiting. By the end of winter, after one of these major fruiting events, the ground underneath the tree can be completely green with the first leaves of hundreds of thousands of germinating seedlings. This “lawn” of green can easily occupy over 100 square metres. Unfortunately, in that situation almost all the seedlings ultimately just fade away. Only those seedlings that are at some distance from the parent tree seem to succeed.

In my opinion, anyone embarking on establishing a planted rainforest plot should definitely consider including at least one of this species.