Metrosidorus excelsa, also known as the New Zealand Christmas tree and in Maori, Pohutukawa, is an interesting tree to style as Bonsai.
I have recently had the opportunity to photograph a few very old Pohutukawa trees in Mt Maunganui in New Zealand. I have no idea how old they are, but their form is quite distinctive when you study the trees in nature. From this I came to the conclusion that they are best suited for informal upright, clump style or root over rock style. They naturally grow aerial roots and form good bark on exposed roots. The red coloured flowers add to the spectacle. The three Bonsai photos are mainly from the http://www.nzbonsai.co.nz website and the http://www.bonsaiforbeginners.com site. The photos following from there are the photos of the trees growing in Mt Maunganui, New Zealand. The first group is typical of the clump style growth that a lot of these trees show.
The next group of trees shows why I think the Pohutukawa is excellent material for root over rock style.
Some of these trees are also seen in nature as examples of an informal upright style.
I have also noticed some branches hanging very low, almost to the point of being a cascade or a semi-cascade.
I have been growing cuttings of another form of Meterosidorus, namely the Metrosideros kermadecensis. This tree has smaller leaves than the excelsa which is great for Bonsai. All these varieties are frost sensitive and needs protection in cold climates.